[PATCH] D90687: [LV] Ignore VF hint when unsafe

[Dave Green via Phabricator via llvm-commits]

dmgreen added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:5237
 
+  auto MaxVF = computeFeasibleMaxVF(TC);
+  if (UserVF) {
----------------
Clamping sounds good.

But I think that there is a difference between the "maximum safe vector width" and "the maximum register bitwidth". The maximum safe distance should be a legality constraint that doesn't depend on the size of the backend registers.

It's perhaps easier to show with examples. This code should produce the same thing (the same width vectors) with and without this patch, I believe, as there is nothing "unsafe" about vectorizing at a higher bitwidth than the vector registers:
https://godbolt.org/z/ePdv3K

(I am trying to not use the term "legal", as it has too many meanings. There is a difference between "legal to vectorize" (as the safety constraints in LoopVectorizationLegality) and "legal vector widths" which just means that the llvm-ir vector can be lowered to a single vector register and I don't think should be very relevant here).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90687/new/

https://reviews.llvm.org/D90687