[WebAssembly] Handle exception specifications

[aheejin]

Summary

Wasm currently does not fully handle exception specifications. Rather
than crashing, this treats throw() in the same way as noexcept, and
ignores and prints a warning for throw(type, ..), for a temporary
measure.

[aheejin]

clang/lib/CodeGen/CGException.cpp

23

One thing I'm not sure about is if it is a good thing to include this in CodeGen. Usually this header is used in Sema/ directory. This depends on which .td file I add warn_wasm_exception_spec_ignored. DiagnosticSemaKinds.td seemed to have a section in exception spec so I added it there, but not entirely sure if that's the best location to add it.

[aheejin]

clang/lib/CodeGen/CGException.cpp

23

Oh, and this warning message was requested by Adobe, but I think it's good to have in general.

[aheejin]

• Delete a debugging(?) line

[aheejin]

clang/test/CodeGenCXX/wasm-eh.cpp

399

This was preexisting just moved

[dschuff]

Actually, would it be possible to not ignore throw() but make it an alias for noexcept(true)? Apparently that is the standard behavior in C++17, so it might make more sense to just implement that now rather than just warning all the time and ignoring it. It would also cover most of the cases that exist, so more users wouldn't need to disable the warning.

clang/lib/CodeGen/CGException.cpp

23

According to the CMakeLists.txt in lib/CodeGen, clangCodeGen depends on clangBasic, so I think it should be ok to include stuff from clang/Basic here (even if it has Sema in the name).
And a warning seems ok, as long as it's possible to suppress it.

[aheejin]

Now this handles throw() in the same way as noexcept, and prints a warning only in case of throw(type, ..), as you suggested.

[aheejin]

Handle throw() in the same way as noexcept

[dschuff]

clang/test/CodeGenCXX/wasm-eh.cpp

399

Is it common in these tests to have RUN lines throughout the file instead of all together up at the top?

[aheejin]

clang/test/CodeGenCXX/wasm-eh.cpp

399

Not sure how common it is, but we have similar examples, such as https://github.com/llvm/llvm-project/blob/master/clang/test/Driver/wasm-toolchain.c. I moved them mainly for readability; because now we can check what prefix WARNING's command line is like and what WARNING's check lines like, and the same for ASSEMBLY. I don't have a strong opinion for this, so if you think it's better to merge them at the top, please let me know, I'll do that as a follow-up.

[aheejin]

Committed and reverted this. Resubmitted this in D80061.

[thakis]

The revert broke check-clang on several bots, e.g. here http://lab.llvm.org:8011/builders/clang-ppc64be-linux-lnt/builds/38159

Can you take a look please? The tree's been red all of today due to this.

[thakis]

clang/test/CodeGenCXX/wasm-eh.cpp

387

Ah, the problem is that this line is missing a -o flag and so it wrote a wasm-eh.ll file to the clang/test/CodeGenCXX directory, which on the next build was interpreted as a test without a run line. I'll land a temporary rm here to remove that file. Please fix this for the reland.

[aheejin]

This is very strange. I haven't committed a file called wasm-eh.ll. (There is a file called wasm-eh.cpp, which existed for more than a year now.) Also there's no wasm-eh.ll in LLVM repo now either. I wonder how this happened..? Also I can't reproduce the bug in my local machine.

[aheejin]

clang/test/CodeGenCXX/wasm-eh.cpp

387

Oh I see... Thanks a lot for doing this.

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_noexcept_needs_constant_expression : Error<
   "argument to noexcept specifier must be a constant expression">;
 def err_exception_spec_not_parsed : Error<
   "exception specification is not available until end of class definition">;
 def err_exception_spec_cycle : Error<
   "exception specification of %0 uses itself">;
 def err_exception_spec_incomplete_type : Error<
   "exception specification needed for member of incomplete class %0">;
+def warn_wasm_exception_spec_ignored : Warning<
+  "exception specifications are currently ignored in wasm">;
 
 // C++ access checking
 def err_class_redeclared_with_different_access : Error<
   "%0 redeclared with '%1' access">;
 def err_access : Error<
   "%1 is a %select{private|protected}0 member of %3">, AccessControl;
 def ext_ms_using_declaration_inaccessible : ExtWarn<
   "using declaration referring to inaccessible member '%0' (which refers "

clang/lib/CodeGen/CGException.cpp

 #include "CGObjCRuntime.h"
 #include "CodeGenFunction.h"
 #include "ConstantEmitter.h"
 #include "TargetInfo.h"
 #include "clang/AST/Mangle.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/AST/StmtObjC.h"
 #include "clang/AST/StmtVisitor.h"
+#include "clang/Basic/DiagnosticSema.h"
 #include "clang/Basic/TargetBuiltins.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/IntrinsicsWebAssembly.h"
 #include "llvm/Support/SaveAndRestore.h"
 
 using namespace clang;
 using namespace CodeGen;
void CodeGenFunction::EmitStartEHSpec(const Decl *D) {
   if (isNoexceptExceptionSpec(EST) && Proto->canThrow() == CT_Cannot) {
     // noexcept functions are simple terminate scopes.
     EHStack.pushTerminate();
   } else if (EST == EST_Dynamic || EST == EST_DynamicNone) {
     // TODO: Revisit exception specifications for the MS ABI.  There is a way to
     // encode these in an object file but MSVC doesn't do anything with it.
     if (getTarget().getCXXABI().isMicrosoft())
       return;
+    // TODO Correctly handle exception specification in wasm
+    if (getTarget().getCXXABI() == TargetCXXABI::WebAssembly) {
+      CGM.getDiags().Report(D->getLocation(),
+                            diag::warn_wasm_exception_spec_ignored);
+      return;
+    }
     unsigned NumExceptions = Proto->getNumExceptions();
     EHFilterScope *Filter = EHStack.pushFilter(NumExceptions);
 
     for (unsigned I = 0; I != NumExceptions; ++I) {
       QualType Ty = Proto->getExceptionType(I);
       QualType ExceptType = Ty.getNonReferenceType().getUnqualifiedType();
       llvm::Value *EHType = CGM.GetAddrOfRTTIDescriptor(ExceptType,
                                                         /*ForEH=*/true);
void CodeGenFunction::EmitEndEHSpec(const Decl *D) {
   ExceptionSpecificationType EST = Proto->getExceptionSpecType();
   if (isNoexceptExceptionSpec(EST) && Proto->canThrow() == CT_Cannot) {
     EHStack.popTerminate();
   } else if (EST == EST_Dynamic || EST == EST_DynamicNone) {
     // TODO: Revisit exception specifications for the MS ABI.  There is a way to
     // encode these in an object file but MSVC doesn't do anything with it.
     if (getTarget().getCXXABI().isMicrosoft())
       return;
+    // TODO Correctly handle exception specification in wasm
+    if (getTarget().getCXXABI() == TargetCXXABI::WebAssembly)
+      return;
     EHFilterScope &filterScope = cast<EHFilterScope>(*EHStack.begin());
     emitFilterDispatchBlock(*this, filterScope);
     EHStack.popFilter();
   }
 }
 
 void CodeGenFunction::EmitCXXTryStmt(const CXXTryStmt &S) {
   EnterCXXTryStmt(S);

clang/test/CodeGenCXX/wasm-eh.cpp

 // REQUIRES: webassembly-registered-target
 // RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -fwasm-exceptions -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 -fwasm-exceptions -target-feature +exception-handling -emit-llvm -o - -std=c++11 | FileCheck %s
-// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -fwasm-exceptions -target-feature +exception-handling -S -o - -std=c++11 | FileCheck %s --check-prefix=ASSEMBLY
 
 void may_throw();
 void dont_throw() noexcept;
 
 struct Cleanup {
   ~Cleanup() { dont_throw(); }
 };
 

 // CHECK:   %[[CLEANUPPAD0:.*]] = cleanuppad within %[[CATCHPAD1]] []
 // CHECK:   cleanupret from %[[CLEANUPPAD0]] unwind label
 
 // CHECK:   %[[CLEANUPPAD1:.*]] = cleanuppad within %[[CATCHPAD0]] []
 // CHECK:   cleanupret from %[[CLEANUPPAD1]] unwind to caller
 
 // CHECK:   unreachable
 
+// Wasm ignores exception specifications at the moment. Checks if a warning
+// message is printed.
+void test9() throw() {
+}
+
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -fwasm-exceptions -target-feature +exception-handling -emit-llvm -std=c++11 2>&1 | FileCheck %s --check-prefix=WARNING
+
+// WARNING: warning: exception specifications are currently ignored in wasm
+
 // Here we only check if the command enables wasm exception handling in the
 // backend so that exception handling instructions can be generated in .s file.
 
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -fwasm-exceptions -target-feature +exception-handling -S -o - -std=c++11 | FileCheck %s --check-prefix=ASSEMBLY
+
 // ASSEMBLY: try
 // ASSEMBLY: catch
 // ASSEMBLY: rethrow
 // ASSEMBLY: end_try