[PATCH] D42724: [X86] Don't make 512-bit vectors legal when preferred vector width is 256 bits and 512 bits aren't required

[Danila Malyutin via Phabricator via llvm-commits]

danilaml added inline comments.
Herald added subscribers: wangpc, sunshaoce, StephenFan, pengfei.
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Comment at: llvm/trunk/lib/Target/X86/X86TargetMachine.cpp:275
+  // Extract required-vector-width attribute.
+  unsigned RequiredVectorWidth = UINT32_MAX;
+  if (F.hasFnAttribute("required-vector-width")) {
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@craig.topper Sorry for commenting on such an old review, but I was investigating some codegen differences for very similar IR and come across this code (the attribute later changed to the `min-legal-vector-width` but otherwise it's the same on main). Is RequiredVectorWidth intended to be initialized to `UINT32_MAX`? What is the rationale? It forces maximum vector width if the function is missing the attribute for some reason, ignoring the `prefer-*` attributes. To me it seems that the conservative approach would be to set it to `0` and increase according to the attribute, since zero length vectors are always legal/"required".


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  https://reviews.llvm.org/D42724/new/

https://reviews.llvm.org/D42724