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[InstCombine] missed fold of icmp-of-casted-shift #51889

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rotateright opened this issue Nov 18, 2021 · 2 comments
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[InstCombine] missed fold of icmp-of-casted-shift #51889

rotateright opened this issue Nov 18, 2021 · 2 comments
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@rotateright
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bugzilla Issues migrated from bugzilla good first issue https://github.com/llvm/llvm-project/contribute llvm:instcombine

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@rotateright
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rotateright commented Nov 18, 2021 •
edited by RKSimon

Bugzilla Link 52547
Version trunk
OS All
CC @RKSimon

Extended Description

Forking this off from bug 52543 - we may be able to generalize this further for other constant values, but here's a missed sub-pattern based on that test:

define i1 @src(i32 %a) {
  %shl = shl i32 1, %a
  %t = trunc i32 %shl to i8
  %r = icmp eq i8 %t, 0
  ret i1 %r
}

define i1 @tgt(i32 %a) {
  %r = icmp ugt i32 %a, 7
  ret i1 %r
}

https://alive2.llvm.org/ce/z/mmkPup

And the 'ne' sibling:
https://alive2.llvm.org/ce/z/ftWEQz
@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 11, 2021
@hasyimibhar
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Potential fix: https://reviews.llvm.org/D115480

@RKSimon RKSimon added good first issue https://github.com/llvm/llvm-project/contribute llvm:instcombine labels Aug 25, 2022
@RKSimon RKSimon mentioned this issue Aug 25, 2022
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llvmbot commented Aug 25, 2022

@llvm/issue-subscribers-good-first-issue

@rotateright rotateright self-assigned this Sep 5, 2022
rotateright added a commit that referenced this issue Sep 8, 2022
@rotateright
[InstCombine] fold icmp of truncated left shift, part 2
444f08c 
(trunc (1 << Y) to iN) == 2**C --> Y == C
(trunc (1 << Y) to iN) != 2**C --> Y != C
https://alive2.llvm.org/ce/z/xnFPo5

Follow-up to d9e1f9d. This was a suggested
enhancement mentioned in issue #51889.
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bugzilla Issues migrated from bugzilla good first issue https://github.com/llvm/llvm-project/contribute llvm:instcombine
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@hasyimibhar @RKSimon @EugeneZelenko @rotateright @llvmbot