[WebAssembly] Move __cpp_exception to libunwind (#185770)

The `__cpp_exception` symbol is now defined in libunwind instead of
compiler-rt. This is moved for a few reasons, but the primary reason is
that compiler-rt is linked duplicate-ly into all shared objects meaning
that it's not suitable for define-once symbols such as
`__cpp_exception`. By moving the definition to the user of the symbol,
libunwind itself, that guarantees that the symbol should be defined
exactly once and only when appropriate. A secondary reason for this
movement is that it avoids the need to compile compiler-rt twice: once
with exception and once without, and instead the same build can be used
for both exceptions-and-not.

compiler-rt/lib/builtins/CMakeLists.txt

@@ -891,7 +891,6 @@ set(s390x_SOURCES
 
 set(wasm_SOURCES
   wasm/__c_longjmp.S
-  wasm/__cpp_exception.S
   ${GENERIC_TF_SOURCES}
   ${GENERIC_SOURCES}
 )

compiler-rt/lib/builtins/wasm/__cpp_exception.S

@@ -1,26 +0,0 @@
-//===-- __cpp_exception.S - Implement __cpp_exception ---------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-//
-// This file implements __cpp_exception which LLVM uses to implement exception
-// handling when Wasm EH is enabled.
-//
-//===----------------------------------------------------------------------===//
-
-#ifdef __wasm_exception_handling__
-
-#ifdef __wasm64__
-#define PTR i64
-#else
-#define PTR i32
-#endif
-
-.globl __cpp_exception
-.tagtype __cpp_exception PTR
-__cpp_exception:
-
-#endif // !__wasm_exception_handling__

libunwind/src/Unwind-wasm.c

@@ -69,6 +69,21 @@ _Unwind_RaiseException(_Unwind_Exception *exception_object) {
   __builtin_wasm_throw(0, exception_object);
 }
 
+// Define the `__cpp_exception` symbol which `__builtin_wasm_throw` above will
+// reference. This is defined here in `libunwind` as the single canonical
+// definition for this API and it's required for users to ensure that there's
+// only one copy of `libunwind` within a wasm module to ensure this is only
+// defined once and exactly once.
+__asm__(".globl __cpp_exception\n"
+#if defined(__wasm32__)
+        ".tagtype __cpp_exception i32\n"
+#elif defined(__wasm64__)
+        ".tagtype __cpp_exception i64\n"
+#else
+#error "Unsupported Wasm architecture"
+#endif
+        "__cpp_exception:\n");
+
 /// Called by __cxa_end_catch.
 _LIBUNWIND_EXPORT void
 _Unwind_DeleteException(_Unwind_Exception *exception_object) {

llvm/utils/gn/secondary/compiler-rt/lib/builtins/sources.gni

@@ -539,7 +539,6 @@ if (current_cpu == "ve") {
 if (current_cpu == "wasm") {
   builtins_sources += [
     "wasm/__c_longjmp.S",
-    "wasm/__cpp_exception.S",
   ]
 }