[llvm-dev] Auto-vectorization option

[Sudakshina Dutta via llvm-dev]

Dear Matt P. Dziubinski,

Thanks a lot for your reply. Although the vectorization is clearly visible
in godbolt, I could not generate it by command line. Does it require some
specific version of llvm/clang ?

Regards
Sudakshina

On Fri, May 14, 2021 at 5:53 PM Matt P. Dziubinski <matdzb at gmail.com> wrote:

> On 5/14/2021 13:30, Sudakshina Dutta via llvm-dev wrote:
> > Dear all,
> >
> > Thanks to all of you. I have executed the following commands on the code
> > given above.
> >
> > clang -O3 -S -c find_max.c -Rpass=vector -Rpass-analysis=vector -o
> > find_max.ll
> >
> > However, the generated code is an assembly code (attached). Is there any
> > way to generate a vectorized IR (.ll) file ?
>
> To get LLVM IR from the frontend (Clang) use -emit-llvm
> -fno-discard-value-names, e.g., https://godbolt.org/z/aWz37qYdW
>
> If you don't need debugger intrinsics (llvm.dbg.*) add -g0, e.g.,
> https://godbolt.org/z/aWz37qYdW
>
> As Sjoerd has mentioned, passing -mllvm -print-before-all to Clang is
> usedful to get pre-vectorized LLVM IR (as well as observe the effects of
> consecutive transformations); Example: https://godbolt.org/z/4za6h6fqo
>
> You can then extract the unoptimized LLVM IR and play with it in "opt"
> (the middle-end optimizer tool) to get the LLVM IR optimized by the
> middle-end passes (including loop vectorizer); note that now you can
> just pass -print-before-all directly: https://llvm.godbolt.org/z/P7E3PGE61
>
> In particular, the LLVM IR displayed under "*** IR Dump Before
> LoopVectorizePass on _Z1fPim ***" is a good baseline for comparisons.
>
> Add "-mllvm -print-module-scope" to get the LLVM IR for the full module
> (translation unit): https://godbolt.org/z/Go7zK8vsW
>
> Then, pass this LLVM (right before LoopVectorizePass) to "opt" using
> options "-loop-vectorize -debug-only=loop-vectorize" to observe the loop
> vectorization pass in action:
> https://llvm.godbolt.org/z/WMa1qosoq
>
> Note that you need a binary built with assertions enabled to use -debug
> options.
>
> Last but not least you can give the optimized LLVM IR to "llc" (the
> backend tool) to get the final assembly:
> https://llvm.godbolt.org/z/hxevcqKEG
>
> Best,
> Matt
>
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