ggml checks every node against the device's supports_op and calls GGML_ABORT
when one is not implemented, because a single-backend graph has nowhere else to
put it: there is no per-op CPU fallback. The whole sd-server dies with SIGABRT
mid-generation and the user gets "the native image renderer stopped
unexpectedly" with no way forward.
Seen on macos-14 arm64 with FLUX.2-klein-4B Q2_K through the cross-platform CI:
the text encoder is already pinned to CPU, and the abort moved into the denoise
loop instead.
ggml_metal_op_encode_impl: error: unsupported op 'MUL_MAT' -> ggml_abort
StableDiffusionGGML::sample -> sample_k_diffusion
A retry on the same backend would abort identically, so the load is restarted
once with --backend cpu (the only flag that changes which backend executes the
graph; --offload-to-cpu moves parameters, not compute) and the generation is
re-submitted. The same checkpoint then renders slower rather than not at all.
Strictly bounded: the signature must carry both the unsupported-op line and
ggml_abort, the device must not already be CPU, and it happens once per load,
so an OOM kill or a genuine crash still surfaces as itself.
9a9999f8cd