[ASan] Don't instrument load/stores with !nosanitize metadata.

This is used to avoid instrumentation of instructions added by UBSan
in Clang frontend (see r213291). This fixes PR20085.

Reviewed in http://reviews.llvm.org/D4544.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213292 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -655,6 +655,9 @@ void AddressSanitizer::instrumentMemIntrinsic(MemIntrinsic *MI) {
 // and set IsWrite/Alignment. Otherwise return NULL.
 static Value *isInterestingMemoryAccess(Instruction *I, bool *IsWrite,
                                         unsigned *Alignment) {
+  // Skip memory accesses inserted by another instrumentation.
+  if (I->getMetadata("nosanitize"))
+    return nullptr;
   if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
     if (!ClInstrumentReads) return nullptr;
     *IsWrite = false;