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

[Alexey Samsonov]

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*?


On Tue, Aug 19, 2014 at 2:09 PM, David Blaikie <dblaikie at gmail.com> wrote:

> 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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> > llvm-commits at cs.uiuc.edu
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> >
>



-- 
Alexey Samsonov
vonosmas at gmail.com
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