llvm/llvm-project

Funnel shift in 64-bit doesn't get matched to an intrinsic

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#138,334 opened on May 2, 2025

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good first issuellvm:instcombine

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Description

Yes I know this has two cases of subpar optimization: One where the funnel shift is 64-bit based, and the other is left funnel shifts that can be converted to a right funnel shift. This one is about the latter. However, this was lifted from one of gcc's test suites.

https://godbolt.org/z/Yd63ed3sv

Contributor guide

Research direction
Examine the LLVM source code for funnel shift recognition, specifically in the target specific lowering or pattern matching for 64 bit operations. Check the relevant test files for funnel shifts.
Tech stack
cpp
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
C++
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65