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

[Alexey Samsonov]

On Wed, Aug 27, 2014 at 4:59 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
>
> On Wed, Aug 27, 2014 at 4:46 PM, Alexey Samsonov <vonosmas at gmail.com>
> wrote:
>
>>
>> On Wed, Aug 27, 2014 at 4:15 PM, David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>>
>>>
>>>
>>> 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.
>>>
>>
>> Well, yes, you can declare a class member of type DenseMap<T*, Foo> if T
>> is forward-declared (which is convenient).
>>
>
> (wait, did you mean DenseMap<T, Foo>? Because DenseMap<T*, Foo> needs T*
> to be defined, and T* is defined even if T is only declared)
>

No, I mean DenseMap<T*, Foo>.
We can forward-declare type T and define a variable of type DenseMap<T*,
Foo>. This definition uses DenseMapInfo<T*>, which uses
PointerLikeTypeTraits<T*>. That is, we
may want to know PointerLikeTypeTraits<T*>::NumLowBitsAvailable even if T
is unknown. However, the actual number of available low bits of pointer T*
depends on the alignment of
type T, and we need the definition of T to calculate the latter.


>
> Sure - but I mean we provide specializations of PointerLikeTypeTraits<T*>
> separate from the definition of T? For the default PointerLikeTypeTraits
> (presumably with a NumLowBitsAvailable of zero) would allow the above. But
> it seems a tricksy to have unrelated bits of code declaring that pointers
> to certain objects have a certain non-zero number of low bits available.
>
> For example PointerLikeTypeTraits<Value*> is in Value.h, Use** is in
> Use.h, etc. So it wouldn't be bad for those to also use alignof<T>.
>
> Hmm, maybe I've lost the plot somewhere along here...
>
>
>
>>
>>> 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
>>>>
>>>
>>>
>>
>>
>> --
>> Alexey Samsonov
>> vonosmas at gmail.com
>>
>
>


-- 
Alexey Samsonov
vonosmas at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140827/1b76c5f5/attachment.html>