[msan] Fix handling of va_arg overflow area on x86_64.

The code was erroneously reading overflow area shadow from the TLS slot,
bypassing the local copy. Reading shadow directly from TLS is wrong, because
it can be overwritten by a nested vararg call, if that happens before va_start.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189104 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -1970,8 +1970,7 @@ struct VarArgAMD64Helper : public VarArgHelper {
       Value *OverflowArgAreaPtr = IRB.CreateLoad(OverflowArgAreaPtrPtr);
       Value *OverflowArgAreaShadowPtr =
         MSV.getShadowPtr(OverflowArgAreaPtr, IRB.getInt8Ty(), IRB);
-      Value *SrcPtr =
-        getShadowPtrForVAArgument(VAArgTLSCopy, IRB, AMD64FpEndOffset);
+      Value *SrcPtr = IRB.CreateConstGEP1_32(VAArgTLSCopy, AMD64FpEndOffset);
       IRB.CreateMemCpy(OverflowArgAreaShadowPtr, SrcPtr, VAArgOverflowSize, 16);
     }
   }