[BugFix][Layout] Fix parallel layout undercoverage

[haoyang9804]

Fixes a wrong-code issue where T.Parallel loops with different extents sharing the same fragment could be silently truncated to the smallest extent.

This change rejects inferred loop layouts whose input shape is smaller than the loop extent, allowing free-mode inference to select a layout that covers all valid iterations.

Regression coverage is added for both register-count and io-aware cost models.

Fixes #2957

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📥 Commits

Reviewing files that changed from the base of the PR and between edb02a6 and 0cd7c1c.

📒 Files selected for processing (5)

• maint/layout_inference/cases/fragment_mixed_extents.py
• maint/layout_inference/expected/fragment_mixed_extents.json
• src/op/parallel.cc
• src/op/parallel.h
• testing/python/transform/test_tilelang_transform_layout_inference.py

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📝 Walkthrough

Walkthrough

Changes

Mixed extent layout inference

Layer / File(s)	Summary
Parallel extent coverage validation src/op/parallel.h, src/op/parallel.cc	Adds a helper that validates candidate layout rank and extent coverage. Candidate selection applies this validation to buffer-derived and plan-derived layouts.
Mixed-extent cases and expected layouts maint/layout_inference/cases/fragment_mixed_extents.py, maint/layout_inference/expected/fragment_mixed_extents.json	Adds read, write, equal-extent, and independent-fragment variants with expected buffer and loop mappings.
Layout coverage regression tests testing/python/transform/test_tilelang_transform_layout_inference.py	Tests both cost models and rejects symbolic loop coverage that cannot be proven.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 0cd7c

This localized change prevents parallel layouts from being truncated and adds regression coverage for the affected cost models; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: leiwang1999

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 24 functions across 4 files. (1 skipped: 1 unsupported.)	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes reject undercovered layouts and add regression coverage for shared fragments, differing extents, reads, writes, and both cost models required by issue #2957.
Out of Scope Changes check	✅ Passed	The source changes, expected layouts, and regression tests directly support the linked issue and stated layout-inference objectives.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: fixing parallel layout undercoverage in layout inference.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Reviewed commit: edb02a6f8a

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Actionable comments posted: 3

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Inline comments:
In `@src/op/parallel.cc`:
- Around line 515-530: Filter candidates that under-cover loop extents before
ChooseBestCandidate selects among them, using the same loop-coverage predicate
as the final check in the surrounding planning flow. Ensure a covering candidate
is chosen when available, while retaining the existing loop extent validation as
a final safety net.
- Around line 515-530: Update the validation loop around loop_vars_ and
loop_layout_->InputShape() to traverse only parallel For nodes, matching each
axis to loop_vars_[i]->var or its corresponding recorded For node instead of
blindly advancing via for_node->body. Skip ordered loops between parallel axes,
and validate every parallel layout extent against the matching parallel loop
extent.

In `@testing/python/transform/test_tilelang_transform_layout_inference.py`:
- Around line 144-179: Update
test_parallel_fragment_layout_covers_mixed_loop_extents to assert that the
collected loop_shapes extents are exactly {100, 256} before checking layout
coverage, ensuring both T.Parallel loops contributed annotations while retaining
the existing structural shape assertions.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between a35008a and edb02a6.

📒 Files selected for processing (4)

• maint/layout_inference/cases/fragment_mixed_extents.py
• maint/layout_inference/expected/fragment_mixed_extents.json
• src/op/parallel.cc
• testing/python/transform/test_tilelang_transform_layout_inference.py

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

[haoyang9804]

Addressed all actionable review feedback in 58da7793 and 0cd7c1c3:

• require proof that every candidate layout covers each parallel-loop extent, including symbolic extents;
• filter under-covered buffer/plan candidates before selection while retaining final validation;
• validate against recorded loop_vars_[i]->dom->extent values;
• assert both {100, 256} loop annotations and cover the larger-read, larger-write, equal-extent, and independent-fragment cases.

Validation completed:

• cmake --build build -j12
• pre-commit run --all-files
• focused layout-inference pytest: 3 passed
• full layout golden harness: all 40 model/case combinations passed
• CuTe parity for the affected case: 20/20
• all four cases passed on Metal runtime

The remaining CodeRabbit docstring-coverage warning is a generic heuristic over internal test/case helpers and C++ methods; the repository formatting and lint gates pass, and the incremental review reports no actionable comments.