3 files changed, 11 insertions, 3 deletions
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 9c64da58c27..fd647a484e6 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/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.
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2e0822794ee..cf4b5cf20a1 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/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);
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 62cd5c8e3db..a775be9ea25 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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;