Add a setjmp/longjmp runtime for a new mode proposed in LLVM

Hopefully, this implementation can replace the current
`-sSUPPORT_LONGJMP=wasm` implementation sooner or later.

system/lib/compiler-rt/wasm_setjmp.c is basically a copy from:
https://github.com/yamt/garbage/blob/wasm-sjlj-alt2/wasm/longjmp/rt.c.

the copyright comment was copy-and-pasted from:
system/lib/compiler-rt/emscripten_exception_builtins.c

The corresponding LLVM change:
llvm/llvm-project#84137

Discussion:
https://docs.google.com/document/d/1ZvTPT36K5jjiedF8MCXbEmYjULJjI723aOAks1IdLLg/edit

Author: yamt

system/lib/compiler-rt/wasm_setjmp.c

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2024 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ *
+ * a runtime implementation for
+ * https://github.com/llvm/llvm-project/pull/84137
+ * https://docs.google.com/document/d/1ZvTPT36K5jjiedF8MCXbEmYjULJjI723aOAks1IdLLg/edit
+ *
+ * a plan is to replace the __USING_WASM_SJLJ__ part of emscripten_setjmp.c
+ * with this.
+ */
+
+#ifdef __USING_WASM_SJLJ__
+
+#include <stddef.h>
+#include <stdint.h>
+
+/*
+ * jmp_buf should have large enough size and alignment to contain
+ * this structure.
+ */
+struct jmp_buf_impl {
+        void *func_invocation_id;
+        uint32_t label;
+
+        /*
+         * this is a temorary storage used by the communication between
+         * __wasm_sjlj_longjmp and WebAssemblyLowerEmscriptenEHSjL-generated
+         * logic.
+         * ideally, this can be replaced with multivalue.
+         */
+        struct arg {
+                void *env;
+                int val;
+        } arg;
+};
+
+void
+__wasm_sjlj_setjmp(void *env, uint32_t label, void *func_invocation_id)
+{
+        struct jmp_buf_impl *jb = env;
+        if (label == 0) { /* ABI contract */
+                __builtin_trap();
+        }
+        if (func_invocation_id == NULL) { /* sanity check */
+                __builtin_trap();
+        }
+        jb->func_invocation_id = func_invocation_id;
+        jb->label = label;
+}
+
+uint32_t
+__wasm_sjlj_test(void *env, void *func_invocation_id)
+{
+        struct jmp_buf_impl *jb = env;
+        if (jb->label == 0) { /* ABI contract */
+                __builtin_trap();
+        }
+        if (func_invocation_id == NULL) { /* sanity check */
+                __builtin_trap();
+        }
+        if (jb->func_invocation_id == func_invocation_id) {
+                return jb->label;
+        }
+        return 0;
+}
+
+void
+__wasm_sjlj_longjmp(void *env, int val)
+{
+        struct jmp_buf_impl *jb = env;
+        struct arg *arg = &jb->arg;
+        /*
+         * C standard:
+         * > The longjmp function cannot cause the setjmp macro to return
+         * > the value 0; if val is 0, the setjmp macro returns the value 1.
+         */
+        if (val == 0) {
+                val = 1;
+        }
+        arg->env = env;
+        arg->val = val;
+        __builtin_wasm_throw(1, arg); /* 1 == C_LONGJMP */
+}
+
+#endif /* __USING_WASM_SJLJ__ */

tools/system_libs.py

@@ -936,6 +936,7 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
         'emscripten_setjmp.c',
         'emscripten_exception_builtins.c',
         'emscripten_tempret.s',
+        'wasm_setjmp.c',
         '__trap.c',
       ])