path: root/yjit_asm.c

// This file is a fragment of the yjit.o compilation unit. See yjit.c.
//
// Note that the definition for some of these functions don't specify
// static inline, but their declaration in yjit_asm.h do. The resulting
// linkage is the same as if they both specify. The relevant sections in
// N1256 is 6.2.2p4, 6.2.2p5, and 6.7.4p5.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <stdint.h>
#include <assert.h>
#include <errno.h>

// For mmapp(), sysconf()
#ifndef _WIN32
#include <unistd.h>
#include <sys/mman.h>
#endif

#include "yjit_asm.h"

// Compute the number of bits needed to encode a signed value
uint32_t sig_imm_size(int64_t imm)
{
    // Compute the smallest size this immediate fits in
    if (imm >= INT8_MIN && imm <= INT8_MAX)
        return 8;
    if (imm >= INT16_MIN && imm <= INT16_MAX)
        return 16;
    if (imm >= INT32_MIN && imm <= INT32_MAX)
        return 32;

    return 64;
}

// Compute the number of bits needed to encode an unsigned value
uint32_t unsig_imm_size(uint64_t imm)
{
    // Compute the smallest size this immediate fits in
    if (imm <= UINT8_MAX)
        return 8;
    else if (imm <= UINT16_MAX)
        return 16;
    else if (imm <= UINT32_MAX)
        return 32;

    return 64;
}

x86opnd_t mem_opnd(uint32_t num_bits, x86opnd_t base_reg, int32_t disp)
{
    bool is_iprel = base_reg.as.reg.reg_type == REG_IP;

    x86opnd_t opnd = {
        OPND_MEM,
        num_bits,
        .as.mem = { base_reg.as.reg.reg_no, 0, 0, false, is_iprel, disp }
    };

    return opnd;
}

x86opnd_t mem_opnd_sib(uint32_t num_bits, x86opnd_t base_reg, x86opnd_t index_reg, int32_t scale, int32_t disp)
{
    uint8_t scale_exp;
    switch (scale) {
      case 8:
        scale_exp = 3;
        break;
      case 4:
        scale_exp = 2;
        break;
      case 2:
        scale_exp = 1;
        break;
      case 1:
        scale_exp = 0;
        break;
      default:
        rb_bug("yjit: scale not one of 1,2,4,8");
        break;
    }

    bool is_iprel = base_reg.as.reg.reg_type == REG_IP;

    x86opnd_t opnd = {
        OPND_MEM,
        num_bits,
        .as.mem = {
            .base_reg_no = base_reg.as.reg.reg_no,
            .idx_reg_no = index_reg.as.reg.reg_no,
            .has_idx = 1,
            .scale_exp = scale_exp,
            .is_iprel = is_iprel,
            .disp = disp
        }
    };

    return opnd;
}

static x86opnd_t resize_opnd(x86opnd_t opnd, uint32_t num_bits)
{
    assert (num_bits % 8 == 0);
    x86opnd_t sub = opnd;
    sub.num_bits = num_bits;
    return sub;
}

x86opnd_t imm_opnd(int64_t imm)
{
    x86opnd_t opnd = {
        OPND_IMM,
        sig_imm_size(imm),
        .as.imm = imm
    };

    return opnd;
}

x86opnd_t const_ptr_opnd(const void *ptr)
{
    x86opnd_t opnd = {
        OPND_IMM,
        64,
        .as.unsig_imm = (uint64_t)ptr
    };

    return opnd;
}

// Align the current write position to a multiple of bytes
static uint8_t *align_ptr(uint8_t *ptr, uint32_t multiple)
{
    // Compute the pointer modulo the given alignment boundary
    uint32_t rem = ((uint32_t)(uintptr_t)ptr) % multiple;

    // If the pointer is already aligned, stop
    if (rem == 0)
        return ptr;

    // Pad the pointer by the necessary amount to align it
    uint32_t pad = multiple - rem;

    return ptr + pad;
}

// Allocate a block of executable memory
static uint8_t *alloc_exec_mem(uint32_t mem_size)
{
    uint8_t *mem_block;

    // On Linux
#if defined(MAP_FIXED_NOREPLACE) && defined(_SC_PAGESIZE)
        // Align the requested address to page size
        uint32_t page_size = (uint32_t)sysconf(_SC_PAGESIZE);
        uint8_t *req_addr = align_ptr((uint8_t*)&alloc_exec_mem, page_size);

        do {
            // Try to map a chunk of memory as executable
            mem_block = (uint8_t*)mmap(
                (void*)req_addr,
                mem_size,
                PROT_READ | PROT_EXEC,
                MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED_NOREPLACE,
                -1,
                0
            );

            // If we succeeded, stop
            if (mem_block != MAP_FAILED) {
                break;
            }

            // +4MB
            req_addr += 4 * 1024 * 1024;
        } while (req_addr < (uint8_t*)&alloc_exec_mem + INT32_MAX);

    // On MacOS and other platforms
#else
        // Try to map a chunk of memory as executable
        mem_block = (uint8_t*)mmap(
            (void*)alloc_exec_mem,
            mem_size,
            PROT_READ | PROT_EXEC,
            MAP_PRIVATE | MAP_ANONYMOUS,
            -1,
            0
        );
#endif

    // Fallback
    if (mem_block == MAP_FAILED) {
        // Try again without the address hint (e.g., valgrind)
        mem_block = (uint8_t*)mmap(
            NULL,
            mem_size,
            PROT_READ | PROT_EXEC,
            MAP_PRIVATE | MAP_ANONYMOUS,
            -1,
            0
            );
    }

    // Check that the memory mapping was successful
    if (mem_block == MAP_FAILED) {
        perror("mmap call failed");
        exit(-1);
    }

    codeblock_t block;
    codeblock_t *cb = &block;

    cb_init(cb, mem_block, mem_size);

    // Fill the executable memory with PUSH DS (0x1E) so that
    // executing uninitialized memory will fault with #UD in
    // 64-bit mode.
    cb_mark_all_writeable(cb);
    memset(mem_block, 0x1E, mem_size);
    cb_mark_all_executable(cb);

    return mem_block;
}

// Initialize a code block object
void cb_init(codeblock_t *cb, uint8_t *mem_block, uint32_t mem_size)
{
    assert (mem_block);
    cb->mem_block_ = mem_block;
    cb->mem_size = mem_size;
    cb->write_pos = 0;
    cb->num_labels = 0;
    cb->num_refs = 0;
    cb->current_aligned_write_pos = ALIGNED_WRITE_POSITION_NONE;
}

// Set the current write position
void cb_set_pos(codeblock_t *cb, uint32_t pos)
{
    // Assert here since while assembler functions do bounds checking, there is
    // nothing stopping users from taking out an out-of-bounds pointer and
    // doing bad accesses with it.
    assert (pos < cb->mem_size);
    cb->write_pos = pos;
}

// Align the current write position to a multiple of bytes
void cb_align_pos(codeblock_t *cb, uint32_t multiple)
{
    // Compute the pointer modulo the given alignment boundary
    uint8_t *ptr = cb_get_write_ptr(cb);
    uint8_t *aligned_ptr = align_ptr(ptr, multiple);
    const uint32_t write_pos = cb->write_pos;

    // Pad the pointer by the necessary amount to align it
    ptrdiff_t pad = aligned_ptr - ptr;
    cb_set_pos(cb, write_pos + (int32_t)pad);
}

// Set the current write position from a pointer
void cb_set_write_ptr(codeblock_t *cb, uint8_t *code_ptr)
{
    intptr_t pos = code_ptr - cb->mem_block_;
    assert (pos < cb->mem_size);
    cb_set_pos(cb, (uint32_t)pos);
}

// Get a direct pointer into the executable memory block
uint8_t *cb_get_ptr(const codeblock_t *cb, uint32_t index)
{
    if (index < cb->mem_size) {
        return &cb->mem_block_[index];
    }
    else {
        return NULL;
    }
}

// Get a direct pointer to the current write position
uint8_t *cb_get_write_ptr(const codeblock_t *cb)
{
    return cb_get_ptr(cb, cb->write_pos);
}

// Write a byte at the current position
void cb_write_byte(codeblock_t *cb, uint8_t byte)
{
    assert (cb->mem_block_);
    if (cb->write_pos < cb->mem_size) {
        cb_mark_position_writeable(cb, cb->write_pos);
        cb->mem_block_[cb->write_pos] = byte;
        cb->write_pos++;
    }
    else {
        cb->dropped_bytes = true;
    }
}

// Write multiple bytes starting from the current position
void cb_write_bytes(codeblock_t *cb, uint32_t num_bytes, ...)
{
    va_list va;
    va_start(va, num_bytes);

    for (uint32_t i = 0; i < num_bytes; ++i)
    {
        uint8_t byte = va_arg(va, int);
        cb_write_byte(cb, byte);
    }

    va_end(va);
}

// Write a signed integer over a given number of bits at the current position
void cb_write_int(codeblock_t *cb, uint64_t val, uint32_t num_bits)
{
    assert (num_bits > 0);
    assert (num_bits % 8 == 0);

    // Switch on the number of bits
    switch (num_bits) {
      case 8:
        cb_write_byte(cb, (uint8_t)val);
        break;

      case 16:
        cb_write_bytes(
            cb,
            2,
            (uint8_t)((val >> 0) & 0xFF),
            (uint8_t)((val >> 8) & 0xFF)
        );
        break;

      case 32:
        cb_write_bytes(
            cb,
            4,
            (uint8_t)((val >>  0) & 0xFF),
            (uint8_t)((val >>  8) & 0xFF),
            (uint8_t)((val >> 16) & 0xFF),
            (uint8_t)((val >> 24) & 0xFF)
        );
        break;

      default:
        {
            // Compute the size in bytes
            uint32_t num_bytes = num_bits / 8;

            // Write out the bytes
            for (uint32_t i = 0; i < num_bytes; ++i)
            {
                uint8_t byte_val = (uint8_t)(val & 0xFF);
                cb_write_byte(cb, byte_val);
                val >>= 8;
            }
        }
    }
}

// Allocate a new label with a given name
uint32_t cb_new_label(codeblock_t *cb, const char *name)
{
    //if (hasASM)
    //    writeString(to!string(label) ~ ":");

    assert (cb->num_labels < MAX_LABELS);

    // Allocate the new label
    uint32_t label_idx = cb->num_labels++;

    // This label doesn't have an address yet
    cb->label_addrs[label_idx] = 0;
    cb->label_names[label_idx] = name;

    return label_idx;
}

// Write a label at the current address
void cb_write_label(codeblock_t *cb, uint32_t label_idx)
{
    assert (label_idx < MAX_LABELS);
    cb->label_addrs[label_idx] = cb->write_pos;
}

// Add a label reference at the current write position
void cb_label_ref(codeblock_t *cb, uint32_t label_idx)
{
    assert (label_idx < MAX_LABELS);
    assert (cb->num_refs < MAX_LABEL_REFS);

    // Keep track of the reference
    cb->label_refs[cb->num_refs] = (labelref_t){ cb->write_pos, label_idx };
    cb->num_refs++;
}

// Link internal label references
void cb_link_labels(codeblock_t *cb)
{
    uint32_t orig_pos = cb->write_pos;

    // For each label reference
    for (uint32_t i = 0; i < cb->num_refs; ++i)
    {
        uint32_t ref_pos = cb->label_refs[i].pos;
        uint32_t label_idx = cb->label_refs[i].label_idx;
        assert (ref_pos < cb->mem_size);
        assert (label_idx < MAX_LABELS);

        uint32_t label_addr = cb->label_addrs[label_idx];
        assert (label_addr < cb->mem_size);

        // Compute the offset from the reference's end to the label
        int64_t offset = (int64_t)label_addr - (int64_t)(ref_pos + 4);

        cb_set_pos(cb, ref_pos);
        cb_write_int(cb, offset, 32);
    }

    cb->write_pos = orig_pos;

    // Clear the label positions and references
    cb->num_labels = 0;
    cb->num_refs = 0;
}

// Check if an operand needs a REX byte to be encoded
static bool rex_needed(x86opnd_t opnd)
{
    if (opnd.type == OPND_NONE || opnd.type == OPND_IMM)
    {
        return false;
    }

    if (opnd.type == OPND_REG)
    {
        return (
            opnd.as.reg.reg_no > 7 ||
            (opnd.num_bits == 8 && opnd.as.reg.reg_no >= 4 && opnd.as.reg.reg_no <= 7)
        );
    }

    if (opnd.type == OPND_MEM)
    {
        return (opnd.as.mem.base_reg_no > 7) || (opnd.as.mem.has_idx && opnd.as.mem.idx_reg_no > 7);
    }

    rb_bug("unreachable");
}

// Check if an SIB byte is needed to encode this operand
static bool sib_needed(x86opnd_t opnd)
{
    if (opnd.type != OPND_MEM)
        return false;

    return (
        opnd.as.mem.has_idx ||
        opnd.as.mem.base_reg_no == RSP.as.reg.reg_no ||
        opnd.as.mem.base_reg_no == R12.as.reg.reg_no
    );
}

// Compute the size of the displacement field needed for a memory operand
static uint32_t disp_size(x86opnd_t opnd)
{
    assert (opnd.type == OPND_MEM);

    // If using RIP as the base, use disp32
    if (opnd.as.mem.is_iprel)
    {
        return 32;
    }

    // Compute the required displacement size
    if (opnd.as.mem.disp != 0)
    {
        uint32_t num_bits = sig_imm_size(opnd.as.mem.disp);
        assert (num_bits <= 32 && "displacement does not fit in 32 bits");

        // x86 can only encode 8-bit and 32-bit displacements
        if (num_bits == 16)
            num_bits = 32;;

        return num_bits;
    }

    // If EBP or RBP or R13 is used as the base, displacement must be encoded
    if (opnd.as.mem.base_reg_no == RBP.as.reg.reg_no ||
        opnd.as.mem.base_reg_no == R13.as.reg.reg_no)
    {
        return 8;
    }

    return 0;
}

// Write the REX byte
static void cb_write_rex(
    codeblock_t *cb,
    bool w_flag,
    uint8_t reg_no,
    uint8_t idx_reg_no,
    uint8_t rm_reg_no
)
{
    // 0 1 0 0 w r x b
    // w - 64-bit operand size flag
    // r - MODRM.reg extension
    // x - SIB.index extension
    // b - MODRM.rm or SIB.base extension
    uint8_t w = w_flag? 1:0;
    uint8_t r = (reg_no & 8)? 1:0;
    uint8_t x = (idx_reg_no & 8)? 1:0;
    uint8_t b = (rm_reg_no & 8)? 1:0;

    // Encode and write the REX byte
    uint8_t rexByte = 0x40 + (w << 3) + (r << 2) + (x << 1) + (b);
    cb_write_byte(cb, rexByte);
}

// Write an opcode byte with an embedded register operand
static void cb_write_opcode(codeblock_t *cb, uint8_t opcode, x86opnd_t reg)
{
    // Write the reg field into the opcode byte
    uint8_t op_byte = opcode | (reg.as.reg.reg_no & 7);
    cb_write_byte(cb, op_byte);
}

// Encode an RM instruction
static void cb_write_rm(
    codeblock_t *cb,
    bool szPref,
    bool rexW,
    x86opnd_t r_opnd,
    x86opnd_t rm_opnd,
    uint8_t opExt,
    uint32_t op_len,
    ...)
{
    assert (op_len > 0 && op_len <= 3);
    assert (r_opnd.type == OPND_REG || r_opnd.type == OPND_NONE);

    // Flag to indicate the REX prefix is needed
    bool need_rex = rexW || rex_needed(r_opnd) || rex_needed(rm_opnd);

    // Flag to indicate SIB byte is needed
    bool need_sib = sib_needed(r_opnd) || sib_needed(rm_opnd);

    // Add the operand-size prefix, if needed
    if (szPref == true)
        cb_write_byte(cb, 0x66);

    // Add the REX prefix, if needed
    if (need_rex)
    {
        // 0 1 0 0 w r x b
        // w - 64-bit operand size flag
        // r - MODRM.reg extension
        // x - SIB.index extension
        // b - MODRM.rm or SIB.base extension

        uint8_t w = rexW? 1:0;

        uint8_t r;
        if (r_opnd.type != OPND_NONE)
            r = (r_opnd.as.reg.reg_no & 8)? 1:0;
        else
            r = 0;

        uint8_t x;
        if (need_sib && rm_opnd.as.mem.has_idx)
            x = (rm_opnd.as.mem.idx_reg_no & 8)? 1:0;
        else
            x = 0;

        uint8_t b;
        if (rm_opnd.type == OPND_REG)
            b = (rm_opnd.as.reg.reg_no & 8)? 1:0;
        else if (rm_opnd.type == OPND_MEM)
            b = (rm_opnd.as.mem.base_reg_no & 8)? 1:0;
        else
            b = 0;

        // Encode and write the REX byte
        uint8_t rex_byte = 0x40 + (w << 3) + (r << 2) + (x << 1) + (b);
        cb_write_byte(cb, rex_byte);
    }

    // Write the opcode bytes to the code block
    va_list va;
    va_start(va, op_len);
    for (uint32_t i = 0; i < op_len; ++i)
    {
        uint8_t byte = va_arg(va, int);
        cb_write_byte(cb, byte);
    }
    va_end(va);

    // MODRM.mod (2 bits)
    // MODRM.reg (3 bits)
    // MODRM.rm  (3 bits)

    assert (
        !(opExt != 0xFF && r_opnd.type != OPND_NONE) &&
        "opcode extension and register operand present"
    );

    // Encode the mod field
    uint8_t mod;
    if (rm_opnd.type == OPND_REG)
    {
        mod = 3;
    }
    else
    {
        uint32_t dsize = disp_size(rm_opnd);
        if (dsize == 0 || rm_opnd.as.mem.is_iprel)
            mod = 0;
        else if (dsize == 8)
            mod = 1;
        else if (dsize == 32)
            mod = 2;
        else
            rb_bug("unreachable");
    }

    // Encode the reg field
    uint8_t reg;
    if (opExt != 0xFF)
        reg = opExt;
    else if (r_opnd.type == OPND_REG)
        reg = r_opnd.as.reg.reg_no & 7;
    else
        reg = 0;

    // Encode the rm field
    uint8_t rm;
    if (rm_opnd.type == OPND_REG)
    {
        rm = rm_opnd.as.reg.reg_no & 7;
    }
    else
    {
        if (need_sib)
            rm = 4;
        else
            rm = rm_opnd.as.mem.base_reg_no & 7;
    }

    // Encode and write the ModR/M byte
    uint8_t rm_byte = (mod << 6) + (reg << 3) + (rm);
    cb_write_byte(cb, rm_byte);

    // Add the SIB byte, if needed
    if (need_sib)
    {
        // SIB.scale (2 bits)
        // SIB.index (3 bits)
        // SIB.base  (3 bits)

        assert (rm_opnd.type == OPND_MEM);

        // Encode the scale value
        uint8_t scale = rm_opnd.as.mem.scale_exp;

        // Encode the index value
        uint8_t index;
        if (!rm_opnd.as.mem.has_idx)
            index = 4;
        else
            index = rm_opnd.as.mem.idx_reg_no & 7;

        // Encode the base register
        uint8_t base = rm_opnd.as.mem.base_reg_no & 7;

        // Encode and write the SIB byte
        uint8_t sib_byte = (scale << 6) + (index << 3) + (base);
        cb_write_byte(cb, sib_byte);
    }

    // Add the displacement
    if (rm_opnd.type == OPND_MEM)
    {
        uint32_t dsize = disp_size(rm_opnd);
        if (dsize > 0)
            cb_write_int(cb, rm_opnd.as.mem.disp, dsize);
    }
}

// Encode a mul-like single-operand RM instruction
static void write_rm_unary(
    codeblock_t *cb,
    const char *mnem,
    uint8_t opMemReg8,
    uint8_t opMemRegPref,
    uint8_t opExt,
    x86opnd_t opnd)
{
    // Write a disassembly string
    //cb.writeASM(mnem, opnd);

    // Check the size of opnd0
    uint32_t opndSize;
    if (opnd.type == OPND_REG || opnd.type == OPND_MEM)
        opndSize = opnd.num_bits;
    else
        rb_bug("yjit: invalid operand");

    assert (opndSize == 8 || opndSize == 16 || opndSize == 32 || opndSize == 64);
    bool szPref = opndSize == 16;
    bool rexW = opndSize == 64;

    if (opndSize == 8)
        cb_write_rm(cb, false, false, NO_OPND, opnd, opExt, 1, opMemReg8);
    else
        cb_write_rm(cb, szPref, rexW, NO_OPND, opnd, opExt, 1, opMemRegPref);
}

// Encode an add-like RM instruction with multiple possible encodings
static void cb_write_rm_multi(
    codeblock_t *cb,
    const char *mnem,
    uint8_t opMemReg8,
    uint8_t opMemRegPref,
    uint8_t opRegMem8,
    uint8_t opRegMemPref,
    uint8_t opMemImm8,
    uint8_t opMemImmSml,
    uint8_t opMemImmLrg,
    uint8_t opExtImm,
    x86opnd_t opnd0,
    x86opnd_t opnd1)
{
    assert (opnd0.type == OPND_REG || opnd0.type == OPND_MEM);

    /*
    // Write disassembly string
    if (!opnd1.isNone)
        cb.writeASM(mnem, opnd0, opnd1);
    else
        cb.writeASM(mnem, opnd0);
    */

    // Check the size of opnd0
    uint32_t opndSize = opnd0.num_bits;

    // Check the size of opnd1
    if (opnd1.type == OPND_REG || opnd1.type == OPND_MEM)
    {
        assert (opnd1.num_bits == opndSize && "operand size mismatch");
    }
    else if (opnd1.type == OPND_IMM)
    {
        assert (opnd1.num_bits <= opndSize);
    }

    assert (opndSize == 8 || opndSize == 16 || opndSize == 32 || opndSize == 64);
    bool szPref = opndSize == 16;
    bool rexW = opndSize == 64;

    // R/M + Reg
    if ((opnd0.type == OPND_MEM && opnd1.type == OPND_REG) ||
        (opnd0.type == OPND_REG && opnd1.type == OPND_REG))
    {
        // R/M is opnd0
        if (opndSize == 8)
            cb_write_rm(cb, false, false, opnd1, opnd0, 0xFF, 1, opMemReg8);
        else
            cb_write_rm(cb, szPref, rexW, opnd1, opnd0, 0xFF, 1, opMemRegPref);
    }

    // Reg + R/M
    else if (opnd0.type == OPND_REG && opnd1.type == OPND_MEM)
    {
        // R/M is opnd1
        if (opndSize == 8)
            cb_write_rm(cb, false, false, opnd0, opnd1, 0xFF, 1, opRegMem8);
        else
            cb_write_rm(cb, szPref, rexW, opnd0, opnd1, 0xFF, 1, opRegMemPref);
    }

    // R/M + Imm
    else if (opnd1.type == OPND_IMM)
    {
        // 8-bit immediate
        if (opnd1.num_bits <= 8)
        {
            if (opndSize == 8)
                cb_write_rm(cb, false, false, NO_OPND, opnd0, opExtImm, 1, opMemImm8);
            else
                cb_write_rm(cb, szPref, rexW, NO_OPND, opnd0, opExtImm, 1, opMemImmSml);

            cb_write_int(cb, opnd1.as.imm, 8);
        }

        // 32-bit immediate
        else if (opnd1.num_bits <= 32)
        {
            assert (opnd1.num_bits <= opndSize && "immediate too large for dst");
            cb_write_rm(cb, szPref, rexW, NO_OPND, opnd0, opExtImm, 1, opMemImmLrg);
            cb_write_int(cb, opnd1.as.imm, (opndSize > 32)? 32:opndSize);
        }

        // Immediate too large
        else
        {
            assert (false && "immediate value too large");
        }
    }

    // Invalid operands
    else
    {
        assert (false && "invalid operand combination");
    }
}

// Encode a single-operand shift instruction
static void cb_write_shift(
    codeblock_t *cb,
    const char *mnem,
    uint8_t opMemOnePref,
    uint8_t opMemClPref,
    uint8_t opMemImmPref,
    uint8_t opExt,
    x86opnd_t opnd0,
    x86opnd_t opnd1)
{
    // Write a disassembly string
    //cb.writeASM(mnem, opnd0, opnd1);

    // Check the size of opnd0
    uint32_t opndSize;
    if (opnd0.type == OPND_REG || opnd0.type == OPND_MEM)
        opndSize = opnd0.num_bits;
    else
        rb_bug("yjit: shift: invalid first operand");

    assert (opndSize == 16 || opndSize == 32 || opndSize == 64);
    bool szPref = opndSize == 16;
    bool rexW = opndSize == 64;

    if (opnd1.type == OPND_IMM)
    {
        if (opnd1.as.imm == 1)
        {
            cb_write_rm(cb, szPref, rexW, NO_OPND, opnd0, opExt, 1, opMemOnePref);
        }
        else
        {
            assert (opnd1.num_bits <= 8);
            cb_write_rm(cb, szPref, rexW, NO_OPND, opnd0, opExt, 1, opMemImmPref);
            cb_write_byte(cb, (uint8_t)opnd1.as.imm);
        }
    }
    /*
    else if (opnd1.isReg && opnd1.reg == CL)
    {
        cb.writeRMInstr!('l', opExt, opMemClPref)(szPref, rexW, opnd0, X86Opnd.NONE);
    }
    */
    else
    {
        assert (false);
    }
}

// Encode a relative jump to a label (direct or conditional)
// Note: this always encodes a 32-bit offset
static void cb_write_jcc(codeblock_t *cb, const char *mnem, uint8_t op0, uint8_t op1, uint32_t label_idx)
{
    //cb.writeASM(mnem, label);

    // Write the opcode
    if (op0 != 0xFF)
        cb_write_byte(cb, op0);
    cb_write_byte(cb, op1);

    // Add a reference to the label
    cb_label_ref(cb, label_idx);

    // Relative 32-bit offset to be patched
    cb_write_int(cb, 0, 32);
}

// Encode a relative jump to a pointer at a 32-bit offset (direct or conditional)
static void cb_write_jcc_ptr(codeblock_t *cb, const char *mnem, uint8_t op0, uint8_t op1, uint8_t *dst_ptr)
{
    //cb.writeASM(mnem, label);

    // Write the opcode
    if (op0 != 0xFF)
        cb_write_byte(cb, op0);
    cb_write_byte(cb, op1);

    // Pointer to the end of this jump instruction
    uint8_t *end_ptr = cb_get_ptr(cb, cb->write_pos + 4);

    // Compute the jump offset
    int64_t rel64 = (int64_t)(dst_ptr - end_ptr);
    if (rel64 >= INT32_MIN && rel64 <= INT32_MAX) {
        // Write the relative 32-bit jump offset
        cb_write_int(cb, (int32_t)rel64, 32);
    }
    else {
        // Offset doesn't fit in 4 bytes. Report error.
        cb->dropped_bytes = true;
    }
}

// Encode a conditional move instruction
static void cb_write_cmov(codeblock_t *cb, const char *mnem, uint8_t opcode1, x86opnd_t dst, x86opnd_t src)
{
    //cb.writeASM(mnem, dst, src);

    assert (dst.type == OPND_REG);
    assert (src.type == OPND_REG || src.type == OPND_MEM);
    assert (dst.num_bits >= 16 && "invalid dst reg size in cmov");

    bool szPref = dst.num_bits == 16;
    bool rexW = dst.num_bits == 64;

    cb_write_rm(cb, szPref, rexW, dst, src, 0xFF, 2, 0x0F, opcode1);
}

// add - Integer addition
void add(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "add",
        0x00, // opMemReg8
        0x01, // opMemRegPref
        0x02, // opRegMem8
        0x03, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x00, // opExtImm
        opnd0,
        opnd1
    );
}

/// and - Bitwise AND
void and(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "and",
        0x20, // opMemReg8
        0x21, // opMemRegPref
        0x22, // opRegMem8
        0x23, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x04, // opExtImm
        opnd0,
        opnd1
    );
}

// call - Call to a pointer with a 32-bit displacement offset
static void call_rel32(codeblock_t *cb, int32_t rel32)
{
    //cb.writeASM("call", rel32);

    // Write the opcode
    cb_write_byte(cb, 0xE8);

    // Write the relative 32-bit jump offset
    cb_write_int(cb, (int32_t)rel32, 32);
}

// call - Call a pointer, encode with a 32-bit offset if possible
void call_ptr(codeblock_t *cb, x86opnd_t scratch_reg, uint8_t *dst_ptr)
{
    assert (scratch_reg.type == OPND_REG);

    // Pointer to the end of this call instruction
    uint8_t *end_ptr = cb_get_ptr(cb, cb->write_pos + 5);

    // Compute the jump offset
    int64_t rel64 = (int64_t)(dst_ptr - end_ptr);

    // If the offset fits in 32-bit
    if (rel64 >= INT32_MIN && rel64 <= INT32_MAX) {
        call_rel32(cb, (int32_t)rel64);
        return;
    }

    // Move the pointer into the scratch register and call
    mov(cb, scratch_reg, const_ptr_opnd(dst_ptr));
    call(cb, scratch_reg);
}

/// call - Call to label with 32-bit offset
void call_label(codeblock_t *cb, uint32_t label_idx)
{
    //cb.writeASM("call", label);

    // Write the opcode
    cb_write_byte(cb, 0xE8);

    // Add a reference to the label
    cb_label_ref(cb, label_idx);

    // Relative 32-bit offset to be patched
    cb_write_int(cb, 0, 32);
}

/// call - Indirect call with an R/M operand
void call(codeblock_t *cb, x86opnd_t opnd)
{
    //cb.writeASM("call", opnd);
    cb_write_rm(cb, false, false, NO_OPND, opnd, 2, 1, 0xFF);
}

/// cmovcc - Conditional move
void cmova(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmova", 0x47, dst, src); }
void cmovae(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovae", 0x43, dst, src); }
void cmovb(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovb", 0x42, dst, src); }
void cmovbe(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovbe", 0x46, dst, src); }
void cmovc(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovc", 0x42, dst, src); }
void cmove(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmove", 0x44, dst, src); }
void cmovg(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovg", 0x4F, dst, src); }
void cmovge(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovge", 0x4D, dst, src); }
void cmovl(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovl", 0x4C, dst, src); }
void cmovle(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovle", 0x4E, dst, src); }
void cmovna(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovna", 0x46, dst, src); }
void cmovnae(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnae", 0x42, dst, src); }
void cmovnb(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnb", 0x43, dst, src); }
void cmovnbe(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnbe", 0x47, dst, src); }
void cmovnc(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnc", 0x43, dst, src); }
void cmovne(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovne", 0x45, dst, src); }
void cmovng(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovng", 0x4E, dst, src); }
void cmovnge(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnge", 0x4C, dst, src); }
void cmovnl(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnl" , 0x4D, dst, src); }
void cmovnle(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnle", 0x4F, dst, src); }
void cmovno(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovno", 0x41, dst, src); }
void cmovnp(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnp", 0x4B, dst, src); }
void cmovns(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovns", 0x49, dst, src); }
void cmovnz(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovnz", 0x45, dst, src); }
void cmovo(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovo", 0x40, dst, src); }
void cmovp(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovp", 0x4A, dst, src); }
void cmovpe(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovpe", 0x4A, dst, src); }
void cmovpo(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovpo", 0x4B, dst, src); }
void cmovs(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovs", 0x48, dst, src); }
void cmovz(codeblock_t *cb, x86opnd_t dst, x86opnd_t src) { cb_write_cmov(cb, "cmovz", 0x44, dst, src); }

/// cmp - Compare and set flags
void cmp(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "cmp",
        0x38, // opMemReg8
        0x39, // opMemRegPref
        0x3A, // opRegMem8
        0x3B, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x07, // opExtImm
        opnd0,
        opnd1
    );
}

/// cdq - Convert doubleword to quadword
void cdq(codeblock_t *cb)
{
    //cb.writeASM("cdq");
    cb_write_byte(cb, 0x99);
}

/// cqo - Convert quadword to octaword
void cqo(codeblock_t *cb)
{
    //cb.writeASM("cqo");
    cb_write_bytes(cb, 2, 0x48, 0x99);
}

/// Interrupt 3 - trap to debugger
void int3(codeblock_t *cb)
{
    //cb.writeASM("INT 3");
    cb_write_byte(cb, 0xCC);
}

/*
// div - Unsigned integer division
alias div = writeRMUnary!(
    "div",
    0xF6, // opMemReg8
    0xF7, // opMemRegPref
    0x06  // opExt
);
*/

/*
/// divsd - Divide scalar double
alias divsd = writeXMM64!(
    "divsd",
    0xF2, // prefix
    0x0F, // opRegMem0
    0x5E  // opRegMem1
);
*/

/*
// idiv - Signed integer division
alias idiv = writeRMUnary!(
    "idiv",
    0xF6, // opMemReg8
    0xF7, // opMemRegPref
    0x07  // opExt
);
*/

/*
/// imul - Signed integer multiplication with two operands
void imul(CodeBlock cb, X86Opnd opnd0, X86Opnd opnd1)
{
    cb.writeASM("imul", opnd0, opnd1);

    assert (opnd0.isReg, "invalid first operand");
    auto opndSize = opnd0.reg.size;

    // Check the size of opnd1
    if (opnd1.isReg)
        assert (opnd1.reg.size is opndSize, "operand size mismatch");
    else if (opnd1.isMem)
        assert (opnd1.mem.size is opndSize, "operand size mismatch");

    assert (opndSize is 16 || opndSize is 32 || opndSize is 64);
    auto szPref = opndSize is 16;
    auto rexW = opndSize is 64;

    cb.writeRMInstr!('r', 0xFF, 0x0F, 0xAF)(szPref, rexW, opnd0, opnd1);
}
*/

/*
/// imul - Signed integer multiplication with three operands (one immediate)
void imul(CodeBlock cb, X86Opnd opnd0, X86Opnd opnd1, X86Opnd opnd2)
{
    cb.writeASM("imul", opnd0, opnd1, opnd2);

    assert (opnd0.isReg, "invalid first operand");
    auto opndSize = opnd0.reg.size;

    // Check the size of opnd1
    if (opnd1.isReg)
        assert (opnd1.reg.size is opndSize, "operand size mismatch");
    else if (opnd1.isMem)
        assert (opnd1.mem.size is opndSize, "operand size mismatch");

    assert (opndSize is 16 || opndSize is 32 || opndSize is 64);
    auto szPref = opndSize is 16;
    auto rexW = opndSize is 64;

    assert (opnd2.isImm, "invalid third operand");
    auto imm = opnd2.imm;

    // 8-bit immediate
    if (imm.immSize <= 8)
    {
        cb.writeRMInstr!('r', 0xFF, 0x6B)(szPref, rexW, opnd0, opnd1);
        cb.writeInt(imm.imm, 8);
    }

    // 32-bit immediate
    else if (imm.immSize <= 32)
    {
        assert (imm.immSize <= opndSize, "immediate too large for dst");
        cb.writeRMInstr!('r', 0xFF, 0x69)(szPref, rexW, opnd0, opnd1);
        cb.writeInt(imm.imm, min(opndSize, 32));
    }

    // Immediate too large
    else
    {
        assert (false, "immediate value too large");
    }
}
*/

/// jcc - relative jumps to a label
void ja_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "ja"  , 0x0F, 0x87, label_idx); }
void jae_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jae" , 0x0F, 0x83, label_idx); }
void jb_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jb"  , 0x0F, 0x82, label_idx); }
void jbe_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jbe" , 0x0F, 0x86, label_idx); }
void jc_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jc"  , 0x0F, 0x82, label_idx); }
void je_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "je"  , 0x0F, 0x84, label_idx); }
void jg_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jg"  , 0x0F, 0x8F, label_idx); }
void jge_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jge" , 0x0F, 0x8D, label_idx); }
void jl_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jl"  , 0x0F, 0x8C, label_idx); }
void jle_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jle" , 0x0F, 0x8E, label_idx); }
void jna_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jna" , 0x0F, 0x86, label_idx); }
void jnae_label(codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnae", 0x0F, 0x82, label_idx); }
void jnb_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnb" , 0x0F, 0x83, label_idx); }
void jnbe_label(codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnbe", 0x0F, 0x87, label_idx); }
void jnc_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnc" , 0x0F, 0x83, label_idx); }
void jne_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jne" , 0x0F, 0x85, label_idx); }
void jng_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jng" , 0x0F, 0x8E, label_idx); }
void jnge_label(codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnge", 0x0F, 0x8C, label_idx); }
void jnl_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnl" , 0x0F, 0x8D, label_idx); }
void jnle_label(codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnle", 0x0F, 0x8F, label_idx); }
void jno_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jno" , 0x0F, 0x81, label_idx); }
void jnp_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnp" , 0x0F, 0x8b, label_idx); }
void jns_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jns" , 0x0F, 0x89, label_idx); }
void jnz_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jnz" , 0x0F, 0x85, label_idx); }
void jo_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jo"  , 0x0F, 0x80, label_idx); }
void jp_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jp"  , 0x0F, 0x8A, label_idx); }
void jpe_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jpe" , 0x0F, 0x8A, label_idx); }
void jpo_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jpo" , 0x0F, 0x8B, label_idx); }
void js_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "js"  , 0x0F, 0x88, label_idx); }
void jz_label  (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jz"  , 0x0F, 0x84, label_idx); }
void jmp_label (codeblock_t *cb, uint32_t label_idx) { cb_write_jcc(cb, "jmp" , 0xFF, 0xE9, label_idx); }

/// jcc - relative jumps to a pointer (32-bit offset)
void ja_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "ja"  , 0x0F, 0x87, ptr); }
void jae_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jae" , 0x0F, 0x83, ptr); }
void jb_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jb"  , 0x0F, 0x82, ptr); }
void jbe_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jbe" , 0x0F, 0x86, ptr); }
void jc_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jc"  , 0x0F, 0x82, ptr); }
void je_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "je"  , 0x0F, 0x84, ptr); }
void jg_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jg"  , 0x0F, 0x8F, ptr); }
void jge_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jge" , 0x0F, 0x8D, ptr); }
void jl_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jl"  , 0x0F, 0x8C, ptr); }
void jle_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jle" , 0x0F, 0x8E, ptr); }
void jna_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jna" , 0x0F, 0x86, ptr); }
void jnae_ptr(codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnae", 0x0F, 0x82, ptr); }
void jnb_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnb" , 0x0F, 0x83, ptr); }
void jnbe_ptr(codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnbe", 0x0F, 0x87, ptr); }
void jnc_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnc" , 0x0F, 0x83, ptr); }
void jne_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jne" , 0x0F, 0x85, ptr); }
void jng_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jng" , 0x0F, 0x8E, ptr); }
void jnge_ptr(codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnge", 0x0F, 0x8C, ptr); }
void jnl_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnl" , 0x0F, 0x8D, ptr); }
void jnle_ptr(codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnle", 0x0F, 0x8F, ptr); }
void jno_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jno" , 0x0F, 0x81, ptr); }
void jnp_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnp" , 0x0F, 0x8b, ptr); }
void jns_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jns" , 0x0F, 0x89, ptr); }
void jnz_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jnz" , 0x0F, 0x85, ptr); }
void jo_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jo"  , 0x0F, 0x80, ptr); }
void jp_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jp"  , 0x0F, 0x8A, ptr); }
void jpe_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jpe" , 0x0F, 0x8A, ptr); }
void jpo_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jpo" , 0x0F, 0x8B, ptr); }
void js_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "js"  , 0x0F, 0x88, ptr); }
void jz_ptr  (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jz"  , 0x0F, 0x84, ptr); }
void jmp_ptr (codeblock_t *cb, uint8_t *ptr) { cb_write_jcc_ptr(cb, "jmp" , 0xFF, 0xE9, ptr); }

/// jmp - Indirect jump near to an R/M operand
void jmp_rm(codeblock_t *cb, x86opnd_t opnd)
{
    //cb.writeASM("jmp", opnd);
    cb_write_rm(cb, false, false, NO_OPND, opnd, 4, 1, 0xFF);
}

// jmp - Jump with relative 32-bit offset
void jmp32(codeblock_t *cb, int32_t offset)
{
    //cb.writeASM("jmp", ((offset > 0)? "+":"-") ~ to!string(offset));
    cb_write_byte(cb, 0xE9);
    cb_write_int(cb, offset, 32);
}

/// lea - Load Effective Address
void lea(codeblock_t *cb, x86opnd_t dst, x86opnd_t src)
{
    //cb.writeASM("lea", dst, src);
    assert (dst.num_bits == 64);
    cb_write_rm(cb, false, true, dst, src, 0xFF, 1, 0x8D);
}

// Does this number fit in 32 bits and stays the same if you zero extend it to 64 bit?
// If the sign bit is clear, sign extension and zero extension yield the same
// result.
static bool
zero_extendable_32bit(uint64_t number)
{
    return number <= UINT32_MAX && (number & (1ull << 31ull)) == 0;
}

/// mov - Data move operation
void mov(codeblock_t *cb, x86opnd_t dst, x86opnd_t src)
{
    // R/M + Imm
    if (src.type == OPND_IMM)
    {
        //cb.writeASM("mov", dst, src);

        // R + Imm
        if (dst.type == OPND_REG)
        {
            assert (
                src.num_bits <= dst.num_bits ||
                unsig_imm_size(src.as.imm) <= dst.num_bits
            );

            // In case the source immediate could be zero extended to be 64
            // bit, we can use the 32-bit operands version of the instruction.
            // For example, we can turn mov(rax, 0x34) into the equivalent
            // mov(eax, 0x34).
            if (dst.num_bits == 64 && zero_extendable_32bit(src.as.unsig_imm)) {
                if (rex_needed(dst))
                    cb_write_rex(cb, false, 0, 0, dst.as.reg.reg_no);
                cb_write_opcode(cb, 0xB8, dst);
                cb_write_int(cb, src.as.imm, 32);
            }
            else {
                if (dst.num_bits == 16)
                    cb_write_byte(cb, 0x66);

                if (rex_needed(dst) || dst.num_bits == 64)
                    cb_write_rex(cb, dst.num_bits == 64, 0, 0, dst.as.reg.reg_no);

                cb_write_opcode(cb, (dst.num_bits == 8)? 0xB0:0xB8, dst);

                cb_write_int(cb, src.as.imm, dst.num_bits);
            }
        }

        // M + Imm
        else if (dst.type == OPND_MEM)
        {
            assert (src.num_bits <= dst.num_bits);

            if (dst.num_bits == 8)
                cb_write_rm(cb, false, false, NO_OPND, dst, 0xFF, 1, 0xC6);
            else
                cb_write_rm(cb, dst.num_bits == 16, dst.num_bits == 64, NO_OPND, dst, 0, 1, 0xC7);

            const uint32_t output_num_bits = (dst.num_bits > 32u) ? 32u : dst.num_bits;
            // assert that we can write whole immediate without loss of information
            assert (sig_imm_size(src.as.imm) <= output_num_bits);
            cb_write_int(cb, src.as.imm, output_num_bits);
        }

        else
        {
            assert (false);
        }
    }
    else
    {
        cb_write_rm_multi(
            cb,
            "mov",
            0x88, // opMemReg8
            0x89, // opMemRegPref
            0x8A, // opRegMem8
            0x8B, // opRegMemPref
            0xC6, // opMemImm8
            0xFF, // opMemImmSml (not available)
            0xFF, // opMemImmLrg
            0xFF, // opExtImm
            dst,
            src
        );
    }
}

/// movsx - Move with sign extension (signed integers)
void movsx(codeblock_t *cb, x86opnd_t dst, x86opnd_t src)
{
    assert (dst.type == OPND_REG);
    assert (src.type == OPND_REG || src.type == OPND_MEM);
    assert (src.num_bits < dst.num_bits);

    //cb.writeASM("movsx", dst, src);

    if (src.num_bits == 8)
    {
        cb_write_rm(cb, dst.num_bits == 16, dst.num_bits == 64, dst, src, 0xFF, 2, 0x0F, 0xBE);
    }
    else if (src.num_bits == 16)
    {
        cb_write_rm(cb, dst.num_bits == 16, dst.num_bits == 64, dst, src, 0xFF, 2, 0x0F, 0xBF);
    }
    else if (src.num_bits == 32)
    {
        cb_write_rm(cb, false, true, dst, src, 0xFF, 1, 0x63);
    }
    else
    {
        assert (false);
    }
}

/*
/// movzx - Move with zero extension (unsigned values)
void movzx(codeblock_t *cb, x86opnd_t dst, x86opnd_t src)
{
    cb.writeASM("movzx", dst, src);

    uint32_t dstSize;
    if (dst.isReg)
        dstSize = dst.reg.size;
    else
        assert (false, "movzx dst must be a register");

    uint32_t srcSize;
    if (src.isReg)
        srcSize = src.reg.size;
    else if (src.isMem)
        srcSize = src.mem.size;
    else
        assert (false);

    assert (
        srcSize < dstSize,
        "movzx: srcSize >= dstSize"
    );

    if (srcSize is 8)
    {
        cb.writeRMInstr!('r', 0xFF, 0x0F, 0xB6)(dstSize is 16, dstSize is 64, dst, src);
    }
    else if (srcSize is 16)
    {
        cb.writeRMInstr!('r', 0xFF, 0x0F, 0xB7)(dstSize is 16, dstSize is 64, dst, src);
    }
    else
    {
        assert (false, "invalid src operand size for movxz");
    }
}
*/

// neg - Integer negation (multiplication by -1)
void neg(codeblock_t *cb, x86opnd_t opnd)
{
    write_rm_unary(
        cb,
        "neg",
        0xF6, // opMemReg8
        0xF7, // opMemRegPref
        0x03,  // opExt
        opnd
    );
}

// nop - Noop, one or multiple bytes long
void nop(codeblock_t *cb, uint32_t length)
{
    switch (length) {
      case 0:
        break;

      case 1:
        //cb.writeASM("nop1");
        cb_write_byte(cb, 0x90);
        break;

      case 2:
        //cb.writeASM("nop2");
        cb_write_bytes(cb, 2, 0x66,0x90);
        break;

      case 3:
        //cb.writeASM("nop3");
        cb_write_bytes(cb, 3, 0x0F,0x1F,0x00);
        break;

      case 4:
        //cb.writeASM("nop4");
        cb_write_bytes(cb, 4, 0x0F,0x1F,0x40,0x00);
        break;

      case 5:
        //cb.writeASM("nop5");
        cb_write_bytes(cb, 5, 0x0F,0x1F,0x44,0x00,0x00);
        break;

      case 6:
        //cb.writeASM("nop6");
        cb_write_bytes(cb, 6, 0x66,0x0F,0x1F,0x44,0x00,0x00);
        break;

      case 7:
        //cb.writeASM("nop7");
        cb_write_bytes(cb, 7, 0x0F,0x1F,0x80,0x00,0x00,0x00,0x00);
        break;

      case 8:
        //cb.writeASM("nop8");
        cb_write_bytes(cb, 8, 0x0F,0x1F,0x84,0x00,0x00,0x00,0x00,0x00);
        break;

      case 9:
        //cb.writeASM("nop9");
        cb_write_bytes(cb, 9, 0x66,0x0F,0x1F,0x84,0x00,0x00,0x00,0x00,0x00);
        break;

      default:
        {
            uint32_t written = 0;
            while (written + 9 <= length)
            {
                nop(cb, 9);
                written += 9;
            }
            nop(cb, length - written);
        }
        break;
    }
}

// not - Bitwise NOT
void not(codeblock_t *cb, x86opnd_t opnd)
{
    write_rm_unary(
        cb,
        "not",
        0xF6, // opMemReg8
        0xF7, // opMemRegPref
        0x02, // opExt
        opnd
    );
}

/// or - Bitwise OR
void or(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "or",
        0x08, // opMemReg8
        0x09, // opMemRegPref
        0x0A, // opRegMem8
        0x0B, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x01, // opExtImm
        opnd0,
        opnd1
    );
}

/// pop - Pop a register off the stack
void pop(codeblock_t *cb, x86opnd_t opnd)
{
    assert (opnd.num_bits == 64);

    //cb.writeASM("pop", opnd);

    if (opnd.type == OPND_REG) {
        if (rex_needed(opnd))
            cb_write_rex(cb, false, 0, 0, opnd.as.reg.reg_no);
        cb_write_opcode(cb, 0x58, opnd);
    }
    else if (opnd.type == OPND_MEM) {
        cb_write_rm(cb, false, false, NO_OPND, opnd, 0, 1, 0x8F);
    }
    else {
        assert(false && "unexpected operand type");
    }
}

/// popfq - Pop the flags register (64-bit)
void popfq(codeblock_t *cb)
{
    //cb.writeASM("popfq");

    // REX.W + 0x9D
    cb_write_bytes(cb, 2, 0x48, 0x9D);
}

/// push - Push an operand on the stack
void push(codeblock_t *cb, x86opnd_t opnd)
{
    assert (opnd.num_bits == 64);

    //cb.writeASM("push", opnd);

    if (opnd.type == OPND_REG) {
      if (rex_needed(opnd))
          cb_write_rex(cb, false, 0, 0, opnd.as.reg.reg_no);
      cb_write_opcode(cb, 0x50, opnd);
    }
    else if (opnd.type == OPND_MEM) {
      cb_write_rm(cb, false, false, NO_OPND, opnd, 6, 1, 0xFF);
    }
    else {
      assert(false && "unexpected operand type");
    }
}

/// pushfq - Push the flags register (64-bit)
void pushfq(codeblock_t *cb)
{
    //cb.writeASM("pushfq");
    cb_write_byte(cb, 0x9C);
}

/// ret - Return from call, popping only the return address
void ret(codeblock_t *cb)
{
    //cb.writeASM("ret");
    cb_write_byte(cb, 0xC3);
}

// sal - Shift arithmetic left
void sal(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_shift(
        cb,
        "sal",
        0xD1, // opMemOnePref,
        0xD3, // opMemClPref,
        0xC1, // opMemImmPref,
        0x04,
        opnd0,
        opnd1
    );
}

/// sar - Shift arithmetic right (signed)
void sar(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_shift(
        cb,
        "sar",
        0xD1, // opMemOnePref,
        0xD3, // opMemClPref,
        0xC1, // opMemImmPref,
        0x07,
        opnd0,
        opnd1
    );
}
// shl - Shift logical left
void shl(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_shift(
        cb,
        "shl",
        0xD1, // opMemOnePref,
        0xD3, // opMemClPref,
        0xC1, // opMemImmPref,
        0x04,
        opnd0,
        opnd1
    );
}

/// shr - Shift logical right (unsigned)
void shr(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_shift(
        cb,
        "shr",
        0xD1, // opMemOnePref,
        0xD3, // opMemClPref,
        0xC1, // opMemImmPref,
        0x05,
        opnd0,
        opnd1
    );
}

/// sub - Integer subtraction
void sub(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "sub",
        0x28, // opMemReg8
        0x29, // opMemRegPref
        0x2A, // opRegMem8
        0x2B, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x05, // opExtImm
        opnd0,
        opnd1
    );
}

/// test - Logical Compare
void test(codeblock_t *cb, x86opnd_t rm_opnd, x86opnd_t test_opnd)
{
    assert (rm_opnd.type == OPND_REG || rm_opnd.type == OPND_MEM);
    assert (test_opnd.type == OPND_REG || test_opnd.type == OPND_IMM);

    // If the second operand is an immediate
    if (test_opnd.type == OPND_IMM)
    {
        x86opnd_t imm_opnd = test_opnd;

        if (imm_opnd.as.imm >= 0)
        {
            assert (unsig_imm_size(imm_opnd.as.unsig_imm) <= 32);
            assert (unsig_imm_size(imm_opnd.as.unsig_imm) <= rm_opnd.num_bits);

            // Use the smallest operand size possible
            rm_opnd = resize_opnd(rm_opnd, unsig_imm_size(imm_opnd.as.unsig_imm));

            if (rm_opnd.num_bits == 8)
            {
                cb_write_rm(cb, false, false, NO_OPND, rm_opnd, 0x00, 1, 0xF6);
                cb_write_int(cb, imm_opnd.as.imm, rm_opnd.num_bits);
            }
            else
            {
                cb_write_rm(cb, rm_opnd.num_bits == 16, false, NO_OPND, rm_opnd, 0x00, 1, 0xF7);
                cb_write_int(cb, imm_opnd.as.imm, rm_opnd.num_bits);
            }
        }
        else
        {
            // This mode only applies to 64-bit R/M operands with 32-bit signed immediates
            assert (imm_opnd.as.imm < 0);
            assert (sig_imm_size(imm_opnd.as.imm) <= 32);
            assert (rm_opnd.num_bits == 64);
            cb_write_rm(cb, false, true, NO_OPND, rm_opnd, 0x00, 1, 0xF7);
            cb_write_int(cb, imm_opnd.as.imm, 32);
        }
    }
    else
    {
        assert (test_opnd.num_bits == rm_opnd.num_bits);

        if (rm_opnd.num_bits == 8)
        {
            cb_write_rm(cb, false, false, test_opnd, rm_opnd, 0xFF, 1, 0x84);
        }
        else
        {
            cb_write_rm(cb, rm_opnd.num_bits == 16, rm_opnd.num_bits == 64, test_opnd, rm_opnd, 0xFF, 1, 0x85);
        }
    }
}

/// Undefined opcode
void ud2(codeblock_t *cb)
{
    cb_write_bytes(cb, 2, 0x0F, 0x0B);
}

/// xchg - Exchange Register/Memory with Register
void xchg(codeblock_t *cb, x86opnd_t rm_opnd, x86opnd_t r_opnd)
{
    assert (rm_opnd.num_bits == 64);
    assert (r_opnd.num_bits == 64);
    assert (rm_opnd.type == OPND_REG);
    assert (r_opnd.type == OPND_REG);

    // If we're exchanging with RAX
    if (rm_opnd.type == OPND_REG && rm_opnd.as.reg.reg_no == RAX.as.reg.reg_no)
    {
        // Write the REX byte
        cb_write_rex(cb, rm_opnd.num_bits == 64, 0, 0, r_opnd.as.reg.reg_no);

        // Write the opcode and register number
        cb_write_byte(cb, 0x90 + (r_opnd.as.reg.reg_no & 7));
    }
    else
    {
        cb_write_rm(cb, rm_opnd.num_bits == 16, rm_opnd.num_bits == 64, r_opnd, rm_opnd, 0xFF, 1, 0x87);
    }
}

/// xor - Exclusive bitwise OR
void xor(codeblock_t *cb, x86opnd_t opnd0, x86opnd_t opnd1)
{
    cb_write_rm_multi(
        cb,
        "xor",
        0x30, // opMemReg8
        0x31, // opMemRegPref
        0x32, // opRegMem8
        0x33, // opRegMemPref
        0x80, // opMemImm8
        0x83, // opMemImmSml
        0x81, // opMemImmLrg
        0x06, // opExtImm
        opnd0,
        opnd1
    );
}

// LOCK - lock prefix for atomic shared memory operations
void cb_write_lock_prefix(codeblock_t *cb)
{
    cb_write_byte(cb, 0xF0);
}

void cb_mark_all_writeable(codeblock_t * cb)
{
    if (mprotect(cb->mem_block_, cb->mem_size, PROT_READ | PROT_WRITE)) {
        fprintf(stderr, "Couldn't make JIT page (%p) writeable, errno: %s", (void *)cb->mem_block_, strerror(errno));
        abort();
    }
}

void cb_mark_position_writeable(codeblock_t * cb, uint32_t write_pos)
{
#ifdef _WIN32
    uint32_t pagesize = 0x1000; // 4KB
#else
    uint32_t pagesize = (uint32_t)sysconf(_SC_PAGESIZE);
#endif
    uint32_t aligned_position = (write_pos / pagesize) * pagesize;

    if (cb->current_aligned_write_pos != aligned_position) {
        cb->current_aligned_write_pos = aligned_position;
        void *const page_addr = cb_get_ptr(cb, aligned_position);
        if (mprotect(page_addr, pagesize, PROT_READ | PROT_WRITE)) {
            fprintf(stderr, "Couldn't make JIT page (%p) writeable, errno: %s", page_addr, strerror(errno));
            abort();
        }
    }
}

void cb_mark_all_executable(codeblock_t * cb)
{
    cb->current_aligned_write_pos = ALIGNED_WRITE_POSITION_NONE;
    if (mprotect(cb->mem_block_, cb->mem_size, PROT_READ | PROT_EXEC)) {
        fprintf(stderr, "Couldn't make JIT page (%p) executable, errno: %s", (void *)cb->mem_block_, strerror(errno));
        abort();
    }
}