[AIROCMLIR-537] Add fusion check for attention with splitKV > 1

[justinrosner]

Motivation

This PR adds some additional fusion checks to ensure that we always return a result of fusible: 0 for attention ops with splitKV > 1. This implements: https://amd-hub.atlassian.net/browse/AIROCMLIR-537

Technical Details

Added a new function called testFusionLegalityAttentionSplitKV that make sure that fusions are disabled with splitKV > 1

Test Plan

• PR CI

Test Result

• PR CI

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[Copilot]

Pull request overview

This PR implements fusion legality checks for attention operations with flash decoding (splitKV > 1) to ensure that fusions are properly disabled when partial results require LSE-based corrections in a subsequent stage.

Changes:

• Added a new fusion legality check function testFusionLegalityAttentionSplitKV to prevent fusion for attention ops with splitKV > 1
• Integrated the new check into the isModuleFusible function to be evaluated alongside other fusion legality checks
• Added test coverage to verify that fusion is disabled when attention operations use splitKV > 1

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
mlir/test/fusion/fusability-attention-splitkv-output-fusion.mlir	New test file that verifies fusion is disabled (fusible:0) for attention ops with splitKV = 4
mlir/lib/Dialect/Rock/utility/fusionUtils.cpp	Implements testFusionLegalityAttentionSplitKV functions (FuncOp and ModuleOp overloads) following established patterns for fusion legality checks
mlir/lib/Dialect/Rock/Tuning/RockTuningImpl.cpp	Integrates the new attention splitKV check into the isModuleFusible function
mlir/include/mlir/Dialect/Rock/utility/fusionUtils.h	Declares the new fusion legality check functions with documentation explaining the constraint

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.