Treat __sync_synchronize and asm("":::"memory") as stronger fences.

lib/CodeGen/TargetInfo.cpp

 //===---- TargetInfo.cpp - Encapsulate target details -----------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // These classes wrap the information about a call or function
@@ skipping 419 matching lines @@
 };
 
 class PNaClTargetCodeGenInfo : public TargetCodeGenInfo {
  public:
   PNaClTargetCodeGenInfo(CodeGen::CodeGenTypes &CGT)
     : TargetCodeGenInfo(new PNaClABIInfo(CGT)) {}
 
   /// For PNaCl we don't want llvm.pow.* intrinsics to be emitted instead
   /// of library function calls.
   bool emitIntrinsicForPow() const { return false; }
+  bool addAsmMemoryAroundSyncSynchronize() const { return true; } // @LOCALMOD
+  bool asmMemoryIsFence() const { return true; } // @LOCALMOD
 };
 
 void PNaClABIInfo::computeInfo(CGFunctionInfo &FI) const {
     FI.getReturnInfo() = classifyReturnType(FI.getReturnType());
 
     for (CGFunctionInfo::arg_iterator it = FI.arg_begin(), ie = FI.arg_end();
          it != ie; ++it)
       it->info = classifyArgumentType(it->type);
   }
 
@@ skipping 4750 matching lines @@
                                                                       HasAVX));
     default:
       return *(TheTargetCodeGenInfo = new X86_64TargetCodeGenInfo(Types,
                                                                   HasAVX));
     }
   }
   case llvm::Triple::hexagon:
     return *(TheTargetCodeGenInfo = new HexagonTargetCodeGenInfo(Types));
   }
 }