[PATCH] D114279: [InstSimplify] Fold X {lshr,udiv} C <u X --> true for nonzero X, non-identity C

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


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Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:2950-2951
+  // If x is nonzero:
+  // x >> C   <u x --> true for C > 0.
+  // x udiv C <u x --> true for C > 1.
+  // TODO: allow non-constant shift amount/divisor
----------------
Spell out the other preds/patterns for completeness? Might want to improve the comments on the block above here while we're at it to show the transform instead of just matched pattern.


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Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:2952
+  // x udiv C <u x --> true for C > 1.
+  // TODO: allow non-constant shift amount/divisor
+  const APInt *C;
----------------
Is that TODO feasible? The lshr half could use isKnownNonZero, but how would we deal with ">1" for udiv?


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Comment at: llvm/test/Transforms/InstSimplify/compare.ll:581
 
 define i1 @lshr_nonzero1(i32 %x) {
 ; CHECK-LABEL: @lshr_nonzero1(
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Put the predicate in the test name rather than incrementing the '1' suffix. That makes it clearer that we're cycling through the preds. Add a couple of negative tests for signed preds?


Repository:
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  https://reviews.llvm.org/D114279/new/

https://reviews.llvm.org/D114279