[wasm] add asyncify based setjmp, fiber, register scan emulation

configure.ac: setup build tools and register objects

main.c: wrap main with rb_wasm_rt_start to handle asyncify unwinds

tool/m4/ruby_wasm_tools.m4: setup default command based on WASI_SDK_PATH
environment variable. checks wasm-opt which is used for asyncify.

tool/wasm-clangw wasm/wasm-opt: a clang wrapper which replaces real
wasm-opt with do-nothing wasm-opt to avoid misoptimization before
asyncify. asyncify is performed at POSTLINK, but clang linker driver
tries to run optimization by wasm-opt unconditionally. inlining pass
at wasm level breaks asyncify's assumption, so should not optimize
before POSTLIK.

wasm/GNUmakefile.in: wasm specific rules to compile objects

wasm/setjmp.c

@@ -0,0 +1,134 @@
+/*
+ This is a WebAssembly userland setjmp/longjmp implementation based on Binaryen's Asyncify.
+ Inspired by Alon Zakai's snippet released under the MIT License:
+ * https://github.com/kripken/talks/blob/991fb1e4b6d7e4b0ea6b3e462d5643f11d422771/jmp.c
+
+ WebAssembly doesn't have context-switching mechanism for now, so emulate it by Asyncify,
+ which transforms WebAssembly binary to unwind/rewind the execution point and store/restore
+ locals.
+
+ The basic concept of this implementation is:
+ 1. setjmp captures the current execution context by unwinding to the root frame, then immediately
+    rewind to the setjmp call using the captured context. The context is saved in jmp_buf.
+ 2. longjmp unwinds to the root frame and rewinds to a setjmp call re-using a passed jmp_buf.
+
+ This implementation also supports switching context across different call stack (non-standard)
+
+ This approach is good at behavior reproducibility and self-containedness compared to Emscripten's
+ JS exception approach. However this is super expensive because Asyncify inserts many glue code to
+ control execution point in userland.
+
+ This implementation will be replaced with future stack-switching feature.
+ */
+#include <stdint.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <stdbool.h>
+#include "wasm/asyncify.h"
+#include "wasm/machine.h"
+#include "wasm/setjmp.h"
+
+#ifdef RB_WASM_ENABLE_DEBUG_LOG
+# include <stdio.h>
+# define RB_WASM_DEBUG_LOG(...) fprintf(stderr, __VA_ARGS__)
+#else
+# define RB_WASM_DEBUG_LOG(...)
+#endif
+
+enum rb_wasm_jmp_buf_state {
+    // Initial state
+    JMP_BUF_STATE_INITIALIZED = 0,
+    // Unwinding to the root or rewinding to the setjmp call
+    // to capture the current execution context
+    JMP_BUF_STATE_CAPTURING   = 1,
+    // Ready for longjmp
+    JMP_BUF_STATE_CAPTURED    = 2,
+    // Unwinding to the root or rewinding to the setjmp call
+    // to restore the execution context
+    JMP_BUF_STATE_RETURNING   = 3,
+};
+
+void
+async_buf_init(struct __rb_wasm_asyncify_jmp_buf* buf)
+{
+    buf->top = &buf->buffer[0];
+    buf->end = &buf->buffer[WASM_SETJMP_STACK_BUFFER_SIZE];
+}
+
+// Global unwinding/rewinding jmpbuf state
+static rb_wasm_jmp_buf *_rb_wasm_active_jmpbuf;
+
+__attribute__((noinline))
+int
+_rb_wasm_setjmp_internal(rb_wasm_jmp_buf *env)
+{
+    RB_WASM_DEBUG_LOG("[%s] env = %p, env->state = %d, _rb_wasm_active_jmpbuf = %p\n", __func__, env, env->state, _rb_wasm_active_jmpbuf);
+    switch (env->state) {
+    case JMP_BUF_STATE_INITIALIZED: {
+        RB_WASM_DEBUG_LOG("[%s] JMP_BUF_STATE_INITIALIZED\n", __func__);
+        env->state = JMP_BUF_STATE_CAPTURING;
+        env->payload = 0;
+        _rb_wasm_active_jmpbuf = env;
+        async_buf_init(&env->setjmp_buf);
+        asyncify_start_unwind(&env->setjmp_buf);
+        return -1; // return a dummy value
+    }
+    case JMP_BUF_STATE_CAPTURING: {
+        asyncify_stop_rewind();
+        RB_WASM_DEBUG_LOG("[%s] JMP_BUF_STATE_CAPTURING\n", __func__);
+        env->state = JMP_BUF_STATE_CAPTURED;
+        _rb_wasm_active_jmpbuf = NULL;
+        return 0;
+    }
+    case JMP_BUF_STATE_RETURNING: {
+        asyncify_stop_rewind();
+        RB_WASM_DEBUG_LOG("[%s] JMP_BUF_STATE_RETURNING\n", __func__);
+        env->state = JMP_BUF_STATE_CAPTURED;
+        _rb_wasm_active_jmpbuf = NULL;
+        return env->payload;
+    }
+    default:
+        assert(0 && "unexpected state");
+    }
+    return 0;
+}
+
+void
+_rb_wasm_longjmp(rb_wasm_jmp_buf* env, int value)
+{
+    RB_WASM_DEBUG_LOG("[%s] env = %p, env->state = %d, value = %d\n", __func__, env, env->state, value);
+    assert(env->state == JMP_BUF_STATE_CAPTURED);
+    assert(value != 0);
+    env->state = JMP_BUF_STATE_RETURNING;
+    env->payload = value;
+    _rb_wasm_active_jmpbuf = env;
+    async_buf_init(&env->longjmp_buf);
+    asyncify_start_unwind(&env->longjmp_buf);
+}
+
+void *
+rb_wasm_handle_jmp_unwind(void)
+{
+    RB_WASM_DEBUG_LOG("[%s] _rb_wasm_active_jmpbuf = %p\n", __func__, _rb_wasm_active_jmpbuf);
+    if (!_rb_wasm_active_jmpbuf) {
+	return NULL;
+    }
+
+    switch (_rb_wasm_active_jmpbuf->state) {
+    case JMP_BUF_STATE_CAPTURING: {
+        RB_WASM_DEBUG_LOG("[%s] JMP_BUF_STATE_CAPTURING\n", __func__);
+        // save the captured Asyncify stack top
+        _rb_wasm_active_jmpbuf->dst_buf_top = _rb_wasm_active_jmpbuf->setjmp_buf.top;
+        break;
+    }
+    case JMP_BUF_STATE_RETURNING: {
+        RB_WASM_DEBUG_LOG("[%s] JMP_BUF_STATE_RETURNING\n", __func__);
+        // restore the saved Asyncify stack top
+        _rb_wasm_active_jmpbuf->setjmp_buf.top = _rb_wasm_active_jmpbuf->dst_buf_top;
+        break;
+    }
+    default:
+        assert(0 && "unexpected state");
+    }
+    return &_rb_wasm_active_jmpbuf->setjmp_buf;
+}