stable-diffusion.cpp/docs/ggml_extensions.md

ggml operator extensions

The patched ggml exposes ggml-ext.h for application-owned operators, kernel overrides, and graph fusion rules. Registrations are in-process and scoped to a backend device. Register them before capability queries or graph allocation; callback code and user data must remain alive until all dependent graphs and backends have been destroyed. Registration names are unique, and duplicate registrations fail rather than replacing live implementations.

Operator definitions

ggml_ext_register_op registers a versioned name, attribute size, and an output metadata inference callback. ggml_ext_op_create creates an ordinary ggml tensor with explicit input dependencies. The initial interface supports one contiguous, out-of-place output with an existing ggml_type.

Attributes are copied into the unused portion of op_params. They are small, plain values; use memcpy when reading attributes that need stricter alignment. Auxiliary tensors must appear in src, not in an attribute pointer or user data. Registered operators use GGML_OP_CUSTOM internally, with an identifiable callback marker. Unsupported devices do not mistake them for CPU callbacks. These process-local descriptors are not a graph serialization format.

Kernel selection

ggml_ext_register_kernel(device, &desc) associates a kernel with a built-in ggml_op, or with a registered custom operator. supports describes correctness constraints; preferred optionally decides whether to override an available native implementation. Higher priorities are considered first. When native execution is unsupported, a supported extension can run regardless of its preference callback.

Capability queries include registered single-node implementations, so the scheduler can select a device even when its native implementation rejects the operator. Callbacks inspect metadata only and must not recursively call the public capability query for the same device and operator. A fusion-only registration does not claim single-node support.

ggml_ext_dev_supports_native_op queries the original device implementation without extension dispatch. Fusion policies can use it to select a supported extension even when their performance preference normally favors native kernels.

prepare creates optional host-side plan state and reports workspace bytes. It can run during memory measurement, before tensor addresses are assigned, and must not launch work or allocate device buffers. Workspace is allocated by ggml before execution and included in scheduler memory estimates and buffer-size reports. execute receives the backend, workspace buffer, and current tensor addresses. destroy releases prepared state. A preparation or execution error is propagated; execution never retries a native implementation after launch.

Workspace is scratch storage shared by sequential extension steps in a plan; its capacity is the maximum request, not the sum across nodes. Contents are not preserved between calls. Queue work in backend execution order, and finish any private-stream use before subsequent backend work can access the workspace. Persistent state belongs in prepare/destroy state or explicit tensor inputs.

Overriding a built-in operator preserves its numerical and aliasing contract, including allocator reuse of input storage. Explicit views/in-place nodes are not intercepted by this first interface. Rebuild the scheduler allocation after changing graph metadata, selection policy, or registrations.

Fusion

A match callback returns the number of consecutive nodes it can replace. The scheduler freezes this selection before allocating tensors. ggml checks compute flags, external uses, output flags, and views; the initial implementation accepts a conservative set of pure operators with only the final node exposed as the fused output. The rule remains responsible for mathematical equivalence, dtype, shape, layout, and hardware constraints. Internal views are accepted when their storage belongs to the fused subgraph. The final output must own storage, except for a terminal CPY, which retains its original destination and write semantics. The extension must handle input/output storage overlap, including allocator reuse, with scratch space when necessary. Other in-place writes are not eligible for extension fusion.

An optional optimize callback runs before matching and allocation. Use ggml_ext_graph_delay to move a producer range after independent nodes. The helper rejects dependencies, unknown side effects, and writes that alias moved inputs or outputs. It can cross independent copies, including dtype conversions. Reordering must preserve edges and tensor metadata. Fusion legality is checked again after reordering.

External inputs stay allocated through the final node. Native graph optimization and execution operate on the remaining contiguous ranges, so native fusion cannot consume nodes selected by an extension. Intermediate allocations are retained in this initial implementation; only their computation is removed.

Automatic extension fusion is enabled in scheduler execution with one pipeline copy and without evaluation callbacks. Multiple pipeline copies and evaluation callbacks retain individual operations. Direct backend graph computation supports single-node extensions; it does not fuse an already allocated graph. Direct calls use temporary plans and synchronize before releasing them; scheduler plans and workspaces are reused across executions.

Use the backend graph APIs or the scheduler. The legacy CPU ggml_graph_compute and backend graph_plan_* interfaces do not dispatch these extensions.

Backend services

Services are discovered through ggml_backend_reg_get_proc_address:

Service	Purpose
ggml_ext_cpu_parallel_v1	Run an application callback on the CPU backend's configured thread pool, with its abort callback.
ggml_ext_cuda_context_v1	Borrow the current CUDA device and stream, ordered with native work.
ggml_ext_cuda_device_v1	Get the physical CUDA ordinal for capability checks before backend creation, including virtual ggml devices.

Workspace is a backend buffer. Its base is a CUDA device pointer on CUDA; callers must use the representation appropriate to their backend. A Vulkan command recording service is not implemented yet.

sd.cpp integration

src/core/ggml_extensions.cpp registers the CPU implementation of sd.rope_apply.v1, an override for existing GGML_OP_ROPE_APPLY nodes, and a RMSNorm -> Mul -> RoPE fusion. The fusion accepts a one-dimensional weight vector. Both interleaved and split-half RoPE, partial rotation, and strided input rows are supported. Model construction uses the custom operator on CPU and CUDA and retains existing backend implementations elsewhere.

src/core/cuda/rope.cu supplies standalone CUDA RoPE, the same builtin override, and RMSNorm -> Mul -> RoPE [-> Scale -> FP16 Copy] fusion. CuTe attention shares its RMSNorm/RoPE kernel. Partial rotation and arbitrary float-aligned input and position-encoding strides are supported by the standalone kernel; RMSNorm fusion requires contiguous position encoding and weights. Copy fusion uses workspace when the destination overlaps an input. These registrations do not require CUTLASS. The old CUDA RoPE dispatch is removed; HIP and MUSA retain their existing implementations.

SD_USE_UPSTREAM_GGML excludes these registrations. The stable-diffusion public C API is unchanged. INT8 ConvRot retains its existing packed activation layout and separate weight scale tensor.

src/core/cuda/cute_attention.cu supplies CuTe Flash Attention on CUDA when CUTLASS headers are configured. It registers an override of FLASH_ATTN_EXT, with the existing sequence-length preference and native fallback. It also registers Q preparation (RMSNorm -> Mul -> RoPE), output scaling, and packed ConvRot output fusion. These kernels use only the public extension API, borrowed CUDA stream, and scheduler-owned workspace; they do not use ggml's CUDA pool or private conversion helpers. The existing CUDA fusion and CuTe disable environment variables are honored.

The combined INT8 GEMM epilogue feeding FP16 Q directly into attention still needs to be externalized. The current extension can fuse RMSNorm/RoPE with attention, while other INT8 GEMM and standalone projection fusions retain their native implementations. Native INT8 projection matchers do not recognize custom RoPE yet, so those graphs currently run GEMM followed by the external RoPE/attention path. Restoring their combined epilogues is part of the remaining migration. Vulkan execution services and kernels also remain pending. This implementation does not yet externalize all model-specific kernels or add dynamic dtypes.