llvm/llvm-project

Failure to recognise unnecessary sext for add+cmp

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#130,088 opened on Mar 6, 2025

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good first issuellvm:instcombinemissed-optimization

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Description

Pulled out #76524 - we fail to recognize that extension is unnecessary and that the trunc is nsw

define i8 @src(i16 noundef %x) {
entry:
  %conv = sext i16 noundef %x to i128
  %add = add i128 %conv, 128
  %or.cond.i = icmp ult i128 %add, 256
  assume i1 %or.cond.i
  %conv1 = trunc i16 noundef %x to i8
  ret i8 %conv1
}
=>
define i8 @tgt(i16 noundef %x) {
entry:
  %add = add i16 noundef %x, 128
  %or.cond.i = icmp ult i16 %add, 256
  assume i1 %or.cond.i
  %conv1 = trunc nsw i16 noundef %x to i8
  ret i8 %conv1
}
Transformation seems to be correct!

https://alive2.llvm.org/ce/z/epn8hd

Contributor guide

Research direction
Study the provided IR transformation and verify using alive2 that the sext is unnecessary. Then implement the transform in the appropriate LLVM pass (e.g., InstCombine) by adding a pattern to recognize when a sext can be eliminated due to subsequent truncation and comparison.
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Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Clear
Prerequisites
C++LLVM IR basics
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35