wasm/fiber.c

   1 /*
   2  This is a ucontext-like userland context switching API for WebAssembly based on Binaryen's Asyncify.
   3
   4  * NOTE:
   5  * This mechanism doesn't take care of stack state. Just save and restore program counter and
   6  * registers (rephrased as locals by Wasm term). So use-site need to save and restore the C stack pointer.
   7  * This Asyncify based implementation is not much efficient and will be replaced with future stack-switching feature.
   8  */
   9
  10 #include <stdlib.h>
  11 #include "wasm/fiber.h"
  12 #include "wasm/asyncify.h"
  13
  14 #ifdef RB_WASM_ENABLE_DEBUG_LOG
  15 # include <stdio.h>
  16 # define RB_WASM_DEBUG_LOG(...) fprintf(stderr, __VA_ARGS__)
  17 #else
  18 # define RB_WASM_DEBUG_LOG(...)
  19 #endif
  20
  21 void
  22 rb_wasm_init_context(rb_wasm_fiber_context *fcp, void (*func)(void *, void *), void *arg0, void *arg1)
  23 {
  24     fcp->asyncify_buf.top = &fcp->asyncify_buf.buffer[0];
  25     fcp->asyncify_buf.end = &fcp->asyncify_buf.buffer[WASM_FIBER_STACK_BUFFER_SIZE];
  26     fcp->is_rewinding = false;
  27     fcp->is_started = false;
  28     fcp->entry_point = func;
  29     fcp->arg0 = arg0;
  30     fcp->arg1 = arg1;
  31     RB_WASM_DEBUG_LOG("[%s] fcp->asyncify_buf %p\n", __func__, &fcp->asyncify_buf);
  32 }
  33
  34 static rb_wasm_fiber_context *_rb_wasm_active_next_fiber;
  35
  36 void
  37 rb_wasm_swapcontext(rb_wasm_fiber_context *ofcp, rb_wasm_fiber_context *fcp)
  38 {
  39     RB_WASM_DEBUG_LOG("[%s] enter ofcp = %p fcp = %p\n", __func__, ofcp, fcp);
  40     if (ofcp->is_rewinding) {
  41         asyncify_stop_rewind();
  42         ofcp->is_rewinding = false;
  43         return;
  44     }
  45     _rb_wasm_active_next_fiber = fcp;
  46     RB_WASM_DEBUG_LOG("[%s] start unwinding asyncify_buf = %p\n", __func__, &ofcp->asyncify_buf);
  47     asyncify_start_unwind(&ofcp->asyncify_buf);
  48 }
  49
  50 void *
  51 rb_wasm_handle_fiber_unwind(void (**new_fiber_entry)(void *, void *),
  52                             void **arg0, void **arg1, bool *is_new_fiber_started)
  53 {
  54     rb_wasm_fiber_context *next_fiber;
  55     if (!_rb_wasm_active_next_fiber) {
  56         RB_WASM_DEBUG_LOG("[%s] no next fiber\n", __func__);
  57         *is_new_fiber_started = false;
  58         return NULL;
  59     }
  60
  61     next_fiber = _rb_wasm_active_next_fiber;
  62     _rb_wasm_active_next_fiber = NULL;
  63
  64     RB_WASM_DEBUG_LOG("[%s] next_fiber->asyncify_buf = %p\n", __func__, &next_fiber->asyncify_buf);
  65
  66     *new_fiber_entry = next_fiber->entry_point;
  67     *arg0 = next_fiber->arg0;
  68     *arg1 = next_fiber->arg1;
  69
  70     if (!next_fiber->is_started) {
  71         RB_WASM_DEBUG_LOG("[%s] new fiber started\n", __func__);
  72         // start a new fiber if not started yet.
  73         next_fiber->is_started = true;
  74         *is_new_fiber_started = true;
  75         return NULL;
  76     } else {
  77         RB_WASM_DEBUG_LOG("[%s] resume a fiber\n", __func__);
  78         // resume a fiber again
  79         next_fiber->is_rewinding = true;
  80         *is_new_fiber_started = false;
  81         return &next_fiber->asyncify_buf;
  82     }
  83 }