[msan] Remove unreachable blocks before instrumenting a function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170883 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -67,6 +67,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/ModuleUtils.h"
 #include "llvm/Type.h"
 
@@ -485,6 +486,13 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   bool runOnFunction() {
     MS.initializeCallbacks(*F.getParent());
     if (!MS.TD) return false;
+
+    // In the presence of unreachable blocks, we may see Phi nodes with
+    // incoming nodes from such blocks. Since InstVisitor skips unreachable
+    // blocks, such nodes will not have any shadow value associated with them.
+    // It's easier to remove unreachable blocks than deal with missing shadow.
+    removeUnreachableBlocks(F);
+
     // Iterate all BBs in depth-first order and create shadow instructions
     // for all instructions (where applicable).
     // For PHI nodes we create dummy shadow PHIs which will be finalized later.