[compiler-rt] [libunwind] [llvm] Fix compiling/using libunwind for wasm targets (PR #185770)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libunwind

Author: Alex Crichton (alexcrichton)

<details>
<summary>Changes</summary>

This commit is an extension of some work in [wasi-sdk] to get C++ exceptions working on WASI targets. This is intended to complement the work in #<!-- -->168449 and together is enough to get C++ exceptions working within wasi-sdk. The changes here are:

* 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.

* A minor compile error is fixed in libunwind's `assembly.h`. Without this the preexisting sources fail to compile, so this is along the lines of #<!-- -->168449 in terms of getting the in-tree sources to compile for wasm exceptions.

---
Full diff: https://github.com/llvm/llvm-project/pull/185770.diff


5 Files Affected:

- (modified) compiler-rt/lib/builtins/CMakeLists.txt (-1) 
- (removed) compiler-rt/lib/builtins/wasm/__cpp_exception.S (-26) 
- (modified) libunwind/src/Unwind-wasm.c (+17) 
- (modified) libunwind/src/assembly.h (+3) 
- (modified) llvm/utils/gn/secondary/compiler-rt/lib/builtins/sources.gni (-1) 


``````````diff
diff --git a/compiler-rt/lib/builtins/CMakeLists.txt b/compiler-rt/lib/builtins/CMakeLists.txt
index 6c27f6d4d529e..f0570a9092f40 100644
--- a/compiler-rt/lib/builtins/CMakeLists.txt
+++ b/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}
 )
diff --git a/compiler-rt/lib/builtins/wasm/__cpp_exception.S b/compiler-rt/lib/builtins/wasm/__cpp_exception.S
deleted file mode 100644
index 0496e1dbf6158..0000000000000
--- a/compiler-rt/lib/builtins/wasm/__cpp_exception.S
+++ /dev/null
@@ -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__
diff --git a/libunwind/src/Unwind-wasm.c b/libunwind/src/Unwind-wasm.c
index 2f4498c3f3989..44d284ba85b8f 100644
--- a/libunwind/src/Unwind-wasm.c
+++ b/libunwind/src/Unwind-wasm.c
@@ -69,6 +69,23 @@ _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) {
diff --git a/libunwind/src/assembly.h b/libunwind/src/assembly.h
index 84c9d526f1d75..2167326605b8a 100644
--- a/libunwind/src/assembly.h
+++ b/libunwind/src/assembly.h
@@ -253,6 +253,9 @@ aliasname:                                                                     \
 #define WEAK_ALIAS(name, aliasname)
 #define NO_EXEC_STACK_DIRECTIVE
 
+#elif defined(__wasm__)
+#define NO_EXEC_STACK_DIRECTIVE
+
 // clang-format on
 #else
 
diff --git a/llvm/utils/gn/secondary/compiler-rt/lib/builtins/sources.gni b/llvm/utils/gn/secondary/compiler-rt/lib/builtins/sources.gni
index 2ac71aa8e8367..c9eeede16e3eb 100644
--- a/llvm/utils/gn/secondary/compiler-rt/lib/builtins/sources.gni
+++ b/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",
   ]
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/185770