[WebAssembly] Generate __clang_call_terminate for Emscripten EH

When an exception thrown ends up calling `std::terminate`, for example,
because an exception is thrown within a `noexcept` function or an
exception is thrown from `__cxa_end_catch` during handling the
previous exception, the libc++abi spec says we are supposed to call
`__cxa_begin_catch` before `std::terminate`:
https://libcxxabi.llvm.org/spec.html
>  When the personality routine encounters a termination condition, it
> will call `__cxa_begin_catch()` to mark the exception as handled and
> then call `terminate()`, which shall not return to its caller.

The default Itanium ABI generates a call to `__clang_call_terminate()`,
which is a function that calls `__cxa_begin_catch` and then
`std::terminate`:
```ll
define void @__clang_call_terminate(ptr noundef %0) {
  %2 = call ptr @__cxa_begin_catch(ptr %0)
  call void @_ZSt9terminatev()
  unreachable
}
```

But we replaced this with just a call to `std::terminate` in
561abd8
because this caused some tricky transformation problems for Wasm EH. The
detailed explanation why is in the commit description, but the summary
is for Wasm EH it needs a `try` with both `catch` and `catch_all` and it
was tricky to deal with.

But that commit replaced `__clang_call_terminate` with `std::terminate`
for all Wasm programs and not only the ones that use Wasm EH. So
Emscripten EH was also affected by that commit. Emscripten EH is not
able to catch foreign exceptions anyway, so this is unnecessary
compromise.

This makes we use `__clang_call_terminate` as in the default Itanium EH
for Emscripten EH. We may later fix Wasm EH too but that requires more
efforts in the backend.

Related issue:
emscripten-core/emscripten#23720

Author: aheejin

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -5150,9 +5150,14 @@ WebAssemblyCXXABI::emitTerminateForUnexpectedException(CodeGenFunction &CGF,
   // Itanium ABI calls __clang_call_terminate(), which __cxa_begin_catch() on
   // the violating exception to mark it handled, but it is currently hard to do
   // with wasm EH instruction structure with catch/catch_all, we just call
-  // std::terminate and ignore the violating exception as in CGCXXABI.
+  // std::terminate and ignore the violating exception as in CGCXXABI in Wasm EH
+  // and calls __clang_call_terminate only in Emscripten EH.
   // TODO Consider code transformation that makes calling __clang_call_terminate
-  // possible.
+  // in Wasm EH possible.
+  if (Exn && !EHPersonality::get(CGF).isWasmPersonality()) {
+    assert(CGF.CGM.getLangOpts().CPlusPlus);
+    return CGF.EmitNounwindRuntimeCall(getClangCallTerminateFn(CGF.CGM), Exn);
+  }
   return CGCXXABI::emitTerminateForUnexpectedException(CGF, Exn);
 }
 

clang/test/CodeGenCXX/wasm-eh.cpp

@@ -6,6 +6,9 @@
 // RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -mllvm -wasm-enable-eh -exception-model=wasm -target-feature +exception-handling -emit-llvm -o - -std=c++11 | FileCheck %s
 // RUN: %clang_cc1 %s -triple wasm64-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -mllvm -wasm-enable-eh -exception-model=wasm -target-feature +exception-handling -emit-llvm -o - -std=c++11 | FileCheck %s
 
+// Test code generation for Wasm EH using WebAssembly EH proposal.
+// (https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md)
+
 void may_throw();
 void dont_throw() noexcept;
 
@@ -381,6 +384,15 @@ void test8() {
 
 // CHECK:   unreachable
 
+void noexcept_throw() noexcept {
+  throw 3;
+}
+
+// CATCH-LABEL: define void @_Z14noexcept_throwv()
+// CHECK: %{{.*}} = cleanuppad within none []
+// CHECK-NEXT:  call void @_ZSt9terminatev()
+
+
 // RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -exception-model=wasm -target-feature +exception-handling -emit-llvm -o - -std=c++11 2>&1 | FileCheck %s --check-prefix=WARNING-DEFAULT
 // RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -exception-model=wasm -target-feature +exception-handling -Wwasm-exception-spec -emit-llvm -o - -std=c++11 2>&1 | FileCheck %s --check-prefix=WARNING-ON
 // RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -exception-model=wasm -target-feature +exception-handling -Wno-wasm-exception-spec -emit-llvm -o - -std=c++11 2>&1 | FileCheck %s --check-prefix=WARNING-OFF

clang/test/CodeGenCXX/wasm-em-eh.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fexceptions -fcxx-exceptions -emit-llvm -o - -std=c++11 2>&1 | FileCheck %s
+
+// Test code generation for Wasm's Emscripten (JavaScript-style) EH.
+
+void noexcept_throw() noexcept {
+  throw 3;
+}
+
+// CATCH-LABEL: define void @_Z14noexcept_throwv()
+// CHECK:       %[[LPAD:.*]] = landingpad { ptr, i32 }
+// CHECK-NEXT:                    catch ptr null
+// CHECK-NEXT:  %[[EXN:.*]] = extractvalue { ptr, i32 } %[[LPAD]], 0
+// CHECK-NEXT:  call void @__clang_call_terminate(ptr %[[EXN]])