[PATCH] D39417: InstCombine: Preserve nuw when reassociating nuw ops

[Matt Arsenault via Phabricator via llvm-commits]

arsenm marked an inline comment as done.
arsenm added inline comments.


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Comment at: lib/Transforms/InstCombine/InstructionCombining.cpp:198-229
 // Return true, if No Signed Wrap should be maintained for I.
 // The No Signed Wrap flag can be kept if the operation "B (I.getOpcode) C",
 // where both B and C should be ConstantInts, results in a constant that does
 // not overflow. This function only handles the Add and Sub opcodes. For
 // all other opcodes, the function conservatively returns false.
 static bool MaintainNoSignedWrap(BinaryOperator &I, Value *B, Value *C) {
   OverflowingBinaryOperator *OBO = dyn_cast<OverflowingBinaryOperator>(&I);
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lebedev.ri wrote:
> Why does `MaintainNoSignedWrap()` need all that logic to prove that no overflow happens,
> but `MaintainNoUnsignedWrap()` does nothing of sorts?
I'm not really sure why MaintainNoSignedWrap has all of those checks. It seems to me like it's looking an overflow that was already illegal?


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  https://reviews.llvm.org/D39417/new/

https://reviews.llvm.org/D39417