Fix SIGILL on non-avx512 GFNI, leopard16

[klauspost]

Broadcast instructions are AVX512, so cannot be used on AVX2 GFNI path.

Fixes #324

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR modifies galois_gen_amd64.s to replace memory-based GFNI broadcast-with-affine (BCST) instruction patterns with explicit VPBROADCASTQ broadcasts followed by register-based VGF2P8AFFINEQB affine operations. Functional behavior is preserved while altering instruction sequences within LEO_MULADD_256 operations.

Changes

Cohort / File(s)	Summary
GFNI Assembly Instruction Patterns galois_gen_amd64.s	Replaced VGF2P8AFFINEQB.BCST memory-based broadcast-affine patterns with explicit VPBROADCASTQ instructions followed by register-based VGF2P8AFFINEQB operations across multiple blocks, maintaining 4-lane broadcast scheme and control flow.

Possibly Related PRs

• leo16: Add simpler (but faster) AVX512 GFNI path #320: Directly related changes to GFNI broadcast and affine instruction sequences in galois_gen_amd64.s, converting memory-based BCST patterns to register-based VPBROADCASTQ variants.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: fixing SIGILL crashes on non-AVX512 systems by removing AVX512-only broadcast instructions from GFNI leopard16 code.
Linked Issues check	✅ Passed	The PR directly addresses issue #324 by removing AVX512-only broadcast instructions from the GFNI assembly path, eliminating illegal instruction faults on AVX2 systems.
Out of Scope Changes check	✅ Passed	All changes are scoped to galois_gen_amd64.s and directly address the root cause: replacing memory-broadcast BCST variants with explicit VPBROADCASTQ + register-based operations.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch no-broadcast

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