[MSVC Compat] Implement -EHc semantics

The -EHc flag implicitly adds a nothrow attribute to any extern "C"
function when exceptions are enabled.

llvm-svn: 261425

clang/include/clang/Basic/LangOptions.def

@@ -110,6 +110,7 @@ LANGOPT(Exceptions        , 1, 0, "exception handling")
 LANGOPT(ObjCExceptions    , 1, 0, "Objective-C exceptions")
 LANGOPT(CXXExceptions     , 1, 0, "C++ exceptions")
 LANGOPT(SjLjExceptions    , 1, 0, "setjmp-longjump exception handling")
+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")
 LANGOPT(RTTIData          , 1, 1, "emit run-time type information data")

clang/include/clang/Driver/CC1Options.td

@@ -511,6 +511,8 @@ def main_file_name : Separate<["-"], "main-file-name">,
 
 def fblocks_runtime_optional : Flag<["-"], "fblocks-runtime-optional">,
   HelpText<"Weakly link in the blocks runtime">;
+def fexternc_nounwind : Flag<["-"], "fexternc-nounwind">,
+  HelpText<"Assume all functions with C linkage do not unwind">;
 def fsjlj_exceptions : Flag<["-"], "fsjlj-exceptions">,
   HelpText<"Use SjLj style exceptions">;
 def split_dwarf_file : Separate<["-"], "split-dwarf-file">,

clang/lib/Driver/Tools.cpp

@@ -5786,8 +5786,7 @@ struct EHFlags {
 /// - s: Cleanup after "synchronous" exceptions, aka C++ exceptions.
 /// - a: Cleanup after "asynchronous" exceptions, aka structured exceptions.
 ///      The 'a' modifier is unimplemented and fundamentally hard in LLVM IR.
-/// - c: Assume that extern "C" functions are implicitly noexcept.  This
-///      modifier is an optimization, so we ignore it for now.
+/// - c: Assume that extern "C" functions are implicitly nounwind.
 /// The default is /EHs-c-, meaning cleanups are disabled.
 static EHFlags parseClangCLEHFlags(const Driver &D, const ArgList &Args) {
   EHFlags EH;
@@ -5897,10 +5896,11 @@ void Clang::AddClangCLArgs(const ArgList &Args, ArgStringList &CmdArgs,
 
   const Driver &D = getToolChain().getDriver();
   EHFlags EH = parseClangCLEHFlags(D, Args);
-  // FIXME: Do something with NoExceptC.
   if (EH.Synch || EH.Asynch) {
     CmdArgs.push_back("-fcxx-exceptions");
     CmdArgs.push_back("-fexceptions");
+    if (EH.NoUnwindC)
+      CmdArgs.push_back("-fexternc-nounwind");
   }
 
   // /EP should expand to -E -P.

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1688,6 +1688,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.ObjCExceptions = Args.hasArg(OPT_fobjc_exceptions);
   Opts.CXXExceptions = Args.hasArg(OPT_fcxx_exceptions);
   Opts.SjLjExceptions = Args.hasArg(OPT_fsjlj_exceptions);
+  Opts.ExternCNoUnwind = Args.hasArg(OPT_fexternc_nounwind);
   Opts.TraditionalCPP = Args.hasArg(OPT_traditional_cpp);
 
   Opts.RTTI = Opts.CPlusPlus && !Args.hasArg(OPT_fno_rtti);

clang/lib/Sema/SemaDecl.cpp

@@ -11582,6 +11582,13 @@ void Sema::AddKnownFunctionAttributes(FunctionDecl *FD) {
     }
   }
 
+  // If C++ exceptions are enabled but we are told extern "C" functions cannot
+  // throw, add an implicit nothrow attribute to any extern "C" function we come
+  // across.
+  if (getLangOpts().CXXExceptions && getLangOpts().ExternCNoUnwind &&
+      FD->isExternC() && !FD->hasAttr<NoThrowAttr>())
+    FD->addAttr(NoThrowAttr::CreateImplicit(Context, FD->getLocation()));
+
   IdentifierInfo *Name = FD->getIdentifier();
   if (!Name)
     return;