[PATCH] D107945: [RISCV] Use RISCV::RVVBitsPerBlock for RGK_ScalableVector in getRegisterBitWidth.

[Sander de Smalen via Phabricator via llvm-commits]

sdesmalen added a comment.

According to the description of `getRegisterBitWidth`, the function returns the width of the largest vector register type, which is probably where SVE and RVV are a bit different. For SVE the maximum vector length is always a multiple of 128bits and bounded by maximum vscale, so we can always return ElementCount of 'vscale x 128'. The LV uses this to determine a suitable VF based on the widest element type. e.g. if the maximum element width is 64bits, the maximum VF would be "vscale x 2", whereas if the max element width is 32bits, the maximum VF would be "vscale x 4". RVV can choose different LMULs, so you may want to return a wider bitwidth as default to get a more suitable vectorization factor, or alternatively experiment with adding a new `RGK_*` enum value to request a smaller/wider bitwidth. The LoopVectorizer also has an option to choose a higher bandwidth "-vectorizer-maximize-bandwidth", which forces the LV to choose a higher bitwidth based on the smallest element type in the loop (instead of the biggest element type).


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https://reviews.llvm.org/D107945