[llvm-dev] Loop vectorizer doesn't try to align vectors on preferred vector alignment

[Benoit Meister via llvm-dev]

On Thu, Apr 12, 2018, 8:40 AM Benoit Meister <meister at reservoir.com> wrote:

> Thank you, Ayal! And thanks for the quote, Mehdi. I believe it says that
> it would be a normal thing for the Loop Vectorizer to conform to the
> backend's preferred alignment constraints as given by the datalayout.
>
> On Thu, Apr 12, 2018, 3:24 AM Zaks, Ayal <ayal.zaks at intel.com> wrote:
>
>> Hi Benoit,
>>
>>
>>
>> > Is there a way (that I missed) to make this happen, or would it
>> require a code change ?
>>
>>
>>
>> It’s the latter… there’s indeed room to make LV (and SLP) vectorizers
>> optimize for larger more efficient alignment, using static analysis and/or
>> dynamic peeling/versioning etc.
>>
>> Cf. LoadStoreVectorizer’s use of getOrEnforceKnownAlignment().
>>
>>
>>
>> Ayal.
>>
>>
>>
>>
>>
>> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *Mehdi
>> AMINI via llvm-dev
>> *Sent:* Thursday, April 12, 2018 01:39
>> *To:* meister at reservoir.com
>> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
>> *Subject:* Re: [llvm-dev] Loop vectorizer doesn't try to align vectors
>> on preferred vector alignment
>>
>>
>>
>> Hi Benoit,
>>
>>
>>
>> Le mer. 11 avr. 2018 à 15:25, Benoit Meister via llvm-dev <
>> llvm-dev at lists.llvm.org> a écrit :
>>
>> Hello,
>>
>>
>>
>> I'm playing around with vectorization in LLVM 6.0.0, and I noticed that
>> when creating a vector load out of a scalar load, the  alignment for the
>> vector load is defined to be the one of the scalar load. For instance,
>> 16-bit vectors get aligned on 2 bytes. This does not correspond to the
>> preferred alignment for vectors that I specified in the data layout (which
>> is bigger).
>>
>>
>>
>> Inspecting lib/Transforms/Vectorize/LoopVectorizer.cpp, there doesn't
>> seem to be an intent of doing so.
>>
>> I looked at this method in particular:
>>
>>
>> void InnerLoopVectorizer::vectorizeMemoryInstruction
>>
>>
>>
>> Is there a way (that I missed) to make this happen, or would it require a
>> code change ? Or did I miss anything obvious ?
>>
>>
>>
>>
>>
>> If the original pointer before vectorization is aligned on 2 bytes
>> boundary, the loop vectorizer can't "promote" it to the new alignment
>> without adding checks (or code versioning) right?
>>
>> Are you in a different case?
>>
>>
>>
>> Also, did you see this description about the alignment in the data layout
>> in LangRef:
>>
>>
>>
>> "The function of the data layout string may not be what you expect.
>> Notably, this is not a specification from the frontend of what alignment
>> the code generator should use.
>>
>> Instead, if specified, the target data layout is required to match what
>> the ultimate *code generator* expects. This string is used by the
>> mid-level optimizers to improve code, and this only works if it matches
>> what the ultimate code generator uses. There is no way to generate IR that
>> does not embed this target-specific detail into the IR. If you don’t
>> specify the string, the default specifications will be used to generate a
>> Data Layout and the optimization phases will operate accordingly and
>> introduce target specificity into the IR with respect to these default
>> specifications."
>>
>> Cheers,
>>
>> --
>>
>> Mehdi
>>
>>
>>
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