llvm/llvm-project

missed fold: (X * 0.0) * constant => X * 0.0

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#85,241 opened on Mar 14, 2024

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floating-pointgood first issuellvm:instcombinemissed-optimization

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Description

https://alive2.llvm.org/ce/z/bKDTJd https://godbolt.org/z/v1qYrb5Ka

define float @src(float %0) {
entry:
  %1 = fmul float %0, 0.000000e+00
  %2 = fmul float %1, 3.000000e+00
  ret float %2
}

define float @tgt(float %0) {
entry:
  %1 = fmul float %0, 0.000000e+00
  ret float %1
}

@jayfoad @regehr

Contributor guide

Research direction
Trace the constant folding logic in the InstCombine pass to identify why (X * 0.0) * constant is not reduced to X * 0.0.
Tech stack
None
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.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitC++LLVM basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40