ruby--ruby/coroutine/copy/Context.c

/*
 *  This file is part of the "Coroutine" project and released under the MIT License.
 *
 *  Created by Samuel Williams on 24/6/2019.
 *  Copyright, 2019, by Samuel Williams.
*/

#include "Context.h"

// http://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html
#ifndef __GNUC__
#define __asm__ asm
#endif

#if defined(__sparc)
__attribute__((noinline))
// https://marc.info/?l=linux-sparc&m=131914569320660&w=2
static void coroutine_flush_register_windows() {
    __asm__
#ifdef __GNUC__
    __volatile__
#endif
#if defined(__sparcv9) || defined(__sparc_v9__) || defined(__arch64__)
#ifdef __GNUC__
    ("flushw" : : : "%o7")
#else
    ("flushw")
#endif
#else
    ("ta 0x03")
#endif
    ;
}
#else
static void coroutine_flush_register_windows() {}
#endif

int coroutine_save_stack(struct coroutine_context * context) {
    void *stack_pointer = &stack_pointer;

    assert(context->stack);
    assert(context->base);

    // At this point, you may need to ensure on architectures that use register windows, that all registers are flushed to the stack.
    coroutine_flush_register_windows();

    // Save stack to private area:
    if (stack_pointer < context->base) {
        size_t size = (char*)context->base - (char*)stack_pointer;
        assert(size <= context->size);

        memcpy(context->stack, stack_pointer, size);
        context->used = size;
    } else {
        size_t size = (char*)stack_pointer - (char*)context->base;
        assert(size <= context->size);

        memcpy(context->stack, context->base, size);
        context->used = size;
    }

    // Save registers / restore point:
    return _setjmp(context->state);
}

__attribute__((noreturn, noinline))
static void coroutine_restore_stack_padded(struct coroutine_context *context, void * buffer) {
    void *stack_pointer = &stack_pointer;

    assert(context->base);

    // Restore stack from private area:
    if (stack_pointer < context->base) {
        void * bottom = (char*)context->base - context->used;
        assert(bottom > stack_pointer);

        memcpy(bottom, context->stack, context->used);
    } else {
        void * top = (char*)context->base + context->used;
        assert(top < stack_pointer);

        memcpy(context->base, context->stack, context->used);
    }

    // Restore registers:
    // The `| (int)buffer` is to force the compiler NOT to elide he buffer and `alloca`.
    _longjmp(context->state, 1 | (int)buffer);
}

static const size_t GAP = 128;

// In order to swap between coroutines, we need to swap the stack and registers.
// `setjmp` and `longjmp` are able to swap registers, but what about swapping stacks? You can use `memcpy` to copy the current stack to a private area and `memcpy` to copy the private stack of the next coroutine to the main stack.
// But if the stack yop are copying in to the main stack is bigger than the currently executing stack, the `memcpy` will clobber the current stack frame (including the context argument). So we use `alloca` to push the current stack frame *beyond* the stack we are about to copy in. This ensures the current stack frame in `coroutine_restore_stack_padded` remains valid for calling `longjmp`.
__attribute__((noreturn))
void coroutine_restore_stack(struct coroutine_context *context) {
    void *stack_pointer = &stack_pointer;
    void *buffer = NULL;
    ssize_t offset = 0;

    // We must ensure that the next stack frame is BEYOND the stack we are restoring:
    if (stack_pointer < context->base) {
        offset = (char*)stack_pointer - ((char*)context->base - context->used) + GAP;
        if (offset > 0) buffer = alloca(offset);
    } else {
        offset = ((char*)context->base + context->used) - (char*)stack_pointer + GAP;
        if (offset > 0) buffer = alloca(offset);
    }

    assert(context->used > 0);

    coroutine_restore_stack_padded(context, buffer);
}

struct coroutine_context *coroutine_transfer(struct coroutine_context *current, struct coroutine_context *target)
{
    struct coroutine_context *previous = target->from;

    // In theory, either this condition holds true, or we should assign the base address to target:
    assert(current->base == target->base);
    // If you are trying to copy the coroutine to a different thread
    // target->base = current->base

    target->from = current;

    assert(current != target);

    // It's possible to come here, even thought the current fiber has been terminated. We are never going to return so we don't bother saving the stack.

    if (current->stack) {
      if (coroutine_save_stack(current) == 0) {
          coroutine_restore_stack(target);
      }
    } else {
        coroutine_restore_stack(target);
    }

    target->from = previous;

    return target;
}