MS ABI: Implement /volatile:ms

The /volatile:ms semantics turn volatile loads and stores into atomic
acquire and release operations.  This distinction is important because
volatile memory operations do not form a happens-before relationship
with non-atomic memory.  This means that a volatile store is not
sufficient for implementing a mutex unlock routine.

Differential Revision: http://reviews.llvm.org/D7580

llvm-svn: 229082

clang/lib/Frontend/CompilerInvocation.cpp

@@ -477,6 +477,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
                                    OPT_fno_data_sections, false);
   Opts.MergeFunctions = Args.hasArg(OPT_fmerge_functions);
 
+  Opts.MSVolatile = Args.hasArg(OPT_fms_volatile);
+
   Opts.VectorizeBB = Args.hasArg(OPT_vectorize_slp_aggressive);
   Opts.VectorizeLoop = Args.hasArg(OPT_vectorize_loops);
   Opts.VectorizeSLP = Args.hasArg(OPT_vectorize_slp);