[PATCH] Use properly aligned pointers for empty/tombstone DenseMap keys.

[Alexey Samsonov]

Hi rsmith, chandlerc,

We should respect alignOf<T*>() when creating empty/tombstone keys
for DenseMap<T*, foo>. Otherwise this can lead to casts on misaligned
addresses, which is undefined behavior, as reported by UBSan.

http://reviews.llvm.org/D4976

Files:
  include/llvm/ADT/DenseMapInfo.h

Index: include/llvm/ADT/DenseMapInfo.h
===================================================================
--- include/llvm/ADT/DenseMapInfo.h
+++ include/llvm/ADT/DenseMapInfo.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_ADT_DENSEMAPINFO_H
 #define LLVM_ADT_DENSEMAPINFO_H
 
+#include "llvm/Support/AlignOf.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
 #include "llvm/Support/type_traits.h"
 
@@ -32,12 +33,20 @@
 struct DenseMapInfo<T*> {
   static inline T* getEmptyKey() {
     uintptr_t Val = static_cast<uintptr_t>(-1);
-    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;
+    const unsigned Alignment =
+        PointerLikeTypeTraits<T*>::NumLowBitsAvailable > alignOf<T*>()
+            ? (unsigned)PointerLikeTypeTraits<T*>::NumLowBitsAvailable
+            : alignOf<T*>();
+    Val <<= Alignment;
     return reinterpret_cast<T*>(Val);
   }
   static inline T* getTombstoneKey() {
     uintptr_t Val = static_cast<uintptr_t>(-2);
-    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;
+    const unsigned Alignment =
+        PointerLikeTypeTraits<T*>::NumLowBitsAvailable > alignOf<T*>()
+            ? (unsigned)PointerLikeTypeTraits<T*>::NumLowBitsAvailable
+            : alignOf<T*>();
+    Val <<= Alignment;
     return reinterpret_cast<T*>(Val);
   }
   static unsigned getHashValue(const T *PtrVal) {
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