[PATCH] D118709: [LV] Use VScaleForTuning to allow wider epilogue VFs.

[Sander de Smalen via Phabricator via llvm-commits]

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When the main loop is e.g. VF=vscale x 1 and the epilogue VF cannot
be any smaller, the vectorizer should try to estimate how many lanes are
executed at runtime and allow a suitable fixed-width VF to be chosen. It
can use VScaleForTuning to figure out what a suitable fixed-width VF could
be. For the case where the main loop VF is VF=vscale x 1, and VScaleForTuning=8,
it could still choose an epilogue VF upto VF=4.

This was a bit tricky to test, so this patch also introduces a wrapper
function to get 'VScaleForTuning' by also considering vscale_range.
If min and max are equal, then that will be the vscale we compile for.
It makes little sense to tune for a different width if the code
will not be portable for other widths.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118709

Files:
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
  llvm/test/Transforms/LoopVectorize/AArch64/sve-epilog-vect.ll

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