[AIMIGRAPHX-1002] Match nop reshapes with broadcasted inputs

[eddieliao]

Motivation

bcast → reduce_mean over axis of size 1 currently fails as the reduce op inside fused_reduce gets optimized out, leaving an submodule with no reduce ins

Technical Details

The matcher for simplify_reshapes needs to be updated to account for such cases in which the strides may not match, but the result is still a nop.

Changelog Category

Add a CHANGELOG.md entry for any option other than Not Applicable

• • Added: New functionality.
• • Changed: Changes to existing functionality.
• • Removed: Functionality or support that has been removed. (Compared to a previous release)
• • Optimized: Component performance that has been optimized or improved.
• • Resolved Issues: Known issues from a previous version that have been resolved.
• • Not Applicable: This PR is not to be included in the changelog.

[Copilot]

Pull request overview

Updates simplify_reshapes matching so that reductions over size-1 axes on broadcasted inputs can be recognized as no-ops even when input/output strides differ (fixing cases like broadcast -> reduce_mean where the reduce gets optimized away incorrectly in downstream fusion scenarios).

Changes:

• Split the “nop reshape” matcher into shape-equality vs lens-equality cases, using lens-only matching for reduce_* ops.
• Add a new matcher combinator match::same_lens(...) to support lens-only equivalence checks.
• Add a regression test covering broadcast -> reduce_mean as a nop-reduction case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/simplify_reshapes_test.cpp	Adds regression test ensuring a nop reduce_mean over a size-1 broadcast axis is eliminated.
src/simplify_reshapes.cpp	Updates nop-reshape matching to use full-shape equality for reshape-like ops and lens-only equality for reduce_*.
src/include/migraphx/matcher.hpp	Introduces same_lens matcher utility used by simplify_reshapes.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/include/migraphx/matcher.hpp	88.89%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4841      +/-   ##
===========================================
+ Coverage    92.80%   92.85%   +0.05%     
===========================================
  Files          584      584              
  Lines        30146    30159      +13     
===========================================
+ Hits         27976    28003      +27     
+ Misses        2170     2156      -14     

Files with missing lines	Coverage Δ
src/simplify_reshapes.cpp	97.95% <100.00%> (+0.01%)	⬆️
src/include/migraphx/matcher.hpp	87.57% <88.89%> (+0.03%)	⬆️

... and 4 files with indirect coverage changes

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[shivadbhavsar]

The core issue to me sounds like the reduce compute_shape methods are not preserving broadcast strides. Is there a potential fix there instead of changing the noop condition in this matcher?

[causten]

Rekicked due to CI internal issue