[joelteply]

#82 adds GGML_OP_MUL_MAT_ID_BACK with a CPU kernel; on Metal the scheduler would run that one
op on the CPU inside a GPU graph. This is the Metal kernel.

• kernels/out_prod.metal: kernel_mul_mat_id_back, in kernel_out_prod's template shape
  <T0, block_q, nl, dequantize_func>. One threadgroup per routed slot (i, t); each thread owns
  columns of that slot's row and walks the expert's rows, so every dst element has one writer
  and no expert is copied or transposed in device memory. nl == 0 reads F32/F16/BF16 element by
  element; nl > 0 dequantizes 16 consecutive elements of a row in registers (ne00 % 16 == 0).
  Instantiated for f32, f16, bf16 (GGML_METAL_HAS_BF16), q4_0, q4_1, q5_0, q5_1, q8_0 and
  q2_K-q6_K: out_prod's set.
• ggml-metal-ops.cpp:
  • ggml_metal_op_mul_mat_id_back dispatches (n_used, n_tokens) threadgroups;
  • ggml_metal_op_mul_mat_id_back_supported refuses the pointer-table gather (src[3]);
  • out_prod's weight-type switch is now one predicate, ggml_metal_frozen_weight_readable,
    shared by both training kernels.
• ggml-metal-device.*:
  • the pipeline getter (kernel_mul_mat_id_back_);
  • supports_op, with bf16 requiring has_bfloat.

Not run here: enrico's Metal driver is wedged, and the Metal toolchain could not be installed
to compile-check the shader. The C++ side compiles (clang -fsyntax-only on ggml-metal-ops.cpp).
Proof is on the M5:

• test-backend-ops test -o MUL_MAT_ID_BACK -b Metal against CPU, for every expert type in
  test_mul_mat_id_back;
• test-backend-ops grad -o MUL_MAT_ID -b Metal.

Co-Authored-By: Claude Opus 5.5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01FRQWzgSfo79JtnwZywHKrE

[joelteply]

PASS at a726c39 (Fable). Verified on the M5 (Apple M5 Max, Metal, serving down), built from this head: test-backend-ops test -o MUL_MAT_ID_BACK -b MTL0 12/12; test-backend-ops grad -o MUL_MAT_ID -b MTL0 19838/19838 (877 MUL_MAT_ID grad cases OK on MTL0). The same counts as my run at a7b1ab7. Logs: ~/.continuum/evidence/2026-10-31-fork83-metal/a726c39d7-{test,grad}.log on the M5.