b10545

ggml-org/llama.cppb10545Aug 21, 2026by github-actions[bot]

AI Summary

Fixes a critical bug in the Metal Tensor API mat-mat kernel where reading out-of-bounds memory caused NaNs when the K dimension was not a multiple of 32.

Key Highlights

  • Metal K-extent clamping for unaligned dimensions
  • Fixes undefined behavior and NaN generation
  • Added tests for unaligned K dimension paths

New Features

  • Metal kernel stability fix

Full Release Notes

<details open>

metal : clamp K extent in tensor API mat-mat kernel for K not a multiple of 32 (#27450)

The Tensor API mat-mat path of kernel_mul_mm (GGML_METAL_HAS_TENSOR) fed a
static K=32 tile to the matmul2d op on every iteration. On the last, partial
K tile (ne00 % 32 != 0) the src1 slice extends past the K extent of the
tensor, and the op reads those out-of-bounds elements (undefined behavior per
the MSL specification, section 2.22.2). Depending on stale memory contents,
this corrupted the result or produced NaN.

Make the matmul2d op use dynamic_extent for K, and clamp the K extent of both
operand tensor views to the remaining valid K range (min(32, K - loop_k)) per
iteration, so the op reads exactly the valid K range on every iteration
(mirroring the tail handling of the MPP matmul2d examples). On K-aligned
inputs the clamp degenerates to the full 32-wide tile: the only difference
from the static-K op is that the dynamic-K op derives K from the operand
extents and edge-checks the tile against the tensor extents (a handful of
integer ops per iteration).

Add test-backend-ops MUL_MAT cases with K not a multiple of 32 to exercise
the unaligned K path.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

</details>

**Website:**
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**Attestations:**
- <https://github.com/ggml-org/llama.cpp/attestations/42040575>

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