split internal.h into files

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.

53 files changed, 3390 insertions, 0 deletions

diff --git a/internal/array.h b/internal/array.h
new file mode 100644
index 0000000000..77f47dc5a2
--- /dev/null
+++ b/internal/array.h
@@ -0,0 +1,88 @@
+#ifndef INTERNAL_ARRAY_H /* -*- C -*- */
+#define INTERNAL_ARRAY_H
+/**
+ * @file
+ * @brief      Internal header for Array.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* array.c */
+
+#ifndef ARRAY_DEBUG
+#define ARRAY_DEBUG (0+RUBY_DEBUG)
+#endif
+
+#ifdef ARRAY_DEBUG
+#define RARRAY_PTR_IN_USE_FLAG FL_USER14
+#define ARY_PTR_USING_P(ary) FL_TEST_RAW((ary), RARRAY_PTR_IN_USE_FLAG)
+#else
+
+/* disable debug function */
+#undef  RARRAY_PTR_USE_START_TRANSIENT
+#undef  RARRAY_PTR_USE_END_TRANSIENT
+#define RARRAY_PTR_USE_START_TRANSIENT(a) ((VALUE *)RARRAY_CONST_PTR_TRANSIENT(a))
+#define RARRAY_PTR_USE_END_TRANSIENT(a)
+#define ARY_PTR_USING_P(ary) 0
+
+#endif
+
+#if USE_TRANSIENT_HEAP
+#define RARY_TRANSIENT_SET(ary) FL_SET_RAW((ary), RARRAY_TRANSIENT_FLAG);
+#define RARY_TRANSIENT_UNSET(ary) FL_UNSET_RAW((ary), RARRAY_TRANSIENT_FLAG);
+#else
+#undef RARRAY_TRANSIENT_P
+#define RARRAY_TRANSIENT_P(a) 0
+#define RARY_TRANSIENT_SET(ary) ((void)0)
+#define RARY_TRANSIENT_UNSET(ary) ((void)0)
+#endif
+
+VALUE rb_ary_last(int, const VALUE *, VALUE);
+void rb_ary_set_len(VALUE, long);
+void rb_ary_delete_same(VALUE, VALUE);
+VALUE rb_ary_tmp_new_fill(long capa);
+VALUE rb_ary_at(VALUE, VALUE);
+VALUE rb_ary_aref1(VALUE ary, VALUE i);
+size_t rb_ary_memsize(VALUE);
+VALUE rb_to_array_type(VALUE obj);
+VALUE rb_check_to_array(VALUE ary);
+VALUE rb_ary_tmp_new_from_values(VALUE, long, const VALUE *);
+VALUE rb_ary_behead(VALUE, long);
+#if defined(__GNUC__) && defined(HAVE_VA_ARGS_MACRO)
+#define rb_ary_new_from_args(n, ...) \
+    __extension__ ({ \
+        const VALUE args_to_new_ary[] = {__VA_ARGS__}; \
+        if (__builtin_constant_p(n)) { \
+            STATIC_ASSERT(rb_ary_new_from_args, numberof(args_to_new_ary) == (n)); \
+        } \
+        rb_ary_new_from_values(numberof(args_to_new_ary), args_to_new_ary); \
+    })
+#endif
+
+static inline VALUE
+rb_ary_entry_internal(VALUE ary, long offset)
+{
+    long len = RARRAY_LEN(ary);
+    const VALUE *ptr = RARRAY_CONST_PTR_TRANSIENT(ary);
+    if (len == 0) return Qnil;
+    if (offset < 0) {
+        offset += len;
+        if (offset < 0) return Qnil;
+    }
+    else if (len <= offset) {
+        return Qnil;
+    }
+    return ptr[offset];
+}
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* array.c (export) */
+void rb_ary_detransient(VALUE a);
+VALUE *rb_ary_ptr_use_start(VALUE ary);
+void rb_ary_ptr_use_end(VALUE ary);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_ARRAY_H */
diff --git a/internal/bignum.h b/internal/bignum.h
new file mode 100644
index 0000000000..ceb048f9fa
--- /dev/null
+++ b/internal/bignum.h
@@ -0,0 +1,167 @@
+#ifndef INTERNAL_BIGNUM_H /* -*- C -*- */
+#define INTERNAL_BIGNUM_H
+/**
+ * @file
+ * @brief      Internal header for Bignums.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#ifndef BDIGIT
+# if SIZEOF_INT*2 <= SIZEOF_LONG_LONG
+#  define BDIGIT unsigned int
+#  define SIZEOF_BDIGIT SIZEOF_INT
+#  define BDIGIT_DBL unsigned LONG_LONG
+#  define BDIGIT_DBL_SIGNED LONG_LONG
+#  define PRI_BDIGIT_PREFIX ""
+#  define PRI_BDIGIT_DBL_PREFIX PRI_LL_PREFIX
+# elif SIZEOF_INT*2 <= SIZEOF_LONG
+#  define BDIGIT unsigned int
+#  define SIZEOF_BDIGIT SIZEOF_INT
+#  define BDIGIT_DBL unsigned long
+#  define BDIGIT_DBL_SIGNED long
+#  define PRI_BDIGIT_PREFIX ""
+#  define PRI_BDIGIT_DBL_PREFIX "l"
+# elif SIZEOF_SHORT*2 <= SIZEOF_LONG
+#  define BDIGIT unsigned short
+#  define SIZEOF_BDIGIT SIZEOF_SHORT
+#  define BDIGIT_DBL unsigned long
+#  define BDIGIT_DBL_SIGNED long
+#  define PRI_BDIGIT_PREFIX "h"
+#  define PRI_BDIGIT_DBL_PREFIX "l"
+# else
+#  define BDIGIT unsigned short
+#  define SIZEOF_BDIGIT (SIZEOF_LONG/2)
+#  define SIZEOF_ACTUAL_BDIGIT SIZEOF_LONG
+#  define BDIGIT_DBL unsigned long
+#  define BDIGIT_DBL_SIGNED long
+#  define PRI_BDIGIT_PREFIX "h"
+#  define PRI_BDIGIT_DBL_PREFIX "l"
+# endif
+#endif
+#ifndef SIZEOF_ACTUAL_BDIGIT
+# define SIZEOF_ACTUAL_BDIGIT SIZEOF_BDIGIT
+#endif
+
+#ifdef PRI_BDIGIT_PREFIX
+# define PRIdBDIGIT PRI_BDIGIT_PREFIX"d"
+# define PRIiBDIGIT PRI_BDIGIT_PREFIX"i"
+# define PRIoBDIGIT PRI_BDIGIT_PREFIX"o"
+# define PRIuBDIGIT PRI_BDIGIT_PREFIX"u"
+# define PRIxBDIGIT PRI_BDIGIT_PREFIX"x"
+# define PRIXBDIGIT PRI_BDIGIT_PREFIX"X"
+#endif
+
+#ifdef PRI_BDIGIT_DBL_PREFIX
+# define PRIdBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"d"
+# define PRIiBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"i"
+# define PRIoBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"o"
+# define PRIuBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"u"
+# define PRIxBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"x"
+# define PRIXBDIGIT_DBL PRI_BDIGIT_DBL_PREFIX"X"
+#endif
+
+#define BIGNUM_EMBED_LEN_NUMBITS 3
+#ifndef BIGNUM_EMBED_LEN_MAX
+# if (SIZEOF_VALUE*RVALUE_EMBED_LEN_MAX/SIZEOF_ACTUAL_BDIGIT) < (1 << BIGNUM_EMBED_LEN_NUMBITS)-1
+#  define BIGNUM_EMBED_LEN_MAX (SIZEOF_VALUE*RVALUE_EMBED_LEN_MAX/SIZEOF_ACTUAL_BDIGIT)
+# else
+#  define BIGNUM_EMBED_LEN_MAX ((1 << BIGNUM_EMBED_LEN_NUMBITS)-1)
+# endif
+#endif
+
+struct RBignum {
+    struct RBasic basic;
+    union {
+        struct {
+            size_t len;
+            BDIGIT *digits;
+        } heap;
+        BDIGIT ary[BIGNUM_EMBED_LEN_MAX];
+    } as;
+};
+#define BIGNUM_SIGN_BIT ((VALUE)FL_USER1)
+/* sign: positive:1, negative:0 */
+#define BIGNUM_SIGN(b) ((RBASIC(b)->flags & BIGNUM_SIGN_BIT) != 0)
+#define BIGNUM_SET_SIGN(b,sign) \
+  ((sign) ? (RBASIC(b)->flags |= BIGNUM_SIGN_BIT) \
+          : (RBASIC(b)->flags &= ~BIGNUM_SIGN_BIT))
+#define BIGNUM_POSITIVE_P(b) BIGNUM_SIGN(b)
+#define BIGNUM_NEGATIVE_P(b) (!BIGNUM_SIGN(b))
+#define BIGNUM_NEGATE(b) (RBASIC(b)->flags ^= BIGNUM_SIGN_BIT)
+
+#define BIGNUM_EMBED_FLAG ((VALUE)FL_USER2)
+#define BIGNUM_EMBED_LEN_MASK \
+    (~(~(VALUE)0U << BIGNUM_EMBED_LEN_NUMBITS) << BIGNUM_EMBED_LEN_SHIFT)
+#define BIGNUM_EMBED_LEN_SHIFT \
+    (FL_USHIFT+3) /* bit offset of BIGNUM_EMBED_LEN_MASK */
+#define BIGNUM_LEN(b) \
+    ((RBASIC(b)->flags & BIGNUM_EMBED_FLAG) ? \
+     (size_t)((RBASIC(b)->flags >> BIGNUM_EMBED_LEN_SHIFT) & \
+              (BIGNUM_EMBED_LEN_MASK >> BIGNUM_EMBED_LEN_SHIFT)) : \
+     RBIGNUM(b)->as.heap.len)
+/* LSB:BIGNUM_DIGITS(b)[0], MSB:BIGNUM_DIGITS(b)[BIGNUM_LEN(b)-1] */
+#define BIGNUM_DIGITS(b) \
+    ((RBASIC(b)->flags & BIGNUM_EMBED_FLAG) ? \
+     RBIGNUM(b)->as.ary : \
+     RBIGNUM(b)->as.heap.digits)
+#define BIGNUM_LENINT(b) rb_long2int(BIGNUM_LEN(b))
+
+#define RBIGNUM(obj) (R_CAST(RBignum)(obj))
+
+/* bignum.c */
+extern const char ruby_digitmap[];
+double rb_big_fdiv_double(VALUE x, VALUE y);
+VALUE rb_big_uminus(VALUE x);
+VALUE rb_big_hash(VALUE);
+VALUE rb_big_odd_p(VALUE);
+VALUE rb_big_even_p(VALUE);
+size_t rb_big_size(VALUE);
+VALUE rb_integer_float_cmp(VALUE x, VALUE y);
+VALUE rb_integer_float_eq(VALUE x, VALUE y);
+VALUE rb_str_convert_to_inum(VALUE str, int base, int badcheck, int raise_exception);
+VALUE rb_big_comp(VALUE x);
+VALUE rb_big_aref(VALUE x, VALUE y);
+VALUE rb_big_abs(VALUE x);
+VALUE rb_big_size_m(VALUE big);
+VALUE rb_big_bit_length(VALUE big);
+VALUE rb_big_remainder(VALUE x, VALUE y);
+VALUE rb_big_gt(VALUE x, VALUE y);
+VALUE rb_big_ge(VALUE x, VALUE y);
+VALUE rb_big_lt(VALUE x, VALUE y);
+VALUE rb_big_le(VALUE x, VALUE y);
+VALUE rb_int_powm(int const argc, VALUE * const argv, VALUE const num);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* bignum.c (export) */
+VALUE rb_big_mul_normal(VALUE x, VALUE y);
+VALUE rb_big_mul_balance(VALUE x, VALUE y);
+VALUE rb_big_mul_karatsuba(VALUE x, VALUE y);
+VALUE rb_big_mul_toom3(VALUE x, VALUE y);
+VALUE rb_big_sq_fast(VALUE x);
+VALUE rb_big_divrem_normal(VALUE x, VALUE y);
+VALUE rb_big2str_poweroftwo(VALUE x, int base);
+VALUE rb_big2str_generic(VALUE x, int base);
+VALUE rb_str2big_poweroftwo(VALUE arg, int base, int badcheck);
+VALUE rb_str2big_normal(VALUE arg, int base, int badcheck);
+VALUE rb_str2big_karatsuba(VALUE arg, int base, int badcheck);
+#if defined(HAVE_LIBGMP) && defined(HAVE_GMP_H)
+VALUE rb_big_mul_gmp(VALUE x, VALUE y);
+VALUE rb_big_divrem_gmp(VALUE x, VALUE y);
+VALUE rb_big2str_gmp(VALUE x, int base);
+VALUE rb_str2big_gmp(VALUE arg, int base, int badcheck);
+#endif
+enum rb_int_parse_flags {
+    RB_INT_PARSE_SIGN       = 0x01,
+    RB_INT_PARSE_UNDERSCORE = 0x02,
+    RB_INT_PARSE_PREFIX     = 0x04,
+    RB_INT_PARSE_ALL        = 0x07,
+    RB_INT_PARSE_DEFAULT    = 0x07
+};
+VALUE rb_int_parse_cstr(const char *str, ssize_t len, char **endp, size_t *ndigits, int base, int flags);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_BIGNUM_H */
diff --git a/internal/bits.h b/internal/bits.h
new file mode 100644
index 0000000000..1551d501fb
--- /dev/null
+++ b/internal/bits.h
@@ -0,0 +1,328 @@
+#ifndef INTERNAL_BITS_H /* -*- C -*- */
+#define INTERNAL_BITS_H
+/**
+ * @file
+ * @brief      Internal header for bitwise integer algorithms.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* The most significant bit of the lower part of half-long integer.
+ * If sizeof(long) == 4, this is 0x8000.
+ * If sizeof(long) == 8, this is 0x80000000.
+ */
+#define HALF_LONG_MSB ((SIGNED_VALUE)1<<((SIZEOF_LONG*CHAR_BIT-1)/2))
+
+#define SIGNED_INTEGER_TYPE_P(int_type) (0 > ((int_type)0)-1)
+#define SIGNED_INTEGER_MAX(sint_type) \
+  (sint_type) \
+  ((((sint_type)1) << (sizeof(sint_type) * CHAR_BIT - 2)) | \
+  ((((sint_type)1) << (sizeof(sint_type) * CHAR_BIT - 2)) - 1))
+#define SIGNED_INTEGER_MIN(sint_type) (-SIGNED_INTEGER_MAX(sint_type)-1)
+#define UNSIGNED_INTEGER_MAX(uint_type) (~(uint_type)0)
+#ifdef HAVE_BUILTIN___BUILTIN_MUL_OVERFLOW_P
+#define MUL_OVERFLOW_P(a, b) \
+    __builtin_mul_overflow_p((a), (b), (__typeof__(a * b))0)
+#elif defined HAVE_BUILTIN___BUILTIN_MUL_OVERFLOW
+#define MUL_OVERFLOW_P(a, b) \
+    RB_GNUC_EXTENSION_BLOCK(__typeof__(a) c; __builtin_mul_overflow((a), (b), &c))
+#endif
+
+#define MUL_OVERFLOW_SIGNED_INTEGER_P(a, b, min, max) ( \
+    (a) == 0 ? 0 : \
+    (a) == -1 ? (b) < -(max) : \
+    (a) > 0 ? \
+      ((b) > 0 ? (max) / (a) < (b) : (min) / (a) > (b)) : \
+      ((b) > 0 ? (min) / (a) < (b) : (max) / (a) > (b)))
+
+#ifdef HAVE_BUILTIN___BUILTIN_MUL_OVERFLOW_P
+/* __builtin_mul_overflow_p can take bitfield */
+/* and GCC permits bitfields for integers other than int */
+#define MUL_OVERFLOW_FIXNUM_P(a, b) RB_GNUC_EXTENSION_BLOCK( \
+    struct { long fixnum : SIZEOF_LONG * CHAR_BIT - 1; } c; \
+    __builtin_mul_overflow_p((a), (b), c.fixnum); \
+)
+#else
+#define MUL_OVERFLOW_FIXNUM_P(a, b) MUL_OVERFLOW_SIGNED_INTEGER_P(a, b, FIXNUM_MIN, FIXNUM_MAX)
+#endif
+
+#ifdef MUL_OVERFLOW_P
+#define MUL_OVERFLOW_LONG_LONG_P(a, b) MUL_OVERFLOW_P(a, b)
+#define MUL_OVERFLOW_LONG_P(a, b)      MUL_OVERFLOW_P(a, b)
+#define MUL_OVERFLOW_INT_P(a, b)       MUL_OVERFLOW_P(a, b)
+#else
+#define MUL_OVERFLOW_LONG_LONG_P(a, b) MUL_OVERFLOW_SIGNED_INTEGER_P(a, b, LLONG_MIN, LLONG_MAX)
+#define MUL_OVERFLOW_LONG_P(a, b)      MUL_OVERFLOW_SIGNED_INTEGER_P(a, b, LONG_MIN, LONG_MAX)
+#define MUL_OVERFLOW_INT_P(a, b)       MUL_OVERFLOW_SIGNED_INTEGER_P(a, b, INT_MIN, INT_MAX)
+#endif
+
+#ifndef swap16
+# ifdef HAVE_BUILTIN___BUILTIN_BSWAP16
+#  define swap16(x) __builtin_bswap16(x)
+# endif
+#endif
+
+#ifndef swap16
+# define swap16(x)      ((uint16_t)((((x)&0xFF)<<8) | (((x)>>8)&0xFF)))
+#endif
+
+#ifndef swap32
+# ifdef HAVE_BUILTIN___BUILTIN_BSWAP32
+#  define swap32(x) __builtin_bswap32(x)
+# endif
+#endif
+
+#ifndef swap32
+# define swap32(x)      ((uint32_t)((((x)&0xFF)<<24)    \
+                        |(((x)>>24)&0xFF)       \
+                        |(((x)&0x0000FF00)<<8)  \
+                        |(((x)&0x00FF0000)>>8)  ))
+#endif
+
+#ifndef swap64
+# ifdef HAVE_BUILTIN___BUILTIN_BSWAP64
+#  define swap64(x) __builtin_bswap64(x)
+# endif
+#endif
+
+#ifndef swap64
+# ifdef HAVE_INT64_T
+#  define byte_in_64bit(n) ((uint64_t)0xff << (n))
+#  define swap64(x)       ((uint64_t)((((x)&byte_in_64bit(0))<<56)      \
+                           |(((x)>>56)&0xFF)                    \
+                           |(((x)&byte_in_64bit(8))<<40)        \
+                           |(((x)&byte_in_64bit(48))>>40)       \
+                           |(((x)&byte_in_64bit(16))<<24)       \
+                           |(((x)&byte_in_64bit(40))>>24)       \
+                           |(((x)&byte_in_64bit(24))<<8)        \
+                           |(((x)&byte_in_64bit(32))>>8)))
+# endif
+#endif
+
+static inline unsigned int
+nlz_int(unsigned int x)
+{
+#if defined(HAVE_BUILTIN___BUILTIN_CLZ)
+    if (x == 0) return SIZEOF_INT * CHAR_BIT;
+    return (unsigned int)__builtin_clz(x);
+#else
+    unsigned int y;
+# if 64 < SIZEOF_INT * CHAR_BIT
+    unsigned int n = 128;
+# elif 32 < SIZEOF_INT * CHAR_BIT
+    unsigned int n = 64;
+# else
+    unsigned int n = 32;
+# endif
+# if 64 < SIZEOF_INT * CHAR_BIT
+    y = x >> 64; if (y) {n -= 64; x = y;}
+# endif
+# if 32 < SIZEOF_INT * CHAR_BIT
+    y = x >> 32; if (y) {n -= 32; x = y;}
+# endif
+    y = x >> 16; if (y) {n -= 16; x = y;}
+    y = x >>  8; if (y) {n -=  8; x = y;}
+    y = x >>  4; if (y) {n -=  4; x = y;}
+    y = x >>  2; if (y) {n -=  2; x = y;}
+    y = x >>  1; if (y) {return n - 2;}
+    return (unsigned int)(n - x);
+#endif
+}
+
+static inline unsigned int
+nlz_long(unsigned long x)
+{
+#if defined(HAVE_BUILTIN___BUILTIN_CLZL)
+    if (x == 0) return SIZEOF_LONG * CHAR_BIT;
+    return (unsigned int)__builtin_clzl(x);
+#else
+    unsigned long y;
+# if 64 < SIZEOF_LONG * CHAR_BIT
+    unsigned int n = 128;
+# elif 32 < SIZEOF_LONG * CHAR_BIT
+    unsigned int n = 64;
+# else
+    unsigned int n = 32;
+# endif
+# if 64 < SIZEOF_LONG * CHAR_BIT
+    y = x >> 64; if (y) {n -= 64; x = y;}
+# endif
+# if 32 < SIZEOF_LONG * CHAR_BIT
+    y = x >> 32; if (y) {n -= 32; x = y;}
+# endif
+    y = x >> 16; if (y) {n -= 16; x = y;}
+    y = x >>  8; if (y) {n -=  8; x = y;}
+    y = x >>  4; if (y) {n -=  4; x = y;}
+    y = x >>  2; if (y) {n -=  2; x = y;}
+    y = x >>  1; if (y) {return n - 2;}
+    return (unsigned int)(n - x);
+#endif
+}
+
+#ifdef HAVE_LONG_LONG
+static inline unsigned int
+nlz_long_long(unsigned LONG_LONG x)
+{
+#if defined(HAVE_BUILTIN___BUILTIN_CLZLL)
+    if (x == 0) return SIZEOF_LONG_LONG * CHAR_BIT;
+    return (unsigned int)__builtin_clzll(x);
+#else
+    unsigned LONG_LONG y;
+# if 64 < SIZEOF_LONG_LONG * CHAR_BIT
+    unsigned int n = 128;
+# elif 32 < SIZEOF_LONG_LONG * CHAR_BIT
+    unsigned int n = 64;
+# else
+    unsigned int n = 32;
+# endif
+# if 64 < SIZEOF_LONG_LONG * CHAR_BIT
+    y = x >> 64; if (y) {n -= 64; x = y;}
+# endif
+# if 32 < SIZEOF_LONG_LONG * CHAR_BIT
+    y = x >> 32; if (y) {n -= 32; x = y;}
+# endif
+    y = x >> 16; if (y) {n -= 16; x = y;}
+    y = x >>  8; if (y) {n -=  8; x = y;}
+    y = x >>  4; if (y) {n -=  4; x = y;}
+    y = x >>  2; if (y) {n -=  2; x = y;}
+    y = x >>  1; if (y) {return n - 2;}
+    return (unsigned int)(n - x);
+#endif
+}
+#endif
+
+#ifdef HAVE_UINT128_T
+static inline unsigned int
+nlz_int128(uint128_t x)
+{
+    uint128_t y;
+    unsigned int n = 128;
+    y = x >> 64; if (y) {n -= 64; x = y;}
+    y = x >> 32; if (y) {n -= 32; x = y;}
+    y = x >> 16; if (y) {n -= 16; x = y;}
+    y = x >>  8; if (y) {n -=  8; x = y;}
+    y = x >>  4; if (y) {n -=  4; x = y;}
+    y = x >>  2; if (y) {n -=  2; x = y;}
+    y = x >>  1; if (y) {return n - 2;}
+    return (unsigned int)(n - x);
+}
+#endif
+
+static inline unsigned int
+nlz_intptr(uintptr_t x)
+{
+#if SIZEOF_UINTPTR_T == SIZEOF_INT
+    return nlz_int(x);
+#elif SIZEOF_UINTPTR_T == SIZEOF_LONG
+    return nlz_long(x);
+#elif SIZEOF_UINTPTR_T == SIZEOF_LONG_LONG
+    return nlz_long_long(x);
+#else
+    #error no known integer type corresponds uintptr_t
+    return /* sane compiler */ ~0;
+#endif
+}
+
+static inline unsigned int
+rb_popcount32(uint32_t x)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_POPCOUNT
+    return (unsigned int)__builtin_popcount(x);
+#else
+    x = (x & 0x55555555) + (x >> 1 & 0x55555555);
+    x = (x & 0x33333333) + (x >> 2 & 0x33333333);
+    x = (x & 0x0f0f0f0f) + (x >> 4 & 0x0f0f0f0f);
+    x = (x & 0x001f001f) + (x >> 8 & 0x001f001f);
+    return (x & 0x0000003f) + (x >>16 & 0x0000003f);
+#endif
+}
+
+static inline int
+rb_popcount64(uint64_t x)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_POPCOUNT
+    return __builtin_popcountll(x);
+#else
+    x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
+    x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
+    x = (x & 0x0707070707070707) + (x >> 4 & 0x0707070707070707);
+    x = (x & 0x001f001f001f001f) + (x >> 8 & 0x001f001f001f001f);
+    x = (x & 0x0000003f0000003f) + (x >>16 & 0x0000003f0000003f);
+    return (x & 0x7f) + (x >>32 & 0x7f);
+#endif
+}
+
+static inline int
+rb_popcount_intptr(uintptr_t x)
+{
+#if SIZEOF_VOIDP == 8
+    return rb_popcount64(x);
+#elif SIZEOF_VOIDP == 4
+    return rb_popcount32(x);
+#endif
+}
+
+static inline int
+ntz_int32(uint32_t x)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_CTZ
+    return __builtin_ctz(x);
+#else
+    return rb_popcount32((~x) & (x-1));
+#endif
+}
+
+static inline int
+ntz_int64(uint64_t x)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_CTZLL
+    return __builtin_ctzll(x);
+#else
+    return rb_popcount64((~x) & (x-1));
+#endif
+}
+
+static inline int
+ntz_intptr(uintptr_t x)
+{
+#if SIZEOF_VOIDP == 8
+    return ntz_int64(x);
+#elif SIZEOF_VOIDP == 4
+    return ntz_int32(x);
+#endif
+}
+
+#if defined(HAVE_UINT128_T) && defined(HAVE_LONG_LONG)
+#   define bit_length(x) \
+    (unsigned int) \
+    (sizeof(x) <= SIZEOF_INT ? SIZEOF_INT * CHAR_BIT - nlz_int((unsigned int)(x)) : \
+     sizeof(x) <= SIZEOF_LONG ? SIZEOF_LONG * CHAR_BIT - nlz_long((unsigned long)(x)) : \
+     sizeof(x) <= SIZEOF_LONG_LONG ? SIZEOF_LONG_LONG * CHAR_BIT - nlz_long_long((unsigned LONG_LONG)(x)) : \
+     SIZEOF_INT128_T * CHAR_BIT - nlz_int128((uint128_t)(x)))
+#elif defined(HAVE_UINT128_T)
+#   define bit_length(x) \
+    (unsigned int) \
+    (sizeof(x) <= SIZEOF_INT ? SIZEOF_INT * CHAR_BIT - nlz_int((unsigned int)(x)) : \
+     sizeof(x) <= SIZEOF_LONG ? SIZEOF_LONG * CHAR_BIT - nlz_long((unsigned long)(x)) : \
+     SIZEOF_INT128_T * CHAR_BIT - nlz_int128((uint128_t)(x)))
+#elif defined(HAVE_LONG_LONG)
+#   define bit_length(x) \
+    (unsigned int) \
+    (sizeof(x) <= SIZEOF_INT ? SIZEOF_INT * CHAR_BIT - nlz_int((unsigned int)(x)) : \
+     sizeof(x) <= SIZEOF_LONG ? SIZEOF_LONG * CHAR_BIT - nlz_long((unsigned long)(x)) : \
+     SIZEOF_LONG_LONG * CHAR_BIT - nlz_long_long((unsigned LONG_LONG)(x)))
+#else
+#   define bit_length(x) \
+    (unsigned int) \
+    (sizeof(x) <= SIZEOF_INT ? SIZEOF_INT * CHAR_BIT - nlz_int((unsigned int)(x)) : \
+     SIZEOF_LONG * CHAR_BIT - nlz_long((unsigned long)(x)))
+#endif
+
+#if USE_FLONUM
+#define RUBY_BIT_ROTL(v, n) (((v) << (n)) | ((v) >> ((sizeof(v) * 8) - n)))
+#define RUBY_BIT_ROTR(v, n) (((v) >> (n)) | ((v) << ((sizeof(v) * 8) - n)))
+#endif
+#endif /* INTERNAL_BITS_H */
diff --git a/internal/class.h b/internal/class.h
new file mode 100644
index 0000000000..8ba64a094b
--- /dev/null
+++ b/internal/class.h
@@ -0,0 +1,143 @@
+#ifndef INTERNAL_CLASS_H /* -*- C -*- */
+#define INTERNAL_CLASS_H
+/**
+ * @file
+ * @brief      Internal header for Class.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct rb_deprecated_classext_struct {
+    char conflict[sizeof(VALUE) * 3];
+};
+
+struct rb_subclass_entry;
+typedef struct rb_subclass_entry rb_subclass_entry_t;
+
+struct rb_subclass_entry {
+    VALUE klass;
+    rb_subclass_entry_t *next;
+};
+
+struct rb_classext_struct {
+    struct st_table *iv_index_tbl;
+    struct st_table *iv_tbl;
+#if SIZEOF_SERIAL_T == SIZEOF_VALUE /* otherwise m_tbl is in struct RClass */
+    struct rb_id_table *m_tbl;
+#endif
+    struct rb_id_table *const_tbl;
+    struct rb_id_table *callable_m_tbl;
+    rb_subclass_entry_t *subclasses;
+    rb_subclass_entry_t **parent_subclasses;
+    /**
+     * In the case that this is an `ICLASS`, `module_subclasses` points to the link
+     * in the module's `subclasses` list that indicates that the klass has been
+     * included. Hopefully that makes sense.
+     */
+    rb_subclass_entry_t **module_subclasses;
+#if SIZEOF_SERIAL_T != SIZEOF_VALUE /* otherwise class_serial is in struct RClass */
+    rb_serial_t class_serial;
+#endif
+    const VALUE origin_;
+    const VALUE refined_class;
+    rb_alloc_func_t allocator;
+    const VALUE includer;
+};
+
+typedef struct rb_classext_struct rb_classext_t;
+
+#undef RClass
+struct RClass {
+    struct RBasic basic;
+    VALUE super;
+    rb_classext_t *ptr;
+#if SIZEOF_SERIAL_T == SIZEOF_VALUE
+    /* Class serial is as wide as VALUE.  Place it here. */
+    rb_serial_t class_serial;
+#else
+    /* Class serial does not fit into struct RClass. Place m_tbl instead. */
+    struct rb_id_table *m_tbl;
+#endif
+};
+
+void rb_class_subclass_add(VALUE super, VALUE klass);
+void rb_class_remove_from_super_subclasses(VALUE);
+int rb_singleton_class_internal_p(VALUE sklass);
+/* class.c */
+VALUE rb_class_boot(VALUE);
+VALUE rb_class_inherited(VALUE, VALUE);
+VALUE rb_make_metaclass(VALUE, VALUE);
+VALUE rb_include_class_new(VALUE, VALUE);
+void rb_class_foreach_subclass(VALUE klass, void (*f)(VALUE, VALUE), VALUE);
+void rb_class_detach_subclasses(VALUE);
+void rb_class_detach_module_subclasses(VALUE);
+void rb_class_remove_from_module_subclasses(VALUE);
+VALUE rb_obj_methods(int argc, const VALUE *argv, VALUE obj);
+VALUE rb_obj_protected_methods(int argc, const VALUE *argv, VALUE obj);
+VALUE rb_obj_private_methods(int argc, const VALUE *argv, VALUE obj);
+VALUE rb_obj_public_methods(int argc, const VALUE *argv, VALUE obj);
+VALUE rb_special_singleton_class(VALUE);
+VALUE rb_singleton_class_clone_and_attach(VALUE obj, VALUE attach);
+VALUE rb_singleton_class_get(VALUE obj);
+
+int rb_class_has_methods(VALUE c);
+void rb_undef_methods_from(VALUE klass, VALUE super);
+
+#define RCLASS_EXT(c) (RCLASS(c)->ptr)
+#define RCLASS_IV_TBL(c) (RCLASS_EXT(c)->iv_tbl)
+#define RCLASS_CONST_TBL(c) (RCLASS_EXT(c)->const_tbl)
+#if SIZEOF_SERIAL_T == SIZEOF_VALUE
+# define RCLASS_M_TBL(c) (RCLASS_EXT(c)->m_tbl)
+#else
+# define RCLASS_M_TBL(c) (RCLASS(c)->m_tbl)
+#endif
+#define RCLASS_CALLABLE_M_TBL(c) (RCLASS_EXT(c)->callable_m_tbl)
+#define RCLASS_IV_INDEX_TBL(c) (RCLASS_EXT(c)->iv_index_tbl)
+#define RCLASS_ORIGIN(c) (RCLASS_EXT(c)->origin_)
+#define RCLASS_REFINED_CLASS(c) (RCLASS_EXT(c)->refined_class)
+#if SIZEOF_SERIAL_T == SIZEOF_VALUE
+# define RCLASS_SERIAL(c) (RCLASS(c)->class_serial)
+#else
+# define RCLASS_SERIAL(c) (RCLASS_EXT(c)->class_serial)
+#endif
+#define RCLASS_INCLUDER(c) (RCLASS_EXT(c)->includer)
+
+#define RCLASS_CLONED     FL_USER6
+#define RICLASS_IS_ORIGIN FL_USER5
+#define RCLASS_REFINED_BY_ANY FL_USER7
+
+static inline void
+RCLASS_SET_ORIGIN(VALUE klass, VALUE origin)
+{
+    RB_OBJ_WRITE(klass, &RCLASS_ORIGIN(klass), origin);
+    if (klass != origin) FL_SET(origin, RICLASS_IS_ORIGIN);
+}
+
+static inline void
+RCLASS_SET_INCLUDER(VALUE iclass, VALUE klass)
+{
+    RB_OBJ_WRITE(iclass, &RCLASS_INCLUDER(iclass), klass);
+}
+
+#undef RCLASS_SUPER
+static inline VALUE
+RCLASS_SUPER(VALUE klass)
+{
+    return RCLASS(klass)->super;
+}
+
+static inline VALUE
+RCLASS_SET_SUPER(VALUE klass, VALUE super)
+{
+    if (super) {
+        rb_class_remove_from_super_subclasses(klass);
+        rb_class_subclass_add(super, klass);
+    }
+    RB_OBJ_WRITE(klass, &RCLASS(klass)->super, super);
+    return super;
+}
+
+#endif /* INTERNAL_CLASS_H */
diff --git a/internal/compar.h b/internal/compar.h
new file mode 100644
index 0000000000..36c5e9c782
--- /dev/null
+++ b/internal/compar.h
@@ -0,0 +1,54 @@
+#ifndef INTERNAL_COMPAR_H /* -*- C -*- */
+#define INTERNAL_COMPAR_H
+/**
+ * @file
+ * @brief      Internal header for Comparable.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#define STRING_P(s) (RB_TYPE_P((s), T_STRING) && CLASS_OF(s) == rb_cString)
+
+#ifdef RUBY_INTEGER_UNIFICATION
+# define rb_cFixnum rb_cInteger
+# define rb_cBignum rb_cInteger
+#endif
+
+enum {
+    cmp_opt_Fixnum,
+    cmp_opt_String,
+    cmp_opt_Float,
+    cmp_optimizable_count
+};
+
+struct cmp_opt_data {
+    unsigned int opt_methods;
+    unsigned int opt_inited;
+};
+
+#define NEW_CMP_OPT_MEMO(type, value) \
+    NEW_PARTIAL_MEMO_FOR(type, value, cmp_opt)
+#define CMP_OPTIMIZABLE_BIT(type) (1U << TOKEN_PASTE(cmp_opt_,type))
+#define CMP_OPTIMIZABLE(data, type) \
+    (((data).opt_inited & CMP_OPTIMIZABLE_BIT(type)) ? \
+     ((data).opt_methods & CMP_OPTIMIZABLE_BIT(type)) : \
+     (((data).opt_inited |= CMP_OPTIMIZABLE_BIT(type)), \
+      rb_method_basic_definition_p(TOKEN_PASTE(rb_c,type), id_cmp) && \
+      ((data).opt_methods |= CMP_OPTIMIZABLE_BIT(type))))
+
+#define OPTIMIZED_CMP(a, b, data) \
+    ((FIXNUM_P(a) && FIXNUM_P(b) && CMP_OPTIMIZABLE(data, Fixnum)) ? \
+     (((long)a > (long)b) ? 1 : ((long)a < (long)b) ? -1 : 0) : \
+     (STRING_P(a) && STRING_P(b) && CMP_OPTIMIZABLE(data, String)) ? \
+     rb_str_cmp(a, b) : \
+     (RB_FLOAT_TYPE_P(a) && RB_FLOAT_TYPE_P(b) && CMP_OPTIMIZABLE(data, Float)) ? \
+     rb_float_cmp(a, b) : \
+     rb_cmpint(rb_funcallv(a, id_cmp, 1, &b), a, b))
+
+/* compar.c */
+VALUE rb_invcmp(VALUE, VALUE);
+
+#endif /* INTERNAL_COMPAR_H */
diff --git a/internal/compile.h b/internal/compile.h
new file mode 100644
index 0000000000..f6091ba076
--- /dev/null
+++ b/internal/compile.h
@@ -0,0 +1,22 @@
+#ifndef INTERNAL_COMPILE_H /* -*- C -*- */
+#define INTERNAL_COMPILE_H
+/**
+ * @file
+ * @brief      Internal header for the compiler.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* compile.c */
+struct rb_block;
+struct rb_iseq_struct;
+int rb_dvar_defined(ID, const struct rb_iseq_struct *);
+int rb_local_defined(ID, const struct rb_iseq_struct *);
+const char * rb_insns_name(int i);
+VALUE rb_insns_name_array(void);
+int rb_vm_insn_addr2insn(const void *);
+
+#endif /* INTERNAL_COMPILE_H */
diff --git a/internal/compilers.h b/internal/compilers.h
new file mode 100644
index 0000000000..697bad5f4e
--- /dev/null
+++ b/internal/compilers.h
@@ -0,0 +1,87 @@
+#ifndef INTERNAL_COMPILERS_H /* -*- C -*- */
+#define INTERNAL_COMPILERS_H
+/**
+ * @file
+ * @brief      Internal header absorbing C compipler differences.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#ifndef MAYBE_UNUSED
+# define MAYBE_UNUSED(x) x
+#endif
+
+#ifndef WARN_UNUSED_RESULT
+# define WARN_UNUSED_RESULT(x) x
+#endif
+
+#ifndef __has_feature
+# define __has_feature(x) 0
+#endif
+
+#ifndef __has_extension
+# define __has_extension __has_feature
+#endif
+
+#define RB_OBJ_BUILTIN_TYPE(obj) rb_obj_builtin_type(obj)
+#define OBJ_BUILTIN_TYPE(obj) RB_OBJ_BUILTIN_TYPE(obj)
+#ifdef __GNUC__
+#define rb_obj_builtin_type(obj) \
+__extension__({ \
+    VALUE arg_obj = (obj); \
+    RB_SPECIAL_CONST_P(arg_obj) ? -1 : \
+        RB_BUILTIN_TYPE(arg_obj); \
+    })
+#else
+static inline int
+rb_obj_builtin_type(VALUE obj)
+{
+    return RB_SPECIAL_CONST_P(obj) ? -1 :
+        RB_BUILTIN_TYPE(obj);
+}
+#endif
+
+/* A macro for defining a flexible array, like: VALUE ary[FLEX_ARY_LEN]; */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define FLEX_ARY_LEN   /* VALUE ary[]; */
+#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
+# define FLEX_ARY_LEN 0 /* VALUE ary[0]; */
+#else
+# define FLEX_ARY_LEN 1 /* VALUE ary[1]; */
+#endif
+
+/*
+ * For declaring bitfields out of non-unsigned int types:
+ *   struct date {
+ *      BITFIELD(enum months, month, 4);
+ *      ...
+ *   };
+ */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define BITFIELD(type, name, size) type name : size
+#else
+# define BITFIELD(type, name, size) unsigned int name : size
+#endif
+
+#if defined(USE_UNALIGNED_MEMBER_ACCESS) && USE_UNALIGNED_MEMBER_ACCESS && \
+    (defined(__clang__) || GCC_VERSION_SINCE(9, 0, 0))
+#include "warnings.h"
+# define UNALIGNED_MEMBER_ACCESS(expr) __extension__({ \
+    COMPILER_WARNING_PUSH; \
+    COMPILER_WARNING_IGNORED(-Waddress-of-packed-member); \
+    typeof(expr) unaligned_member_access_result = (expr); \
+    COMPILER_WARNING_POP; \
+    unaligned_member_access_result; \
+})
+#else
+# define UNALIGNED_MEMBER_ACCESS(expr) expr
+#endif
+#define UNALIGNED_MEMBER_PTR(ptr, mem) UNALIGNED_MEMBER_ACCESS(&(ptr)->mem)
+
+#undef RB_OBJ_WRITE
+#define RB_OBJ_WRITE(a, slot, b) UNALIGNED_MEMBER_ACCESS(rb_obj_write((VALUE)(a), (VALUE *)(slot), (VALUE)(b), __FILE__, __LINE__))
+
+#endif /* INTERNAL_COMPILERS_H */
diff --git a/internal/complex.h b/internal/complex.h
new file mode 100644
index 0000000000..7a363fc0e5
--- /dev/null
+++ b/internal/complex.h
@@ -0,0 +1,28 @@
+#ifndef INTERNAL_COMPLEX_H /* -*- C -*- */
+#define INTERNAL_COMPLEX_H
+/**
+ * @file
+ * @brief      Internal header for Complex.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct RComplex {
+    struct RBasic basic;
+    VALUE real;
+    VALUE imag;
+};
+
+#define RCOMPLEX(obj) (R_CAST(RComplex)(obj))
+
+/* shortcut macro for internal only */
+#define RCOMPLEX_SET_REAL(cmp, r) RB_OBJ_WRITE((cmp), &((struct RComplex *)(cmp))->real,(r))
+#define RCOMPLEX_SET_IMAG(cmp, i) RB_OBJ_WRITE((cmp), &((struct RComplex *)(cmp))->imag,(i))
+
+/* complex.c */
+VALUE rb_dbl_complex_new_polar_pi(double abs, double ang);
+
+#endif /* INTERNAL_COMPLEX_H */
diff --git a/internal/cont.h b/internal/cont.h
new file mode 100644
index 0000000000..066a28238c
--- /dev/null
+++ b/internal/cont.h
@@ -0,0 +1,19 @@
+#ifndef INTERNAL_CONT_H /* -*- C -*- */
+#define INTERNAL_CONT_H
+/**
+ * @file
+ * @brief      Internal header for Fiber.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct rb_thread_struct;
+/* cont.c */
+VALUE rb_obj_is_fiber(VALUE);
+void rb_fiber_reset_root_local_storage(struct rb_thread_struct *);
+void ruby_register_rollback_func_for_ensure(VALUE (*ensure_func)(VALUE), VALUE (*rollback_func)(VALUE));
+
+#endif /* INTERNAL_CONT_H */
diff --git a/internal/debug.h b/internal/debug.h
new file mode 100644
index 0000000000..8fa75fd8c6
--- /dev/null
+++ b/internal/debug.h
@@ -0,0 +1,34 @@
+#ifndef INTERNAL_DEBUG_H /* -*- C -*- */
+#define INTERNAL_DEBUG_H
+/**
+ * @file
+ * @brief      Internal header for debugging.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* MRI debug support */
+void rb_obj_info_dump(VALUE obj);
+void rb_obj_info_dump_loc(VALUE obj, const char *file, int line, const char *func);
+void ruby_debug_breakpoint(void);
+
+// show obj data structure without any side-effect
+#define rp(obj) rb_obj_info_dump_loc((VALUE)(obj), __FILE__, __LINE__, __func__)
+
+// same as rp, but add message header
+#define rp_m(msg, obj) do { \
+    fprintf(stderr, "%s", (msg)); \
+    rb_obj_info_dump((VALUE)obj); \
+} while (0)
+
+// `ruby_debug_breakpoint()` does nothing,
+// but breakpoint is set in run.gdb, so `make gdb` can stop here.
+#define bp() ruby_debug_breakpoint()
+
+/* debug.c */
+PRINTF_ARGS(void ruby_debug_printf(const char*, ...), 1, 2);
+
+#endif /* INTERNAL_DEBUG_H */
diff --git a/internal/dir.h b/internal/dir.h
new file mode 100644
index 0000000000..9162d41d7a
--- /dev/null
+++ b/internal/dir.h
@@ -0,0 +1,16 @@
+#ifndef INTERNAL_DIR_H /* -*- C -*- */
+#define INTERNAL_DIR_H
+/**
+ * @file
+ * @brief      Internal header for Dir.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* dir.c */
+VALUE rb_dir_getwd_ospath(void);
+
+#endif /* INTERNAL_DIR_H */
diff --git a/internal/enc.h b/internal/enc.h
new file mode 100644
index 0000000000..afd8642627
--- /dev/null
+++ b/internal/enc.h
@@ -0,0 +1,24 @@
+#ifndef INTERNAL_ENC_H /* -*- C -*- */
+#define INTERNAL_ENC_H
+/**
+ * @file
+ * @brief      Internal header for Encoding.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+
+/* us_ascii.c */
+#ifdef RUBY_ENCODING_H
+extern rb_encoding OnigEncodingUS_ASCII;
+#endif
+
+/* utf_8.c */
+#ifdef RUBY_ENCODING_H
+extern rb_encoding OnigEncodingUTF_8;
+#endif
+
+#endif /* INTERNAL_ENC_H */
diff --git a/internal/encoding.h b/internal/encoding.h
new file mode 100644
index 0000000000..79fbadaf50
--- /dev/null
+++ b/internal/encoding.h
@@ -0,0 +1,28 @@
+#ifndef INTERNAL_ENCODING_H /* -*- C -*- */
+#define INTERNAL_ENCODING_H
+/**
+ * @file
+ * @brief      Internal header for Encoding.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* encoding.c */
+ID rb_id_encoding(void);
+#ifdef RUBY_ENCODING_H
+rb_encoding *rb_enc_get_from_index(int index);
+rb_encoding *rb_enc_check_str(VALUE str1, VALUE str2);
+#endif
+int rb_encdb_replicate(const char *alias, const char *orig);
+int rb_encdb_alias(const char *alias, const char *orig);
+int rb_encdb_dummy(const char *name);
+void rb_encdb_declare(const char *name);
+void rb_enc_set_base(const char *name, const char *orig);
+int rb_enc_set_dummy(int index);
+void rb_encdb_set_unicode(int index);
+PUREFUNC(int rb_data_is_encoding(VALUE obj));
+
+#endif /* INTERNAL_ENCODING_H */
diff --git a/internal/enum.h b/internal/enum.h
new file mode 100644
index 0000000000..e4e710bad0
--- /dev/null
+++ b/internal/enum.h
@@ -0,0 +1,18 @@
+#ifndef INTERNAL_ENUM_H /* -*- C -*- */
+#define INTERNAL_ENUM_H
+/**
+ * @file
+ * @brief      Internal header for Enumerable.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* enum.c */
+extern VALUE rb_cArithSeq;
+VALUE rb_f_send(int argc, VALUE *argv, VALUE recv);
+VALUE rb_nmin_run(VALUE obj, VALUE num, int by, int rev, int ary);
+
+#endif /* INTERNAL_ENUM_H */
diff --git a/internal/enumerator.h b/internal/enumerator.h
new file mode 100644
index 0000000000..5f0f6333b6
--- /dev/null
+++ b/internal/enumerator.h
@@ -0,0 +1,20 @@
+#ifndef INTERNAL_ENUMERATOR_H /* -*- C -*- */
+#define INTERNAL_ENUMERATOR_H
+/**
+ * @file
+ * @brief      Internal header for Enumerator.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* enumerator.c (export) */
+VALUE rb_arith_seq_new(VALUE obj, VALUE meth, int argc, VALUE const *argv,
+                       rb_enumerator_size_func *size_fn,
+                       VALUE beg, VALUE end, VALUE step, int excl);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_ENUMERATOR_H */
diff --git a/internal/error.h b/internal/error.h
new file mode 100644
index 0000000000..5eddc7670f
--- /dev/null
+++ b/internal/error.h
@@ -0,0 +1,72 @@
+#ifndef INTERNAL_ERROR_H /* -*- C -*- */
+#define INTERNAL_ERROR_H
+/**
+ * @file
+ * @brief      Internal header for Exception.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* error.c */
+extern VALUE rb_eEAGAIN;
+extern VALUE rb_eEWOULDBLOCK;
+extern VALUE rb_eEINPROGRESS;
+void rb_report_bug_valist(VALUE file, int line, const char *fmt, va_list args);
+NORETURN(void rb_async_bug_errno(const char *,int));
+const char *rb_builtin_type_name(int t);
+const char *rb_builtin_class_name(VALUE x);
+PRINTF_ARGS(void rb_warn_deprecated(const char *fmt, const char *suggest, ...), 1, 3);
+#ifdef RUBY_ENCODING_H
+VALUE rb_syntax_error_append(VALUE, VALUE, int, int, rb_encoding*, const char*, va_list);
+PRINTF_ARGS(void rb_enc_warn(rb_encoding *enc, const char *fmt, ...), 2, 3);
+PRINTF_ARGS(void rb_sys_enc_warning(rb_encoding *enc, const char *fmt, ...), 2, 3);
+PRINTF_ARGS(void rb_syserr_enc_warning(int err, rb_encoding *enc, const char *fmt, ...), 3, 4);
+#endif
+
+typedef enum {
+    RB_WARN_CATEGORY_NONE,
+    RB_WARN_CATEGORY_DEPRECATED,
+    RB_WARN_CATEGORY_EXPERIMENTAL,
+} rb_warning_category_t;
+rb_warning_category_t rb_warning_category_from_name(VALUE category);
+bool rb_warning_category_enabled_p(rb_warning_category_t category);
+
+#define rb_raise_cstr(etype, mesg) \
+    rb_exc_raise(rb_exc_new_str(etype, rb_str_new_cstr(mesg)))
+#define rb_raise_static(etype, mesg) \
+    rb_exc_raise(rb_exc_new_str(etype, rb_str_new_static(mesg, rb_strlen_lit(mesg))))
+
+VALUE rb_name_err_new(VALUE mesg, VALUE recv, VALUE method);
+#define rb_name_err_raise_str(mesg, recv, name) \
+    rb_exc_raise(rb_name_err_new(mesg, recv, name))
+#define rb_name_err_raise(mesg, recv, name) \
+    rb_name_err_raise_str(rb_fstring_cstr(mesg), (recv), (name))
+VALUE rb_nomethod_err_new(VALUE mesg, VALUE recv, VALUE method, VALUE args, int priv);
+VALUE rb_key_err_new(VALUE mesg, VALUE recv, VALUE name);
+#define rb_key_err_raise(mesg, recv, name) \
+    rb_exc_raise(rb_key_err_new(mesg, recv, name))
+PRINTF_ARGS(VALUE rb_warning_string(const char *fmt, ...), 1, 2);
+NORETURN(void rb_vraise(VALUE, const char *, va_list));
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* error.c (export) */
+int rb_bug_reporter_add(void (*func)(FILE *, void *), void *data);
+NORETURN(void rb_unexpected_type(VALUE,int));
+#undef Check_Type
+#define Check_Type(v, t) \
+    (!RB_TYPE_P((VALUE)(v), (t)) || \
+     ((t) == RUBY_T_DATA && RTYPEDDATA_P(v)) ? \
+     rb_unexpected_type((VALUE)(v), (t)) : (void)0)
+
+static inline int
+rb_typeddata_is_instance_of_inline(VALUE obj, const rb_data_type_t *data_type)
+{
+    return RB_TYPE_P(obj, T_DATA) && RTYPEDDATA_P(obj) && (RTYPEDDATA_TYPE(obj) == data_type);
+}
+#define rb_typeddata_is_instance_of rb_typeddata_is_instance_of_inline
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_ERROR_H */
diff --git a/internal/eval.h b/internal/eval.h
new file mode 100644
index 0000000000..5fc3b38e90
--- /dev/null
+++ b/internal/eval.h
@@ -0,0 +1,30 @@
+#ifndef INTERNAL_EVAL_H /* -*- C -*- */
+#define INTERNAL_EVAL_H
+/**
+ * @file
+ * @brief      Internal header for the evaluator.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @note       There  also  is  eval_intern.h, which  is  evaluator's  internal
+ *             header (related to this file, but not the same role).
+ */
+
+/* eval.c */
+VALUE rb_refinement_module_get_refined_class(VALUE module);
+extern ID ruby_static_id_signo, ruby_static_id_status;
+void rb_class_modify_check(VALUE);
+#define id_signo ruby_static_id_signo
+#define id_status ruby_static_id_status
+NORETURN(VALUE rb_f_raise(int argc, VALUE *argv));
+
+/* eval_error.c */
+VALUE rb_get_backtrace(VALUE info);
+
+/* eval_jump.c */
+void rb_call_end_proc(VALUE data);
+void rb_mark_end_proc(void);
+
+#endif /* INTERNAL_EVAL_H */
diff --git a/internal/file.h b/internal/file.h
new file mode 100644
index 0000000000..f4a0794dc6
--- /dev/null
+++ b/internal/file.h
@@ -0,0 +1,55 @@
+#ifndef INTERNAL_FILE_H /* -*- C -*- */
+#define INTERNAL_FILE_H
+/**
+ * @file
+ * @brief      Internal header for File.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* file.c */
+extern const char ruby_null_device[];
+VALUE rb_home_dir_of(VALUE user, VALUE result);
+VALUE rb_default_home_dir(VALUE result);
+VALUE rb_realpath_internal(VALUE basedir, VALUE path, int strict);
+#ifdef RUBY_ENCODING_H
+VALUE rb_check_realpath(VALUE basedir, VALUE path, rb_encoding *origenc);
+#endif
+void rb_file_const(const char*, VALUE);
+int rb_file_load_ok(const char *);
+VALUE rb_file_expand_path_fast(VALUE, VALUE);
+VALUE rb_file_expand_path_internal(VALUE, VALUE, int, int, VALUE);
+VALUE rb_get_path_check_to_string(VALUE);
+VALUE rb_get_path_check_convert(VALUE);
+int ruby_is_fd_loadable(int fd);
+
+#ifdef RUBY_FUNCTION_NAME_STRING
+# if defined __GNUC__ && __GNUC__ >= 4
+#   pragma GCC visibility push(default)
+# endif
+NORETURN(void rb_sys_fail_path_in(const char *func_name, VALUE path));
+NORETURN(void rb_syserr_fail_path_in(const char *func_name, int err, VALUE path));
+# if defined __GNUC__ && __GNUC__ >= 4
+#   pragma GCC visibility pop
+# endif
+# define rb_sys_fail_path(path) rb_sys_fail_path_in(RUBY_FUNCTION_NAME_STRING, path)
+# define rb_syserr_fail_path(err, path) rb_syserr_fail_path_in(RUBY_FUNCTION_NAME_STRING, (err), (path))
+#else
+# define rb_sys_fail_path(path) rb_sys_fail_str(path)
+# define rb_syserr_fail_path(err, path) rb_syserr_fail_str((err), (path))
+#endif
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* file.c (export) */
+#if defined HAVE_READLINK && defined RUBY_ENCODING_H
+VALUE rb_readlink(VALUE path, rb_encoding *enc);
+#endif
+#ifdef __APPLE__
+VALUE rb_str_normalize_ospath(const char *ptr, long len);
+#endif
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_FILE_H */
diff --git a/internal/fixnum.h b/internal/fixnum.h
new file mode 100644
index 0000000000..08ca7bcec7
--- /dev/null
+++ b/internal/fixnum.h
@@ -0,0 +1,150 @@
+#ifndef INTERNAL_FIXNUM_H /* -*- C -*- */
+#define INTERNAL_FIXNUM_H
+/**
+ * @file
+ * @brief      Internal header for Fixnums.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#if HAVE_LONG_LONG && SIZEOF_LONG * 2 <= SIZEOF_LONG_LONG
+# define DLONG LONG_LONG
+# define DL2NUM(x) LL2NUM(x)
+#elif defined(HAVE_INT128_T)
+# define DLONG int128_t
+# define DL2NUM(x) (RB_FIXABLE(x) ? LONG2FIX(x) : rb_int128t2big(x))
+VALUE rb_int128t2big(int128_t n);
+#endif
+
+static inline long
+rb_overflowed_fix_to_int(long x)
+{
+    return (long)((unsigned long)(x >> 1) ^ (1LU << (SIZEOF_LONG * CHAR_BIT - 1)));
+}
+
+static inline VALUE
+rb_fix_plus_fix(VALUE x, VALUE y)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_ADD_OVERFLOW
+    long lz;
+    /* NOTE
+     * (1) `LONG2FIX(FIX2LONG(x)+FIX2LONG(y))`
+     +     = `((lx*2+1)/2 + (ly*2+1)/2)*2+1`
+     +     = `lx*2 + ly*2 + 1`
+     +     = `(lx*2+1) + (ly*2+1) - 1`
+     +     = `x + y - 1`
+     * (2) Fixnum's LSB is always 1.
+     *     It means you can always run `x - 1` without overflow.
+     * (3) Of course `z = x + (y-1)` may overflow.
+     *     At that time true value is
+     *     * positive: 0b0 1xxx...1, and z = 0b1xxx...1
+     *     * nevative: 0b1 0xxx...1, and z = 0b0xxx...1
+     *     To convert this true value to long,
+     *     (a) Use arithmetic shift
+     *         * positive: 0b11xxx...
+     *         * negative: 0b00xxx...
+     *     (b) invert MSB
+     *         * positive: 0b01xxx...
+     *         * negative: 0b10xxx...
+     */
+    if (__builtin_add_overflow((long)x, (long)y-1, &lz)) {
+        return rb_int2big(rb_overflowed_fix_to_int(lz));
+    }
+    else {
+        return (VALUE)lz;
+    }
+#else
+    long lz = FIX2LONG(x) + FIX2LONG(y);
+    return LONG2NUM(lz);
+#endif
+}
+
+static inline VALUE
+rb_fix_minus_fix(VALUE x, VALUE y)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_SUB_OVERFLOW
+    long lz;
+    if (__builtin_sub_overflow((long)x, (long)y-1, &lz)) {
+        return rb_int2big(rb_overflowed_fix_to_int(lz));
+    }
+    else {
+        return (VALUE)lz;
+    }
+#else
+    long lz = FIX2LONG(x) - FIX2LONG(y);
+    return LONG2NUM(lz);
+#endif
+}
+
+/* arguments must be Fixnum */
+static inline VALUE
+rb_fix_mul_fix(VALUE x, VALUE y)
+{
+    long lx = FIX2LONG(x);
+    long ly = FIX2LONG(y);
+#ifdef DLONG
+    return DL2NUM((DLONG)lx * (DLONG)ly);
+#else
+    if (MUL_OVERFLOW_FIXNUM_P(lx, ly)) {
+        return rb_big_mul(rb_int2big(lx), rb_int2big(ly));
+    }
+    else {
+        return LONG2FIX(lx * ly);
+    }
+#endif
+}
+
+/*
+ * This behaves different from C99 for negative arguments.
+ * Note that div may overflow fixnum.
+ */
+static inline void
+rb_fix_divmod_fix(VALUE a, VALUE b, VALUE *divp, VALUE *modp)
+{
+    /* assume / and % comply C99.
+     * ldiv(3) won't be inlined by GCC and clang.
+     * I expect / and % are compiled as single idiv.
+     */
+    long x = FIX2LONG(a);
+    long y = FIX2LONG(b);
+    long div, mod;
+    if (x == FIXNUM_MIN && y == -1) {
+        if (divp) *divp = LONG2NUM(-FIXNUM_MIN);
+        if (modp) *modp = LONG2FIX(0);
+        return;
+    }
+    div = x / y;
+    mod = x % y;
+    if (y > 0 ? mod < 0 : mod > 0) {
+        mod += y;
+        div -= 1;
+    }
+    if (divp) *divp = LONG2FIX(div);
+    if (modp) *modp = LONG2FIX(mod);
+}
+
+/* div() for Ruby
+ * This behaves different from C99 for negative arguments.
+ */
+static inline VALUE
+rb_fix_div_fix(VALUE x, VALUE y)
+{
+    VALUE div;
+    rb_fix_divmod_fix(x, y, &div, NULL);
+    return div;
+}
+
+/* mod() for Ruby
+ * This behaves different from C99 for negative arguments.
+ */
+static inline VALUE
+rb_fix_mod_fix(VALUE x, VALUE y)
+{
+    VALUE mod;
+    rb_fix_divmod_fix(x, y, NULL, &mod);
+    return mod;
+}
+#endif /* INTERNAL_FIXNUM_H */
diff --git a/internal/gc.h b/internal/gc.h
new file mode 100644
index 0000000000..cf59eaed0b
--- /dev/null
+++ b/internal/gc.h
@@ -0,0 +1,79 @@
+#ifndef INTERNAL_GC_H /* -*- C -*- */
+#define INTERNAL_GC_H
+/**
+ * @file
+ * @brief      Internal header for GC.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* gc.c */
+extern VALUE *ruby_initial_gc_stress_ptr;
+extern int ruby_disable_gc;
+void *ruby_mimmalloc(size_t size) RUBY_ATTR_MALLOC;
+void ruby_mimfree(void *ptr);
+void rb_objspace_set_event_hook(const rb_event_flag_t event);
+#if USE_RGENGC
+void rb_gc_writebarrier_remember(VALUE obj);
+#else
+#define rb_gc_writebarrier_remember(obj) 0
+#endif
+void ruby_gc_set_params(void);
+void rb_copy_wb_protected_attribute(VALUE dest, VALUE obj);
+
+#if defined(HAVE_MALLOC_USABLE_SIZE) || defined(HAVE_MALLOC_SIZE) || defined(_WIN32)
+#define ruby_sized_xrealloc(ptr, new_size, old_size) ruby_xrealloc(ptr, new_size)
+#define ruby_sized_xrealloc2(ptr, new_count, element_size, old_count) ruby_xrealloc2(ptr, new_count, element_size)
+#define ruby_sized_xfree(ptr, size) ruby_xfree(ptr)
+#define SIZED_REALLOC_N(var,type,n,old_n) REALLOC_N(var, type, n)
+#else
+RUBY_SYMBOL_EXPORT_BEGIN
+void *ruby_sized_xrealloc(void *ptr, size_t new_size, size_t old_size) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2));
+void *ruby_sized_xrealloc2(void *ptr, size_t new_count, size_t element_size, size_t old_count) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2, 3));
+void ruby_sized_xfree(void *x, size_t size);
+RUBY_SYMBOL_EXPORT_END
+#define SIZED_REALLOC_N(var,type,n,old_n) ((var)=(type*)ruby_sized_xrealloc2((void*)(var), (n), sizeof(type), (old_n)))
+#endif
+
+/* optimized version of NEWOBJ() */
+#undef NEWOBJF_OF
+#undef RB_NEWOBJ_OF
+#define RB_NEWOBJ_OF(obj,type,klass,flags) \
+  type *(obj) = (type*)(((flags) & FL_WB_PROTECTED) ? \
+                        rb_wb_protected_newobj_of(klass, (flags) & ~FL_WB_PROTECTED) : \
+                        rb_wb_unprotected_newobj_of(klass, flags))
+#define NEWOBJ_OF(obj,type,klass,flags) RB_NEWOBJ_OF(obj,type,klass,flags)
+
+#ifdef __has_attribute
+#if __has_attribute(alloc_align)
+__attribute__((__alloc_align__(1)))
+#endif
+#endif
+void *rb_aligned_malloc(size_t, size_t) RUBY_ATTR_MALLOC RUBY_ATTR_ALLOC_SIZE((2));
+
+size_t rb_size_mul_or_raise(size_t, size_t, VALUE); /* used in compile.c */
+size_t rb_size_mul_add_or_raise(size_t, size_t, size_t, VALUE); /* used in iseq.h */
+void *rb_xmalloc_mul_add(size_t, size_t, size_t) RUBY_ATTR_MALLOC;
+void *rb_xrealloc_mul_add(const void *, size_t, size_t, size_t);
+void *rb_xmalloc_mul_add_mul(size_t, size_t, size_t, size_t) RUBY_ATTR_MALLOC;
+void *rb_xcalloc_mul_add_mul(size_t, size_t, size_t, size_t) RUBY_ATTR_MALLOC;
+
+RUBY_SYMBOL_EXPORT_BEGIN
+const char *rb_objspace_data_type_name(VALUE obj);
+
+/* gc.c (export) */
+VALUE rb_wb_protected_newobj_of(VALUE, VALUE);
+VALUE rb_wb_unprotected_newobj_of(VALUE, VALUE);
+
+size_t rb_obj_memsize_of(VALUE);
+void rb_gc_verify_internal_consistency(void);
+
+#define RB_OBJ_GC_FLAGS_MAX 6
+size_t rb_obj_gc_flags(VALUE, ID[], size_t);
+void rb_gc_mark_values(long n, const VALUE *values);
+void rb_gc_mark_vm_stack_values(long n, const VALUE *values);
+RUBY_SYMBOL_EXPORT_END
+#endif /* INTERNAL_GC_H */
diff --git a/internal/hash.h b/internal/hash.h
new file mode 100644
index 0000000000..3f27bb8d8c
--- /dev/null
+++ b/internal/hash.h
@@ -0,0 +1,143 @@
+#ifndef INTERNAL_HASH_H /* -*- C -*- */
+#define INTERNAL_HASH_H
+/**
+ * @file
+ * @brief      Internal header for Hash.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+enum ruby_rhash_flags {
+    RHASH_PASS_AS_KEYWORDS = FL_USER1,                                   /* FL 1 */
+    RHASH_PROC_DEFAULT = FL_USER2,                                       /* FL 2 */
+    RHASH_ST_TABLE_FLAG = FL_USER3,                                      /* FL 3 */
+#define RHASH_AR_TABLE_MAX_SIZE SIZEOF_VALUE
+    RHASH_AR_TABLE_SIZE_MASK = (FL_USER4|FL_USER5|FL_USER6|FL_USER7),    /* FL 4..7 */
+    RHASH_AR_TABLE_SIZE_SHIFT = (FL_USHIFT+4),
+    RHASH_AR_TABLE_BOUND_MASK = (FL_USER8|FL_USER9|FL_USER10|FL_USER11), /* FL 8..11 */
+    RHASH_AR_TABLE_BOUND_SHIFT = (FL_USHIFT+8),
+
+    // we can not put it in "enum" because it can exceed "int" range.
+#define RHASH_LEV_MASK (FL_USER13 | FL_USER14 | FL_USER15 |                /* FL 13..19 */ \
+                        FL_USER16 | FL_USER17 | FL_USER18 | FL_USER19)
+
+#if USE_TRANSIENT_HEAP
+    RHASH_TRANSIENT_FLAG = FL_USER12,                                    /* FL 12 */
+#endif
+
+    RHASH_LEV_SHIFT = (FL_USHIFT + 13),
+    RHASH_LEV_MAX = 127, /* 7 bits */
+
+    RHASH_ENUM_END
+};
+
+#define RHASH_AR_TABLE_SIZE_RAW(h) \
+  ((unsigned int)((RBASIC(h)->flags & RHASH_AR_TABLE_SIZE_MASK) >> RHASH_AR_TABLE_SIZE_SHIFT))
+
+void rb_hash_st_table_set(VALUE hash, st_table *st);
+
+#if 0 /* for debug */
+int rb_hash_ar_table_p(VALUE hash);
+struct ar_table_struct *rb_hash_ar_table(VALUE hash);
+st_table *rb_hash_st_table(VALUE hash);
+#define RHASH_AR_TABLE_P(hash)       rb_hash_ar_table_p(hash)
+#define RHASH_AR_TABLE(h)            rb_hash_ar_table(h)
+#define RHASH_ST_TABLE(h)            rb_hash_st_table(h)
+#else
+#define RHASH_AR_TABLE_P(hash)       (!FL_TEST_RAW((hash), RHASH_ST_TABLE_FLAG))
+#define RHASH_AR_TABLE(hash)         (RHASH(hash)->as.ar)
+#define RHASH_ST_TABLE(hash)         (RHASH(hash)->as.st)
+#endif
+
+#define RHASH(obj)                   (R_CAST(RHash)(obj))
+#define RHASH_ST_SIZE(h)             (RHASH_ST_TABLE(h)->num_entries)
+#define RHASH_ST_TABLE_P(h)          (!RHASH_AR_TABLE_P(h))
+#define RHASH_ST_CLEAR(h)            (FL_UNSET_RAW(h, RHASH_ST_TABLE_FLAG), RHASH(h)->as.ar = NULL)
+
+#define RHASH_AR_TABLE_SIZE_MASK     (VALUE)RHASH_AR_TABLE_SIZE_MASK
+#define RHASH_AR_TABLE_SIZE_SHIFT    RHASH_AR_TABLE_SIZE_SHIFT
+#define RHASH_AR_TABLE_BOUND_MASK    (VALUE)RHASH_AR_TABLE_BOUND_MASK
+#define RHASH_AR_TABLE_BOUND_SHIFT   RHASH_AR_TABLE_BOUND_SHIFT
+
+#if USE_TRANSIENT_HEAP
+#define RHASH_TRANSIENT_P(hash)   FL_TEST_RAW((hash), RHASH_TRANSIENT_FLAG)
+#define RHASH_SET_TRANSIENT_FLAG(h)   FL_SET_RAW(h, RHASH_TRANSIENT_FLAG)
+#define RHASH_UNSET_TRANSIENT_FLAG(h) FL_UNSET_RAW(h, RHASH_TRANSIENT_FLAG)
+#else
+#define RHASH_TRANSIENT_P(hash)   0
+#define RHASH_SET_TRANSIENT_FLAG(h)   ((void)0)
+#define RHASH_UNSET_TRANSIENT_FLAG(h) ((void)0)
+#endif
+
+#if   SIZEOF_VALUE / RHASH_AR_TABLE_MAX_SIZE == 2
+typedef uint16_t ar_hint_t;
+#elif SIZEOF_VALUE / RHASH_AR_TABLE_MAX_SIZE == 1
+typedef unsigned char ar_hint_t;
+#else
+#error unsupported
+#endif
+
+struct RHash {
+    struct RBasic basic;
+    union {
+        st_table *st;
+        struct ar_table_struct *ar; /* possibly 0 */
+    } as;
+    const VALUE ifnone;
+    union {
+        ar_hint_t ary[RHASH_AR_TABLE_MAX_SIZE];
+        VALUE word;
+    } ar_hint;
+};
+
+#ifdef RHASH_IFNONE
+#  undef RHASH_IFNONE
+#  undef RHASH_SIZE
+
+#  define RHASH_IFNONE(h)    (RHASH(h)->ifnone)
+#  define RHASH_SIZE(h)      (RHASH_AR_TABLE_P(h) ? RHASH_AR_TABLE_SIZE_RAW(h) : RHASH_ST_SIZE(h))
+#endif /* ifdef RHASH_IFNONE */
+
+/* hash.c */
+#if RHASH_CONVERT_TABLE_DEBUG
+struct st_table *rb_hash_tbl_raw(VALUE hash, const char *file, int line);
+#define RHASH_TBL_RAW(h) rb_hash_tbl_raw(h, __FILE__, __LINE__)
+#else
+struct st_table *rb_hash_tbl_raw(VALUE hash);
+#define RHASH_TBL_RAW(h) rb_hash_tbl_raw(h)
+#endif
+
+VALUE rb_hash_new_with_size(st_index_t size);
+VALUE rb_hash_has_key(VALUE hash, VALUE key);
+VALUE rb_hash_default_value(VALUE hash, VALUE key);
+VALUE rb_hash_set_default_proc(VALUE hash, VALUE proc);
+long rb_dbl_long_hash(double d);
+st_table *rb_init_identtable(void);
+VALUE rb_hash_compare_by_id_p(VALUE hash);
+VALUE rb_to_hash_type(VALUE obj);
+VALUE rb_hash_key_str(VALUE);
+VALUE rb_hash_keys(VALUE hash);
+VALUE rb_hash_values(VALUE hash);
+VALUE rb_hash_rehash(VALUE hash);
+VALUE rb_hash_resurrect(VALUE hash);
+int rb_hash_add_new_element(VALUE hash, VALUE key, VALUE val);
+VALUE rb_hash_set_pair(VALUE hash, VALUE pair);
+
+int rb_hash_stlike_lookup(VALUE hash, st_data_t key, st_data_t *pval);
+int rb_hash_stlike_delete(VALUE hash, st_data_t *pkey, st_data_t *pval);
+RUBY_SYMBOL_EXPORT_BEGIN
+int rb_hash_stlike_foreach(VALUE hash, st_foreach_callback_func *func, st_data_t arg);
+RUBY_SYMBOL_EXPORT_END
+int rb_hash_stlike_foreach_with_replace(VALUE hash, st_foreach_check_callback_func *func, st_update_callback_func *replace, st_data_t arg);
+int rb_hash_stlike_update(VALUE hash, st_data_t key, st_update_callback_func func, st_data_t arg);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* hash.c (export) */
+VALUE rb_hash_delete_entry(VALUE hash, VALUE key);
+VALUE rb_ident_hash_new(void);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_HASH_H */
diff --git a/internal/imemo.h b/internal/imemo.h
new file mode 100644
index 0000000000..682582a40e
--- /dev/null
+++ b/internal/imemo.h
@@ -0,0 +1,212 @@
+#ifndef INTERNAL_IMEMO_H /* -*- C -*- */
+#define INTERNAL_IMEMO_H
+/**
+ * @file
+ * @brief      IMEMO: Internal memo object.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#ifndef IMEMO_DEBUG
+#define IMEMO_DEBUG 0
+#endif
+
+struct RIMemo {
+    VALUE flags;
+    VALUE v0;
+    VALUE v1;
+    VALUE v2;
+    VALUE v3;
+};
+
+enum imemo_type {
+    imemo_env            =  0,
+    imemo_cref           =  1, /*!< class reference */
+    imemo_svar           =  2, /*!< special variable */
+    imemo_throw_data     =  3,
+    imemo_ifunc          =  4, /*!< iterator function */
+    imemo_memo           =  5,
+    imemo_ment           =  6,
+    imemo_iseq           =  7,
+    imemo_tmpbuf         =  8,
+    imemo_ast            =  9,
+    imemo_parser_strterm = 10
+};
+#define IMEMO_MASK   0x0f
+
+static inline enum imemo_type
+imemo_type(VALUE imemo)
+{
+    return (RBASIC(imemo)->flags >> FL_USHIFT) & IMEMO_MASK;
+}
+
+static inline int
+imemo_type_p(VALUE imemo, enum imemo_type imemo_type)
+{
+    if (LIKELY(!RB_SPECIAL_CONST_P(imemo))) {
+        /* fixed at compile time if imemo_type is given. */
+        const VALUE mask = (IMEMO_MASK << FL_USHIFT) | RUBY_T_MASK;
+        const VALUE expected_type = (imemo_type << FL_USHIFT) | T_IMEMO;
+        /* fixed at runtime. */
+        return expected_type == (RBASIC(imemo)->flags & mask);
+    }
+    else {
+        return 0;
+    }
+}
+
+VALUE rb_imemo_new(enum imemo_type type, VALUE v1, VALUE v2, VALUE v3, VALUE v0);
+
+/* FL_USER0 to FL_USER3 is for type */
+#define IMEMO_FL_USHIFT (FL_USHIFT + 4)
+#define IMEMO_FL_USER0 FL_USER4
+#define IMEMO_FL_USER1 FL_USER5
+#define IMEMO_FL_USER2 FL_USER6
+#define IMEMO_FL_USER3 FL_USER7
+#define IMEMO_FL_USER4 FL_USER8
+
+/* CREF (Class REFerence) is defined in method.h */
+
+/*! SVAR (Special VARiable) */
+struct vm_svar {
+    VALUE flags;
+    const VALUE cref_or_me; /*!< class reference or rb_method_entry_t */
+    const VALUE lastline;
+    const VALUE backref;
+    const VALUE others;
+};
+
+
+#define THROW_DATA_CONSUMED IMEMO_FL_USER0
+
+/*! THROW_DATA */
+struct vm_throw_data {
+    VALUE flags;
+    VALUE reserved;
+    const VALUE throw_obj;
+    const struct rb_control_frame_struct *catch_frame;
+    int throw_state;
+};
+
+#define THROW_DATA_P(err) RB_TYPE_P((VALUE)(err), T_IMEMO)
+
+/* IFUNC (Internal FUNCtion) */
+
+struct vm_ifunc_argc {
+#if SIZEOF_INT * 2 > SIZEOF_VALUE
+    signed int min: (SIZEOF_VALUE * CHAR_BIT) / 2;
+    signed int max: (SIZEOF_VALUE * CHAR_BIT) / 2;
+#else
+    int min, max;
+#endif
+};
+
+/*! IFUNC (Internal FUNCtion) */
+struct vm_ifunc {
+    VALUE flags;
+    VALUE reserved;
+    rb_block_call_func_t func;
+    const void *data;
+    struct vm_ifunc_argc argc;
+};
+
+#define IFUNC_NEW(a, b, c) ((struct vm_ifunc *)rb_imemo_new(imemo_ifunc, (VALUE)(a), (VALUE)(b), (VALUE)(c), 0))
+struct vm_ifunc *rb_vm_ifunc_new(rb_block_call_func_t func, const void *data, int min_argc, int max_argc);
+static inline struct vm_ifunc *
+rb_vm_ifunc_proc_new(rb_block_call_func_t func, const void *data)
+{
+    return rb_vm_ifunc_new(func, data, 0, UNLIMITED_ARGUMENTS);
+}
+
+typedef struct rb_imemo_tmpbuf_struct {
+    VALUE flags;
+    VALUE reserved;
+    VALUE *ptr; /* malloc'ed buffer */
+    struct rb_imemo_tmpbuf_struct *next; /* next imemo */
+    size_t cnt; /* buffer size in VALUE */
+} rb_imemo_tmpbuf_t;
+
+#define rb_imemo_tmpbuf_auto_free_pointer() rb_imemo_new(imemo_tmpbuf, 0, 0, 0, 0)
+rb_imemo_tmpbuf_t *rb_imemo_tmpbuf_parser_heap(void *buf, rb_imemo_tmpbuf_t *old_heap, size_t cnt);
+
+#define RB_IMEMO_TMPBUF_PTR(v) \
+    ((void *)(((const struct rb_imemo_tmpbuf_struct *)(v))->ptr))
+
+static inline void *
+rb_imemo_tmpbuf_set_ptr(VALUE v, void *ptr)
+{
+    return ((rb_imemo_tmpbuf_t *)v)->ptr = ptr;
+}
+
+static inline VALUE
+rb_imemo_tmpbuf_auto_free_pointer_new_from_an_RString(VALUE str)
+{
+    const void *src;
+    VALUE imemo;
+    rb_imemo_tmpbuf_t *tmpbuf;
+    void *dst;
+    size_t len;
+
+    SafeStringValue(str);
+    /* create tmpbuf to keep the pointer before xmalloc */
+    imemo = rb_imemo_tmpbuf_auto_free_pointer();
+    tmpbuf = (rb_imemo_tmpbuf_t *)imemo;
+    len = RSTRING_LEN(str);
+    src = RSTRING_PTR(str);
+    dst = ruby_xmalloc(len);
+    memcpy(dst, src, len);
+    tmpbuf->ptr = dst;
+    return imemo;
+}
+
+void rb_strterm_mark(VALUE obj);
+
+/*! MEMO
+ *
+ * @see imemo_type
+ * */
+struct MEMO {
+    VALUE flags;
+    VALUE reserved;
+    const VALUE v1;
+    const VALUE v2;
+    union {
+        long cnt;
+        long state;
+        const VALUE value;
+        void (*func)(void);
+    } u3;
+};
+
+#define MEMO_V1_SET(m, v) RB_OBJ_WRITE((m), &(m)->v1, (v))
+#define MEMO_V2_SET(m, v) RB_OBJ_WRITE((m), &(m)->v2, (v))
+
+#define MEMO_CAST(m) ((struct MEMO *)m)
+
+#define MEMO_NEW(a, b, c) ((struct MEMO *)rb_imemo_new(imemo_memo, (VALUE)(a), (VALUE)(b), (VALUE)(c), 0))
+
+#define roomof(x, y) (((x) + (y) - 1) / (y))
+#define type_roomof(x, y) roomof(sizeof(x), sizeof(y))
+#define MEMO_FOR(type, value) ((type *)RARRAY_PTR(value))
+#define NEW_MEMO_FOR(type, value) \
+  ((value) = rb_ary_tmp_new_fill(type_roomof(type, VALUE)), MEMO_FOR(type, value))
+#define NEW_PARTIAL_MEMO_FOR(type, value, member) \
+  ((value) = rb_ary_tmp_new_fill(type_roomof(type, VALUE)), \
+   rb_ary_set_len((value), offsetof(type, member) / sizeof(VALUE)), \
+   MEMO_FOR(type, value))
+
+/* ment is in method.h */
+
+RUBY_SYMBOL_EXPORT_BEGIN
+#if IMEMO_DEBUG
+VALUE rb_imemo_new_debug(enum imemo_type type, VALUE v1, VALUE v2, VALUE v3, VALUE v0, const char *file, int line);
+#define rb_imemo_new(type, v1, v2, v3, v0) rb_imemo_new_debug(type, v1, v2, v3, v0, __FILE__, __LINE__)
+#else
+VALUE rb_imemo_new(enum imemo_type type, VALUE v1, VALUE v2, VALUE v3, VALUE v0);
+#endif
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_IMEMO_H */
diff --git a/internal/inits.h b/internal/inits.h
new file mode 100644
index 0000000000..c9ef513728
--- /dev/null
+++ b/internal/inits.h
@@ -0,0 +1,51 @@
+#ifndef INTERNAL_INITS_H /* -*- C -*- */
+#define INTERNAL_INITS_H
+/**
+ * @file
+ * @brief      Internal header aggregating init functions.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* class.c */
+void Init_class_hierarchy(void);
+
+/* dmyext.c */
+void Init_enc(void);
+void Init_ext(void);
+
+/* file.c */
+void Init_File(void);
+
+/* gc.c */
+void Init_heap(void);
+
+/* localeinit.c */
+int Init_enc_set_filesystem_encoding(void);
+
+/* newline.c */
+void Init_newline(void);
+
+/* vm.c */
+void Init_BareVM(void);
+void Init_vm_objects(void);
+
+/* vm_backtrace.c */
+void Init_vm_backtrace(void);
+
+/* vm_eval.c */
+void Init_vm_eval(void);
+
+/* vm_insnhelper.c */
+void Init_vm_stack_canary(void);
+
+/* vm_method.c */
+void Init_eval_method(void);
+
+/* inits.c */
+void rb_call_inits(void);
+
+#endif /* INTERNAL_INITS_H */
diff --git a/internal/io.h b/internal/io.h
new file mode 100644
index 0000000000..00bca80fef
--- /dev/null
+++ b/internal/io.h
@@ -0,0 +1,31 @@
+#ifndef INTERNAL_IO_H /* -*- C -*- */
+#define INTERNAL_IO_H
+/**
+ * @file
+ * @brief      Internal header for IO.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* io.c */
+void ruby_set_inplace_mode(const char *);
+void rb_stdio_set_default_encoding(void);
+VALUE rb_io_flush_raw(VALUE, int);
+#ifdef RUBY_IO_H
+size_t rb_io_memsize(const rb_io_t *);
+#endif
+int rb_stderr_tty_p(void);
+void rb_io_fptr_finalize_internal(void *ptr);
+#define rb_io_fptr_finalize rb_io_fptr_finalize_internal
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* io.c (export) */
+void rb_maygvl_fd_fix_cloexec(int fd);
+int rb_gc_for_fd(int err);
+void rb_write_error_str(VALUE mesg);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_IO_H */
diff --git a/internal/load.h b/internal/load.h
new file mode 100644
index 0000000000..f4bf42cd28
--- /dev/null
+++ b/internal/load.h
@@ -0,0 +1,18 @@
+#ifndef INTERNAL_LOAD_H /* -*- C -*- */
+#define INTERNAL_LOAD_H
+/**
+ * @file
+ * @brief      Internal header for require.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* load.c */
+VALUE rb_get_expanded_load_path(void);
+int rb_require_internal(VALUE fname);
+NORETURN(void rb_load_fail(VALUE, const char*));
+
+#endif /* INTERNAL_LOAD_H */
diff --git a/internal/loadpath.h b/internal/loadpath.h
new file mode 100644
index 0000000000..df8078924c
--- /dev/null
+++ b/internal/loadpath.h
@@ -0,0 +1,17 @@
+#ifndef INTERNAL_LOADPATH_H /* -*- C -*- */
+#define INTERNAL_LOADPATH_H
+/**
+ * @file
+ * @brief      Internal header for $LOAD_PATH.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* loadpath.c */
+extern const char ruby_exec_prefix[];
+extern const char ruby_initial_load_paths[];
+
+#endif /* INTERNAL_LOADPATH_H */
diff --git a/internal/math.h b/internal/math.h
new file mode 100644
index 0000000000..51d398f61e
--- /dev/null
+++ b/internal/math.h
@@ -0,0 +1,23 @@
+#ifndef INTERNAL_MATH_H /* -*- C -*- */
+#define INTERNAL_MATH_H
+/**
+ * @file
+ * @brief      Internal header for Math.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* math.c */
+VALUE rb_math_atan2(VALUE, VALUE);
+VALUE rb_math_cos(VALUE);
+VALUE rb_math_cosh(VALUE);
+VALUE rb_math_exp(VALUE);
+VALUE rb_math_hypot(VALUE, VALUE);
+VALUE rb_math_log(int argc, const VALUE *argv);
+VALUE rb_math_sin(VALUE);
+VALUE rb_math_sinh(VALUE);
+
+#endif /* INTERNAL_MATH_H */
diff --git a/internal/missing.h b/internal/missing.h
new file mode 100644
index 0000000000..c27999e317
--- /dev/null
+++ b/internal/missing.h
@@ -0,0 +1,18 @@
+#ifndef INTERNAL_MISSING_H /* -*- C -*- */
+#define INTERNAL_MISSING_H
+/**
+ * @file
+ * @brief      Internal header corresponding missing.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* missing/setproctitle.c */
+#ifndef HAVE_SETPROCTITLE
+extern void ruby_init_setproctitle(int argc, char *argv[]);
+#endif
+
+#endif /* INTERNAL_MISSING_H */
diff --git a/internal/mjit.h b/internal/mjit.h
new file mode 100644
index 0000000000..71d25e52ae
--- /dev/null
+++ b/internal/mjit.h
@@ -0,0 +1,27 @@
+#ifndef INTERNAL_MJIT_H /* -*- C -*- */
+#define INTERNAL_MJIT_H
+/**
+ * @file
+ * @brief      Internal header for MJIT.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* mjit.c */
+
+#if USE_MJIT
+extern bool mjit_enabled;
+VALUE mjit_pause(bool wait_p);
+VALUE mjit_resume(void);
+void mjit_finish(bool close_handle_p);
+#else
+#define mjit_enabled 0
+static inline VALUE mjit_pause(bool wait_p){ return Qnil; } // unreachable
+static inline VALUE mjit_resume(void){ return Qnil; } // unreachable
+static inline void mjit_finish(bool close_handle_p){}
+#endif
+
+#endif /* INTERNAL_MJIT_H */
diff --git a/internal/numeric.h b/internal/numeric.h
new file mode 100644
index 0000000000..3e64b82daf
--- /dev/null
+++ b/internal/numeric.h
@@ -0,0 +1,213 @@
+#ifndef INTERNAL_NUMERIC_H /* -*- C -*- */
+#define INTERNAL_NUMERIC_H
+/**
+ * @file
+ * @brief      Internal header for Numeric.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct RFloat {
+    struct RBasic basic;
+    double float_value;
+};
+
+#define RFLOAT(obj)  (R_CAST(RFloat)(obj))
+
+/* numeric.c */
+
+#define FIXNUM_POSITIVE_P(num) ((SIGNED_VALUE)(num) > (SIGNED_VALUE)INT2FIX(0))
+#define FIXNUM_NEGATIVE_P(num) ((SIGNED_VALUE)(num) < 0)
+#define FIXNUM_ZERO_P(num) ((num) == INT2FIX(0))
+
+#define INT_NEGATIVE_P(x) (FIXNUM_P(x) ? FIXNUM_NEGATIVE_P(x) : BIGNUM_NEGATIVE_P(x))
+
+#define FLOAT_ZERO_P(x) (RFLOAT_VALUE(x) == 0.0)
+
+#ifndef ROUND_DEFAULT
+# define ROUND_DEFAULT RUBY_NUM_ROUND_HALF_UP
+#endif
+enum ruby_num_rounding_mode {
+    RUBY_NUM_ROUND_HALF_UP,
+    RUBY_NUM_ROUND_HALF_EVEN,
+    RUBY_NUM_ROUND_HALF_DOWN,
+    RUBY_NUM_ROUND_DEFAULT = ROUND_DEFAULT
+};
+#define ROUND_TO(mode, even, up, down) \
+    ((mode) == RUBY_NUM_ROUND_HALF_EVEN ? even : \
+     (mode) == RUBY_NUM_ROUND_HALF_UP ? up : down)
+#define ROUND_FUNC(mode, name) \
+    ROUND_TO(mode, name##_half_even, name##_half_up, name##_half_down)
+#define ROUND_CALL(mode, name, args) \
+    ROUND_TO(mode, name##_half_even args, \
+             name##_half_up args, name##_half_down args)
+
+int rb_num_to_uint(VALUE val, unsigned int *ret);
+VALUE ruby_num_interval_step_size(VALUE from, VALUE to, VALUE step, int excl);
+double ruby_float_step_size(double beg, double end, double unit, int excl);
+int ruby_float_step(VALUE from, VALUE to, VALUE step, int excl, int allow_endless);
+double ruby_float_mod(double x, double y);
+int rb_num_negative_p(VALUE);
+VALUE rb_int_succ(VALUE num);
+VALUE rb_int_uminus(VALUE num);
+VALUE rb_float_uminus(VALUE num);
+VALUE rb_int_plus(VALUE x, VALUE y);
+VALUE rb_float_plus(VALUE x, VALUE y);
+VALUE rb_int_minus(VALUE x, VALUE y);
+VALUE rb_int_mul(VALUE x, VALUE y);
+VALUE rb_float_mul(VALUE x, VALUE y);
+VALUE rb_float_div(VALUE x, VALUE y);
+VALUE rb_int_idiv(VALUE x, VALUE y);
+VALUE rb_int_modulo(VALUE x, VALUE y);
+VALUE rb_int2str(VALUE num, int base);
+VALUE rb_fix_plus(VALUE x, VALUE y);
+VALUE rb_fix_aref(VALUE fix, VALUE idx);
+VALUE rb_int_gt(VALUE x, VALUE y);
+int rb_float_cmp(VALUE x, VALUE y);
+VALUE rb_float_gt(VALUE x, VALUE y);
+VALUE rb_int_ge(VALUE x, VALUE y);
+enum ruby_num_rounding_mode rb_num_get_rounding_option(VALUE opts);
+double rb_int_fdiv_double(VALUE x, VALUE y);
+VALUE rb_int_pow(VALUE x, VALUE y);
+VALUE rb_float_pow(VALUE x, VALUE y);
+VALUE rb_int_cmp(VALUE x, VALUE y);
+VALUE rb_int_equal(VALUE x, VALUE y);
+VALUE rb_int_divmod(VALUE x, VALUE y);
+VALUE rb_int_and(VALUE x, VALUE y);
+VALUE rb_int_lshift(VALUE x, VALUE y);
+VALUE rb_int_div(VALUE x, VALUE y);
+VALUE rb_int_abs(VALUE num);
+VALUE rb_int_odd_p(VALUE num);
+int rb_int_positive_p(VALUE num);
+int rb_int_negative_p(VALUE num);
+VALUE rb_num_pow(VALUE x, VALUE y);
+VALUE rb_float_ceil(VALUE num, int ndigits);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* numeric.c (export) */
+VALUE rb_int_positive_pow(long x, unsigned long y);
+RUBY_SYMBOL_EXPORT_END
+
+
+static inline VALUE
+rb_num_compare_with_zero(VALUE num, ID mid)
+{
+    VALUE zero = INT2FIX(0);
+    VALUE r = rb_check_funcall(num, mid, 1, &zero);
+    if (r == Qundef) {
+        rb_cmperr(num, zero);
+    }
+    return r;
+}
+
+static inline int
+rb_num_positive_int_p(VALUE num)
+{
+    const ID mid = '>';
+
+    if (FIXNUM_P(num)) {
+        if (rb_method_basic_definition_p(rb_cInteger, mid))
+            return FIXNUM_POSITIVE_P(num);
+    }
+    else if (RB_TYPE_P(num, T_BIGNUM)) {
+        if (rb_method_basic_definition_p(rb_cInteger, mid))
+            return BIGNUM_POSITIVE_P(num);
+    }
+    return RTEST(rb_num_compare_with_zero(num, mid));
+}
+
+
+static inline int
+rb_num_negative_int_p(VALUE num)
+{
+    const ID mid = '<';
+
+    if (FIXNUM_P(num)) {
+        if (rb_method_basic_definition_p(rb_cInteger, mid))
+            return FIXNUM_NEGATIVE_P(num);
+    }
+    else if (RB_TYPE_P(num, T_BIGNUM)) {
+        if (rb_method_basic_definition_p(rb_cInteger, mid))
+            return BIGNUM_NEGATIVE_P(num);
+    }
+    return RTEST(rb_num_compare_with_zero(num, mid));
+}
+
+
+VALUE rb_float_abs(VALUE flt);
+VALUE rb_float_equal(VALUE x, VALUE y);
+VALUE rb_float_eql(VALUE x, VALUE y);
+VALUE rb_flo_div_flo(VALUE x, VALUE y);
+
+static inline double
+rb_float_flonum_value(VALUE v)
+{
+#if USE_FLONUM
+    if (v != (VALUE)0x8000000000000002) { /* LIKELY */
+        union {
+            double d;
+            VALUE v;
+        } t;
+
+        VALUE b63 = (v >> 63);
+        /* e: xx1... -> 011... */
+        /*    xx0... -> 100... */
+        /*      ^b63           */
+        t.v = RUBY_BIT_ROTR((2 - b63) | (v & ~(VALUE)0x03), 3);
+        return t.d;
+    }
+#endif
+    return 0.0;
+}
+
+static inline double
+rb_float_noflonum_value(VALUE v)
+{
+    return ((struct RFloat *)v)->float_value;
+}
+
+static inline double
+rb_float_value_inline(VALUE v)
+{
+    if (FLONUM_P(v)) {
+        return rb_float_flonum_value(v);
+    }
+    return rb_float_noflonum_value(v);
+}
+
+static inline VALUE
+rb_float_new_inline(double d)
+{
+#if USE_FLONUM
+    union {
+        double d;
+        VALUE v;
+    } t;
+    int bits;
+
+    t.d = d;
+    bits = (int)((VALUE)(t.v >> 60) & 0x7);
+    /* bits contains 3 bits of b62..b60. */
+    /* bits - 3 = */
+    /*   b011 -> b000 */
+    /*   b100 -> b001 */
+
+    if (t.v != 0x3000000000000000 /* 1.72723e-77 */ &&
+        !((bits-3) & ~0x01)) {
+        return (RUBY_BIT_ROTL(t.v, 3) & ~(VALUE)0x01) | 0x02;
+    }
+    else if (t.v == (VALUE)0) {
+        /* +0.0 */
+        return 0x8000000000000002;
+    }
+    /* out of range */
+#endif
+    return rb_float_new_in_heap(d);
+}
+
+#define rb_float_value(v) rb_float_value_inline(v)
+#define rb_float_new(d)   rb_float_new_inline(d)
+#endif /* INTERNAL_NUMERIC_H */
diff --git a/internal/object.h b/internal/object.h
new file mode 100644
index 0000000000..f67293769a
--- /dev/null
+++ b/internal/object.h
@@ -0,0 +1,45 @@
+#ifndef INTERNAL_OBJECT_H /* -*- C -*- */
+#define INTERNAL_OBJECT_H
+/**
+ * @file
+ * @brief      Internal header for Object.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+
+/* object.c */
+void rb_obj_copy_ivar(VALUE dest, VALUE obj);
+CONSTFUNC(VALUE rb_obj_equal(VALUE obj1, VALUE obj2));
+CONSTFUNC(VALUE rb_obj_not(VALUE obj));
+VALUE rb_class_search_ancestor(VALUE klass, VALUE super);
+NORETURN(void rb_undefined_alloc(VALUE klass));
+double rb_num_to_dbl(VALUE val);
+VALUE rb_obj_dig(int argc, VALUE *argv, VALUE self, VALUE notfound);
+VALUE rb_immutable_obj_clone(int, VALUE *, VALUE);
+VALUE rb_obj_not_equal(VALUE obj1, VALUE obj2);
+VALUE rb_convert_type_with_id(VALUE,int,const char*,ID);
+VALUE rb_check_convert_type_with_id(VALUE,int,const char*,ID);
+int rb_bool_expected(VALUE, const char *);
+
+struct RBasicRaw {
+    VALUE flags;
+    VALUE klass;
+};
+
+#define RBASIC_CLEAR_CLASS(obj)        memset(&(((struct RBasicRaw *)((VALUE)(obj)))->klass), 0, sizeof(VALUE))
+#define RBASIC_SET_CLASS_RAW(obj, cls) memcpy(&((struct RBasicRaw *)((VALUE)(obj)))->klass, &(cls), sizeof(VALUE))
+#define RBASIC_SET_CLASS(obj, cls)     do { \
+    VALUE _obj_ = (obj); \
+    RB_OBJ_WRITE(_obj_, &((struct RBasicRaw *)(_obj_))->klass, cls); \
+} while (0)
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* object.c (export) */
+int rb_opts_exception_p(VALUE opts, int default_value);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_OBJECT_H */
diff --git a/internal/parse.h b/internal/parse.h
new file mode 100644
index 0000000000..f70e8e75a3
--- /dev/null
+++ b/internal/parse.h
@@ -0,0 +1,31 @@
+#ifndef INTERNAL_PARSE_H /* -*- C -*- */
+#define INTERNAL_PARSE_H
+/**
+ * @file
+ * @brief      Internal header for the parser.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* parse.y */
+#ifndef USE_SYMBOL_GC
+#define USE_SYMBOL_GC 1
+#endif
+VALUE rb_parser_set_yydebug(VALUE, VALUE);
+RUBY_SYMBOL_EXPORT_BEGIN
+VALUE rb_parser_set_context(VALUE, const struct rb_iseq_struct *, int);
+RUBY_SYMBOL_EXPORT_END
+void *rb_parser_load_file(VALUE parser, VALUE name);
+int rb_is_const_name(VALUE name);
+int rb_is_class_name(VALUE name);
+int rb_is_instance_name(VALUE name);
+int rb_is_local_name(VALUE name);
+PUREFUNC(int rb_is_const_sym(VALUE sym));
+PUREFUNC(int rb_is_attrset_sym(VALUE sym));
+ID rb_make_internal_id(void);
+void rb_gc_free_dsymbol(VALUE);
+
+#endif /* INTERNAL_PARSE_H */
diff --git a/internal/proc.h b/internal/proc.h
new file mode 100644
index 0000000000..2831afc95c
--- /dev/null
+++ b/internal/proc.h
@@ -0,0 +1,24 @@
+#ifndef INTERNAL_PROC_H /* -*- C -*- */
+#define INTERNAL_PROC_H
+/**
+ * @file
+ * @brief      Internal header for Proc.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct rb_block;
+
+/* proc.c */
+VALUE rb_proc_location(VALUE self);
+st_index_t rb_hash_proc(st_index_t hash, VALUE proc);
+int rb_block_arity(void);
+int rb_block_min_max_arity(int *max);
+VALUE rb_func_proc_new(rb_block_call_func_t func, VALUE val);
+VALUE rb_func_lambda_new(rb_block_call_func_t func, VALUE val, int min_argc, int max_argc);
+VALUE rb_block_to_s(VALUE self, const struct rb_block *block, const char *additional_info);
+
+#endif /* INTERNAL_PROC_H */
diff --git a/internal/process.h b/internal/process.h
new file mode 100644
index 0000000000..8258f97f1b
--- /dev/null
+++ b/internal/process.h
@@ -0,0 +1,95 @@
+#ifndef INTERNAL_PROCESS_H /* -*- C -*- */
+#define INTERNAL_PROCESS_H
+/**
+ * @file
+ * @brief      Internal header for Process.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* process.c */
+#define RB_MAX_GROUPS (65536)
+
+struct waitpid_state;
+struct rb_execarg {
+    union {
+        struct {
+            VALUE shell_script;
+        } sh;
+        struct {
+            VALUE command_name;
+            VALUE command_abspath; /* full path string or nil */
+            VALUE argv_str;
+            VALUE argv_buf;
+        } cmd;
+    } invoke;
+    VALUE redirect_fds;
+    VALUE envp_str;
+    VALUE envp_buf;
+    VALUE dup2_tmpbuf;
+    unsigned use_shell : 1;
+    unsigned pgroup_given : 1;
+    unsigned umask_given : 1;
+    unsigned unsetenv_others_given : 1;
+    unsigned unsetenv_others_do : 1;
+    unsigned close_others_given : 1;
+    unsigned close_others_do : 1;
+    unsigned chdir_given : 1;
+    unsigned new_pgroup_given : 1;
+    unsigned new_pgroup_flag : 1;
+    unsigned uid_given : 1;
+    unsigned gid_given : 1;
+    unsigned exception : 1;
+    unsigned exception_given : 1;
+    struct waitpid_state *waitpid_state; /* for async process management */
+    rb_pid_t pgroup_pgid; /* asis(-1), new pgroup(0), specified pgroup (0<V). */
+    VALUE rlimit_limits; /* Qfalse or [[rtype, softlim, hardlim], ...] */
+    mode_t umask_mask;
+    rb_uid_t uid;
+    rb_gid_t gid;
+    int close_others_maxhint;
+    VALUE fd_dup2;
+    VALUE fd_close;
+    VALUE fd_open;
+    VALUE fd_dup2_child;
+    VALUE env_modification; /* Qfalse or [[k1,v1], ...] */
+    VALUE path_env;
+    VALUE chdir_dir;
+};
+
+/* argv_str contains extra two elements.
+ * The beginning one is for /bin/sh used by exec_with_sh.
+ * The last one for terminating NULL used by execve.
+ * See rb_exec_fillarg() in process.c. */
+#define ARGVSTR2ARGV(argv_str) ((char **)RB_IMEMO_TMPBUF_PTR(argv_str) + 1)
+
+static inline size_t
+ARGVSTR2ARGC(VALUE argv_str)
+{
+    size_t i = 0;
+    char *const *p = ARGVSTR2ARGV(argv_str);
+    while (p[i++])
+        ;
+    return i - 1;
+}
+
+rb_pid_t rb_fork_ruby(int *status);
+void rb_last_status_clear(void);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* process.c (export) */
+int rb_exec_async_signal_safe(const struct rb_execarg *e, char *errmsg, size_t errmsg_buflen);
+rb_pid_t rb_fork_async_signal_safe(int *status, int (*chfunc)(void*, char *, size_t), void *charg, VALUE fds, char *errmsg, size_t errmsg_buflen);
+VALUE rb_execarg_new(int argc, const VALUE *argv, int accept_shell, int allow_exc_opt);
+struct rb_execarg *rb_execarg_get(VALUE execarg_obj); /* dangerous.  needs GC guard. */
+int rb_execarg_addopt(VALUE execarg_obj, VALUE key, VALUE val);
+void rb_execarg_parent_start(VALUE execarg_obj);
+void rb_execarg_parent_end(VALUE execarg_obj);
+int rb_execarg_run_options(const struct rb_execarg *e, struct rb_execarg *s, char* errmsg, size_t errmsg_buflen);
+VALUE rb_execarg_extract_options(VALUE execarg_obj, VALUE opthash);
+void rb_execarg_setenv(VALUE execarg_obj, VALUE env);
+RUBY_SYMBOL_EXPORT_END
+#endif /* INTERNAL_PROCESS_H */
diff --git a/internal/random.h b/internal/random.h
new file mode 100644
index 0000000000..6498769c86
--- /dev/null
+++ b/internal/random.h
@@ -0,0 +1,16 @@
+#ifndef INTERNAL_RANDOM_H /* -*- C -*- */
+#define INTERNAL_RANDOM_H
+/**
+ * @file
+ * @brief      Internal header for Random.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* random.c */
+int ruby_fill_random_bytes(void *, size_t, int);
+
+#endif /* INTERNAL_RANDOM_H */
diff --git a/internal/range.h b/internal/range.h
new file mode 100644
index 0000000000..a9101cd3f9
--- /dev/null
+++ b/internal/range.h
@@ -0,0 +1,18 @@
+#ifndef INTERNAL_RANGE_H /* -*- C -*- */
+#define INTERNAL_RANGE_H
+/**
+ * @file
+ * @brief      Internal header for Range.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* range.c */
+#define RANGE_BEG(r) (RSTRUCT(r)->as.ary[0])
+#define RANGE_END(r) (RSTRUCT(r)->as.ary[1])
+#define RANGE_EXCL(r) (RSTRUCT(r)->as.ary[2])
+
+#endif /* INTERNAL_RANGE_H */
diff --git a/internal/rational.h b/internal/rational.h
new file mode 100644
index 0000000000..993d056927
--- /dev/null
+++ b/internal/rational.h
@@ -0,0 +1,47 @@
+#ifndef INTERNAL_RATIONAL_H /* -*- C -*- */
+#define INTERNAL_RATIONAL_H
+/**
+ * @file
+ * @brief      Internal header for Rational.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+struct RRational {
+    struct RBasic basic;
+    VALUE num;
+    VALUE den;
+};
+
+#define RRATIONAL(obj) (R_CAST(RRational)(obj))
+#define RRATIONAL_SET_NUM(rat, n) RB_OBJ_WRITE((rat), &((struct RRational *)(rat))->num,(n))
+#define RRATIONAL_SET_DEN(rat, d) RB_OBJ_WRITE((rat), &((struct RRational *)(rat))->den,(d))
+
+/* rational.c */
+VALUE rb_rational_canonicalize(VALUE x);
+VALUE rb_rational_uminus(VALUE self);
+VALUE rb_rational_plus(VALUE self, VALUE other);
+VALUE rb_rational_mul(VALUE self, VALUE other);
+VALUE rb_lcm(VALUE x, VALUE y);
+VALUE rb_rational_reciprocal(VALUE x);
+VALUE rb_cstr_to_rat(const char *, int);
+VALUE rb_rational_abs(VALUE self);
+VALUE rb_rational_cmp(VALUE self, VALUE other);
+VALUE rb_rational_pow(VALUE self, VALUE other);
+VALUE rb_numeric_quo(VALUE x, VALUE y);
+VALUE rb_float_numerator(VALUE x);
+VALUE rb_float_denominator(VALUE x);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* rational.c (export) */
+VALUE rb_gcd(VALUE x, VALUE y);
+VALUE rb_gcd_normal(VALUE self, VALUE other);
+#if defined(HAVE_LIBGMP) && defined(HAVE_GMP_H)
+VALUE rb_gcd_gmp(VALUE x, VALUE y);
+#endif
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_RATIONAL_H */
diff --git a/internal/re.h b/internal/re.h
new file mode 100644
index 0000000000..7676030684
--- /dev/null
+++ b/internal/re.h
@@ -0,0 +1,25 @@
+#ifndef INTERNAL_RE_H /* -*- C -*- */
+#define INTERNAL_RE_H
+/**
+ * @file
+ * @brief      Internal header for Regexp.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* re.c */
+VALUE rb_reg_compile(VALUE str, int options, const char *sourcefile, int sourceline);
+VALUE rb_reg_check_preprocess(VALUE);
+long rb_reg_search0(VALUE, VALUE, long, int, int);
+VALUE rb_reg_match_p(VALUE re, VALUE str, long pos);
+bool rb_reg_start_with_p(VALUE re, VALUE str);
+void rb_backref_set_string(VALUE string, long pos, long len);
+void rb_match_unbusy(VALUE);
+int rb_match_count(VALUE match);
+int rb_match_nth_defined(int nth, VALUE match);
+VALUE rb_reg_new_ary(VALUE ary, int options);
+
+#endif /* INTERNAL_RE_H */
diff --git a/internal/sanitizers.h b/internal/sanitizers.h
new file mode 100644
index 0000000000..4c6ad28e71
--- /dev/null
+++ b/internal/sanitizers.h
@@ -0,0 +1,179 @@
+#ifndef INTERNAL_SANITIZERS_H /* -*- C -*- */
+#define INTERNAL_SANITIZERS_H
+/**
+ * @file
+ * @brief      Internal header for ASAN / MSAN / etc.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+
+#if 0
+#elif defined(NO_SANITIZE) && __has_feature(memory_sanitizer)
+# define ATTRIBUTE_NO_ADDRESS_SAFETY_ANALYSIS(x) \
+    NO_SANITIZE("memory", NO_SANITIZE("address", NOINLINE(x)))
+#elif defined(NO_SANITIZE)
+# define ATTRIBUTE_NO_ADDRESS_SAFETY_ANALYSIS(x) \
+    NO_SANITIZE("address", NOINLINE(x))
+#elif defined(NO_SANITIZE_ADDRESS)
+# define ATTRIBUTE_NO_ADDRESS_SAFETY_ANALYSIS(x) \
+    NO_SANITIZE_ADDRESS(NOINLINE(x))
+#elif defined(NO_ADDRESS_SAFETY_ANALYSIS)
+# define ATTRIBUTE_NO_ADDRESS_SAFETY_ANALYSIS(x) \
+    NO_ADDRESS_SAFETY_ANALYSIS(NOINLINE(x))
+#else
+# define ATTRIBUTE_NO_ADDRESS_SAFETY_ANALYSIS(x) x
+#endif
+
+#if defined(NO_SANITIZE) && defined(__GNUC__) &&! defined(__clang__)
+/* GCC warns about unknown sanitizer, which is annoying. */
+#undef NO_SANITIZE
+#define NO_SANITIZE(x, y) \
+    COMPILER_WARNING_PUSH; \
+    COMPILER_WARNING_IGNORED(-Wattributes); \
+    __attribute__((__no_sanitize__(x))) y; \
+    COMPILER_WARNING_POP
+#endif
+
+#ifndef NO_SANITIZE
+# define NO_SANITIZE(x, y) y
+#endif
+
+#ifdef HAVE_VALGRIND_MEMCHECK_H
+# include <valgrind/memcheck.h>
+# ifndef VALGRIND_MAKE_MEM_DEFINED
+#  define VALGRIND_MAKE_MEM_DEFINED(p, n) VALGRIND_MAKE_READABLE((p), (n))
+# endif
+# ifndef VALGRIND_MAKE_MEM_UNDEFINED
+#  define VALGRIND_MAKE_MEM_UNDEFINED(p, n) VALGRIND_MAKE_WRITABLE((p), (n))
+# endif
+#else
+# define VALGRIND_MAKE_MEM_DEFINED(p, n) 0
+# define VALGRIND_MAKE_MEM_UNDEFINED(p, n) 0
+#endif
+
+#ifndef MJIT_HEADER
+
+#ifdef HAVE_SANITIZER_ASAN_INTERFACE_H
+# include <sanitizer/asan_interface.h>
+#endif
+
+#if !__has_feature(address_sanitizer)
+# define __asan_poison_memory_region(x, y)
+# define __asan_unpoison_memory_region(x, y)
+# define __asan_region_is_poisoned(x, y) 0
+#endif
+
+#ifdef HAVE_SANITIZER_MSAN_INTERFACE_H
+# if __has_feature(memory_sanitizer)
+#  include <sanitizer/msan_interface.h>
+# endif
+#endif
+
+#if !__has_feature(memory_sanitizer)
+# define __msan_allocated_memory(x, y) ((void)(x), (void)(y))
+# define __msan_poison(x, y) ((void)(x), (void)(y))
+# define __msan_unpoison(x, y) ((void)(x), (void)(y))
+# define __msan_unpoison_string(x) ((void)(x))
+#endif
+
+/*!
+ * This function asserts that a (continuous) memory region from ptr to size
+ * being "poisoned".  Both read / write access to such memory region are
+ * prohibited until properly unpoisoned.  The region must be previously
+ * allocated (do not pass a freed pointer here), but not necessarily be an
+ * entire object that the malloc returns.  You can punch hole a part of a
+ * gigantic heap arena.  This is handy when you do not free an allocated memory
+ * region to reuse later: poison when you keep it unused, and unpoison when you
+ * reuse.
+ *
+ * \param[in]  ptr   pointer to the beginning of the memory region to poison.
+ * \param[in]  size  the length of the memory region to poison.
+ */
+static inline void
+asan_poison_memory_region(const volatile void *ptr, size_t size)
+{
+    __msan_poison(ptr, size);
+    __asan_poison_memory_region(ptr, size);
+}
+
+/*!
+ * This is a variant of asan_poison_memory_region that takes a VALUE.
+ *
+ * \param[in]  obj   target object.
+ */
+static inline void
+asan_poison_object(VALUE obj)
+{
+    MAYBE_UNUSED(struct RVALUE *) ptr = (void *)obj;
+    asan_poison_memory_region(ptr, SIZEOF_VALUE);
+}
+
+#if !__has_feature(address_sanitizer)
+#define asan_poison_object_if(ptr, obj) ((void)(ptr), (void)(obj))
+#else
+#define asan_poison_object_if(ptr, obj) do { \
+        if (ptr) asan_poison_object(obj); \
+    } while (0)
+#endif
+
+/*!
+ * This function predicates if the given object is fully addressable or not.
+ *
+ * \param[in]  obj        target object.
+ * \retval     0          the given object is fully addressable.
+ * \retval     otherwise  pointer to first such byte who is poisoned.
+ */
+static inline void *
+asan_poisoned_object_p(VALUE obj)
+{
+    MAYBE_UNUSED(struct RVALUE *) ptr = (void *)obj;
+    return __asan_region_is_poisoned(ptr, SIZEOF_VALUE);
+}
+
+/*!
+ * This function asserts that a (formally poisoned) memory region from ptr to
+ * size is now addressable.  Write access to such memory region gets allowed.
+ * However read access might or might not be possible depending on situations,
+ * because the region can have contents of previous usages.  That information
+ * should be passed by the malloc_p flag.  If that is true, the contents of the
+ * region is _not_ fully defined (like the return value of malloc behaves).
+ * Reading from there is NG; write something first.  If malloc_p is false on
+ * the other hand, that memory region is fully defined and can be read
+ * immediately.
+ *
+ * \param[in]  ptr       pointer to the beginning of the memory region to unpoison.
+ * \param[in]  size      the length of the memory region.
+ * \param[in]  malloc_p  if the memory region is like a malloc's return value or not.
+ */
+static inline void
+asan_unpoison_memory_region(const volatile void *ptr, size_t size, bool malloc_p)
+{
+    __asan_unpoison_memory_region(ptr, size);
+    if (malloc_p) {
+        __msan_allocated_memory(ptr, size);
+    }
+    else {
+        __msan_unpoison(ptr, size);
+    }
+}
+
+/*!
+ * This is a variant of asan_unpoison_memory_region that takes a VALUE.
+ *
+ * \param[in]  obj       target object.
+ * \param[in]  malloc_p  if the memory region is like a malloc's return value or not.
+ */
+static inline void
+asan_unpoison_object(VALUE obj, bool newobj_p)
+{
+    MAYBE_UNUSED(struct RVALUE *) ptr = (void *)obj;
+    asan_unpoison_memory_region(ptr, SIZEOF_VALUE, newobj_p);
+}
+
+#endif
+
+#endif /* INTERNAL_SANITIZERS_H */
diff --git a/internal/serial.h b/internal/serial.h
new file mode 100644
index 0000000000..224ab47ac6
--- /dev/null
+++ b/internal/serial.h
@@ -0,0 +1,30 @@
+#ifndef INTERNAL_SERIAL_H /* -*- C -*- */
+#define INTERNAL_SERIAL_H
+/**
+ * @file
+ * @brief      Internal header for rb_serial_t.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#if defined(HAVE_LONG_LONG)
+typedef unsigned LONG_LONG rb_serial_t;
+# define SERIALT2NUM ULL2NUM
+# define PRI_SERIALT_PREFIX PRI_LL_PREFIX
+# define SIZEOF_SERIAL_T SIZEOF_LONG_LONG
+#elif defined(HAVE_UINT64_T)
+typedef uint64_t rb_serial_t;
+# define SERIALT2NUM SIZET2NUM
+# define PRI_SERIALT_PREFIX PRI_64_PREFIX
+# define SIZEOF_SERIAL_T SIZEOF_UINT64_T
+#else
+typedef unsigned long rb_serial_t;
+# define SERIALT2NUM ULONG2NUM
+# define PRI_SERIALT_PREFIX PRI_LONG_PREFIX
+# define SIZEOF_SERIAL_T SIZEOF_LONG
+#endif
+
+#endif /* INTERNAL_SERIAL_H */
diff --git a/internal/signal.h b/internal/signal.h
new file mode 100644
index 0000000000..0d501f7895
--- /dev/null
+++ b/internal/signal.h
@@ -0,0 +1,22 @@
+#ifndef INTERNAL_SIGNAL_H /* -*- C -*- */
+#define INTERNAL_SIGNAL_H
+/**
+ * @file
+ * @brief      Internal header for SignalException.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* signal.c */
+extern int ruby_enable_coredump;
+int rb_get_next_signal(void);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* signal.c (export) */
+int rb_grantpt(int fd);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_SIGNAL_H */
diff --git a/internal/static_assert.h b/internal/static_assert.h
new file mode 100644
index 0000000000..05b79b4ba3
--- /dev/null
+++ b/internal/static_assert.h
@@ -0,0 +1,21 @@
+#ifndef INTERNAL_STATIC_ASSERT_H /* -*- C -*- */
+#define INTERNAL_STATIC_ASSERT_H
+/**
+ * @file
+ * @brief      C11 shim for _Static_assert.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
+# define STATIC_ASSERT(name, expr) _Static_assert(expr, #name ": " #expr)
+#elif GCC_VERSION_SINCE(4, 6, 0) || __has_extension(c_static_assert)
+# define STATIC_ASSERT(name, expr) RB_GNUC_EXTENSION _Static_assert(expr, #name ": " #expr)
+#else
+# define STATIC_ASSERT(name, expr) typedef int static_assert_##name##_check[1 - 2*!(expr)]
+#endif
+
+#endif /* INTERNAL_STATIC_ASSERT_H */
diff --git a/internal/stdbool.h b/internal/stdbool.h
new file mode 100644
index 0000000000..4c103b43ce
--- /dev/null
+++ b/internal/stdbool.h
@@ -0,0 +1,20 @@
+#ifndef INTERNAL_STDBOOL_H /* -*- C -*- */
+#define INTERNAL_STDBOOL_H
+/**
+ * @file
+ * @brief      Thin wrapper to <stdbool.h>
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+#include "ruby/config.h" /* for HAVE_STDBOOL_H */
+
+#ifdef HAVE_STDBOOL_H
+# include <stdbool.h>
+#else
+# include "missing/stdbool.h"
+#endif
+
+#endif /* INTERNAL_STDBOOL_H */
diff --git a/internal/string.h b/internal/string.h
new file mode 100644
index 0000000000..2135490029
--- /dev/null
+++ b/internal/string.h
@@ -0,0 +1,92 @@
+#ifndef INTERNAL_STRING_H /* -*- C -*- */
+#define INTERNAL_STRING_H
+/**
+ * @file
+ * @brief      Internal header for String.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+
+/* string.c */
+VALUE rb_fstring(VALUE);
+VALUE rb_fstring_new(const char *ptr, long len);
+#define rb_fstring_lit(str) rb_fstring_new((str), rb_strlen_lit(str))
+#define rb_fstring_literal(str) rb_fstring_lit(str)
+VALUE rb_fstring_cstr(const char *str);
+#ifdef HAVE_BUILTIN___BUILTIN_CONSTANT_P
+# define rb_fstring_cstr(str) RB_GNUC_EXTENSION_BLOCK(  \
+    (__builtin_constant_p(str)) ?               \
+        rb_fstring_new((str), (long)strlen(str)) : \
+        rb_fstring_cstr(str) \
+)
+#endif
+#ifdef RUBY_ENCODING_H
+VALUE rb_fstring_enc_new(const char *ptr, long len, rb_encoding *enc);
+#define rb_fstring_enc_lit(str, enc) rb_fstring_enc_new((str), rb_strlen_lit(str), (enc))
+#define rb_fstring_enc_literal(str, enc) rb_fstring_enc_lit(str, enc)
+#endif
+int rb_str_buf_cat_escaped_char(VALUE result, unsigned int c, int unicode_p);
+int rb_str_symname_p(VALUE);
+VALUE rb_str_quote_unprintable(VALUE);
+VALUE rb_id_quote_unprintable(ID);
+#define QUOTE(str) rb_str_quote_unprintable(str)
+#define QUOTE_ID(id) rb_id_quote_unprintable(id)
+char *rb_str_fill_terminator(VALUE str, const int termlen);
+void rb_str_change_terminator_length(VALUE str, const int oldtermlen, const int termlen);
+VALUE rb_str_locktmp_ensure(VALUE str, VALUE (*func)(VALUE), VALUE arg);
+VALUE rb_str_chomp_string(VALUE str, VALUE chomp);
+#ifdef RUBY_ENCODING_H
+VALUE rb_external_str_with_enc(VALUE str, rb_encoding *eenc);
+VALUE rb_str_cat_conv_enc_opts(VALUE newstr, long ofs, const char *ptr, long len,
+                               rb_encoding *from, int ecflags, VALUE ecopts);
+VALUE rb_enc_str_scrub(rb_encoding *enc, VALUE str, VALUE repl);
+VALUE rb_str_initialize(VALUE str, const char *ptr, long len, rb_encoding *enc);
+#endif
+#define STR_NOEMBED      FL_USER1
+#define STR_SHARED       FL_USER2 /* = ELTS_SHARED */
+#define STR_EMBED_P(str) (!FL_TEST_RAW((str), STR_NOEMBED))
+#define STR_SHARED_P(s)  FL_ALL_RAW((s), STR_NOEMBED|ELTS_SHARED)
+#define is_ascii_string(str) (rb_enc_str_coderange(str) == ENC_CODERANGE_7BIT)
+#define is_broken_string(str) (rb_enc_str_coderange(str) == ENC_CODERANGE_BROKEN)
+size_t rb_str_memsize(VALUE);
+VALUE rb_sym_proc_call(ID mid, int argc, const VALUE *argv, int kw_splat, VALUE passed_proc);
+VALUE rb_sym_to_proc(VALUE sym);
+char *rb_str_to_cstr(VALUE str);
+VALUE rb_str_eql(VALUE str1, VALUE str2);
+VALUE rb_obj_as_string_result(VALUE str, VALUE obj);
+const char *ruby_escaped_char(int c);
+VALUE rb_str_opt_plus(VALUE, VALUE);
+
+/* expect tail call optimization */
+static inline VALUE
+rb_str_eql_internal(const VALUE str1, const VALUE str2)
+{
+    const long len = RSTRING_LEN(str1);
+    const char *ptr1, *ptr2;
+
+    if (len != RSTRING_LEN(str2)) return Qfalse;
+    if (!rb_str_comparable(str1, str2)) return Qfalse;
+    if ((ptr1 = RSTRING_PTR(str1)) == (ptr2 = RSTRING_PTR(str2)))
+        return Qtrue;
+    if (memcmp(ptr1, ptr2, len) == 0)
+        return Qtrue;
+    return Qfalse;
+}
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* string.c (export) */
+VALUE rb_str_tmp_frozen_acquire(VALUE str);
+void rb_str_tmp_frozen_release(VALUE str, VALUE tmp);
+#ifdef RUBY_ENCODING_H
+/* internal use */
+VALUE rb_setup_fake_str(struct RString *fake_str, const char *name, long len, rb_encoding *enc);
+#endif
+VALUE rb_str_upto_each(VALUE, VALUE, int, int (*each)(VALUE, VALUE), VALUE);
+VALUE rb_str_upto_endless_each(VALUE, int (*each)(VALUE, VALUE), VALUE);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_STRING_H */
diff --git a/internal/struct.h b/internal/struct.h
new file mode 100644
index 0000000000..f63b45007e
--- /dev/null
+++ b/internal/struct.h
@@ -0,0 +1,88 @@
+#ifndef INTERNAL_STRUCT_H /* -*- C -*- */
+#define INTERNAL_STRUCT_H
+/**
+ * @file
+ * @brief      Internal header for Struct.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#define RSTRUCT_EMBED_LEN_MAX RSTRUCT_EMBED_LEN_MAX
+#define RSTRUCT_EMBED_LEN_MASK RSTRUCT_EMBED_LEN_MASK
+#define RSTRUCT_EMBED_LEN_SHIFT RSTRUCT_EMBED_LEN_SHIFT
+
+enum {
+    RSTRUCT_EMBED_LEN_MAX = RVALUE_EMBED_LEN_MAX,
+    RSTRUCT_EMBED_LEN_MASK = (RUBY_FL_USER2|RUBY_FL_USER1),
+    RSTRUCT_EMBED_LEN_SHIFT = (RUBY_FL_USHIFT+1),
+    RSTRUCT_TRANSIENT_FLAG = FL_USER3,
+
+    RSTRUCT_ENUM_END
+};
+
+#if USE_TRANSIENT_HEAP
+#define RSTRUCT_TRANSIENT_P(st) FL_TEST_RAW((obj), RSTRUCT_TRANSIENT_FLAG)
+#define RSTRUCT_TRANSIENT_SET(st) FL_SET_RAW((st), RSTRUCT_TRANSIENT_FLAG)
+#define RSTRUCT_TRANSIENT_UNSET(st) FL_UNSET_RAW((st), RSTRUCT_TRANSIENT_FLAG)
+#else
+#define RSTRUCT_TRANSIENT_P(st) 0
+#define RSTRUCT_TRANSIENT_SET(st) ((void)0)
+#define RSTRUCT_TRANSIENT_UNSET(st) ((void)0)
+#endif
+
+struct RStruct {
+    struct RBasic basic;
+    union {
+        struct {
+            long len;
+            const VALUE *ptr;
+        } heap;
+        const VALUE ary[RSTRUCT_EMBED_LEN_MAX];
+    } as;
+};
+
+#undef RSTRUCT_LEN
+#undef RSTRUCT_PTR
+#undef RSTRUCT_SET
+#undef RSTRUCT_GET
+#define RSTRUCT_EMBED_LEN(st)                               \
+    (long)((RBASIC(st)->flags >> RSTRUCT_EMBED_LEN_SHIFT) & \
+           (RSTRUCT_EMBED_LEN_MASK >> RSTRUCT_EMBED_LEN_SHIFT))
+#define RSTRUCT_LEN(st) rb_struct_len(st)
+#define RSTRUCT_LENINT(st) rb_long2int(RSTRUCT_LEN(st))
+#define RSTRUCT_CONST_PTR(st) rb_struct_const_ptr(st)
+#define RSTRUCT_PTR(st) ((VALUE *)RSTRUCT_CONST_PTR(RB_OBJ_WB_UNPROTECT_FOR(STRUCT, st)))
+#define RSTRUCT_SET(st, idx, v) RB_OBJ_WRITE(st, &RSTRUCT_CONST_PTR(st)[idx], (v))
+#define RSTRUCT_GET(st, idx)    (RSTRUCT_CONST_PTR(st)[idx])
+#define RSTRUCT(obj) (R_CAST(RStruct)(obj))
+
+/* struct.c */
+VALUE rb_struct_init_copy(VALUE copy, VALUE s);
+VALUE rb_struct_lookup(VALUE s, VALUE idx);
+VALUE rb_struct_s_keyword_init(VALUE klass);
+
+static inline long
+rb_struct_len(VALUE st)
+{
+    return (RBASIC(st)->flags & RSTRUCT_EMBED_LEN_MASK) ?
+        RSTRUCT_EMBED_LEN(st) : RSTRUCT(st)->as.heap.len;
+}
+
+static inline const VALUE *
+rb_struct_const_ptr(VALUE st)
+{
+    return FIX_CONST_VALUE_PTR((RBASIC(st)->flags & RSTRUCT_EMBED_LEN_MASK) ?
+        RSTRUCT(st)->as.ary : RSTRUCT(st)->as.heap.ptr);
+}
+
+static inline const VALUE *
+rb_struct_const_heap_ptr(VALUE st)
+{
+    /* TODO: check embed on debug mode */
+    return RSTRUCT(st)->as.heap.ptr;
+}
+
+#endif /* INTERNAL_STRUCT_H */
diff --git a/internal/symbol.h b/internal/symbol.h
new file mode 100644
index 0000000000..8423cb4dc8
--- /dev/null
+++ b/internal/symbol.h
@@ -0,0 +1,29 @@
+#ifndef INTERNAL_SYMBOL_H /* -*- C -*- */
+#define INTERNAL_SYMBOL_H
+/**
+ * @file
+ * @brief      Internal header for Symbol.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* symbol.c */
+#ifdef RUBY_ENCODING_H
+VALUE rb_sym_intern(const char *ptr, long len, rb_encoding *enc);
+#endif
+VALUE rb_sym_intern_ascii(const char *ptr, long len);
+VALUE rb_sym_intern_ascii_cstr(const char *ptr);
+#ifdef __GNUC__
+#define rb_sym_intern_ascii_cstr(ptr) __extension__ ( \
+{                                               \
+    (__builtin_constant_p(ptr)) ?               \
+        rb_sym_intern_ascii((ptr), (long)strlen(ptr)) : \
+        rb_sym_intern_ascii_cstr(ptr); \
+})
+#endif
+VALUE rb_to_symbol_type(VALUE obj);
+
+#endif /* INTERNAL_SYMBOL_H */
diff --git a/internal/thread.h b/internal/thread.h
new file mode 100644
index 0000000000..568351e6cb
--- /dev/null
+++ b/internal/thread.h
@@ -0,0 +1,44 @@
+#ifndef INTERNAL_THREAD_H /* -*- C -*- */
+#define INTERNAL_THREAD_H
+/**
+ * @file
+ * @brief      Internal header for Thread.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* thread.c */
+#define COVERAGE_INDEX_LINES    0
+#define COVERAGE_INDEX_BRANCHES 1
+#define COVERAGE_TARGET_LINES    1
+#define COVERAGE_TARGET_BRANCHES 2
+#define COVERAGE_TARGET_METHODS  4
+#define COVERAGE_TARGET_ONESHOT_LINES 8
+
+VALUE rb_obj_is_mutex(VALUE obj);
+VALUE rb_suppress_tracing(VALUE (*func)(VALUE), VALUE arg);
+void rb_thread_execute_interrupts(VALUE th);
+VALUE rb_get_coverages(void);
+int rb_get_coverage_mode(void);
+VALUE rb_default_coverage(int);
+VALUE rb_thread_shield_new(void);
+VALUE rb_thread_shield_wait(VALUE self);
+VALUE rb_thread_shield_release(VALUE self);
+VALUE rb_thread_shield_destroy(VALUE self);
+int rb_thread_to_be_killed(VALUE thread);
+void rb_mutex_allow_trap(VALUE self, int val);
+VALUE rb_uninterruptible(VALUE (*b_proc)(VALUE), VALUE data);
+VALUE rb_mutex_owned_p(VALUE self);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* Temporary.  This API will be removed (renamed). */
+VALUE rb_thread_io_blocking_region(rb_blocking_function_t *func, void *data1, int fd);
+
+/* thread.c (export) */
+int ruby_thread_has_gvl_p(void); /* for ext/fiddle/closure.c */
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_THREAD_H */
diff --git a/internal/time.h b/internal/time.h
new file mode 100644
index 0000000000..b20f24e1cf
--- /dev/null
+++ b/internal/time.h
@@ -0,0 +1,30 @@
+#ifndef INTERNAL_TIME_H /* -*- C -*- */
+#define INTERNAL_TIME_H
+/**
+ * @file
+ * @brief      Internal header for Time.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#if SIGNEDNESS_OF_TIME_T < 0    /* signed */
+# define TIMET_MAX SIGNED_INTEGER_MAX(time_t)
+# define TIMET_MIN SIGNED_INTEGER_MIN(time_t)
+#elif SIGNEDNESS_OF_TIME_T > 0  /* unsigned */
+# define TIMET_MAX UNSIGNED_INTEGER_MAX(time_t)
+# define TIMET_MIN ((time_t)0)
+#endif
+#define TIMET_MAX_PLUS_ONE (2*(double)(TIMET_MAX/2+1))
+
+/* time.c */
+struct timeval rb_time_timeval(VALUE);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* time.c (export) */
+void ruby_reset_leap_second_info(void);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_TIME_H */
diff --git a/internal/transcode.h b/internal/transcode.h
new file mode 100644
index 0000000000..798c9265c2
--- /dev/null
+++ b/internal/transcode.h
@@ -0,0 +1,19 @@
+#ifndef INTERNAL_TRANSCODE_H /* -*- C -*- */
+#define INTERNAL_TRANSCODE_H
+/**
+ * @file
+ * @brief      Internal header for Encoding::Converter.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* transcode.c */
+extern VALUE rb_cEncodingConverter;
+#ifdef RUBY_ENCODING_H
+size_t rb_econv_memsize(rb_econv_t *);
+#endif
+
+#endif /* INTERNAL_TRANSCODE_H */
diff --git a/internal/util.h b/internal/util.h
new file mode 100644
index 0000000000..c1ba2a4323
--- /dev/null
+++ b/internal/util.h
@@ -0,0 +1,25 @@
+#ifndef INTERNAL_UTIL_H /* -*- C -*- */
+#define INTERNAL_UTIL_H
+/**
+ * @file
+ * @brief      Internal header corresponding util.c.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    DO NOT ADD RANDOM GARBAGE HERE THIS FILE IS FOR util.c
+ */
+
+/* util.c */
+char *ruby_dtoa(double d_, int mode, int ndigits, int *decpt, int *sign, char **rve);
+char *ruby_hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign, char **rve);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* util.c (export) */
+extern const signed char ruby_digit36_to_number_table[];
+extern const char ruby_hexdigits[];
+extern unsigned long ruby_scan_digits(const char *str, ssize_t len, int base, size_t *retlen, int *overflow);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_UTIL_H */
diff --git a/internal/variable.h b/internal/variable.h
new file mode 100644
index 0000000000..1de31f6b92
--- /dev/null
+++ b/internal/variable.h
@@ -0,0 +1,59 @@
+#ifndef INTERNAL_VARIABLE_H /* -*- C -*- */
+#define INTERNAL_VARIABLE_H
+/**
+ * @file
+ * @brief      Internal header for variables.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* global variable */
+
+struct rb_global_entry {
+    struct rb_global_variable *var;
+    ID id;
+};
+
+struct rb_global_entry *rb_global_entry(ID);
+VALUE rb_gvar_get(struct rb_global_entry *);
+VALUE rb_gvar_set(struct rb_global_entry *, VALUE);
+VALUE rb_gvar_defined(struct rb_global_entry *);
+
+/* variable.c */
+#if USE_TRANSIENT_HEAP
+#define ROBJECT_TRANSIENT_FLAG    FL_USER13
+#define ROBJ_TRANSIENT_P(obj)     FL_TEST_RAW((obj), ROBJECT_TRANSIENT_FLAG)
+#define ROBJ_TRANSIENT_SET(obj)   FL_SET_RAW((obj), ROBJECT_TRANSIENT_FLAG)
+#define ROBJ_TRANSIENT_UNSET(obj) FL_UNSET_RAW((obj), ROBJECT_TRANSIENT_FLAG)
+#else
+#define ROBJ_TRANSIENT_P(obj)     0
+#define ROBJ_TRANSIENT_SET(obj)   ((void)0)
+#define ROBJ_TRANSIENT_UNSET(obj) ((void)0)
+#endif
+void rb_gc_mark_global_tbl(void);
+size_t rb_generic_ivar_memsize(VALUE);
+VALUE rb_search_class_path(VALUE);
+VALUE rb_attr_delete(VALUE, ID);
+VALUE rb_ivar_lookup(VALUE obj, ID id, VALUE undef);
+void rb_autoload_str(VALUE mod, ID id, VALUE file);
+VALUE rb_autoload_at_p(VALUE, ID, int);
+void rb_deprecate_constant(VALUE mod, const char *name);
+NORETURN(VALUE rb_mod_const_missing(VALUE,VALUE));
+rb_gvar_getter_t *rb_gvar_getter_function_of(const struct rb_global_entry *);
+rb_gvar_setter_t *rb_gvar_setter_function_of(const struct rb_global_entry *);
+bool rb_gvar_is_traced(const struct rb_global_entry *);
+void rb_gvar_readonly_setter(VALUE v, ID id, VALUE *_);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* variable.c (export) */
+void rb_mark_generic_ivar(VALUE);
+void rb_mv_generic_ivar(VALUE src, VALUE dst);
+VALUE rb_const_missing(VALUE klass, VALUE name);
+int rb_class_ivar_set(VALUE klass, ID vid, VALUE value);
+void rb_iv_tbl_copy(VALUE dst, VALUE src);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_VARIABLE_H */
diff --git a/internal/vm.h b/internal/vm.h
new file mode 100644
index 0000000000..e74b5c9a86
--- /dev/null
+++ b/internal/vm.h
@@ -0,0 +1,166 @@
+#ifndef INTERNAL_VM_H /* -*- C -*- */
+#define INTERNAL_VM_H
+/**
+ * @file
+ * @brief      Internal header for RubyVM.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* vm_insnhelper.h */
+rb_serial_t rb_next_class_serial(void);
+
+/* vm.c */
+VALUE rb_obj_is_thread(VALUE obj);
+void rb_vm_mark(void *ptr);
+PUREFUNC(VALUE rb_vm_top_self(void));
+void rb_vm_inc_const_missing_count(void);
+const void **rb_vm_get_insns_address_table(void);
+VALUE rb_source_location(int *pline);
+const char *rb_source_location_cstr(int *pline);
+MJIT_STATIC void rb_vm_pop_cfunc_frame(void);
+int rb_vm_add_root_module(ID id, VALUE module);
+void rb_vm_check_redefinition_by_prepend(VALUE klass);
+int rb_vm_check_optimizable_mid(VALUE mid);
+VALUE rb_yield_refine_block(VALUE refinement, VALUE refinements);
+MJIT_STATIC VALUE ruby_vm_special_exception_copy(VALUE);
+PUREFUNC(st_table *rb_vm_fstring_table(void));
+
+/* vm_eval.c */
+VALUE rb_adjust_argv_kw_splat(int *, const VALUE **, int *);
+VALUE rb_current_realfilepath(void);
+VALUE rb_check_block_call(VALUE, ID, int, const VALUE *, rb_block_call_func_t, VALUE);
+typedef void rb_check_funcall_hook(int, VALUE, ID, int, const VALUE *, VALUE);
+VALUE rb_check_funcall_with_hook(VALUE recv, ID mid, int argc, const VALUE *argv,
+                                 rb_check_funcall_hook *hook, VALUE arg);
+VALUE rb_check_funcall_with_hook_kw(VALUE recv, ID mid, int argc, const VALUE *argv,
+                                 rb_check_funcall_hook *hook, VALUE arg, int kw_splat);
+const char *rb_type_str(enum ruby_value_type type);
+VALUE rb_check_funcall_default(VALUE, ID, int, const VALUE *, VALUE);
+VALUE rb_yield_1(VALUE val);
+VALUE rb_yield_force_blockarg(VALUE values);
+VALUE rb_lambda_call(VALUE obj, ID mid, int argc, const VALUE *argv,
+                     rb_block_call_func_t bl_proc, int min_argc, int max_argc,
+                     VALUE data2);
+
+/* vm_insnhelper.c */
+VALUE rb_equal_opt(VALUE obj1, VALUE obj2);
+VALUE rb_eql_opt(VALUE obj1, VALUE obj2);
+
+/* vm_method.c */
+enum method_missing_reason {
+    MISSING_NOENTRY   = 0x00,
+    MISSING_PRIVATE   = 0x01,
+    MISSING_PROTECTED = 0x02,
+    MISSING_FCALL     = 0x04,
+    MISSING_VCALL     = 0x08,
+    MISSING_SUPER     = 0x10,
+    MISSING_MISSING   = 0x20,
+    MISSING_NONE      = 0x40
+};
+struct rb_callable_method_entry_struct;
+struct rb_method_definition_struct;
+struct rb_execution_context_struct;
+struct rb_control_frame_struct;
+struct rb_calling_info;
+struct rb_call_data;
+/* I have several reasons to chose 64 here:
+ *
+ * - A cache line must be a power-of-two size.
+ * - Setting this to anything less than or equal to 32 boosts nothing.
+ * - I have never seen an architecture that has 128 byte L1 cache line.
+ * - I know Intel Core and Sparc T4 at least uses 64.
+ * - I know jemalloc internally has this exact same `#define CACHE_LINE 64`.
+ *   https://github.com/jemalloc/jemalloc/blob/dev/include/jemalloc/internal/jemalloc_internal_types.h
+ */
+#define CACHELINE 64
+struct rb_call_cache {
+    /* inline cache: keys */
+    rb_serial_t method_state;
+    rb_serial_t class_serial[
+        (CACHELINE
+         - sizeof(rb_serial_t)                                   /* method_state */
+         - sizeof(struct rb_callable_method_entry_struct *)      /* me */
+         - sizeof(uintptr_t)                                     /* method_serial */
+         - sizeof(enum method_missing_reason)                    /* aux */
+         - sizeof(VALUE (*)(                                     /* call */
+               struct rb_execution_context_struct *e,
+               struct rb_control_frame_struct *,
+               struct rb_calling_info *,
+               const struct rb_call_data *)))
+        / sizeof(rb_serial_t)
+    ];
+
+    /* inline cache: values */
+    const struct rb_callable_method_entry_struct *me;
+    uintptr_t method_serial; /* me->def->method_serial */
+
+    VALUE (*call)(struct rb_execution_context_struct *ec,
+                  struct rb_control_frame_struct *cfp,
+                  struct rb_calling_info *calling,
+                  struct rb_call_data *cd);
+
+    union {
+        unsigned int index; /* used by ivar */
+        enum method_missing_reason method_missing_reason; /* used by method_missing */
+    } aux;
+};
+STATIC_ASSERT(cachelined, sizeof(struct rb_call_cache) <= CACHELINE);
+struct rb_call_info {
+    /* fixed at compile time */
+    ID mid;
+    unsigned int flag;
+    int orig_argc;
+};
+struct rb_call_data {
+    struct rb_call_cache cc;
+    struct rb_call_info ci;
+};
+RUBY_FUNC_EXPORTED
+RUBY_FUNC_NONNULL(1, VALUE rb_funcallv_with_cc(struct rb_call_data*, VALUE, ID, int, const VALUE*));
+RUBY_FUNC_EXPORTED
+RUBY_FUNC_NONNULL(1, bool rb_method_basic_definition_p_with_cc(struct rb_call_data *, VALUE, ID));
+
+#ifdef __GNUC__
+# define rb_funcallv(recv, mid, argc, argv) \
+    __extension__({ \
+        static struct rb_call_data rb_funcallv_data; \
+        rb_funcallv_with_cc(&rb_funcallv_data, recv, mid, argc, argv); \
+    })
+# define rb_method_basic_definition_p(klass, mid) \
+    __extension__({ \
+        static struct rb_call_data rb_mbdp; \
+        (klass == Qfalse) ? /* hidden object cannot be overridden */ true : \
+            rb_method_basic_definition_p_with_cc(&rb_mbdp, klass, mid); \
+    })
+#endif
+
+/* vm_dump.c */
+void rb_print_backtrace(void);
+
+/* vm_backtrace.c */
+VALUE rb_vm_thread_backtrace(int argc, const VALUE *argv, VALUE thval);
+VALUE rb_vm_thread_backtrace_locations(int argc, const VALUE *argv, VALUE thval);
+
+VALUE rb_make_backtrace(void);
+void rb_backtrace_print_as_bugreport(void);
+int rb_backtrace_p(VALUE obj);
+VALUE rb_backtrace_to_str_ary(VALUE obj);
+VALUE rb_backtrace_to_location_ary(VALUE obj);
+void rb_backtrace_each(VALUE (*iter)(VALUE recv, VALUE str), VALUE output);
+
+#define RUBY_DTRACE_CREATE_HOOK(name, arg) \
+    RUBY_DTRACE_HOOK(name##_CREATE, arg)
+#define RUBY_DTRACE_HOOK(name, arg) \
+do { \
+    if (UNLIKELY(RUBY_DTRACE_##name##_ENABLED())) { \
+        int dtrace_line; \
+        const char *dtrace_file = rb_source_location_cstr(&dtrace_line); \
+        if (!dtrace_file) dtrace_file = ""; \
+        RUBY_DTRACE_##name(arg, dtrace_file, dtrace_line); \
+    } \
+} while (0)
+#endif /* INTERNAL_VM_H */
diff --git a/internal/warnings.h b/internal/warnings.h
new file mode 100644
index 0000000000..957583968d
--- /dev/null
+++ b/internal/warnings.h
@@ -0,0 +1,50 @@
+#ifndef INTERNAL_WARNINGS_H /* -*- C -*- */
+#define INTERNAL_WARNINGS_H
+/**
+ * @file
+ * @brief      Internal header to suppress / mandate warnings.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+#if defined(_MSC_VER)
+# define COMPILER_WARNING_PUSH          __pragma(warning(push))
+# define COMPILER_WARNING_POP           __pragma(warning(pop))
+# define COMPILER_WARNING_ERROR(flag)   __pragma(warning(error: flag)))
+# define COMPILER_WARNING_IGNORED(flag) __pragma(warning(suppress: flag)))
+
+#elif defined(__clang__) /* clang 2.6 already had this feature */
+# define COMPILER_WARNING_PUSH          _Pragma("clang diagnostic push")
+# define COMPILER_WARNING_POP           _Pragma("clang diagnostic pop")
+# define COMPILER_WARNING_SPECIFIER(kind, msg) \
+    clang diagnostic kind # msg
+# define COMPILER_WARNING_ERROR(flag) \
+    COMPILER_WARNING_PRAGMA(COMPILER_WARNING_SPECIFIER(error, flag))
+# define COMPILER_WARNING_IGNORED(flag) \
+    COMPILER_WARNING_PRAGMA(COMPILER_WARNING_SPECIFIER(ignored, flag))
+
+#elif GCC_VERSION_SINCE(4, 6, 0)
+/* https://gcc.gnu.org/onlinedocs/gcc-4.6.4/gcc/Diagnostic-Pragmas.html */
+# define COMPILER_WARNING_PUSH          _Pragma("GCC diagnostic push")
+# define COMPILER_WARNING_POP           _Pragma("GCC diagnostic pop")
+# define COMPILER_WARNING_SPECIFIER(kind, msg) \
+    GCC diagnostic kind # msg
+# define COMPILER_WARNING_ERROR(flag) \
+    COMPILER_WARNING_PRAGMA(COMPILER_WARNING_SPECIFIER(error, flag))
+# define COMPILER_WARNING_IGNORED(flag) \
+    COMPILER_WARNING_PRAGMA(COMPILER_WARNING_SPECIFIER(ignored, flag))
+
+#else /* other compilers to follow? */
+# define COMPILER_WARNING_PUSH          /* nop */
+# define COMPILER_WARNING_POP           /* nop */
+# define COMPILER_WARNING_ERROR(flag)   /* nop */
+# define COMPILER_WARNING_IGNORED(flag) /* nop */
+#endif
+
+#define COMPILER_WARNING_PRAGMA(str) COMPILER_WARNING_PRAGMA_(str)
+#define COMPILER_WARNING_PRAGMA_(str) _Pragma(#str)
+
+#endif /* INTERNAL_WARNINGS_H */