[WebAssembly] Add exception handling option

[aheejin]

Summary

Add exception handling option to clang.

[sbc100]

Don't we already have -fexceptions and -fno-exceptions for this?

[dschuff]

This flag turns on CPU features (i.e. there's one for each new wasm feature proposal that has to be feature-detected), so I think this makes sense to be consistent with all the others. I could imagine enabling exceptions in the frontend but having some kind of emulation in the backend (like we do today for emscripten). More generally the semantics -fno-exceptions unfortunately doesn't exactly match the kind of behavior people typically want because it doesn't allow you to compile code that has try/catch/throw at all. In PNaCl and emscripten the default behavior is instead to compile that code but to lower it away and turn throw into abort. Also IIRC even with -fno-exceptions the code still has to allow exceptions to propagate which means you end up with invokes everywhere instead of calls, so you are still paying most of the costs of enabling EH. So that bit is a little bit complex and we'll probably want to think a bit more carefully about what options we want to have. But a machine flag for enabling the CPU feature makes sense to start.

cfe/trunk/docs/ClangCommandLineReference.rst

 WebAssembly
 -----------
 .. option:: -mnontrapping-fptoint, -mno-nontrapping-fptoint
 
 .. option:: -msign-ext, -mno-sign-ext
 
 .. option:: -msimd128, -mno-simd128
 
+.. option:: -mexception-handling, -mno-exception-handling
+
 X86
 ---
 .. option:: -m3dnow, -mno-3dnow
 
 .. option:: -m3dnowa, -mno-3dnowa
 
 .. option:: -madx, -mno-adx
 

cfe/trunk/include/clang/Driver/Options.td

def ffixed_x18 : Flag<["-"], "ffixed-x18">, Group<m_aarch64_Features_Group>,
   HelpText<"Reserve the x18 register (AArch64 only)">;
 
 def msimd128 : Flag<["-"], "msimd128">, Group<m_wasm_Features_Group>;
 def mno_simd128 : Flag<["-"], "mno-simd128">, Group<m_wasm_Features_Group>;
 def mnontrapping_fptoint : Flag<["-"], "mnontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def mno_nontrapping_fptoint : Flag<["-"], "mno-nontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def msign_ext : Flag<["-"], "msign-ext">, Group<m_wasm_Features_Group>;
 def mno_sign_ext : Flag<["-"], "mno-sign-ext">, Group<m_wasm_Features_Group>;
+def mexception_handing : Flag<["-"], "mexception-handling">, Group<m_wasm_Features_Group>;
+def mno_exception_handing : Flag<["-"], "mno-exception-handling">, Group<m_wasm_Features_Group>;
 
 def mamdgpu_debugger_abi : Joined<["-"], "mamdgpu-debugger-abi=">,
   Flags<[HelpHidden]>,
   Group<m_Group>,
   HelpText<"Generate additional code for specified <version> of debugger ABI (AMDGPU only)">,
   MetaVarName<"<version>">;
 def mxnack : Flag<["-"], "mxnack">, Group<m_amdgpu_Features_Group>,
   HelpText<"Enable XNACK (AMDGPU only)">;

cfe/trunk/lib/Basic/Targets/WebAssembly.h

class LLVM_LIBRARY_VISIBILITY WebAssemblyTargetInfo : public TargetInfo {
 
   enum SIMDEnum {
     NoSIMD,
     SIMD128,
   } SIMDLevel;
 
   bool HasNontrappingFPToInt;
   bool HasSignExt;
+  bool HasExceptionHandling;
 
 public:
   explicit WebAssemblyTargetInfo(const llvm::Triple &T, const TargetOptions &)
       : TargetInfo(T), SIMDLevel(NoSIMD), HasNontrappingFPToInt(false),
-        HasSignExt(false) {
+        HasSignExt(false), HasExceptionHandling(false) {
     NoAsmVariants = true;
     SuitableAlign = 128;
     LargeArrayMinWidth = 128;
     LargeArrayAlign = 128;
     SimdDefaultAlign = 128;
     SigAtomicType = SignedLong;
     LongDoubleWidth = LongDoubleAlign = 128;
     LongDoubleFormat = &llvm::APFloat::IEEEquad();

cfe/trunk/lib/Basic/Targets/WebAssembly.cpp

 static constexpr llvm::StringLiteral ValidCPUNames[] = {
     {"mvp"}, {"bleeding-edge"}, {"generic"}};
 
 bool WebAssemblyTargetInfo::hasFeature(StringRef Feature) const {
   return llvm::StringSwitch<bool>(Feature)
       .Case("simd128", SIMDLevel >= SIMD128)
       .Case("nontrapping-fptoint", HasNontrappingFPToInt)
       .Case("sign-ext", HasSignExt)
+      .Case("exception-handling", HasExceptionHandling)
       .Default(false);
 }
 
 bool WebAssemblyTargetInfo::isValidCPUName(StringRef Name) const {
   return llvm::find(ValidCPUNames, Name) != std::end(ValidCPUNames);
 }
 
 void WebAssemblyTargetInfo::fillValidCPUList(
for (const auto &Feature : Features) {
     if (Feature == "+sign-ext") {
       HasSignExt = true;
       continue;
     }
     if (Feature == "-sign-ext") {
       HasSignExt = false;
       continue;
     }
+    if (Feature == "+exception-handling") {
+      HasExceptionHandling = true;
+      continue;
+    }
+    if (Feature == "-exception-handling") {
+      HasExceptionHandling = false;
+      continue;
+    }
 
     Diags.Report(diag::err_opt_not_valid_with_opt)
         << Feature << "-target-feature";
     return false;
   }
   return true;
 }