[clang] Add -fignore-exceptions

Summary:

This is trying to implement the functionality proposed in:
http://lists.llvm.org/pipermail/cfe-dev/2017-April/053417.html
An exception can throw, but no cleanup is going to happen.
A module compiled with exceptions on, can catch the exception throws
from module compiled with -fignore-exceptions.

The use cases for enabling this option are:
1. Performance analysis of EH instrumentation overhead
2. The ability to QA non EH functionality when EH functionality is not available.
3. User of EH enabled headers knows the calls won't throw in their program and
   wants the performance gain from ignoring EH construct.

The implementation tried to accomplish that by removing any landing pad code
 that might get generated.

Reviewed by: aaron.ballman

Differential Revision: https://reviews.llvm.org/D72644

clang/include/clang/Basic/LangOptions.def

@@ -132,6 +132,7 @@ LANGOPT(DWARFExceptions   , 1, 0, "dwarf exception handling")
 LANGOPT(SjLjExceptions    , 1, 0, "setjmp-longjump exception handling")
 LANGOPT(SEHExceptions     , 1, 0, "SEH .xdata exception handling")
 LANGOPT(WasmExceptions    , 1, 0, "WebAssembly exception handling")
+LANGOPT(IgnoreExceptions  , 1, 0, "ignore exceptions")
 LANGOPT(ExternCNoUnwind   , 1, 0, "Assume extern C functions don't unwind")
 LANGOPT(TraditionalCPP    , 1, 0, "traditional CPP emulation")
 LANGOPT(RTTI              , 1, 1, "run-time type information")

clang/include/clang/Driver/Options.td

@@ -932,6 +932,8 @@ def fseh_exceptions : Flag<["-"], "fseh-exceptions">, Group<f_Group>,
   Flags<[CC1Option]>, HelpText<"Use SEH style exceptions">;
 def fwasm_exceptions : Flag<["-"], "fwasm-exceptions">, Group<f_Group>,
   Flags<[CC1Option]>, HelpText<"Use WebAssembly style exceptions">;
+def fignore_exceptions : Flag<["-"], "fignore-exceptions">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Enable support for ignoring exception handling constructs">;
 def fexcess_precision_EQ : Joined<["-"], "fexcess-precision=">,
     Group<clang_ignored_gcc_optimization_f_Group>;
 def : Flag<["-"], "fexpensive-optimizations">, Group<clang_ignored_gcc_optimization_f_Group>;

clang/lib/CodeGen/CGException.cpp

@@ -703,12 +703,12 @@ llvm::BasicBlock *CodeGenFunction::getInvokeDestImpl() {
   assert(EHStack.requiresLandingPad());
   assert(!EHStack.empty());
 
-  // If exceptions are disabled and SEH is not in use, then there is no invoke
-  // destination. SEH "works" even if exceptions are off. In practice, this
-  // means that C++ destructors and other EH cleanups don't run, which is
+  // If exceptions are disabled/ignored and SEH is not in use, then there is no
+  // invoke destination. SEH "works" even if exceptions are off. In practice,
+  // this means that C++ destructors and other EH cleanups don't run, which is
   // consistent with MSVC's behavior.
   const LangOptions &LO = CGM.getLangOpts();
-  if (!LO.Exceptions) {
+  if (!LO.Exceptions || LO.IgnoreExceptions) {
     if (!LO.Borland && !LO.MicrosoftExt)
       return nullptr;
     if (!currentFunctionUsesSEHTry())
@@ -751,7 +751,9 @@ llvm::BasicBlock *CodeGenFunction::getInvokeDestImpl() {
 
 llvm::BasicBlock *CodeGenFunction::EmitLandingPad() {
   assert(EHStack.requiresLandingPad());
-
+  assert(!CGM.getLangOpts().IgnoreExceptions &&
+         "LandingPad should not be emitted when -fignore-exceptions are in "
+         "effect.");
   EHScope &innermostEHScope = *EHStack.find(EHStack.getInnermostEHScope());
   switch (innermostEHScope.getKind()) {
   case EHScope::Terminate:

clang/lib/Driver/ToolChains/Clang.cpp

@@ -463,6 +463,11 @@ static void addExceptionArgs(const ArgList &Args, types::ID InputType,
     }
   }
 
+  // OPT_fignore_exceptions means exception could still be thrown,
+  // but no clean up or catch would happen in current module.
+  // So we do not set EH to false.
+  Args.AddLastArg(CmdArgs, options::OPT_fignore_exceptions);
+
   if (EH)
     CmdArgs.push_back("-fexceptions");
 }

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2773,6 +2773,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   if (Args.hasArg(OPT_fno_threadsafe_statics))
     Opts.ThreadsafeStatics = 0;
   Opts.Exceptions = Args.hasArg(OPT_fexceptions);
+  Opts.IgnoreExceptions = Args.hasArg(OPT_fignore_exceptions);
   Opts.ObjCExceptions = Args.hasArg(OPT_fobjc_exceptions);
   Opts.CXXExceptions = Args.hasArg(OPT_fcxx_exceptions);
 

clang/test/CodeGen/ignore-exceptions.cpp

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 %s -triple powerpc64-linux -fexceptions -fcxx-exceptions -fignore-exceptions -emit-llvm -o - | FileCheck %s
+
+struct A {
+  ~A(){}
+};
+
+void f(void) {
+// CHECK-NOT: personality i8* bitcast (i32 (...)* @__gcc_personality_v0 to i8*)
+  A a;
+  try {
+    throw 1;
+  } catch(...) {
+  }
+// CHECK:  %a = alloca %struct.A, align 1
+// CHECK:  %exception = call i8* @__cxa_allocate_exception(i64 4) #1
+// CHECK:  %0 = bitcast i8* %exception to i32*
+// CHECK:  store i32 1, i32* %0, align 16
+// CHECK:  call void @__cxa_throw(i8* %exception, i8* bitcast (i8** @_ZTIi to i8*), i8* null) #2
+// CHECK:  unreachable
+
+// CHECK-NOT: invoke
+// CHECK-NOT: landingpad
+// CHECK-NOT: __cxa_begin_catch
+// CHECK-NOT: __cxa_end_catch
+}