[PATCH] D143883: [InstCombine] canonicalize urem as cmp+select

[Nikita Popov via Phabricator via llvm-commits]

nikic requested changes to this revision.
nikic added a comment.
This revision now requires changes to proceed.

Needs more tests for preconditions, e.g. what happens if the constant is not 1?

Note that there is also a more general form of this fold, see https://github.com/llvm/llvm-project/blob/697a162fa63df328ec9ca334636c5e85390b2bf0/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp#L806. The tricky bit here is that we would nee `X < 2*Y`, which is hard to establish as a precondition in this context.



================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp:1791
+  // Value *X;
+  if (match(Op0, m_NUWAdd(m_Value(X), m_One())) &&
+      simplifyICmpWithDominatingAssume(ICmpInst::ICMP_ULT, X, Op1, SQ, &I)) {
----------------
Why the nuw requirement? Seems to be fine without it? https://alive2.llvm.org/ce/z/uo7HMz


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp:1792
+  if (match(Op0, m_NUWAdd(m_Value(X), m_One())) &&
+      simplifyICmpWithDominatingAssume(ICmpInst::ICMP_ULT, X, Op1, SQ, &I)) {
+    Value *Cmp = Builder.CreateICmpEQ(Op0, Op1);
----------------
Why is this specific to assumes? Can we do a generic simplifyICmp? (If we do so, Op0 needs to be frozen.)


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  https://reviews.llvm.org/D143883/new/

https://reviews.llvm.org/D143883