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

[David Blaikie]

On Tue, Aug 19, 2014 at 1:30 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
> 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*>();

Should this be built into PointerLikeTypeTraits instead?

> +    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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