path: root/cont.c

/**********************************************************************

  cont.c - 

  $Author$
  created at: Thu May 23 09:03:43 2007

  Copyright (C) 2007 Koichi Sasada

**********************************************************************/

#include "ruby/ruby.h"
#include "vm_core.h"
#include "gc.h"
#include "eval_intern.h"

#define CAPTURE_JUST_VALID_VM_STACK 1

enum context_type {
    CONTINUATION_CONTEXT = 0,
    FIBER_CONTEXT = 1,
    ROOT_FIBER_CONTEXT = 2
};

typedef struct rb_context_struct {
    enum context_type type;
    VALUE self;
    int argc;
    VALUE value;
    VALUE *vm_stack;
#ifdef CAPTURE_JUST_VALID_VM_STACK
    int vm_stack_slen;  /* length of stack (head of th->stack) */
    int vm_stack_clen;  /* length of control frames (tail of th->stack) */
#endif
    VALUE *machine_stack;
    VALUE *machine_stack_src;
#ifdef __ia64
    VALUE *machine_register_stack;
    VALUE *machine_register_stack_src;
    int machine_register_stack_size;
#endif
    rb_thread_t saved_thread;
    rb_jmpbuf_t jmpbuf;
    int machine_stack_size;
} rb_context_t;

enum fiber_status {
    CREATED,
    RUNNING,
    TERMINATED
};

typedef struct rb_fiber_struct {
    rb_context_t cont;
    VALUE prev;
    enum fiber_status status;
    struct rb_fiber_struct *prev_fiber;
    struct rb_fiber_struct *next_fiber;
} rb_fiber_t;

static VALUE rb_cContinuation;
static VALUE rb_cFiber;
static VALUE rb_eFiberError;

#define GetContPtr(obj, ptr)  \
  Data_Get_Struct(obj, rb_context_t, ptr)

#define GetFiberPtr(obj, ptr)  do {\
  ptr = (rb_fiber_t*)DATA_PTR(obj);\
  if (!ptr) rb_raise(rb_eFiberError, "uninitialized fiber");\
} while(0)

NOINLINE(static VALUE cont_capture(volatile int *stat));

void rb_thread_mark(rb_thread_t *th);

static void
cont_mark(void *ptr)
{
    RUBY_MARK_ENTER("cont");
    if (ptr) {
	rb_context_t *cont = ptr;
	rb_gc_mark(cont->value);
	rb_thread_mark(&cont->saved_thread);

	if (cont->vm_stack) {
#ifdef CAPTURE_JUST_VALID_VM_STACK
	    rb_gc_mark_locations(cont->vm_stack,
				 cont->vm_stack + cont->vm_stack_slen + cont->vm_stack_clen);
#else
	    rb_gc_mark_localtion(cont->vm_stack,
				 cont->vm_stack, cont->saved_thread.stack_size);
#endif
	}

	if (cont->machine_stack) {
	    rb_gc_mark_locations(cont->machine_stack,
				 cont->machine_stack + cont->machine_stack_size);
	}
#ifdef __ia64
	if (cont->machine_register_stack) {
	    rb_gc_mark_locations(cont->machine_register_stack,
				 cont->machine_register_stack + cont->machine_register_stack_size);
	}
#endif
    }
    RUBY_MARK_LEAVE("cont");
}

static void
cont_free(void *ptr)
{
    RUBY_FREE_ENTER("cont");
    if (ptr) {
	rb_context_t *cont = ptr;
	RUBY_FREE_UNLESS_NULL(cont->saved_thread.stack); fflush(stdout);
	RUBY_FREE_UNLESS_NULL(cont->machine_stack);
#ifdef __ia64
	RUBY_FREE_UNLESS_NULL(cont->machine_register_stack);
#endif
	RUBY_FREE_UNLESS_NULL(cont->vm_stack);

	/* free rb_cont_t or rb_fiber_t */
	ruby_xfree(ptr);
    }
    RUBY_FREE_LEAVE("cont");
}

static void
fiber_mark(void *ptr)
{
    RUBY_MARK_ENTER("cont");
    if (ptr) {
	rb_fiber_t *fib = ptr;
	rb_gc_mark(fib->prev);
	cont_mark(&fib->cont);
    }
    RUBY_MARK_LEAVE("cont");
}

static void
fiber_link_join(rb_fiber_t *fib)
{
    VALUE current_fibval = rb_fiber_current();
    rb_fiber_t *current_fib;
    GetFiberPtr(current_fibval, current_fib);

    /* join fiber link */
    fib->next_fiber = current_fib->next_fiber;
    fib->prev_fiber = current_fib;
    current_fib->next_fiber->prev_fiber = fib;
    current_fib->next_fiber = fib;
}

static void
fiber_link_remove(rb_fiber_t *fib)
{
    fib->prev_fiber->next_fiber = fib->next_fiber;
    fib->next_fiber->prev_fiber = fib->prev_fiber;
}

static void
fiber_free(void *ptr)
{
    RUBY_FREE_ENTER("fiber");
    if (ptr) {
	rb_fiber_t *fib = ptr;

	if (fib->cont.type != ROOT_FIBER_CONTEXT) {
	    st_free_table(fib->cont.saved_thread.local_storage);
	}
	fiber_link_remove(fib);

	cont_free(&fib->cont);
    }
    RUBY_FREE_LEAVE("fiber");
}

static void
cont_save_machine_stack(rb_thread_t *th, rb_context_t *cont)
{
    int size;
    rb_thread_t *sth = &cont->saved_thread;

    SET_MACHINE_STACK_END(&th->machine_stack_end);
#ifdef __ia64
    th->machine_register_stack_end = rb_ia64_bsp();
#endif

    if (th->machine_stack_start > th->machine_stack_end) {
	size = cont->machine_stack_size = th->machine_stack_start - th->machine_stack_end;
	cont->machine_stack_src = th->machine_stack_end;
    }
    else {
	size = cont->machine_stack_size = th->machine_stack_end - th->machine_stack_start;
	cont->machine_stack_src = th->machine_stack_start;
    }

    if (cont->machine_stack) {
	REALLOC_N(cont->machine_stack, VALUE, size);
    }
    else {
	cont->machine_stack = ALLOC_N(VALUE, size);
    }

    FLUSH_REGISTER_WINDOWS;
    MEMCPY(cont->machine_stack, cont->machine_stack_src, VALUE, size);

#ifdef __ia64
    rb_ia64_flushrs();
    size = cont->machine_register_stack_size = th->machine_register_stack_end - th->machine_register_stack_start;
    cont->machine_register_stack_src = th->machine_register_stack_start;
    if (cont->machine_register_stack) {
	REALLOC_N(cont->machine_register_stack, VALUE, size);
    }
    else {
	cont->machine_register_stack = ALLOC_N(VALUE, size);
    }

    MEMCPY(cont->machine_register_stack, cont->machine_register_stack_src, VALUE, size);
#endif

    sth->machine_stack_start = sth->machine_stack_end = 0;
#ifdef __ia64
    sth->machine_register_stack_start = sth->machine_register_stack_end = 0;
#endif
}

static void
cont_init(rb_context_t *cont)
{
    rb_thread_t *th = GET_THREAD();

    /* save thread context */
    cont->saved_thread = *th;
}

static rb_context_t *
cont_new(VALUE klass)
{
    rb_context_t *cont;
    volatile VALUE contval;

    contval = Data_Make_Struct(klass, rb_context_t, cont_mark, cont_free, cont);
    cont->self = contval;
    cont_init(cont);
    return cont;
}

void rb_vm_stack_to_heap(rb_thread_t *th);

static VALUE
cont_capture(volatile int *stat)
{
    rb_context_t *cont;
    rb_thread_t *th = GET_THREAD(), *sth;
    volatile VALUE contval;

    rb_vm_stack_to_heap(th);
    cont = cont_new(rb_cContinuation);
    contval = cont->self;
    sth = &cont->saved_thread;

#ifdef CAPTURE_JUST_VALID_VM_STACK
    cont->vm_stack_slen = th->cfp->sp + th->mark_stack_len - th->stack;
    cont->vm_stack_clen = th->stack + th->stack_size - (VALUE*)th->cfp;
    cont->vm_stack = ALLOC_N(VALUE, cont->vm_stack_slen + cont->vm_stack_clen);
    MEMCPY(cont->vm_stack, th->stack, VALUE, cont->vm_stack_slen);
    MEMCPY(cont->vm_stack + cont->vm_stack_slen, (VALUE*)th->cfp, VALUE, cont->vm_stack_clen);
#else
    cont->vm_stack = ALLOC_N(VALUE, th->stack_size);
    MEMCPY(cont->vm_stack, th->stack, VALUE, th->stack_size);
#endif
    sth->stack = 0;

    cont_save_machine_stack(th, cont);

    if (ruby_setjmp(cont->jmpbuf)) {
	VALUE value;

	value = cont->value;
	if (cont->argc == -1) rb_exc_raise(value);
	cont->value = Qnil;
	*stat = 1;
	return value;
    }
    else {
	*stat = 0;
	return cont->self;
    }
}

NOINLINE(NORETURN(static void cont_restore_1(rb_context_t *)));

static void
cont_restore_1(rb_context_t *cont)
{
    rb_thread_t *th = GET_THREAD(), *sth = &cont->saved_thread;

    /* restore thread context */
    if (cont->type == CONTINUATION_CONTEXT) {
	/* continuation */
	VALUE fib;

	th->fiber = sth->fiber;
	fib = th->fiber ? th->fiber : th->root_fiber;

	if (fib) {
	    rb_context_t *fcont;
	    GetContPtr(fib, fcont);
	    th->stack_size = fcont->saved_thread.stack_size;
	    th->stack = fcont->saved_thread.stack;
	}
#ifdef CAPTURE_JUST_VALID_VM_STACK
	MEMCPY(th->stack, cont->vm_stack, VALUE, cont->vm_stack_slen);
	MEMCPY(th->stack + sth->stack_size - cont->vm_stack_clen,
	       cont->vm_stack + cont->vm_stack_slen, VALUE, cont->vm_stack_clen);
#else
	MEMCPY(th->stack, cont->vm_stack, VALUE, sth->stack_size);
#endif
    }
    else {
	/* fiber */
	th->stack = sth->stack;
	th->stack_size = sth->stack_size;
	th->local_storage = sth->local_storage;
	th->fiber = cont->self;
    }

    th->cfp = sth->cfp;
    th->safe_level = sth->safe_level;
    th->raised_flag = sth->raised_flag;
    th->state = sth->state;
    th->status = sth->status;
    th->tag = sth->tag;
    th->trap_tag = sth->trap_tag;
    th->errinfo = sth->errinfo;
    th->first_proc = sth->first_proc;

    /* restore machine stack */
#ifdef _M_AMD64
    {
	/* workaround for x64 SEH */
	jmp_buf buf;
	setjmp(buf);
	((_JUMP_BUFFER*)(&cont->jmpbuf))->Frame =
	    ((_JUMP_BUFFER*)(&buf))->Frame;
    }
#endif
    if (cont->machine_stack_src) {
	FLUSH_REGISTER_WINDOWS;
	MEMCPY(cont->machine_stack_src, cont->machine_stack,
	       VALUE, cont->machine_stack_size);
    }

#ifdef __ia64
    if (cont->machine_register_stack_src) {
	MEMCPY(cont->machine_register_stack_src, cont->machine_register_stack,
	       VALUE, cont->machine_register_stack_size);
    }
#endif

    ruby_longjmp(cont->jmpbuf, 1);
}

NORETURN(NOINLINE(static void cont_restore_0(rb_context_t *, VALUE *)));

#ifdef __ia64
#define C(a) rse_##a##0, rse_##a##1, rse_##a##2, rse_##a##3, rse_##a##4
#define E(a) rse_##a##0= rse_##a##1= rse_##a##2= rse_##a##3= rse_##a##4
static volatile int C(a), C(b), C(c), C(d), C(e);
static volatile int C(f), C(g), C(h), C(i), C(j);
static volatile int C(k), C(l), C(m), C(n), C(o);
static volatile int C(p), C(q), C(r), C(s), C(t);
int rb_dummy_false = 0;
NORETURN(NOINLINE(static void register_stack_extend(rb_context_t *, VALUE *)));
static void
register_stack_extend(rb_context_t *cont, VALUE *curr_bsp)
{
    if (rb_dummy_false) {
        /* use registers as much as possible */
        E(a) = E(b) = E(c) = E(d) = E(e) =
        E(f) = E(g) = E(h) = E(i) = E(j) =
        E(k) = E(l) = E(m) = E(n) = E(o) =
        E(p) = E(q) = E(r) = E(s) = E(t) = 0;
        E(a) = E(b) = E(c) = E(d) = E(e) =
        E(f) = E(g) = E(h) = E(i) = E(j) =
        E(k) = E(l) = E(m) = E(n) = E(o) =
        E(p) = E(q) = E(r) = E(s) = E(t) = 0;
    }
    if (curr_bsp < cont->machine_register_stack_src+cont->machine_register_stack_size) {
        register_stack_extend(cont, (VALUE*)rb_ia64_bsp());
    }
    cont_restore_1(cont);
}
#undef C
#undef E
#endif

static void
cont_restore_0(rb_context_t *cont, VALUE *addr_in_prev_frame)
{
    if (cont->machine_stack_src) {
#define STACK_PAD_SIZE 1024
	VALUE space[STACK_PAD_SIZE];

#if STACK_GROW_DIRECTION < 0 /* downward */
	if (addr_in_prev_frame > cont->machine_stack_src) {
	    cont_restore_0(cont, &space[0]);
	}
#elif STACK_GROW_DIRECTION > 0 /* upward */
	if (addr_in_prev_frame < cont->machine_stack_src + cont->machine_stack_size) {
	    cont_restore_0(cont, &space[STACK_PAD_SIZE-1]);
	}
#else
	if (addr_in_prev_frame > &space[0]) {
	    /* Stack grows downward */
	    if (addr_in_prev_frame > cont->machine_stack_src) {
		cont_restore_0(cont, &space[0]);
	    }
	}
	else {
	    /* Stack grows upward */
	    if (addr_in_prev_frame < cont->machine_stack_src + cont->machine_stack_size) {
		cont_restore_0(cont, &space[STACK_PAD_SIZE-1]);
	    }
	}
#endif
    }
#ifdef __ia64
    register_stack_extend(cont, (VALUE*)rb_ia64_bsp());
#else
    cont_restore_1(cont);
#endif
}

/*
 *  Document-class: Continuation
 *
 *  Continuation objects are generated by
 *  <code>Kernel#callcc</code>. They hold a return address and execution
 *  context, allowing a nonlocal return to the end of the
 *  <code>callcc</code> block from anywhere within a program.
 *  Continuations are somewhat analogous to a structured version of C's
 *  <code>setjmp/longjmp</code> (although they contain more state, so
 *  you might consider them closer to threads).
 *     
 *  For instance:
 *     
 *     arr = [ "Freddie", "Herbie", "Ron", "Max", "Ringo" ]
 *     callcc{|$cc|}
 *     puts(message = arr.shift)
 *     $cc.call unless message =~ /Max/
 *     
 *  <em>produces:</em>
 *     
 *     Freddie
 *     Herbie
 *     Ron
 *     Max
 *     
 *  This (somewhat contrived) example allows the inner loop to abandon
 *  processing early:
 *     
 *     callcc {|cont|
 *       for i in 0..4
 *         print "\n#{i}: "
 *         for j in i*5...(i+1)*5
 *           cont.call() if j == 17
 *           printf "%3d", j
 *         end
 *       end
 *     }
 *     print "\n"
 *     
 *  <em>produces:</em>
 *     
 *     0:   0  1  2  3  4
 *     1:   5  6  7  8  9
 *     2:  10 11 12 13 14
 *     3:  15 16
 */

/*
 *  call-seq:
 *     callcc {|cont| block }   =>  obj
 *  
 *  Generates a <code>Continuation</code> object, which it passes to the
 *  associated block. Performing a <em>cont</em><code>.call</code> will
 *  cause the <code>callcc</code> to return (as will falling through the
 *  end of the block). The value returned by the <code>callcc</code> is
 *  the value of the block, or the value passed to
 *  <em>cont</em><code>.call</code>. See class <code>Continuation</code>
 *  for more details. Also see <code>Kernel::throw</code> for
 *  an alternative mechanism for unwinding a call stack.
 */

static VALUE
rb_callcc(VALUE self)
{
    volatile int called;
    volatile VALUE val = cont_capture(&called);

    if (called) {
	return val;
    }
    else {
	return rb_yield(val);
    }
}

static VALUE
make_passing_arg(int argc, VALUE *argv)
{
    switch(argc) {
      case 0:
	return Qnil;
      case 1:
	return argv[0];
      default:
	return rb_ary_new4(argc, argv);
    }
}

/*
 *  call-seq:
 *     cont.call(args, ...)
 *     cont[args, ...]
 *  
 *  Invokes the continuation. The program continues from the end of the
 *  <code>callcc</code> block. If no arguments are given, the original
 *  <code>callcc</code> returns <code>nil</code>. If one argument is
 *  given, <code>callcc</code> returns it. Otherwise, an array
 *  containing <i>args</i> is returned.
 *     
 *     callcc {|cont|  cont.call }           #=> nil
 *     callcc {|cont|  cont.call 1 }         #=> 1
 *     callcc {|cont|  cont.call 1, 2, 3 }   #=> [1, 2, 3]
 */

static VALUE
rb_cont_call(int argc, VALUE *argv, VALUE contval)
{
    rb_context_t *cont;
    rb_thread_t *th = GET_THREAD();
    GetContPtr(contval, cont);

    if (cont->saved_thread.self != th->self) {
	rb_raise(rb_eRuntimeError, "continuation called across threads");
    }
    if (cont->saved_thread.trap_tag != th->trap_tag) {
	rb_raise(rb_eRuntimeError, "continuation called across trap");
    }
    if (cont->saved_thread.fiber) {
	rb_context_t *fcont;
	GetContPtr(cont->saved_thread.fiber, fcont);

	if (th->fiber != cont->saved_thread.fiber) {
	    rb_raise(rb_eRuntimeError, "continuation called across fiber");
	}
    }

    cont->argc = argc;
    cont->value = make_passing_arg(argc, argv);

    cont_restore_0(cont, &contval);
    return Qnil; /* unreachable */
}

/*********/
/* fiber */
/*********/

/*
 *  Document-class: Fiber
 *
 *  Fibers are primitives for implementing light weight cooperative
 *  concurrency in Ruby. Basically they are a means of creating code blocks 
 *  that can be paused and resumed, much like threads. The main difference 
 *  is that they are never preempted and that the scheduling must be done by 
 *  the programmer and not the VM. 
 *
 *  As opposed to other stackless light weight concurrency models, each fiber
 *  comes with a small 4KB stack. This enables the fiber to be paused from deeply
 *  nested function calls within the fiber block.
 *
 *  When a fiber is created it will not run automatically. Rather it must be 
 *  be explicitly asked to run using the <code>Fiber#resume</code> method. 
 *  The code running inside the fiber can give up control by calling 
 *  <code>Fiber.yield</code> in which case it yields control back to caller 
 *  (the caller of the <code>Fiber#resume</code>).
 * 
 *  Upon yielding or termination the Fiber returns the value of the last 
 *  executed expression
 *  
 *  For instance:
 *  
 *    fiber = Fiber.new do
 *      Fiber.yield 1
 *      2
 *    end
 *
 *    puts fiber.resume
 *    puts fiber.resume
 *    puts fiber.resume
 *    
 *  <em>produces</em>
 *    
 *    1
 *    2
 *    FiberError: dead fiber called
 *     
 *  The <code>Fiber#resume</code> method accepts an arbitary number of
 *  parameters, if it is the first call to <code>resume</code> then they
 *  will be passed as block arguments. Otherwise they will be the return
 *  value of the call to <code>Fiber.yield</code>
 *
 *  Example:
 *  
 *    fiber = Fiber.new do |first|
 *      second = Fiber.yield first + 2
 *    end
 *
 *    puts fiber.resume 10
 *    puts fiber.resume 14
 *    puts fiber.resume 18
 *
 *  <em>produces</em>
 *    
 *    12
 *    14
 *    FiberError: dead fiber called
 *
 */

#define FIBER_VM_STACK_SIZE (4 * 1024)

static VALUE
fiber_alloc(VALUE klass)
{
    return Data_Wrap_Struct(klass, fiber_mark, fiber_free, 0);
}

static rb_fiber_t*
fiber_t_alloc(VALUE fibval)
{
    rb_fiber_t *fib = ALLOC(rb_fiber_t);

    memset(fib, 0, sizeof(rb_fiber_t));
    fib->cont.self = fibval;
    fib->cont.type = FIBER_CONTEXT;
    cont_init(&fib->cont);
    fib->prev = Qnil;
    fib->status = CREATED;

    DATA_PTR(fibval) = fib;

    return fib;
}

static VALUE
fiber_init(VALUE fibval, VALUE proc)
{
    rb_fiber_t *fib = fiber_t_alloc(fibval);
    rb_context_t *cont = &fib->cont;
    rb_thread_t *th = &cont->saved_thread;

    fiber_link_join(fib);

    /* initialize cont */
    cont->vm_stack = 0;

    th->stack = 0;
    th->stack_size = FIBER_VM_STACK_SIZE;
    th->stack = ALLOC_N(VALUE, th->stack_size);

    th->cfp = (void *)(th->stack + th->stack_size);
    th->cfp--;
    th->cfp->pc = 0;
    th->cfp->sp = th->stack + 1;
    th->cfp->bp = 0;
    th->cfp->lfp = th->stack;
    *th->cfp->lfp = 0;
    th->cfp->dfp = th->stack;
    th->cfp->self = Qnil;
    th->cfp->flag = 0;
    th->cfp->iseq = 0;
    th->cfp->proc = 0;
    th->cfp->block_iseq = 0;
    th->tag = 0;
    th->local_storage = st_init_numtable();

    th->first_proc = proc;

    MEMCPY(&cont->jmpbuf, &th->root_jmpbuf, rb_jmpbuf_t, 1);

    return fibval;
}

static VALUE
rb_fiber_init(VALUE fibval)
{
    return fiber_init(fibval, rb_block_proc());
}

VALUE
rb_fiber_new(VALUE (*func)(ANYARGS), VALUE obj)
{
    return fiber_init(fiber_alloc(rb_cFiber), rb_proc_new(func, obj));
}

static VALUE
return_fiber(void)
{
    rb_fiber_t *fib;
    VALUE curr = rb_fiber_current();
    GetFiberPtr(curr, fib);

    if (fib->prev == Qnil) {
	rb_thread_t *th = GET_THREAD();

	if (th->root_fiber != curr) {
	    return th->root_fiber;
	}
	else {
	    rb_raise(rb_eFiberError, "can't yield from root fiber");
	}
    }
    else {
	VALUE prev = fib->prev;
	fib->prev = Qnil;
	return prev;
    }
}

VALUE rb_fiber_transfer(VALUE fib, int argc, VALUE *argv);

static void
rb_fiber_terminate(rb_fiber_t *fib)
{
    VALUE value = fib->cont.value;
    fib->status = TERMINATED;
    rb_fiber_transfer(return_fiber(), 1, &value);
}

void
rb_fiber_start(void)
{
    rb_thread_t *th = GET_THREAD();
    rb_fiber_t *fib;
    rb_context_t *cont;
    rb_proc_t *proc;
    int state;

    GetFiberPtr(th->fiber, fib);
    cont = &fib->cont;

    TH_PUSH_TAG(th);
    if ((state = EXEC_TAG()) == 0) {
	int argc;
	VALUE *argv, args;
	GetProcPtr(cont->saved_thread.first_proc, proc);
	args = cont->value;
	argv = (argc = cont->argc) > 1 ? RARRAY_PTR(args) : &args;
	cont->value = Qnil;
	th->errinfo = Qnil;
	th->local_lfp = proc->block.lfp;
	th->local_svar = Qnil;

	fib->status = RUNNING;
	cont->value = rb_vm_invoke_proc(th, proc, proc->block.self, argc, argv, 0);
    }
    TH_POP_TAG();

    if (state) {
	if (TAG_RAISE) {
	    th->thrown_errinfo = th->errinfo;
	}
	else {
	    th->thrown_errinfo =
	      rb_vm_make_jump_tag_but_local_jump(state, th->errinfo);
	}
	RUBY_VM_SET_INTERRUPT(th);
    }

    rb_fiber_terminate(fib);
    rb_bug("rb_fiber_start: unreachable");
}

static rb_fiber_t *
root_fiber_alloc(rb_thread_t *th)
{
    rb_fiber_t *fib;

    /* no need to allocate vm stack */
    fib = fiber_t_alloc(fiber_alloc(rb_cFiber));
    fib->cont.type = ROOT_FIBER_CONTEXT;
    fib->prev_fiber = fib->next_fiber = fib;

    return fib;
}

VALUE
rb_fiber_current()
{
    rb_thread_t *th = GET_THREAD();
    if (th->fiber == 0) {
	/* save root */
	rb_fiber_t *fib = root_fiber_alloc(th);
	th->root_fiber = th->fiber = fib->cont.self;
    }
    return th->fiber;
}

static VALUE
fiber_store(rb_fiber_t *next_fib)
{
    rb_thread_t *th = GET_THREAD();
    rb_fiber_t *fib;

    if (th->fiber) {
	GetFiberPtr(th->fiber, fib);
	fib->cont.saved_thread = *th;
    }
    else {
	/* create current fiber */
	fib = root_fiber_alloc(th);
	th->root_fiber = th->fiber = fib->cont.self;
    }

    cont_save_machine_stack(th, &fib->cont);

    if (ruby_setjmp(fib->cont.jmpbuf)) {
	/* restored */
	GetFiberPtr(th->fiber, fib);
	if (fib->cont.argc == -1) rb_exc_raise(fib->cont.value);
	return fib->cont.value;
    }
    else {
	return Qundef;
    }
}

static inline VALUE
fiber_switch(VALUE fibval, int argc, VALUE *argv, int is_resume)
{
    VALUE value;
    rb_fiber_t *fib;
    rb_context_t *cont;
    rb_thread_t *th = GET_THREAD();

    GetFiberPtr(fibval, fib);
    cont = &fib->cont;

    if (cont->saved_thread.self != th->self) {
	rb_raise(rb_eFiberError, "fiber called across threads");
    }
    else if (cont->saved_thread.trap_tag != th->trap_tag) {
	rb_raise(rb_eFiberError, "fiber called across trap");
    }
    else if (fib->status == TERMINATED) {
	value = rb_exc_new2(rb_eFiberError, "dead fiber called");
	if (th->fiber != fibval) rb_exc_raise(value);
	fibval = fib->prev;
	if (NIL_P(fibval)) fibval = th->root_fiber;
	GetFiberPtr(fibval, fib);
	cont = &fib->cont;
	cont->argc = -1;
	cont->value = value;
	cont_restore_0(cont, &value);
    }

    if (is_resume) {
	fib->prev = rb_fiber_current();
    }

    cont->argc = argc;
    cont->value = make_passing_arg(argc, argv);

    if ((value = fiber_store(fib)) == Qundef) {
	cont_restore_0(cont, &value);
	rb_bug("rb_fiber_resume: unreachable");
    }

    RUBY_VM_CHECK_INTS();

    return value;
}

VALUE
rb_fiber_transfer(VALUE fib, int argc, VALUE *argv)
{
    return fiber_switch(fib, argc, argv, 0);
}

VALUE
rb_fiber_resume(VALUE fibval, int argc, VALUE *argv)
{
    rb_fiber_t *fib;
    GetFiberPtr(fibval, fib);

    if (fib->prev != Qnil) {
	rb_raise(rb_eFiberError, "double resume");
    }

    return fiber_switch(fibval, argc, argv, 1);
}

VALUE
rb_fiber_yield(int argc, VALUE *argv)
{
    return rb_fiber_transfer(return_fiber(), argc, argv);
}

/*
 *  call-seq:
 *     fiber.alive? -> true or false
 *  
 *  Returns true if the fiber can still be resumed (or transferred to).
 *  After finishing execution of the fiber block this method will always
 *  return false.
 */
VALUE
rb_fiber_alive_p(VALUE fibval)
{
    rb_fiber_t *fib;
    GetFiberPtr(fibval, fib);
    return fib->status != TERMINATED;
}

/*
 *  call-seq:
 *     fiber.resume(args, ...) -> obj
 *  
 *  Resumes the fiber from the point at which the last <code>Fiber.yield</code>
 *  was called, or starts running it if it is the first call to 
 *  <code>resume</code>. Arguments passed to resume will be the value of
 *  the <code>Fiber.yield</code> expression or will be passed as block 
 *  parameters to the fiber's block if this is the first <code>resume</code>.
 *  
 *  Alternatively, when resume is called it evaluates to the arguments passed
 *  to the next <code>Fiber.yield</code> statement inside the fiber's block
 *  or to the block value if it runs to completion without any
 *  <code>Fiber.yield</code>
 */
static VALUE
rb_fiber_m_resume(int argc, VALUE *argv, VALUE fib)
{
    return rb_fiber_resume(fib, argc, argv);
}

/*
 *  call-seq:
 *     fiber.transfer(args, ...) -> obj
 *  
 *  Transfer control to another fiber, resuming it from where it last
 *  stopped or starting it if it was not resumed before. The calling 
 *  fiber will be suspended much like in a call to <code>Fiber.yield</code>.
 *  
 *  The fiber which recieves the transfer call is treats it much like 
 *  a resume call. Arguments passed to transfer are treated like those
 *  passed to resume.
 *     
 *  You cannot resume a fiber that transferred control to another one.
 *  This will cause a double resume error. You need to transfer control
 *  back to this fiber before it can yield and resume.
 */
static VALUE
rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fib)
{
    return rb_fiber_transfer(fib, argc, argv);
}

/*
 *  call-seq:
 *     Fiber.yield(args, ...) -> obj
 *  
 *  Yields control back to the context that resumed the fiber, passing
 *  along any arguments that were passed to it. The fiber will resume
 *  processing at this point when <code>resume</code> is called next.
 *  Any arguments passed to the next <code>resume</code> will be the
 *  value that this <code>Fiber.yield</code> expression evaluates to.
 */
static VALUE
rb_fiber_s_yield(int argc, VALUE *argv, VALUE klass)
{
    return rb_fiber_yield(argc, argv);
}

/*
 *  call-seq:
 *     Fiber.current() -> fiber
 *  
 *  Returns the current fiber. You need to <code>require 'fiber'</code>
 *  before using this method. If you are not running in the context of
 *  a fiber this method will return the root fiber.
 */
static VALUE
rb_fiber_s_current(VALUE klass)
{
    return rb_fiber_current();
}

void
Init_Cont(void)
{
    rb_cFiber = rb_define_class("Fiber", rb_cObject);
    rb_define_alloc_func(rb_cFiber, fiber_alloc);
    rb_eFiberError = rb_define_class("FiberError", rb_eStandardError);
    rb_define_singleton_method(rb_cFiber, "yield", rb_fiber_s_yield, -1);
    rb_define_method(rb_cFiber, "initialize", rb_fiber_init, 0);
    rb_define_method(rb_cFiber, "resume", rb_fiber_m_resume, -1);
}

void
ruby_Init_Continuation_body(void)
{
    rb_cContinuation = rb_define_class("Continuation", rb_cObject);
    rb_undef_alloc_func(rb_cContinuation);
    rb_undef_method(CLASS_OF(rb_cContinuation), "new");
    rb_define_method(rb_cContinuation, "call", rb_cont_call, -1);
    rb_define_method(rb_cContinuation, "[]", rb_cont_call, -1);
    rb_define_global_function("callcc", rb_callcc, 0);
}

void
ruby_Init_Fiber_as_Coroutine(void)
{
    rb_define_method(rb_cFiber, "transfer", rb_fiber_m_transfer, -1);
    rb_define_method(rb_cFiber, "alive?", rb_fiber_alive_p, 0);
    rb_define_singleton_method(rb_cFiber, "current", rb_fiber_s_current, 0);
}