[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.
• This ignores and prints a warning for throw(type, ..), for a temporary measure. This warning is controlled by -Wwasm-exception-spec, which is on by default. You can suppress the warning by using -Wno-wasm-exception-spec.

[aheejin]

• rst file fix

[aheejin]

This is an improved version of D79655. The warning now can be suppressed by -Wno-wasm-exception-spec. To do this, we needed a diagnostic group.

[aheejin]

• Add missing -o

[aheejin]

• Add missing - after -o

clang/docs/DiagnosticsReference.rst

 -------------------
 **Diagnostic text:**
 
 +------------------------------------------------------------------------+
 |:warning:`warning:` |nbsp| :diagtext:`zero size arrays are an extension`|
 +------------------------------------------------------------------------+
 
 
+-Wwasm-exception-spec
+---------------------
+**Diagnostic text:**
+
++----------------------------------------------------------------------------------------------------------------+
+|:warning:`warning:` |nbsp| :diagtext:`dynamic exception specifications with types are currently ignored in wasm`|
++----------------------------------------------------------------------------------------------------------------+

clang/include/clang/Basic/DiagnosticGroups.td

3. As a per-translation unit limit using the ``clang max_tokens_total`` pragma,
    .. code-block: c++
       #pragma clang max_tokens_total 1234
 
 These limits can be helpful in limiting code growth through included files.
 
 Setting a token limit of zero means no limit.
 }];
 }
+
+def WebAssemblyExceptionSpec : DiagGroup<"wasm-exception-spec">;

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_dynamic_exception_spec_ignored : ExtWarn<
+  "dynamic exception specifications with types are currently ignored in wasm">,
+  InGroup<WebAssemblyExceptionSpec>;
 
 // 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;
+    // In wasm we currently treat 'throw()' in the same way as 'noexcept'. In
+    // case of throw with types, we ignore it and print a warning for now.
+    // TODO Correctly handle exception specification in wasm
+    if (getTarget().getCXXABI() == TargetCXXABI::WebAssembly) {
+      if (EST == EST_DynamicNone)
+        EHStack.pushTerminate();
+      else
+        CGM.getDiags().Report(D->getLocation(),
+                              diag::warn_wasm_dynamic_exception_spec_ignored)
+            << FD->getExceptionSpecSourceRange();
+      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;
+    // In wasm we currently treat 'throw()' in the same way as 'noexcept'. In
+    // case of throw with types, we ignore it and print a warning for now.
+    // TODO Correctly handle exception specification in wasm
+    if (getTarget().getCXXABI() == TargetCXXABI::WebAssembly) {
+      if (EST == EST_DynamicNone)
+        EHStack.popTerminate();
+      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
 // https://reviews.llvm.org/D79655 temporarily added a RUN line that was missing
 // a -o flag and wrote to the source dir. The file it wrote was then interpreted
 // as a test without RUN line, breaking bots. FIXME: Remove this rm line once
 // it's been in the tree long enough to clean up everyone's build dirs.
 // Removing this June 2020 should be fine.
 // RUN: rm -f %S/wasm-eh.ll
 // 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
 
+// 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 2>&1 | FileCheck %s --check-prefix=WARNING-DEFAULT
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -fms-extensions -fexceptions -fcxx-exceptions -fwasm-exceptions -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 -fwasm-exceptions -target-feature +exception-handling -Wno-wasm-exception-spec -emit-llvm -o - -std=c++11 2>&1 | FileCheck %s --check-prefix=WARNING-OFF
+
+// Wasm ignores dynamic exception specifications with types at the moment. This
+// is controlled by -Wwasm-exception-spec, which is on by default. This warning
+// can be suppressed with -Wno-wasm-exception-spec.
+// Checks if a warning message is correctly printed or not printed depending on
+// the options.
+void test9() throw(int) {
+}
+// WARNING-DEFAULT: warning: dynamic exception specifications with types are currently ignored in wasm
+// WARNING-ON: warning: dynamic exception specifications with types are currently ignored in wasm
+// WARNING-OFF-NOT: warning: dynamic exception specifications with types are currently ignored in wasm
+
+// Wasm curremtly treats 'throw()' in the same way as 'noexept'. Check if the
+// same warning message is printed as if when a 'noexcept' function throws.
+void test10() throw() {
+  throw 3;
+}
+// WARNING-DEFAULT: warning: 'test10' has a non-throwing exception specification but can still throw
+// WARNING-DEFAULT: function declared non-throwing here
+
 // 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