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

[David Blaikie]

On Wed, Aug 27, 2014 at 4:09 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:

>
> On Tue, Aug 19, 2014 at 4:20 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> On Tue, Aug 19, 2014 at 2:59 PM, Alexey Samsonov <vonosmas at gmail.com>
>> wrote:
>> > 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*?
>>
>> I /think/ that'd be a reasonable thing to do. If anyone cares about
>> the number of low bits available, presumably they care even if those
>> bits are just provided by the natural alignment.
>>
>
> OK, I was confused =/ PointerLikeTypeTraits<T*>::NumLowBitsAvailable
> provides information
> about the contents of T*, so what actually matters is alignOf<T>, not
> alignOf<T*>. However,
> alignOf<T> requires that T is defined, which is probably a restriction we
> don't want to impose.
>

Do we ever provide PointerLikeTypeTraits<T*> for T somewhere other than
where T is defined? That seems subtle/easily surprising.

I suppose there's the case where PointerLikeTypeTraits<T*> is the default
implementation - and in that case we don't want to impose the limitation
that T is defined? Is there any way we can do that conditionally?

- David


>
>
>>
>> - David
>>
>> >
>> >
>> > 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) {
>> >> >
>> >> > _______________________________________________
>> >> > llvm-commits mailing list
>> >> > llvm-commits at cs.uiuc.edu
>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>> >> >
>> >
>> >
>> >
>> >
>> > --
>> > Alexey Samsonov
>> > vonosmas at gmail.com
>>
>
>
>
> --
> Alexey Samsonov
> vonosmas at gmail.com
>
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