[wasm] allocate Asyncify setjmp buffer in heap

`rb_jmpbuf_t` type is considerably large due to inline-allocated
Asyncify buffer, and it leads to stack overflow even with small number
of C-method call frames. This commit allocates the Asyncify buffer used
by `rb_wasm_setjmp` in heap to mitigate the issue.

This patch introduces a new type `rb_vm_tag_jmpbuf_t` to abstract the
representation of a jump buffer, and init/deinit hook points to manage
lifetime of the buffer. These changes are effectively NFC for non-wasm
platforms.

eval_intern.h

@@ -110,9 +110,11 @@ extern int select_large_fdset(int, fd_set *, fd_set *, fd_set *, struct timeval
   _tag.tag = Qundef; \
   _tag.prev = _ec->tag; \
   _tag.lock_rec = rb_ec_vm_lock_rec(_ec); \
+  rb_vm_tag_jmpbuf_init(&_tag.buf); \
 
 #define EC_POP_TAG() \
   _ec->tag = _tag.prev; \
+  rb_vm_tag_jmpbuf_deinit(&_tag.buf); \
 } while (0)
 
 #define EC_TMPPOP_TAG() \
@@ -161,7 +163,7 @@ rb_ec_tag_jump(const rb_execution_context_t *ec, enum ruby_tag_type st)
 {
     RUBY_ASSERT(st != TAG_NONE);
     ec->tag->state = st;
-    ruby_longjmp(ec->tag->buf, 1);
+    ruby_longjmp(RB_VM_TAG_JMPBUF_GET(ec->tag->buf), 1);
 }
 
 /*
@@ -169,7 +171,7 @@ rb_ec_tag_jump(const rb_execution_context_t *ec, enum ruby_tag_type st)
   [ISO/IEC 9899:1999] 7.13.1.1
 */
 #define EC_EXEC_TAG() \
-    (UNLIKELY(ruby_setjmp(_tag.buf)) ? rb_ec_tag_state(VAR_FROM_MEMORY(_ec)) : (EC_REPUSH_TAG(), 0))
+    (UNLIKELY(ruby_setjmp(RB_VM_TAG_JMPBUF_GET(_tag.buf))) ? rb_ec_tag_state(VAR_FROM_MEMORY(_ec)) : (EC_REPUSH_TAG(), 0))
 
 #define EC_JUMP_TAG(ec, st) rb_ec_tag_jump(ec, st)
 

vm.c

@@ -2462,7 +2462,7 @@ vm_exec(rb_execution_context_t *ec)
 
     rb_wasm_try_catch_init(&try_catch, vm_exec_bottom_main, vm_exec_bottom_rescue, &ctx);
 
-    rb_wasm_try_catch_loop_run(&try_catch, &_tag.buf);
+    rb_wasm_try_catch_loop_run(&try_catch, &RB_VM_TAG_JMPBUF_GET(_tag.buf));
 
     result = ctx.result;
 #else

vm_core.h

@@ -884,6 +884,61 @@ typedef RUBY_JMP_BUF rb_jmpbuf_t;
 typedef void *rb_jmpbuf_t[5];
 #endif
 
+/*
+  `rb_vm_tag_jmpbuf_t` type represents a buffer used to
+  long jump to a C frame associated with `rb_vm_tag`.
+
+  Use-site of `rb_vm_tag_jmpbuf_t` is responsible for calling the
+  following functions:
+  - `rb_vm_tag_jmpbuf_init` once `rb_vm_tag_jmpbuf_t` is allocated.
+  - `rb_vm_tag_jmpbuf_deinit` once `rb_vm_tag_jmpbuf_t` is no longer necessary.
+
+  `RB_VM_TAG_JMPBUF_GET` transforms a `rb_vm_tag_jmpbuf_t` into a
+  `rb_jmpbuf_t` to be passed to `rb_setjmp/rb_longjmp`.
+*/
+#if defined(__wasm__) && !defined(__EMSCRIPTEN__)
+/*
+  WebAssembly target with Asyncify-based SJLJ needs
+  to capture the execution context by unwind/rewind-ing
+  call frames into a jump buffer. The buffer space tends
+  to be considerably large unlike other architectures'
+  register-based buffers.
+  Therefore, we allocates the buffer on the heap on such
+  environments.
+*/
+typedef rb_jmpbuf_t *rb_vm_tag_jmpbuf_t;
+
+#define RB_VM_TAG_JMPBUF_GET(buf) (*buf)
+
+static inline void
+rb_vm_tag_jmpbuf_init(rb_vm_tag_jmpbuf_t *jmpbuf)
+{
+  *jmpbuf = malloc(sizeof(rb_jmpbuf_t));
+}
+
+static inline void
+rb_vm_tag_jmpbuf_deinit(const rb_vm_tag_jmpbuf_t *jmpbuf)
+{
+  free(*jmpbuf);
+}
+#else
+typedef rb_jmpbuf_t rb_vm_tag_jmpbuf_t;
+
+#define RB_VM_TAG_JMPBUF_GET(buf) (buf)
+
+static inline void
+rb_vm_tag_jmpbuf_init(rb_vm_tag_jmpbuf_t *jmpbuf)
+{
+  // no-op
+}
+
+static inline void
+rb_vm_tag_jmpbuf_deinit(const rb_vm_tag_jmpbuf_t *jmpbuf)
+{
+  // no-op
+}
+#endif
+
 /*
   the members which are written in EC_PUSH_TAG() should be placed at
   the beginning and the end, so that entire region is accessible.
@@ -891,7 +946,7 @@ typedef void *rb_jmpbuf_t[5];
 struct rb_vm_tag {
     VALUE tag;
     VALUE retval;
-    rb_jmpbuf_t buf;
+    rb_vm_tag_jmpbuf_t buf;
     struct rb_vm_tag *prev;
     enum ruby_tag_type state;
     unsigned int lock_rec;
@@ -899,7 +954,7 @@ struct rb_vm_tag {
 
 STATIC_ASSERT(rb_vm_tag_buf_offset, offsetof(struct rb_vm_tag, buf) > 0);
 STATIC_ASSERT(rb_vm_tag_buf_end,
-              offsetof(struct rb_vm_tag, buf) + sizeof(rb_jmpbuf_t) <
+              offsetof(struct rb_vm_tag, buf) + sizeof(rb_vm_tag_jmpbuf_t) <
               sizeof(struct rb_vm_tag));
 
 struct rb_unblock_callback {