tessl 0.2.0

Metal 4 GEMM and encode runtime for Apple silicon, built on Metal Performance Primitives TensorOps
Documentation

tessl is a Rust GPU runtime substrate that executes high-performance matrix multiplication on Apple silicon through Metal 4 and Metal Performance Primitives (MPP) matmul2d, targeting the neural accelerators on Apple M-series hardware.

The name is short for tessellation — the design centers around how matrix operations are partitioned into tile geometries and the order in which those tiles are traversed.
Status 0.2.0 — Metal 4 / MPP TensorOps verified on M5 Pro
API docs docs.rs/tessl — built on aarch64-apple-darwin with all features
Tests 228 passing, including doc tests (cargo test --release -- --test-threads=1)
Kernel coverage All 44 promoted kernels have a numeric test, not only a name check
Platform Apple silicon, macOS 26+, Xcode 26 Metal Toolchain
License MIT OR Apache-2.0

Key Highlights

  • Pure Metal 4 Architecture: Built strictly on Metal 4 primitives (MTL4CommandBuffer, MTL4ComputeCommandEncoder, MTL4ArgumentTable, MTLResidencySet). Legacy MTLCommandQueue and command buffer paths are deliberately absent.
  • Hardware-Accelerated GEMM: Direct integration with MPP TensorOps matmul2d across NN, TN, and NT layouts in f32, bf16 (with f32 accumulate), and tf32-relaxed precision modes.
  • Cooperative Register Accumulators: High-throughput cooperative destination kernels (get_destination_cooperative_tensor) holding f32 accumulators in GPU registers across the entire $K$-reduction, eliminating device memory round-trips for NN, TN, NT, and accumulating paths.
  • In-Kernel Grid Swizzling & Bounds Checking: Column-panel tile swizzling for large grids ($\ge 2048$ tiles) bounding operand rereads, combined with origin-shifted slice bounds checking for ragged edges.
  • Zero-Wait Execution Pipeline: Packed command encoding with bump-allocated constant arenas (16 MiB) and MTLSharedEvent synchronization—host threads never block mid-step.
  • Neural-Network Kernel Library: 18 Metal source files providing 72 kernel entry points — RMSNorm, gated MLP activations (SiLU / GELU-tanh), flash attention (sliding-window $h{=}128/256$, global $h{=}512$), fused RMSNorm+QKV+RoPE, MLX-format Q4 GEMV/GEMM, Q8 GEMV, KV-cache stores, embedding lookup, row-wise softmax/sum/max, and softcap sampling. All reachable through 62 shape-checked entry points in tessl::nn, not as raw pipeline-name strings.
  • Fused GEMM Epilogue: C = activation(alpha * A@B + beta * C_prev + bias) in a single dispatch, applied while the accumulator is still in registers — measured 1.6–2.4× cheaper than the same work as a separate pass over $C$.
  • Mixed Precision & Quantization: f32, bf16, tf32-relaxed, IEEE binary16 (DType::F16), and an exact int8 x int8 -> int32 GEMM with fused per-column dequantization (nn::gemm_i8_dequant).
  • Strided Batched GEMM: gemm_batched with explicit per-operand batch strides, so a batch dimension is expressed rather than inferred from a rank-2 shape.
  • Decode ICB Capture & Replay: Low-latency Indirect Command Buffer (ICB) capture and ping-pong execution with freeze-binds and range-batching for decode-shaped inference workloads.

[!IMPORTANT] Platform Requirements:

  • OS: macOS 26+
  • Toolchain: Xcode 26 with the Metal Toolchain component (xcodebuild -downloadComponent MetalToolchain).
  • Hardware: Apple Silicon GPU with Neural Accelerators (Apple M-series) for the MPP TensorOps path. A portable simdgroup_matrix fallback is available for A/B testing, but is 2–3× slower.

System Architecture

graph TD
    subgraph Consumers["Downstream Consumers"]
        Gemma["gemma-metal<br/>(Gemma 4 Inference)"]
        Arch02["tessl-arch02<br/>(Value Residual Training)"]
    end

    subgraph TesslAPI["tessl Public API"]
        GpuRt["GpuRuntime"]
        GemmFn["gemm() / gemm_epilogue() / gemm_batched()"]
        NnFn["nn::* (RMSNorm, attention, softmax, Q4/Q8)"]
        TensorObj["Tensor / GpuBuffer"]
        IcbObj["DecodeIcb / PingPongCbReplay"]
    end

    subgraph CoreEngine["tessl Core Runtime Substrate"]
        RuntimeMod["runtime.rs<br/>MTL4 Buffers, Pools & Const Arena"]
        GemmMod["gemm.rs<br/>Validation, Layouts & Coop Dispatch"]
        DispatchMod["dispatch.rs<br/>Binder & Argument Table Encode"]
        IcbMod["decode_icb.rs / cb_replay.rs<br/>ICB Capture, Tape Replay & Coalescing"]
        NnMod["nn.rs<br/>Typed API over the NN kernel library"]
        MtlTensorMod["mtl_tensor.rs<br/>Quantized MTLTensor Prep (WWDC26-330)"]
    end

    subgraph Metal4Layer["Metal 4 Driver & Hardware Layer"]
        CmdBuf["MTL4CommandBuffer / Allocator"]
        ArgTable["MTL4ArgumentTable (31-slot)"]
        ResSet["MTLResidencySet (Hot / Cold Pools)"]
        SharedEvt["MTLSharedEvent (Zero-wait Sync)"]
    end

    subgraph Shaders["Compiled Metallib Shaders"]
        TensorOpsMetal["matmul_tensorops.metal (MPP matmul2d)"]
        SimdMetal["matmul_simdgroup.metal (Fallback)"]
        UtilsMetal["utils.metal (Elementwise & Softcap)"]
    end

    Gemma -->|Links & Overlays| TesslAPI
    Arch02 -->|DEP_TESSL_KERNELS| TesslAPI
    TesslAPI --> CoreEngine
    CoreEngine --> Metal4Layer
    Metal4Layer --> Shaders

Performance vs. PyTorch MPS & MLX

Apple M5 Pro, python3 bench/paired_cross_runtime.py --rounds 5 --lanes torch,mlx against torch 2.13 (MPS) and MLX, re-measured 2026-09-01. The harness interleaves tessl and the baseline round by round so thermal throttling and frequency scaling hit both lanes alike — see Benchmarking.

Geomean of per-shape medians over 5 rounds across an 8-shape ladder:

Comparison Geomean Worst shape Best shape Shapes below 1.0
bf16 vs. PyTorch MPS bf16 1.03× 0.86× 1.16× 4 of 8
f32 exact vs. PyTorch MPS f32 1.12× 0.87× 1.78× 2 of 8
tf32-relaxed vs. PyTorch MPS f32 2.11× 1.76× 2.45× 0 of 8
bf16 vs. MLX bf16 2.55× 1.12× 3.67× 0 of 8

[!IMPORTANT] bf16 against MPS is parity, not a win. An earlier version of this table claimed 1.11× with a worst shape of 1.01×, which reads as "never loses". Re-run with the same harness it is 1.03× and it loses on half the ladder, by as much as 0.86× at 8192×3072×768. The number that is worth something is the tf32 lane at 2.11×, which wins on every shape — and the MLX comparison at 2.55×, also clean. Apple's own bf16 GEMM is well tuned; matching it is the honest claim.

Peak observed throughput, cargo run --release --bin bench_gemm_sweep, medians over 50 iterations after 10 warmup:

Precision Peak Shape
bf16 26,642 GFLOP/s square_4096
tf32-relaxed 16,293 GFLOP/s mlp_up
f32 exact 6,431 GFLOP/s square_2048

[!WARNING] Benchmarking rigor.

  • Clock drift. Single-run cross-runtime numbers fluctuate 15–20% on identical work as the power governor moves. Use the paired sweep (bench/paired_cross_runtime.py); for kernel-vs-kernel A/B use bench_gemm_tile_tune or bench_gemm_tnnt_tune behind TESSL_GEMM_TUNE=1.
  • Dispatch floor. Below ~2 GFLOP of total work both runtimes sit on a ~0.25 ms host submit-and-wait floor, which measures driver dispatch latency rather than shader throughput.
  • These figures are reproducible, and were reproduced. The f32 and tf32 peaks previously published here (10,897 and 18,040 GFLOP/s) are not: the crate's own committed sweep in bench/results/ records 6,606 for f32, and a fresh run gives 6,431. Two independent sources agreeing against a third is why they were replaced rather than averaged.

Metal 4 Memory & Residency Hierarchy

tessl manages GPU memory allocations explicitly to eliminate mid-command buffer host stalls and memory thrashing.

flowchart TD
    subgraph DeviceMemory["Unified System Memory (Metal 4 Device)"]
        subgraph Pools["tessl Managed Pools"]
            Hot["Hot Pool<br/>(Weights & Persistent State)<br/>Resident for lifetime of run"]
            Cold["Cold Pool<br/>(Intermediate Activations)<br/>Recycled + removeAllocation after CB"]
            Bump["Bump Pool<br/>(Per-step Ephemeral Slabs)<br/>Cursor reset on sync"]
        end

        subgraph Arenas["Low-Latency Arenas"]
            ConstArena["Constant Arena (16 MiB Bump)<br/>Scalar & Uniform Table Offsets"]
        end
    end

    subgraph DriverResidency["Metal 4 Driver Residency Management"]
        ResSet["MTLResidencySet"]
        ArgTable["MTL4ArgumentTable"]
    end

    Hot -->|Registered Once| ResSet
    Cold -->|Dynamic Register / Evict| ResSet
    Bump -->|Pre-allocated Slabs| ResSet
    ConstArena -->|Direct Table Offsets| ArgTable
  • BufferKind::Hot: Persistent allocations (model weights, optimizer state, KV cache banks). Added to the MTLResidencySet once at initialization and retained across steps.
  • BufferKind::Cold: Intermediate activations. Managed via an active freelist pool with a default 2 GiB cap (DEFAULT_POOL_CACHE_BYTES). Unused slabs are evicted via removeAllocation upon command buffer completion.
  • BufferKind::Bump: Ephemeral scratch memory allocated linearly from pre-committed slabs. Bump cursors are reset at synchronization points without individual buffer deallocations.
  • Constant Arena (16 MiB): Eliminates per-dispatch host allocation overhead for scalars and small metadata buffers by writing directly into a shared staging buffer at 16-byte aligned offsets.

GEMM Pipeline & Cooperative Destination Execution

flowchart TD
    Start["gemm(a, b, c, backend)"] --> Validate{"validate_gemm()<br/>Rank-2, Non-empty, Bounds &lt;= 2^31,<br/>Same Runtime, No In/Out Overlap"}
    Validate -- Fail --> Err["Return Err(String)"]
    Validate -- Pass --> BackendCheck{"Backend?"}

    BackendCheck -- SimdGroup --> SimdGroupKernel["matmul_simdgroup<br/>(Portable SIMD Fallback)"]
    BackendCheck -- TensorOps --> LayoutCheck{"Layout Resolution"}

    LayoutCheck -- "TN / NT Layout" --> SplitKCheck{"prefer_tn_splitk?<br/>(K &gt;= 2048, M,N &lt;= 384,<br/>min(M,N) &lt;= 128)"}
    SplitKCheck -- Yes --> SplitKKernel["matmul2d_tensorops_tn/nt_splitk_*<br/>(Split-K partial reductions)"]
    SplitKCheck -- No --> CoopTN["matmul2d_tensorops_tn/nt_bf16_f32<br/>(128x64 sg4 Cooperative Destination)"]

    LayoutCheck -- "NN Layout" --> PrecisionCheck{"Precision Mode"}
    
    PrecisionCheck -- "f32 exact" --> F32Exact["matmul2d_tensorops_f32<br/>(Tile: 32x32, 1 simdgroup)"]
    
    PrecisionCheck -- "bf16 / tf32-relaxed" --> NNTable{"nn_coop_kernel()<br/>N &lt;= 512?"}
    
    NNTable -- "N &lt;= 512 (Narrow)" --> NNNarrow["matmul2d_tensorops_*_64x64_sg4<br/>• TILE_COOP_NARROW (64x64, 4 simdgroups)<br/>• Register accumulator, cT.store<br/>• Edge bounds-checked slices"]
    
    NNTable -- "N &gt; 512 (Default)" --> NNDefault["matmul2d_tensorops_*<br/>• TILE_COOP_DEFAULT (128x64, 4 simdgroups)<br/>• Column-panel swizzle if grid &gt;= 2048 tiles<br/>• Register accumulator, cT.store<br/>• Edge bounds-checked slices"]

Cooperative Destination Advantages

  1. Register Accumulation: op.template get_destination_cooperative_tensor<...>() maintains the full f32 accumulator in hardware SIMDgroup registers across the entire $K$-reduction loop.
  2. Zero Pre-Zero Overhead: Register accumulators are initialized via .set(i, 0.0f) in shader code. The host-side zero_f32(C) pre-pass is completely eliminated.
  3. Single Store to Memory: Device memory $C$ is written exactly once (cT.store(tC)) at threadgroup termination.
  4. Ragged Edge Handling: Boundary tiles use origin-shifted full-extent tensor slices (mA.slice(...), mB.slice(...), mC.slice(...)), executing the same cooperative register accumulation without dropping tail elements.
  5. Column-Panel Grid Swizzling: For large dispatch grids ($\text{tiles}_n \times \text{tiles}_m \ge 2048$), threadgroups are swizzled into 8-tile-row bands to bound operand $B$ cache rereads, delivering $+11%$ throughput at $4096^3$.

Indirect Command Buffer (ICB) Decode Pipeline

For auto-regressive generation where kernel execution times approach dispatch overheads, tessl provides Indirect Command Buffer (ICB) capture and tape replay.

sequenceDiagram
    autonumber
    participant Host as Host Runtime / Client
    participant Binder as Binder / Dispatcher
    participant Tape as DecodeIcb Capture Tape
    participant ICB as Metal 4 MTLIndirectCommandBuffer
    participant GPU as Apple Silicon GPU

    Note over Host,GPU: 1. Capture Phase (First Token / Warmup)
    Host->>Binder: begin_decode_icb_capture()
    loop Model Layers (Decode Graph)
        Host->>Binder: bind_buffer(), set_pipeline(), dispatch()
        Binder->>Tape: Record Command (PSO, ArgTable, Buffers, Grid Size)
    end
    Host->>Tape: take_decode_icb_capture() -> Bake ICB Tape
    Tape->>ICB: Encode ICB Commands (freeze-binds / range-batching)

    Note over Host,GPU: 2. Steady-State Replay Phase (Subsequent Tokens)
    loop Each Decode Token
        Host->>Tape: try_replay_icb(runtime)
        Tape->>ICB: executeCommandsInBuffer:withRange: (Zero setArgumentTable host tax)
        Host->>GPU: Submit MTL4CommandBuffer (Ping-Pong buffers)
        GPU-->>Host: Signal MTLSharedEvent (Zero-wait async execution)
    end

Quickstart

cargo add tessl

Both snippets below are compiled and run as examples, so they cannot drift from the API:

cargo run --release --example gemm      # the GEMM quickstart
cargo run --release --example nn_layer  # RMSNorm -> gate/up -> GELU -> residual

Basic GEMM

use tessl::{gemm, GemmBackend, GpuRuntime};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let rt = GpuRuntime::new()?;

    let a = rt.alloc_tensor_f32(&[4096, 2304])?;
    let b = rt.alloc_tensor_f32(&[2304, 768])?;
    let c = rt.alloc_tensor_f32(&[4096, 768])?;

    gemm(&a, &b, &c, GemmBackend::TensorOps)?;   // C = A @ B via MPP TensorOps
    rt.synchronize()?;
    Ok(())
}

Consuming kernels from a downstream crate

tessl sets links = "tessl" and exports DEP_TESSL_KERNELS, so a crate that compiles its own metallib can build against the canonical kernel sources rather than keeping a copy that silently drifts. In a downstream build.rs:

let tessl_kernels = std::path::PathBuf::from(std::env::var("DEP_TESSL_KERNELS").unwrap());
let matmul_shader = tessl_kernels.join("matmul_tensorops.metal");
// ... compile matmul_shader into your own metallib

To overlay a custom metallib at runtime:

use std::path::Path;

let rt = GpuRuntime::from_metallib_path(Path::new("/path/to/custom.metallib"))?;

// Or overlay onto tessl's default library. Pipeline names must be unique across
// the primary library and every overlay: `pipeline()` resolves the primary
// first, so a duplicate name in an overlay is silently unreachable.
let rt = GpuRuntime::new()?;
rt.add_metallib(Path::new("/path/to/custom_overlay.metallib"))?;

Neural-Network Kernels

Every kernel is reached through a typed, shape-checked function in tessl::nn. None of them require the caller to name a pipeline string or hand-compute a threadgroup grid.

Group Entry points Notes
Normalization rms_norm_f32 Fused weight multiply; eps applied inside the kernel.
MLP mlp_silu, mlp_gelu_tanh, gate_up_* GeluTanh uses a clamped precise::tanh; plain tanh lowers to air.fast_tanh at -O2 and returns NaN past roughly |10|.
Attention flash_attn_swa_h128/h256, flash_attn_global_h512 Sliding-window and global variants, selected by AttnHeadDim.
Fused prologue rms_qkv_rope RMSNorm → QKV projection → RoPE in one dispatch.
Reductions softmax_rows_f32, row_sum_f32, row_max_f32 Max-subtracted softmax; a fully masked row returns uniform, not NaN.
Quantized Q4Bank, Q4MlxBank, gemv_q8, gemm_i8_dequant Both the signed-int4 and the MLX unsigned-affine conventions.
Cache / IO kv_store, embed_lookup, softcap_sample

Argument validation is not advisory. Every entry point checks buffer capacity and dimension products before encoding anything, and returns Err without dispatching — tests/nn_adversarial.rs asserts all three properties (error returned, no panic, dispatch count still zero) across the whole surface.

Fused GEMM Epilogue

gemm_epilogue computes C = activation(alpha * A@B + beta * C_prev + bias) in one dispatch.

Every term there is otherwise a separate kernel that reads all of C and writes all of C. A bias plus an activation costs two extra full round-trips through device memory — on a bandwidth-bound machine, most of what the GEMM saved. Applied inside the cooperative-destination kernel the accumulator is still in registers, so C is written exactly once and read at most once, only when beta != 0.

use tessl::{gemm_epilogue, Activation, Epilogue, GemmBackend};

gemm_epilogue(&a, &b, &c, GemmBackend::TensorOps, Epilogue {
    alpha: 1.0,
    beta: 0.0,                 // skips reading C entirely
    bias: Some(&bias),         // per-column, length N
    activation: Activation::GeluTanh,
})?;

Bias is per-column and broadcasts across rows through a row-stride-0 tensor view, so the same cooperative load that fetches C_prev fetches the bias with no separate indexing.

Shape gemm Fused gemm + one pass over C Epilogue cost vs. one pass
512³ 0.377 ms 0.558 ms 0.661 ms 0.181 ms 1.57× cheaper
1024³ 0.471 ms 0.584 ms 0.746 ms 0.113 ms 2.43× cheaper
2048×2048×512 0.916 ms 1.139 ms 1.297 ms 0.223 ms 1.71× cheaper

cargo run --release --example epilogue_cost. The comparison arm is gemm plus a single add_inplace_f32 sweep — strictly less work than a real bias broadcast, and half the work of bias plus a separate activation. Fusing beats even that lower bound at every shape. All three arms are GPU-side in one interleaved run, so machine load during measurement affects them alike.

It requires the cooperative-destination path — bf16 operands, or f32 with relaxed precision. The exact-f32 and simdgroup kernels write C straight from the matmul with no register accumulator, so there is nothing to fuse into; those are refused rather than silently falling back to separate dispatches, which would make the call quietly slower than the unfused code it replaced.

Verification

# Full suite. GPU tests are not thread-safe across concurrent OS threads
# sharing default command encoders, so --test-threads=1 is required.
cargo test --release -- --test-threads=1

# Validate static TileGeom definitions against compiled Metal kernel constants
python3 scripts/audit_gemm_tiles.py

# Quick shape fuzz (160 cases) runs as part of the ordinary suite:
cargo test --release --lib -- --test-threads=1 --nocapture gemm_fuzz_quick

# Deep soak (2500 cases), #[ignore]d so it stays out of the default run:
cargo test --release --lib -- --ignored --test-threads=1 --nocapture gemm_fuzz_deep

# Replay a specific failing seed:
STRESS_SEED=0xdeadbeef cargo test --release --lib -- --test-threads=1 gemm_fuzz_quick

Static tile audit (scripts/audit_gemm_tiles.py) cross-references every Rust TileGeom struct against the constexpr int SM/SN parameters compiled into matmul_tensorops.metal, including macro-instantiated kernels (NN_COOP_KERNEL, TN_NT_COOP_KERNEL). A mismatch would make the host dispatch incorrect threadgroup grids, silently leaving output tiles unwritten.

Shape fuzzer gemm_fuzz_quick / gemm_fuzz_deep check numerical correctness across non-standard dimensions, reporting the failing seed for replay via STRESS_SEED.

[!NOTE] An earlier version of this section claimed the fuzzer "asserts its own coverage — the test panics if any selectable NN kernel is exercised in fewer than 1% of fuzz iterations". No such assertion is implemented. It named a test (gemm_randomized_shape_fuzz) and environment variables (GEMM_FUZZ_SEED, GEMM_FUZZ_CASES) that do not exist either, so the documented command ran zero tests and reported success. Per-kernel coverage accounting would be worth adding; until it is, the fuzzer checks correctness on the shapes it happens to draw and nothing more.

Benchmarking & Tuning Binaries

Tuning and A/B measurement kernels (92 variants) are excluded from the default metallib to keep release binaries light (0.20 MB vs. 1.07 MB):

TESSL_GEMM_TUNE=1 cargo build --release --bins
Binary Purpose
bench_gemm_tile_tune Exhaustive tile geometry ($SM \times SN$) and $BK$ ladder benchmark.
bench_gemm_tnnt_tune TN/NT tile sweep; the paired, round-interleaved A/B comparison lane.
bench_gemm_sweep Cross-runtime sweep (f32, tf32, bf16) with JSON telemetry output.
bench_nn_kernels Throughput of the nn library, timed both batched and solo so the dispatch floor is visible rather than hidden.
probe_gemm_parity Bit-exact verification probe comparing TensorOps against the reference SIMD path.
bench/paired_cross_runtime.py Python harness driving paired tessl vs. PyTorch MPS / MLX evaluation.

Environment Variables

All runtime configuration uses the canonical TESSL_* prefix. Legacy METAL_RUNTIME_* and METAL_NATIVE_* variants are still accepted.

Variable Default Description
TESSL_GEMM_TUNE 0 Compiles the 92-kernel A/B tuning suite into the metallib (build-time).
TESSL_GEMM_ACCUM 0 Enables native TensorOps multiply_accumulate for TN/NT accumulate paths.
TESSL_GEMM_ACCUM_DX 0 Enables the hardware accumulate path specifically for $dX$ NT GEMM.
TESSL_GEMM_INTERIOR 0 Enables interior-offset tile optimizations for f32 GEMM.
TESSL_HAZARD_BARRIERS 0 (barriers on) Unsafe, do not enable. 1 removes the always-on Dispatch→Dispatch device barrier. The sense is the opposite of what this row said until 2026-08-31, and following the old wording to "enforce barriers" removed them. Enabling it requires the caller to place an explicit Binder::barrier at every RAW edge, and tessl's own ops do not: measured on an M5 Pro, gemm_tn_accum_train 64×64×128 under async encode produced wrong results in 300 of 300 repetitions with this set.
TESSL_COARSE_BARRIERS inherits TESSL_HAZARD_BARRIERS Replaces per-RAW barriers with coarse phase-level synchronization.
TESSL_MID_COMMIT=N 0 Overlaps host command encoding with GPU execution every $N$ dispatches.
TESSL_DECODE_ICB 0 Enables the Indirect Command Buffer capture and execution path.
TESSL_ICB_FREEZE_BINDS 0 Freezes argument table buffer bindings directly into ICB commands.
TESSL_ICB_RANGE_BATCH 0 Coalesces contiguous ICB command ranges into single execution dispatches.
TESSL_SKIP_AOT 0 Bypasses the build.rs AOT shader compile and reuses an existing default.metallib. Panics if that file is absent rather than baking a path that fails at every GpuRuntime::new().

Feature Flags

Feature Default Description
quant-prep Disabled Compiles mtl_tensor for native quantized MTLTensor bindings (WWDC26-330). Off by default because it is exactly that — prep: try_quant_tensorops_prefill_gemm returns an error and nothing calls it. Kept compiling behind a flag rather than shipped as public API that does not work.

Documentation

Document Topic & Scope
API reference Every public type, entry point and feature flag on docs.rs, rendered from the source of the released version. Start at the crate root for the platform requirements, the two quickstarts and the module map.
Architecture Deep dive into kernel selection, cooperative destination register mechanics, $K$-reduction bandwidth analysis, and TN/NT layout optimizations.
Benchmarking The paired measurement protocol, GPU thermal and frequency scaling mitigation, and five measurement pitfalls.
Verification Static tile geometry audit, randomized shape fuzzing, and fault injection test suites.
Tuning log Empirical $BK$ ladder benchmarks, root causes, and the landed M5 Pro speedups.
Changelog Release history.

Known Gaps

Recorded rather than implied. Every kernel is wired to a typed Rust API, the suite is warning-free, and there are no stubs; these are capabilities the crate does not have.

Gap Why it matters Why not yet
Int4 TensorOps GEMM Half the weight bandwidth of int8. TensorOps itself accepts int4b_format — the block is the shader-side tensor constructor for a sub-byte element type, not the objc2 binding this table used to blame. nn::gemm_i8_dequant ships the int8 case.
No CPU fallback Without a Metal 4 device, nothing runs. Deliberate. This is an Apple-silicon runtime, and a silent CPU path would make every "GPU" benchmark here meaningless.
GPU CI on hosted runners Whether the suite runs unattended, or only on hardware I own. Measured, not assumed: it does not. On macos-26 the Metal Toolchain installs and all 18 kernel sources compile and lint — the check job does exactly that every push — but the device probe fails, so the shaders build there and cannot execute. The suite therefore runs on a gated self-hosted M5 runner. CI covers build, clippy, rustdoc and the static tile audit on every push; the 228 tests do not run unattended.
Benchmark numbers in CI The GFLOP/s figures above are reproducible only by hand. Hosted runners are virtualised and shared, so a timing from one describes the runner. The bench job runs the sweep on bare-metal Apple silicon and is gated behind a repository variable until such a runner is registered.

Requirements

  • OS: Apple Silicon, macOS 26 or newer
  • Toolchain: Xcode 26 with the Metal Toolchain (xcodebuild -downloadComponent MetalToolchain)
  • Language: Rust 1.82+

License

Dual-licensed under either of:

at your option.