[llvm-dev] LoopVectorize module - some possible enhancements

[Nuzman, Dorit via llvm-dev]

Hi Alex, 

About the gather/scatter intrinsics mechanism: it is already supported (they are part of LLVM-IR), but each target has to decide whether to allow using them for auto-vectorization.
On X86, avx2 gathers are not enabled for vectorization due to cost considerations. With avx512, Gathers and Scatters are enabled for vectorization (so if you use -march=skx or -mavx512bw there's a chance you'll see them used).

Dorit

-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Renato Golin via llvm-dev
Sent: Tuesday, August 23, 2016 15:00
To: Alex Susu <alex.e.susu at gmail.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] LoopVectorize module - some possible enhancements

Hi Alex,

Intel was doing the scatter/gather support for AVX, so I'm copying Elena who should know more about this.

cheers,
--renato

On 21 August 2016 at 21:15, Alex Susu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>   Hello, Michael,
>     I'd like to ask if we can enhance the LoopVectorize LLVM module (I 
> am currently using a version from Jul 2016).
>
>     More exactly:
>     - do you envision to support in the near future LLVM IR gather and 
> scatter intrinsics (as described at 
> http://llvm.org/docs/LangRef.html#llvm-masked-gather-intrinsics and 
> scatter)?
>       I see you have defined some methods that should use them like:
>         - bool isLegalMaskedGather(Type *DataType);
>         - void 
> InnerLoopVectorizer::vectorizeMemoryInstruction(Instruction
> *Instr) which defines a bool CreateGatherScatter, etc
>       I gave to clang a simple vector add C program with step/stride 2 
> with flag "-avx2", but the resulting vector code does NOT use gather nor scatter.
>
>     - did you try to consider pathological cases of loops such as:
>       for (int i = 0; i < N; i += 2) {
>           C[i/2] = A[i/2] + B[i/2];
>       }
>      which does NOT get vectorized with my version of LoopVectorize, 
> although it's simple to reason it's trivial to vectorize.
>
>     One more question: how can I obtain an expression for the bounds 
> and the step of the original loop? For example, when I print the 
> ScalarEvolution object in 
> LoopVectorizationLegality::isConsecutivePtr(), I can get the value 
> reported in "Exits" for the indvars.iv instruction which is the upper bound for the loop getting vectorized.
>
>
>   Thank you,
>     Alex
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