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Metal GPU Code Skill

Write production-quality Metal code with correct patterns, optimal performance, and clear explanations.

When to Read References

For detailed API topology, Metal 4 specifics, and Apple Silicon optimization patterns, read:

/mnt/skills/user/metal-gpu/references/metal-api-guide.md

Core Principles

1. Always start with the device: MTLCreateSystemDefaultDevice() — every Metal workflow begins here
2. Command pattern: Device → Command Queue → Command Buffer → Command Encoder → Commit
3. Shaders are MSL (Metal Shading Language): C++14-based, with Metal-specific types and attributes
4. Resource management matters: Use appropriate storage modes, avoid unnecessary copies
5. Triple buffering for render loops to keep CPU and GPU in parallel

Quick Reference: Metal Command Pipeline

MTLDevice
  └─ makeCommandQueue() → MTLCommandQueue
       └─ makeCommandBuffer() → MTLCommandBuffer
            ├─ makeRenderCommandEncoder(descriptor:) → MTLRenderCommandEncoder
            ├─ makeComputeCommandEncoder() → MTLComputeCommandEncoder
            └─ makeBlitCommandEncoder() → MTLBlitCommandEncoder

Writing Shaders (MSL)

Use Metal Shading Language. Always include:

• #include <metal_stdlib> and using namespace metal;
• Correct attribute qualifiers: [[vertex_id]], [[position]], [[stage_in]], [[buffer(n)]], [[texture(n)]]
• Proper address space qualifiers: device, constant, threadgroup, thread

Vertex Shader Pattern

#include <metal_stdlib>
using namespace metal;

struct VertexIn {
    float3 position [[attribute(0)]];
    float3 normal   [[attribute(1)]];
    float2 texCoord [[attribute(2)]];
};

struct VertexOut {
    float4 position [[position]];
    float3 normal;
    float2 texCoord;
};

vertex VertexOut vertex_main(VertexIn in [[stage_in]],
                             constant float4x4 &mvp [[buffer(1)]]) {
    VertexOut out;
    out.position = mvp * float4(in.position, 1.0);
    out.normal = in.normal;
    out.texCoord = in.texCoord;
    return out;
}

Fragment Shader Pattern

fragment float4 fragment_main(VertexOut in [[stage_in]],
                              texture2d<float> albedo [[texture(0)]],
                              sampler texSampler [[sampler(0)]]) {
    float4 color = albedo.sample(texSampler, in.texCoord);
    return color;
}

Compute Kernel Pattern

kernel void compute_main(device float *input  [[buffer(0)]],
                         device float *output [[buffer(1)]],
                         uint id [[thread_position_in_grid]]) {
    output[id] = input[id] * 2.0;
}

Swift-Side Setup Patterns

Render Pipeline Setup

let device = MTLCreateSystemDefaultDevice()!
let commandQueue = device.makeCommandQueue()!

// Load shaders
let library = device.makeDefaultLibrary()!
let vertexFunction = library.makeFunction(name: "vertex_main")
let fragmentFunction = library.makeFunction(name: "fragment_main")

// Pipeline descriptor
let pipelineDescriptor = MTLRenderPipelineDescriptor()
pipelineDescriptor.vertexFunction = vertexFunction
pipelineDescriptor.fragmentFunction = fragmentFunction
pipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm

// Vertex descriptor
let vertexDescriptor = MTLVertexDescriptor()
vertexDescriptor.attributes[0].format = .float3  // position
vertexDescriptor.attributes[0].offset = 0
vertexDescriptor.attributes[0].bufferIndex = 0
vertexDescriptor.layouts[0].stride = MemoryLayout<SIMD3<Float>>.stride
pipelineDescriptor.vertexDescriptor = vertexDescriptor

let pipelineState = try! device.makeRenderPipelineState(descriptor: pipelineDescriptor)

Compute Pipeline Setup

let computeFunction = library.makeFunction(name: "compute_main")!
let computePipeline = try! device.makeComputePipelineState(function: computeFunction)

let commandBuffer = commandQueue.makeCommandBuffer()!
let encoder = commandBuffer.makeComputeCommandEncoder()!
encoder.setComputePipelineState(computePipeline)
encoder.setBuffer(inputBuffer, offset: 0, index: 0)
encoder.setBuffer(outputBuffer, offset: 0, index: 1)

let gridSize = MTLSize(width: elementCount, height: 1, depth: 1)
let threadGroupSize = MTLSize(
    width: min(computePipeline.maxTotalThreadsPerThreadgroup, elementCount),
    height: 1, depth: 1
)
encoder.dispatchThreads(gridSize, threadsPerThreadgroup: threadGroupSize)
encoder.endEncoding()
commandBuffer.commit()

MetalKit View Rendering

import MetalKit

class Renderer: NSObject, MTKViewDelegate {
    let device: MTLDevice
    let commandQueue: MTLCommandQueue
    let pipelineState: MTLRenderPipelineState

    func draw(in view: MTKView) {
        guard let drawable = view.currentDrawable,
              let descriptor = view.currentRenderPassDescriptor else { return }

        let commandBuffer = commandQueue.makeCommandBuffer()!
        let encoder = commandBuffer.makeRenderCommandEncoder(descriptor: descriptor)!

        encoder.setRenderPipelineState(pipelineState)
        // Set buffers, draw primitives...
        encoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: 3)

        encoder.endEncoding()
        commandBuffer.present(drawable)
        commandBuffer.commit()
    }
}

Performance Best Practices

1. Storage modes: Use .shared on Apple Silicon (unified memory), .private for GPU-only data, .managed on Intel Macs
2. Triple buffering: Rotate 3 buffers with a semaphore to avoid CPU/GPU stalls
3. Avoid per-frame allocations: Reuse buffers and command encoders
4. Use dispatchThreads over dispatchThreadgroups when possible (Apple Silicon)
5. Prefer tile-based deferred rendering patterns on Apple GPUs — use imageblocks and tile shaders
6. Compile pipelines ahead of time: Pipeline creation is expensive, do it at load time
7. Use Metal GPU frame capture in Xcode to profile and debug

Common Mistakes to Avoid

• Forgetting encoder.endEncoding() before committing
• Mismatched buffer indices between Swift and MSL
• Using wrong pixel format for render targets
• Not handling nil from optional Metal API calls
• Blocking the main thread waiting for GPU completion — use addCompletedHandler instead
• Forgetting to set the vertex descriptor when using [[stage_in]]

Metal 4 Notes

Metal 4 introduces a modernized core API. Key changes:

• New compilation API for finer shader compilation control
• Updated command encoding patterns
• See references/metal-api-guide.md for the full Metal 4 API topology

Frameworks Ecosystem

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