<div dir="ltr"><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Aug 27, 2014 at 5:36 PM, Alexey Samsonov <span dir="ltr"><<a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a>></span> wrote:<br>
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<div class="">On Wed, Aug 27, 2014 at 4:59 PM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br>
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<div>On Wed, Aug 27, 2014 at 4:46 PM, Alexey Samsonov <span dir="ltr"><<a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a>></span> wrote:<br>
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<div>On Wed, Aug 27, 2014 at 4:15 PM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br>
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<div>On Wed, Aug 27, 2014 at 4:09 PM, Alexey Samsonov <span dir="ltr"><<a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a>></span> wrote:<br>
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<div>On Tue, Aug 19, 2014 at 4:20 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:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div>On Tue, Aug 19, 2014 at 2:59 PM, Alexey Samsonov <<a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a>> wrote:<br>
> PointerLikeTypeTraits::NumLowBitsAvailable have different values for<br>
> different kinds of pointers (e.g. certain pointers can be cache-line<br>
> aligned). Do you suggest to bump the default value of NumLowBitsAvailable<br>
> for generic T* to return the natural alignment of T*?<br>
<br>
</div>I /think/ that'd be a reasonable thing to do. If anyone cares about<br>
the number of low bits available, presumably they care even if those<br>
bits are just provided by the natural alignment.<br></blockquote><div><br></div></div><div>OK, I was confused =/ PointerLikeTypeTraits<T*>::NumLowBitsAvailable provides information</div><div>about the contents of T*, so what actually matters is alignOf<T>, not alignOf<T*>. However,</div>
<div>alignOf<T> requires that T is defined, which is probably a restriction we don't want to impose.</div></div></div></div></blockquote><div><br></div></div><div>Do we ever provide PointerLikeTypeTraits<T*> for T somewhere other than where T is defined? That seems subtle/easily surprising.<br>
</div></div></div></div></blockquote><div><br></div></div><div>Well, yes, you can declare a class member of type DenseMap<T*, Foo> if T is forward-declared (which is convenient).</div></div></div></div></blockquote>
</div><div><br>(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) </div></div></div></div></blockquote><div><br></div></div><div>
No, I mean DenseMap<T*, Foo>.</div>
<div>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</div><div>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</div>
<div>type T, and we need the definition of T to calculate the latter.</div></div></div></div></blockquote><div><br></div><div>OK, so we can fix the immediate problems with UBSan by a smaller change to DenseMapInfo<AssertingVH<T>>. I've updated <a href="http://reviews.llvm.org/D4976">http://reviews.llvm.org/D4976</a>.</div>
<div>Does it look reasonable?</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">
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<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br>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.<br>
<br>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>.<br><br>Hmm, maybe I've lost the plot somewhere along here... <br>
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<br>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?<span><font color="#888888"><br>
<br>- David</font></span></div><div><div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">
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<span><font color="#888888"><br>
- David<br>
</font></span><div><div><br>
><br>
><br>
> On Tue, Aug 19, 2014 at 2:09 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<br>
>><br>
>> On Tue, Aug 19, 2014 at 1:30 PM, Alexey Samsonov <<a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a>><br>
>> 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>
>> Should this be built into PointerLikeTypeTraits instead?<br>
>><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>
>> > _______________________________________________<br>
>> > llvm-commits mailing list<br>
>> > <a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank">llvm-commits@cs.uiuc.edu</a><br>
>> > <a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits</a><br>
>> ><br>
><br>
><br>
><br>
><br>
> --<br>
> Alexey Samsonov<br>
> <a href="mailto:vonosmas@gmail.com" target="_blank">vonosmas@gmail.com</a><br>
</div></div></blockquote></div></div></div><span><font color="#888888"><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>
</font></span></div></div>
</blockquote></div></div></div><br></div></div>
</blockquote></div></div></div><span><font color="#888888"><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>
</font></span></div></div>
</blockquote></div></div></div><br></div></div>
</blockquote></div></div></div><span class=""><font color="#888888"><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>
</font></span></div></div>
</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></div>