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[InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0 #71533

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XChy opened this issue Nov 7, 2023 · 5 comments · May be fixed by #92658
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[InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0 #71533

XChy opened this issue Nov 7, 2023 · 5 comments · May be fixed by #92658
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llvm:instcombine missed-optimization

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@XChy
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XChy commented Nov 7, 2023

When I played with InstCombine, I generated something missed-optimized.
No real-world motivation, but it seems to be easy to implement.
Missed example.
Proof.
@XChy XChy changed the title [InstCombine] Missed optimization for select a%2, (a/2*2)*(a/2*2), 0 [InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0 Nov 7, 2023
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jf-botto commented Nov 8, 2023

I'd love to work on this if possible?

@XChy
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XChy commented Nov 8, 2023

I'd love to work on this if possible?

Sure, thanks for contribution.

@nikic nikic mentioned this issue Nov 11, 2023
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nikic commented Nov 11, 2023

Note that the mul here isn't really relevant. Here is a simpler pattern that is also missed: https://alive2.llvm.org/ce/z/dYwE-Q

The general issue is making use of KnownBits information inside select arms.

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Thank you @nikic,
So what you're saying is, the fix for this is optimising the RHS/LHS of a select by taking into consideration any KnownBits that can be inferred from the condition of the select?

@XChy XChy mentioned this issue Nov 26, 2023
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jf-botto commented Dec 1, 2023

Making good progress on this, just have some edge cases that need tidying up.

@jf-botto jf-botto linked a pull request May 18, 2024 that will close this issue
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[InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0 jf-botto/llvm-project
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@nikic @jf-botto @XChy