[llvm-dev] [RFC] Add TargetTransformInfo::isAllocaPtrValueNonZero and let ValueTracking depend on TargetTransformInfo

[Hal Finkel via llvm-dev]

On 12/14/2017 03:49 PM, Liu, Yaxun (Sam) wrote:
>
> Hal,
>
> Thanks for your suggestion. I think that makes sense.
>
> Currently, non-zero alloca address space is already represented by 
> data layout, e.g., the last component of the data layout of 
> amdgcn---amdgiz target is -A5, which means alloca is in address space 
> 5. How about adding a letter z to -A5 to indicate alloca may have zero 
> value? i.e. -A5 means alloca is in address space 5 and always has 
> non-zero value, -A5z means alloca is in address space 5 and may have 
> zero value.
>
> Then we can add function DataLayout::isAllocaPtrValueNonZero() to 
> check whether alloca always has non-zero value.
>

I believe that we should phrase this in a way not directly connected to 
alloca, but to dereferenceability. We currently assume that 
dereferenceable pointers can't be null in address space 0, but we don't 
make that assumption about other address spaces (where null might be a 
valid address). I think that we should address this problem, and then 
simply apply the result to alloca.

We currently have, for address spaces:

> |p[n]:<size>:<abi>:<pref>|
>     This specifies the/size/of a pointer and
>     its|<abi>|and|<pref>|erred alignments for address space|n|. All
>     sizes are in bits. The address space,|n|, is optional, and if not
>     specified, denotes the default address space 0. The value
>     of|n|must be in the range [1,2^23). 

How about we allow the addition of 'n' (for non-zero) after the 
address-space number. This, implied by default for address space zero 
but not others, will imply that dereferenceable address are non-null. We 
can then apply this in isKnownNonZero in the places where we currently 
check for address space zero.

Would that address your use case? Or can you have null dereferenceable 
pointers in that address space, just not ones from alloca?

  -Hal

> Sam
>
> *From:*Hal Finkel [mailto:hfinkel at anl.gov]
> *Sent:* Thursday, December 14, 2017 4:06 PM
> *To:* Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>; llvm-dev 
> <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [RFC] Add 
> TargetTransformInfo::isAllocaPtrValueNonZero and let ValueTracking 
> depend on TargetTransformInfo
>
> Hi, Sam,
>
> Our general design has been that TTI is for cost modeling, not for 
> providing semantic information. This piece of information can go in 
> DataLayout, and I think we should put it there. Especially given that 
> this affects our canonicalization process, or preference should be to 
> avoid TTI. The more than TTI affects our canonical form, the more 
> fragmented it becomes, and the harder it is to understand. Putting it 
> in DataLayout also prevents us from having to propagate TTI into a 
> bunch of ValueTracking functions (and maximally retains our ability to 
> process IR without backends compiled in).
>
> Thanks again,
> Hal
>
> On 12/14/2017 02:32 PM, Liu, Yaxun (Sam) via llvm-dev wrote:
>
>     Some optimizations depend on whether alloca instruction always has
>     non-zero value. Currently, this checking is done by
>     isKnownNonZero() in ValueTracking, and it assumes alloca in
>     address space 0 always has non-zero value but alloca in non-zero
>     address spaces does not always have non-zero value.
>
>     However, this assumption is incorrect for certain targets. For
>     example, amdgcn---amdgiz target has alloca in address space 5, and
>     its alloca always has non-zero value. This assumption causes some
>     optimizations disabled for amdgcn---amdgiz target.
>
>     After discussions at https://reviews.llvm.org/D40670
>     <https://reviews.llvm.org/D40670>, I propose to introduce
>     TargetTransformInfo::isAllocaPtrValueNonZero for representing
>     whether alloca instruction always has non-zero value, and add a
>     TargetTransformInfo argument to ValueTracking functions e.g.
>     isKnownNonZero().
>
>     As a result, passes using ValueTracking will require
>     TargetTransformInfo.
>
>     Comments are welcome.
>
>     Thanks.
>
>     Sam
>
>
>
>
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>
>
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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