<div dir="ltr">PointerLikeTypeTraits::NumLowBitsAvailable have different values for different kinds of pointers (e.g. certain pointers can be cache-line aligned). Do you suggest to bump the default value of NumLowBitsAvailable for generic T* to return the natural alignment of T*?</div>
<div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Aug 19, 2014 at 2:09 PM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">On Tue, Aug 19, 2014 at 1:30 PM, Alexey Samsonov <<a href="mailto:vonosmas@gmail.com">vonosmas@gmail.com</a>> wrote:<br>

> Hi rsmith, chandlerc,<br>
><br>
> We should respect alignOf<T*>() when creating empty/tombstone keys<br>
> for DenseMap<T*, foo>. Otherwise this can lead to casts on misaligned<br>
> addresses, which is undefined behavior, as reported by UBSan.<br>
><br>
> <a href="http://reviews.llvm.org/D4976" target="_blank">http://reviews.llvm.org/D4976</a><br>
><br>
> Files:<br>
>   include/llvm/ADT/DenseMapInfo.h<br>
><br>
> Index: include/llvm/ADT/DenseMapInfo.h<br>
> ===================================================================<br>
> --- include/llvm/ADT/DenseMapInfo.h<br>
> +++ include/llvm/ADT/DenseMapInfo.h<br>
> @@ -14,6 +14,7 @@<br>
>  #ifndef LLVM_ADT_DENSEMAPINFO_H<br>
>  #define LLVM_ADT_DENSEMAPINFO_H<br>
><br>
> +#include "llvm/Support/AlignOf.h"<br>
>  #include "llvm/Support/PointerLikeTypeTraits.h"<br>
>  #include "llvm/Support/type_traits.h"<br>
><br>
> @@ -32,12 +33,20 @@<br>
>  struct DenseMapInfo<T*> {<br>
>    static inline T* getEmptyKey() {<br>
>      uintptr_t Val = static_cast<uintptr_t>(-1);<br>
> -    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;<br>
> +    const unsigned Alignment =<br>
> +        PointerLikeTypeTraits<T*>::NumLowBitsAvailable > alignOf<T*>()<br>
> +            ? (unsigned)PointerLikeTypeTraits<T*>::NumLowBitsAvailable<br>
> +            : alignOf<T*>();<br>
<br>
</div></div>Should this be built into PointerLikeTypeTraits instead?<br>
<div class=""><br>
> +    Val <<= Alignment;<br>
>      return reinterpret_cast<T*>(Val);<br>
>    }<br>
>    static inline T* getTombstoneKey() {<br>
>      uintptr_t Val = static_cast<uintptr_t>(-2);<br>
> -    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;<br>
> +    const unsigned Alignment =<br>
> +        PointerLikeTypeTraits<T*>::NumLowBitsAvailable > alignOf<T*>()<br>
> +            ? (unsigned)PointerLikeTypeTraits<T*>::NumLowBitsAvailable<br>
> +            : alignOf<T*>();<br>
> +    Val <<= Alignment;<br>
>      return reinterpret_cast<T*>(Val);<br>
>    }<br>
>    static unsigned getHashValue(const T *PtrVal) {<br>
><br>
</div>> _______________________________________________<br>
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><br>
</blockquote></div><br><br clear="all"><div><br></div>-- <br><div dir="ltr">Alexey Samsonov<br><a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a></div>
</div>