[llvm-dev] Vector evolution?

[Sanjay Patel via llvm-dev]

I have no idea how well this code was tested, but some researchers already
wrote an LLVM pass that does exactly what is requested:
https://github.com/revec/llvm-revec
See the linked paper that they published for their experimental results.

On Tue, Sep 1, 2020 at 11:51 AM Florian Hahn via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> > On Sep 1, 2020, at 16:23, Alexandre Bique <bique.alexandre at gmail.com>
> wrote:
> >
> > On Tue, Sep 1, 2020 at 5:10 PM Florian Hahn <florian_hahn at apple.com>
> wrote:
> >> The loop vectorizer does not really handle loops that already operate
> on vectors, so that is why the loop using v4f32 does not get widened.
> >>
> >> Arguably the user explicitly asked for 4xfloat vectors in the v4f32
> version, so that is what gets generated.
> >
> > In my case I have tons of legacy code written for SSE2 and if the
> > compiler can make a better and correct version of it, why not?
>
> Right, that’s also a reasonable argument.
>
> Cases like those should get unrolled sufficiently already and all that
> needs to be done is combining multiple instructions to their wider
> versions. This can probably be done relatively easily in either the SLP
> vectorizer [1]  or VectorCombine [2[
>
> [1]
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
> [2]
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Vectorize/VectorCombine.cp
>
> >> (Those kinds of issues are better to discuss on https://bugs.llvm.org/
> IMO, because it is easier to keep track of the progress on the issue).
> >
> > That is noted, but I can't think of it as a bug unless I understand the
> issue.
>
> I would not say it is a bug, but rather a missing transform. The naming of
> https://bugs.llvm.org/ might imply that it is only for bugs, but in
> practice it is used to collect missed optimizations, suggestions for new
> features and so on as well.
>
> Cheers,
> Florian
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