/metal-kernel

name: metal-kernel
description: Write Metal/MPS kernels for PyTorch operators. Use when adding MPS device support to operators, implementing Metal shaders, or porting CUDA kernels to Apple Silicon. Covers native_functions.yaml dispatch, host-side operators, and Metal kernel implementation.

Metal Kernel Writing Guide

This skill guides you through implementing Metal kernels for PyTorch operators on Apple Silicon.

**Important:** The goal of this skill is to use native Metal capabilities via the `c10/metal/` infrastructure, NOT MPSGraph. Native Metal kernels provide better control, performance, and maintainability.

Overview

There are two workflows covered by this skill:

1. **Adding new MPS support** - Implementing a new operator from scratch 2. **Migrating from MPSGraph** - Converting existing MPSGraph-based operators to native Metal

Both workflows involve: 1. **Update dispatch** in `aten/src/ATen/native/native_functions.yaml` 2. **Write Metal kernel** in `aten/src/ATen/native/mps/kernels/` 3. **Implement host-side stub** in `aten/src/ATen/native/mps/operations/`

Step 1: Update native_functions.yaml

**Location:** `aten/src/ATen/native/native_functions.yaml`

For New Operators

Find the operator entry and add MPS dispatch:

# Simple MPS-specific implementation
- func: my_op(Tensor self) -> Tensor
  dispatch:
    CPU: my_op_cpu
    CUDA: my_op_cuda
    MPS: my_op_mps

# Shared implementation across devices (preferred for structured kernels)
- func: my_op.out(Tensor self, *, Tensor(a!) out) -> Tensor(a!)
  dispatch:
    CPU, CUDA, MPS: my_op_out

# Structured kernel (preferred for new ops)
- func: my_op.out(Tensor self, *, Tensor(a!) out) -> Tensor(a!)
  structured: True
  structured_inherits: TensorIteratorBase
  dispatch:
    CPU, CUDA, MPS: my_op_out

For Migrating from MPSGraph

When migrating an existing operator from MPSGraph to native Metal, **consolidate the dispatch entry**:

# BEFORE (MPSGraph-based, separate dispatch)
- func: atan2.out(Tensor self, Tensor other, *, Tensor(a!) out) -> Tensor(a!)
  structured: True
  structured_inherits: TensorIteratorBase
  dispatch:
    CPU, CUDA: atan2_out
    MPS: atan2_out_mps  # Separate MPS implementation

# AFTER (native Metal, shared dispatch via stub)
- func: atan2.out(Tensor self, Tensor other, *, Tensor(a!) out) -> Tensor(a!)
  structured: True
  structured_inherits: TensorIteratorBase
  dispatch:
    CPU, CUDA, MPS: atan2_out  # MPS now uses the same stub mechanism

**Key change:** Replace `MPS: my_op_out_mps` with adding `MPS` to the shared dispatch line (e.g., `CPU, CUDA, MPS: my_op_out`).

**Update every overload.** A single op typically has several `native_functions.yaml` entries — functional / inplace / `.out`, plus `Tensor` and `Scalar` variants. Each entry has its own `dispatch:` block, and each one must be moved over. Any entry left pointing at `MPS: my_op_mps` still routes that overload to the MPSGraph code, so callers can silently land on the old path depending on which overload they hit. Before declaring the migration done, grep the legacy function name and confirm no entry still references it.

**Dispatch naming conventions:**

• `MPS: function_name_mps` - MPS-specific implementation (old MPSGraph pattern)
• `CPU, CUDA, MPS: function_name` - Shared stub implementation (native Metal pattern)

Step 2: Implement Metal Kernel

**Location:** `aten/src/ATen/native/mps/kernels/`

Unary Kernel Pattern

// MyKernel.metal
#include <c10/metal/indexing.h>
#include <c10/metal/utils.h>
#include <metal_stdlib>

using namespace metal;
using namespace c10::metal;

// Define operation functor
struct my_op_functor {
  template <typename T>
  inline T operator()(const T x) {
    return /* your operation */;
  }
};

// Register for supported types
REGISTER_UNARY_OP(my_op, float, float);
REGISTER_UNARY_OP(my_op, half, half);
REGISTER_UNARY_OP(my_op, bfloat, bfloat);

Binary Kernel Pattern

struct my_binary_functor {
  template <typename T>
  inline T operator()(const T a, const T b) {
    return /* your operation */;
  }
};

REGISTER_BINARY_OP(my_binary, float, float);
REGISTER_BINARY_OP(my_binary, half, half);

Binary Kernel Type Registration Macros

For binary operations, use the convenience macros defined in `BinaryKernel.metal`:

// Floating-point types only (float, half, bfloat)
REGISTER_FLOAT_BINARY_OP(my_op);

// Integral types with float output (for math ops like atan2, copysign)
// Registers: long->float, int->float, short->float, uchar->float, char->float, bool->float
REGISTER_INT2FLOAT_BINARY_OP(my_op);

// Integral types with same-type output (for bitwise/logical ops)
// Registers: long, int, short, uchar, char, bool
REGISTER_INTEGER_BINARY_OP(my_op);

// Floating-point with opmath precision (for ops needing higher precision)
REGISTER_OPMATH_FLOAT_BINARY_OP(my_op);

**Common patterns:**

• Math functions (atan2, copysign, logaddexp): Use both `REGISTER_FLOAT_BINARY_OP` and `REGISTER_INT2FLOAT_BINARY_OP`
• Comparison/logical ops (maximum, minimum): Use both `REGISTER_FLOAT_BINARY_OP` and `REGISTER_INTEGER_BINARY_OP`
• Arithmetic ops (add, sub, mul): Use both `REGISTER_FLOAT_BINARY_OP` and `REGISTER_INTEGER_BINARY_OP`

**Example for atan2 (supports both float and int inputs):**

struct atan2_functor {
  template <typename T, enable_if_t<is_floating_point_v<T>, bool> = true>
  inline T operator()(const T a, const T b) {
    return static_cast<T>(precise::atan2(float(a), float(b)));
  }
  template <typename T, enable_if_t<is_integral_v<T>, bool> = true>
  inline float operator()(const T a, const T b) {
    return precise::atan2(float(a), float(b));
  }
};

REGISTER_FLOAT_BINARY_OP(atan2);
REGISTER_INT2FLOAT_BINARY_OP(atan2);

With Scalar Parameter

struct my_alpha_functor {
  template <typename T>
  inline T operator()(const T a, const T b, const T alpha) {
    return a + c10::metal::mul(alpha, b);
  }
};

REG