llvm/llvm-project

AMDGPU should implement IR version of isFMAFasterThanFMulAndFAdd

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#108,751 opened on Sep 15, 2024

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backend:AMDGPUgood first issuemissed-optimization

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Description

TargetLowering has an IR variant of isFMAFasterThanFMulAndFAdd, and AMDGPU does not yet implement it.

As far as I can tell, this is only ever used in target code to implement isProfitableToHoist (e.g. AArch64 here https://github.com/llvm/llvm-project/blob/3ae71d154e5dfb5e5a5d27b3699b27ce2b55f44d/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L16086)

So for this to be effective, isProfitableToHoist would also need to be implemented similarly.

Contributor guide

Research direction
Study the existing implementation of isFMAFasterThanFMulAndFAdd in AArch64 (in AArch64ISelLowering.cpp) and the IR variant in TargetLowering. Understand the AMDGPU hardware FMA characteristics. Then implement the IR version for AMDGPU in AMDGPUISelLowering.cpp. Also implement isProfitableToHoist similarly. Test with relevant lit tests.
Tech stack
ccpp
Domain
compilers
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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
C++
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35