llvm/llvm-project

Missing FP optimization opportunities for fcmp ord operations

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#49,175 opened on Apr 4, 2021

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bugzillafloating-pointgood first issuellvm:optimizationsmissed-optimization

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Description
Bugzilla Link 49831
Version trunk
OS All
Blocks llvm/llvm-project#49274
CC @RKSimon,@rotateright

Extended Description

; Excerpted from llvm/test/Transforms/InstCombine/and-fcmp.ll

define i1 @_commute_logical(i1 %z, float %c, float %d) {
  %ord1 = fcmp ninf ord float %c, 0.0
  %and = select i1 %ord1, i1 %z, i1 false
  %ord2 = fcmp ninf reassoc ord float %d, 0.0
  %r = select i1 %ord2, i1 %and, i1 false
  ret i1 %r
}

This can be optimized to

  %c.fr = freeze float %c
  %ord1 = fcmp ord float %c.fr, %d
  %r = select i1 %ord1, i1 %z, i1 false

Alive2 link: https://alive2.llvm.org/ce/z/QfkJak

Contributor guide

Research direction
Explore the InstCombine pass in LLVM, particularly the handling of fcmp ord operations. Look at the test file llvm/test/Transforms/InstCombine/and fcmp.ll and the Alive2 link provided. Check if there is any existing implementation or related patches from the linked bugzilla. Implement the optimization described, ensuring no undefined behavior by using freeze instruction. Ensure correctness with Alive2.
Tech stack
cpp
Domain
performance
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
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.
Blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
LLVM IRInstCombine passFP optimization
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15