[libunwind][WebAssembly] Support Wasm EH

This adds Wasm-specific libunwind port to support Wasm exception
handling (https://github.com/WebAssembly/exception-handling).

Wasm EH requires `__USING_WASM_EXCEPTIONS__` to be defined. This adds
`Unwind-wasm.c`, which defines libunwind APIs for Wasm. This also adds a
`thread_local` struct of type `_Unwind_LandingPadContext`, which serves
as a medium for input/output data between the user code and the
personality function. How all these work is explained in
https://github.com/WebAssembly/tool-conventions/blob/main/EHScheme.md.
(The doc is old and "You Shouldn't Prune Unreachable Resumes" section
doesn't apply anymore, but otherwise it should be good)

The bulk of these changes was added back in Mar 2020 in
emscripten-core/emscripten#10577 to emscripten
repo and has been used ever since. Now we'd like to upstream this so
that other toolchains that don't use emscripten libraries, e.g., WASI,
can use this too.

Companion patch: D158918

Reviewed By: dschuff, #libunwind, phosek

Differential Revision: https://reviews.llvm.org/D158919

Author: aheejin

libunwind/src/Unwind-wasm.c

@@ -1,11 +1,21 @@
-#define NDEBUG
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//
+//  Implements Wasm exception handling proposal
+//  (https://github.com/WebAssembly/exception-handling) based C++ exceptions
+//
+//===----------------------------------------------------------------------===//
 
 #include "config.h"
 #include "unwind.h"
 #include <stdbool.h>
 #include <threads.h>
 
-#if 1 //def __USING_WASM_EXCEPTIONS__
+#ifdef __USING_WASM_EXCEPTIONS__
 
 _Unwind_Reason_Code __gxx_personality_wasm0(int version, _Unwind_Action actions,
                                             uint64_t exceptionClass,
@@ -43,17 +53,17 @@ _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
 
   // Call personality function. Wasm does not have two-phase unwinding, so we
   // only do the cleanup phase.
-  _Unwind_Reason_Code ret = __gxx_personality_wasm0(
+  return __gxx_personality_wasm0(
       1, _UA_SEARCH_PHASE, exception_object->exception_class, exception_object,
       (struct _Unwind_Context *)&__wasm_lpad_context);
-  return ret;
 }
 
 /// Called by __cxa_throw.
 _LIBUNWIND_EXPORT _Unwind_Reason_Code
 _Unwind_RaiseException(_Unwind_Exception *exception_object) {
   _LIBUNWIND_TRACE_API("_Unwind_RaiseException(exception_object=%p)",
                        (void *)exception_object);
+  // Use Wasm EH's 'throw' instruction.
   __builtin_wasm_throw(0, exception_object);
 }
 
@@ -74,9 +84,8 @@ _LIBUNWIND_EXPORT void _Unwind_SetGR(struct _Unwind_Context *context, int index,
                        (void *)context, index, value);
   // We only use this function to set __wasm_lpad_context.selector field, which
   // is index 1 in the personality function.
-  if (index != 1)
-    return;
-  ((struct _Unwind_LandingPadContext *)context)->selector = value;
+  if (index == 1)
+    ((struct _Unwind_LandingPadContext *)context)->selector = value;
 }
 
 /// Called by personality handler to get instruction pointer.
@@ -90,7 +99,7 @@ _LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
   return result;
 }
 
-/// Not used in wasm.
+/// Not used in Wasm.
 _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *context,
                                      uintptr_t value) {}
 
@@ -103,7 +112,7 @@ _Unwind_GetLanguageSpecificData(struct _Unwind_Context *context) {
   return result;
 }
 
-/// Not used in wasm.
+/// Not used in Wasm.
 _LIBUNWIND_EXPORT uintptr_t
 _Unwind_GetRegionStart(struct _Unwind_Context *context) {
   return 0;

libunwind/src/config.h

@@ -85,7 +85,7 @@
   __asm__(".globl " SYMBOL_NAME(aliasname));                                   \
   __asm__(SYMBOL_NAME(aliasname) " = " SYMBOL_NAME(name));                     \
   _LIBUNWIND_ALIAS_VISIBILITY(SYMBOL_NAME(aliasname))
-#elif defined(__ELF__) || defined(_AIX)
+#elif defined(__ELF__) || defined(_AIX) || defined(__wasm__)
 #define _LIBUNWIND_WEAK_ALIAS(name, aliasname)                                 \
   extern "C" _LIBUNWIND_EXPORT __typeof(name) aliasname                        \
       __attribute__((weak, alias(#name)));