[PATCH] asan: do not instrument direct inbounds accesses to stack variables

[Kostya Serebryany]

================
Comment at: lib/Transforms/Instrumentation/AddressSanitizer.cpp:882
@@ +881,3 @@
+  // A direct inbounds access to a stack variable is always valid.
+  if (isa<AllocaInst>(Addr->stripInBoundsConstantOffsets())) {
+    NumOptimizedAccessesToStackVar++;
----------------
mega cool. 
Let's put it under a flag e.g. asan-opt-stack-inbounds (off by default) and play with it more. 

================
Comment at: lib/Transforms/Instrumentation/AddressSanitizer.cpp:886
@@ +885,3 @@
+  }
+/*
+  This does not give the desired result.
----------------
remove this then?

http://reviews.llvm.org/D7583

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