OneDNN BRGeMM Micro-Kernel Integration for BF16 MatMul

[bbhattar]

This PR integrates OneDNN BRGeMM (Batch-Reduced General Matrix Multiply) micro-kernels as an alternative compute path for BF16 MatMul on Intel Xeon platforms with AMX or AVX-512 BF16 support.

What

When enabled via the GEMMA_ONEDNN_BRGEMM compile-time flag, BF16×BF16 MatMul operations are dispatched to JIT-compiled BRGeMM kernels instead of the Highway SIMD path. This targets Gemma model workloads (FFW projections, attention) on Intel Xeon Scalable (SPR/EMR) processors. At this point support has been added to both CMake and Bazel build systems.

How to Enable

# CMake
cmake -DGEMMA_ONEDNN_BRGEMM=ON ..

# Bazel
bazel build --define gemma_onednn_brgemm=1 ...

Runtime Fallback

When GEMMA_ONEDNN_BRGEMM is enabled at compile time, the BRGeMM path activates for BF16×BF16 operations whose dimensions meet AMX tile constraints (M, N, K ≥ 32 and K % 32 == 0). All other cases — non-BF16 types, smaller or non-aligned dimensions, mixed precision — fall through to the standard Highway SIMD MatMul path automatically.

Changes

File	Description
ops/brgemm.h	Types, caches, thread-local buffers, UseOneDnnBrgemm(), autotuning candidates
ops/brgemm-inl.h	DoMatMul_BRGeMM(): kernel JIT/caching, B-packing with hugepages, tiled parallel execution
ops/matmul-inl.h	BRGeMM dispatch block in MatMul() guarded by #if GEMMA_ONEDNN_BRGEMM
ops/matmul.h	#include "ops/brgemm.h", brgemm_autotune field in MMPerKey
ops/bench_matmul.cc	Check brgemm_autotune.Best() to avoid infinite loop when BRGeMM handles dispatch
CMakeLists.txt	GEMMA_ONEDNN_BRGEMM option, FetchContent for OneDNN v3.11, conditional target linking
BUILD.bazel	config_setting for gemma_onednn_brgemm, conditional OneDNN dep and defines for x86_64
MODULE.bazel	OneDNN v3.11 http_archive dependency
bazel/onednn.BUILD	Bazel build rules for OneDNN
util/zones.h	kBRGeMM caller enum for thread pool dispatch
util/zones.cc	CallerName mapping for kBRGeMM

Testing

• matmul_test passes with and without GEMMA_ONEDNN_BRGEMM (all original test shapes, types, and correctness checks preserved)
• bench_matmul runs successfully with BRGeMM enabled
• No changes to existing tests; zero impact when OneDNN is not enabled or on non-x86 platforms

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