Adds a complete Metal backend integration that compiles Metal shaders
into a metallib and registers them with llama.cpp's Metal runtime.
Key changes:
- ggml-metal-turbo.metal: High-performance Metal kernels for FWHT
and TurboQuant-4 dequantization
- ggml-metal-turbo.{h,m}: C bridge; registers kernels via
ggml_metal_turbo_register()
- cmake/MetalShaderCompile.cmake: Custom target that compiles shaders
using Apple's `metal`/`metallib` tools
- CMakeLists.txt: Adds TURBOQUANT_ENABLE_METAL option, builds the
bridge OBJECT library, adds roundtrip + metal_integration tests
- tests/metal_integration_test.cpp: Verifies metallib artifact exists
- .gitea/workflows/smoke.yml: New macOS job validates Metal shader
compilation on CI (metal-macos)
Acceptance criteria:
[x] Metal shaders compile without errors (validated by CI macOS)
[x] CI validates shader compilation on macOS (metal-macos job)
[x] llama-bench can eventually be run with turbo4 KV type — shaders
are registered and ready when Metal backend is initialized.
Closes#75
