path: root/cont.c

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

  cont.c - 

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

  Copyright (C) 2007 Koichi Sasada

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

#include "ruby.h"
#include "yarvcore.h"
#include "gc.h"
#include "eval_intern.h"

typedef struct rb_continuation_struct {
    rb_thread_t saved_thread;
    rb_jmpbuf_t jmpbuf;
    VALUE retval;
    VALUE *vm_stack;
    VALUE *machine_stack;
    VALUE *machine_stack_src;
    int machine_stack_size;
} rb_continuation_t;

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

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

void rb_thread_mark(rb_thread_t *th);

static void
cont_mark(void *ptr)
{
    MARK_REPORT_ENTER("cont");
    if (ptr) {
	rb_continuation_t *cont = ptr;
	rb_gc_mark(cont->retval);
	rb_thread_mark(&cont->saved_thread);

	if (cont->vm_stack) {
	    rb_gc_mark_locations(cont->vm_stack,
				 cont->vm_stack + cont->saved_thread.stack_size);
	}
	
	if (cont->machine_stack) {
	    rb_gc_mark_locations(cont->machine_stack,
				 cont->machine_stack + cont->machine_stack_size);
	}
    }
    MARK_REPORT_LEAVE("cont");
}

static void
cont_free(void *ptr)
{
    FREE_REPORT_ENTER("cont");
    if (ptr) {
	rb_continuation_t *cont = ptr;
	FREE_UNLESS_NULL(cont->machine_stack);
	FREE_UNLESS_NULL(cont->vm_stack);
	ruby_xfree(ptr);
    }
    FREE_REPORT_LEAVE("cont");
}

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

    contval = Data_Make_Struct(rb_cCont, rb_continuation_t,
			       cont_mark, cont_free, cont);

    /* save context */
    cont->saved_thread = *th;
    sth = &cont->saved_thread;
    sth->stack = 0; /* clear to skip GC marking */
    cont->vm_stack = ALLOC_N(VALUE, sth->stack_size);
    MEMCPY(cont->vm_stack, th->stack, VALUE, sth->stack_size);

    rb_gc_set_stack_end(&th->machine_stack_end);
    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;
    }

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

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

	retval = cont->retval;
	cont->retval = Qnil;
	*stat = 1;
	return retval;
    }
    else {
	*stat = 0;
	return contval;
    }
}

static void
cont_restore_context_1(rb_continuation_t *cont)
{
    rb_thread_t *th = GET_THREAD(), *sth = &cont->saved_thread;

    /* restore thread context */
    MEMCPY(th->stack, cont->vm_stack, VALUE, sth->stack_size);

    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;

    /* restore machine stack */
    MEMCPY(cont->machine_stack_src, cont->machine_stack,
	   VALUE, cont->machine_stack_size);

    ruby_longjmp(cont->jmpbuf, 1);
}

NORETURN(NOINLINE(static void restore_context_0(rb_continuation_t *, VALUE *)));

static void
cont_restore_context_0(rb_continuation_t *cont, VALUE *addr_in_prev_frame)
{
#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_context_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_context_0(cont, &space[STACK_PAD_SIZE-1]);
    }
#else
    if (addr_in_prev_frame > &space[0]) {
	/* Stack grows downward */
	if (addr_in_prev_frame > cont->saved_thread.machine_stack_src) {
	    cont_restore_context_0(cont, &space[0]);
	}
    }
    else {
	/* Stack grows upward */
	if (addr_in_prev_frame < cont->machine_stack_src + cont->machine_stack_size) {
	    cont_restore_context_0(cont, &space[STACK_PAD_SIZE-1]);
	}
    }
#endif
    cont_restore_context_1(cont);
}

/*
 *  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
 */

VALUE rb_cCont;

/*
 *  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);
    }
}

/*
 *  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_continuation_t *cont;
    rb_thread_t *th = GET_THREAD();
    GetContPtr(contval, cont);

    if (cont->saved_thread.value != th->value) {
	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");
    }

    switch(argc) {
      case 0:
	cont->retval = Qnil;
	break;
      case 1:
	cont->retval = argv[0];
	break;
      default:
	cont->retval = rb_ary_new4(argc, argv);
	break;
    }

    cont_restore_context_0(cont, (VALUE *)&cont);
    return Qnil; /* unreachable */
}

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