[PATCH] D64988: [InstCombine] 'Rem' formation from subtraction of rounded-down value (PR42673)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: RKSimon, spatel, ZaMaZaN4iK.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.

We can fold
`x - ((x / y) * y)`
to
`x % y`

https://rise4fun.com/Alive/8Rp

To be noted, we also prefer `x - (x % y)` to `(x / y) * y`,
and these tests show that we miss this transform
if division is not one-use. The caveat is that we could
still transform if we know that there already is `(x % y)`;
but unlike back-end's `SelectionDAG::getNodeIfExists()`,
i'm not sure how to nicely check that in middle-end.

https://godbolt.org/z/rWjNl4
https://bugs.llvm.org/show_bug.cgi?id=42673


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D64988

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/srem-via-sdiv-mul-sub.ll
  llvm/test/Transforms/InstCombine/urem-via-udiv-mul-sub.ll

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