[Bugfix] Fix MiniMax M3 index_topk kernel for non-power-of-2 num_idx_heads (#49157)

[Tejas-Raj01]

Purpose

This PR fixes Issue #49157 where the MiniMax M3 index_topk Triton kernel fails to compile and execute when num_idx_heads is not a power of 2.

Specifically, this addresses:

1. Replaced the invalid tl.math.next_power_of_2 function call with the correct triton.next_power_of_2 for BLOCK_SIZE_HQ calculation.
2. Updated the q_mask logic to properly bound both h_offsets and q_offsets during execution to prevent out-of-bounds memory access.
3. Applied these fixes to both the CUDA (common) and ROCm (amd) paths.

Fixes #49157.

Test Plan

Added a dedicated regression test test_minimax_m3_decode_index_regression_non_power_of_2 that explicitly uses a non-power-of-2 configuration (num_idx_heads = 6).

Test command:

FLASHINFER_DISABLE_VERSION_CHECK=1 python -m pytest tests/kernels/attention/test_minimax_m3.py::test_minimax_m3_decode_index_regression_non_power_of_2 -v

[claude]

Claude Code Review

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[Tejas-Raj01]

@ @mgoin @tlrmchlsmth @WoosukKwon @yewentao256 @zyongye @AndreasKaratzas @njhill

ready for review.

[mergify]

Hi @Tejas-Raj01, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[mergify]

Hi @Tejas-Raj01, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[ehfd]

@mgoin can we get this approved?

[ehfd]

@mgoin @LucasWilkinson @MatthewBonanni @zyongye

[Tejas-Raj01]

Hi team, following up on PR #49199 ([Bugfix] Fix MiniMax M3 index_topk kernel for non-power-of-2 num_idx_heads).

Quick context:

• Fixes Issue [Bug]: MiniMax M3 decode index kernel fails when num_idx_heads is not a power of two #49157 where the MiniMax M3 index_topk Triton kernel fails to compile and execute for non-power-of-2 num_idx_heads.
• Replaces tl.math.next_power_of_2 with triton.next_power_of_2 and updates the q_mask logic to prevent out-of-bounds memory access.
• Applies fixes to both the CUDA and ROCm paths, and includes a dedicated regression test.
• The ready label was added previously, and all recent pre-commit formatting issues have been resolved and synced with main.

Could someone take a look and approve? There are 3 expected checks currently running.
cc @mgoin @zyongye @tlrmchlsmth @yewentao256 @AndreasKaratzas @WoosukKwon @dllehr-amd @hongxiayang @tjtanaa

[Tejas-Raj01]

@mgoin @ehfd @njhill,
I have updated the branch to sync with main. All the relevant MiniMax M3 and CUDA/ROCm kernel tests passed successfully in the previous run; the only failure was an unrelated flaky Qwen3/Gemma test. Could someone provide an approving review so this can merge once the new CI finishes?

[ehfd]

@pavanimajety Could you perhaps approve? Follow-up to #48929.