Validating Pytorch Custom Ops

Validate PyTorch Implementations and Custom Operators

Use this workflow whenever adding a backend under attn_gym/linear/<variant>/impl/ or attn_gym/sparse/<variant>/impl/.

Choose the least opaque integration

1. Eager/reference implementation: keep ordinary PyTorch code traceable. Do not create a custom operator merely because the public function must support torch.compile.
2. Triton implementation: direct Triton kernels can work under torch.compile. Prefer torch.library.triton_op with torch.library.wrap_triton when a stable operator boundary is useful and compiler subsystems should remain able to inspect the implementation.
3. CuTeDSL or external implementation: use a private registered operator (torch.library.define/impl, below) when the launcher cannot be traced correctly.

The documented public function remains an ordinary Python function that owns semantic validation, mode selection, and backend dispatch. Compiling only a private launcher does not prove that the public operation is compile-friendly.

Operator boundary CPU overhead

Every registered-operator boundary costs CPU time per call. Microbenchmark (torch 2.14 nightly, B200, identical trivial launcher, CPU wall-clock per call, medians over interleaved rounds):

Nesting one registered op inside another pays the boundary again (nested custom_op 7.1 us, nested define/impl 3.7 us). Wrapping the real kda_l2norm_fwd launcher in custom_op added ~11 us per forward call over invoking the launcher directly.

Consequences for launch-bound paths (small kernels, high call rate):

• Registration exists for compile and fake-tensor consumers; eager execution gains nothing from it. The eager floor is a plain autograd.Function over the launcher with no torch.library registration (real l2norm op: ~17 us cheaper per gradient-tracking forward and ~58 us per fwd+bwd than the custom_op boundary), at the cost of fullgraph=True support.
• Use the define/impl pattern below for repo operators; torch.library.custom_op costs ~3 us more per call for schema inference and mutation checking and is only preferable for prototypes.
• Prefer autograd.Function over register_autograd for hot training ops; it was the cheapest measured autograd boundary. Registering a Python kernel at the Autograd dispatch key (torch.library.impl(op, "Autograd")) makes the raw op differentiable but runs Python on every call, including inference; use it only when direct torch.ops calls must be differentiable.
• Give each hot path at most one registered-operator boundary. Nested custom ops double the dispatch tax; share the Python launcher between entrypoint ops instead of calling one op from another.
• One contract, one operator pair. When several backends (Triton portable, CuTeDSL fast) or variants (a transform kind, a mode) implement the same public semantics, register a single fwd/bwd pair, pass the variant as a schema argument (str kind, bool, float), and select the backend inside the CUDA implementation on real tensors. Do not create one op pair per backend or thread a use_<backend> flag through autograd.Function.apply: selection that runs under Dynamo tracing specializes into the graph, trips the functools.lru_cache warning for cached device-property helpers, and multiplies fakes, handles, and Functions. Make every backend return the same output contract (e.g. already-reduced parameter gradients) so one fake describes all of them. attn_gym::_gate_transform_{fwd,bwd} is the reference shape.
• A define/impl op has no autograd kernel: backprop through a direct call warns and produces no gradient, so route all differentiable use through the autograd.Function wrapper.
• torch.library.opcheck accepts torch.ops.attn_gym.op.default, so the validation workflow below is unchanged.

Public result contract

Private backends should return tensors or a fixed tensor tuple. The public API may wrap that result in a documented NamedTuple so users can use attributes or unpack it:

from typing import NamedTuple

import torch


class OperationOutput(NamedTuple):
    output: torch.Tensor
    final_state: torch.Tensor | None = None


def operation(...) -> OperationOutput:
    output, final_state = backend_forward(...)
    return OperationOutput(output, final_state)

A registered operator's output structure and arity must agree with its schema. Do not switch between returning a tensor and a tuple based on an argument. Represent optional outputs explicitly and match them in the fake implementation.

Opaque operator pattern (define/impl + autograd.Function)

The repo standard for opaque kernel boundaries. Write the schema string yourself; nothing is inferred from annotations:

import torch
from torch import Tensor

torch.library.define(
    "attn_gym::example_fwd",
    "(Tensor query, Tensor key, Tensor value, Tensor? initial_state) -> (Tensor, Tensor)",
)
torch.library.define("attn_gym::example_bwd", "(Tensor query, Tensor grad_output) -> Tensor")


def _example_fwd_cuda(
    query: Tensor, key: Tensor, value: Tensor, initial_state: Tensor | None
) -> tuple[Tensor, Tensor]:
    return launch_backend(query, key, value, initial_state)


# Register with an explicit call, not as a decorator: the decorator form returns None,
# and keeping the launcher callable lets benchmarks and tests bypass the boundary.
torch.library.impl("attn_gym::example_fwd", "CUDA", _example_fwd_cuda)


@torch.library.register_fake("attn_gym::example_fwd")
def _example_fwd_fake(
    query: Tensor, key: Tensor, value: Tensor, initial_state: Tensor | None
) -> tuple[Tensor, Tensor]:
    return torch.empty_like(value), query.new_empty(query.shape[:2])


_example_fwd = torch.ops.attn_gym.example_fwd.default
_example_bwd = torch.ops.attn_gym.example_bwd.default  # impl/fake registered the same way


class _Example(torch.autograd.Function):
    @staticmethod
    def forward(ctx, query: Tensor, key: Tensor, value: Tensor) -> Tensor:
        output, state = _example_fwd(query, key, value, None)
        ctx.save_for_backward(query)
        return output

    @staticmethod
    @torch.autograd.function.once_differentiable
    def backward(ctx, grad_output: Tensor):
        (query,) = ctx.saved_tensors
        return _example_bwd(query, grad_output), None, None

Schema rules:

• Optional tensors are Tensor?; scalars are int, float, bool; defaults may be embedded (Tensor? cu_seqlens=None).
• Encode mutation explicitly with alias annotations: (Tensor(a!) state, Tensor value) -> ().
• The schema language is a superset of what custom_op annotation inference accepts, adding e.g. ScalarType, Layout, MemoryFormat, Generator, SymInt, and int[2]. Neither flow accepts arbitrary Python objects, dataclasses, or callables; flatten configs to scalars or specialize per-config outside the op.
• Optional outputs parse as Tensor? and, as of torch 2.15 nightly (verified 2026-08-16: plain fullgraph compile, autograd.Function wrapping + backward, and opcheck all pass for both the tensor and None branches), no longer hard-fail the compile stack; on older releases the None return handling was unreliable. Keep the fixed-arity convention anyway: for "N or N+1 returns depending on a flag", define two fixed-arity schemas sharing one launcher (see kda_chunk_fwd / kda_chunk_fwd_with_state) and let the autograd.Function branch on the flag; its output arity is free to vary. The bool flag specializes into separate graphs regardless, so a merged optional-output schema saves nothing under compile, while a Tensor? return forces None-narrowing on every caller and extra schema boxing costs real time on launch-bound eager paths (collapsing schema pairs measured 3.22% slower on the dense forward probe, PR #314).
• Optional inputs are fully compile-safe: the merged kda_chunk_bwd schema takes Tensor? cu_seqlens, Tensor? chunk_offsets for both dense and ragged and passes the strict fullgraph matrix. The forward keeps separate dense/ragged ops as a measured eager-dispatch decision, not a compile requirement.
• Tensors needed only by the backward (autograd tapes, packing metadata) should be op outputs saved by the autograd.Function via ctx.save_for_backward, not part of the wrapper's user-facing return. Unlike register_autograd, the Function is not limited to saving user-visible outputs.
• Bind torch.ops.attn_gym.<op>.default to a module-level name and call that; resolving the torch.ops attribute chain per call adds overhead.

The fake implementation describes output metadata without running the kernel. It must:

• return the same output structure as the real implementation;
• preserve shape, dtype, device, layout, and relevant strides. For outputs the kernel allocates fresh, use x.new_empty(shape) / torch.empty(...), never torch.empty_like(x): empty_like preserves the strides of a dense-but-permuted input while the kernel writes a contiguous result, and Inductor then fails assert_size_stride. Include a transposed parameter (p.t().contiguous().t()) in the opcheck inputs to pin this;
• avoid reading tensor values, storage, data pointers, or calling .item();
• use symbolic shape arithmetic rather than data-dependent Python branches.

For genuinely data-dependent output dimensions, use torch.library.get_ctx() and new_dynamic_size() rather than inspecting input data.

Registration must exist before graph capture. Keep registration deterministic and lightweight; do not rely on registration side effects occurring inside a compiled region. Continue importing optional kernel dependencies lazily.

CuTeDSL layout and alignment contracts

Expose the weakest layout contract the kernel actually needs. Do not require full contiguity when only one mode must be contiguous, and do not treat alignment as a runtime repair mechanism: assumed_align is a promise to codegen that may enable wide loads.

Use the shared helpers in attn_gym._backends.cute:

• tensor_supports_contiguous_dim(tensor, dim=-1, alignment_bytes=...) checks that one mode has stride 1 and that every slice origin satisfies the promised byte alignment;
• make_fake_strided_tensor(dtype, shape, contiguous_dim=-1, ...) creates the matching TVM-FFI fake signature with dynamic other strides and honest minimum alignment;
• tensor_supports_tma(tensor) is the 16-byte CUDA/TMA specialization of the same predicate;
• requires_int64_abi(...) still selects the independent wide-stride/address specialization.

Prefer two measured signatures when vectorization needs stronger assumptions:

1. a compact/aligned fast signature advertising the vector width;
2. a general signature with the required contiguous mode, dynamic outer strides, and only element-size alignment.

Normalize only unsupported layouts. A noncontiguous required mode may need a compact copy, but an outer-strided or misaligned tensor belongs on the general signature if the generated loads are safe. Note that .contiguous() returns an already-contiguous tensor unchanged and therefore does not fix a misaligned storage offset.

For every new CuTe tensor ABI, test:

• compact aligned input selects the fast path;
• arbitrary aligned outer strides with the required contiguous mode;
• a noncontiguous required mode and its documented fallback or rejection;
• contiguous storage with a deliberately misaligned offset;
• partial logical tiles (odd token counts and non-divisible head/channel groups);
• every supported dtype, fake/opcheck, dynamic fullgraph reuse, and forced-int64 equivalence.

Express views through native slicing and local_tile/partition algebra. Reconstruct a tensor from .iterator only for a deliberate storage reinterpretation such as a TMA mode permutation, and explain that contract at the call site.

Mutation and aliasing

Declare every mutated argument in the schema string:

torch.library.define("attn_gym::update_state", "(Tensor(a!) state, Tensor value) -> ()")

Do not declare an operator functional if its launcher writes into an input, including recurrent state, cache, workspace, or output buffers. Prefer functional operators when practical.

Autograd

Training backends attach autograd with a torch.autograd.Function outside the registered ops, as in the pattern above: forward calls the forward op (autograd is already disabled inside Function.forward, so no redispatch guard is needed), backward calls the backward op, and the public API routes through Function.apply. Notes:

• Mark first-order-only backwards with @torch.autograd.function.once_differentiable.
• Return one gradient entry per forward input and None for nondifferentiable inputs.
• Use ctx.mark_non_differentiable(...) for auxiliary outputs and ctx.set_materialize_grads(False) when the backward handles None grads.
• The raw forward op is not differentiable; direct torch.ops calls that backprop through it warn and produce no gradient. Route all differentiable use through the wrapper.
• Do not use CustomOpDef.register_autograd (slowest measured path) or an Autograd dispatch-key kernel (taxes every inference call) unless raw op calls must be differentiable.
• If training is unsupported, reject requiring-gradient inputs clearly instead of silently detaching them.

Compiler-visible Triton pattern

import torch
import triton
import triton.language as tl
from torch import Tensor


@triton.jit
def add_kernel(x_ptr, y_ptr, output_ptr, size, BLOCK: tl.constexpr):
    block = tl.program_id(0)
    offsets = block * BLOCK + tl.arange(0, BLOCK)
    mask = offsets < size
    x = tl.load(x_ptr + offsets, mask=mask)
    y = tl.load(y_ptr + offsets, mask=mask)
    tl.store(output_ptr + offsets, x + y, mask=mask)


@torch.library.triton_op("attn_gym::add", mutates_args={})
def add(x: Tensor, y: Tensor) -> Tensor:
    output = torch.empty_like(x)
    size = x.numel()
    grid = lambda meta: (triton.cdiv(size, meta["BLOCK"]),)
    torch.library.wrap_triton(add_kernel)[grid](
        x,
        y,
        output,
        size,
        BLOCK=256,
    )
    return output

Keep cross-variant Triton infrastructure in attn_gym._backends.triton only after it has multiple callers. Variant-specific kernels, schemas, fake behavior, and autograd remain in the variant backend.

Large-layout address width

Optimized kernels must preserve an int32 default path and select a separate int64 specialization when relative element offsets can exceed signed int32. Inventory every input, output, optional tensor, manually reconstructed view, and manual load/store offset; checking only the primary input or numel() is insufficient.

For Triton, use attn_gym._backends.triton.requires_int64_offsets in a @triton.heuristics USE_INT64_OFFSETS constexpr. It checks reachable storage cosize for the project's nonnegative-strided layouts. In the wide branch, cast program IDs, loaded token/chunk origins, and other address indices to tl.int64 before the first potentially overflowing multiply or addition. Casting a completed offset or pointer is too late. Bounded routing arrays may stay int32, but widen their loaded values before using them in wide pointer arithmetic.

For CuTeDSL TVM-FFI, use attn_gym._backends.cute.requires_int64_abi. In addition to reachable cosize, it checks every declared ABI stride because a size-one dimension can carry an unreachable stride larger than INT32_MAX that the compiled signature must still represent. Carry use_int64_offsets through the op, compile-cache key, and stable kernel name; use matching cute.sym_int64 fake-signature fields in the wide variant. Widen values before multiplication when constructing layouts or adding to iterators.

Required validation has three layers:

1. Predicate tests on meta/fake tensors, including an oversized singleton stride without allocating the unreachable storage.
2. Forced-int64 equivalence with the normal int32 specialization on small forward and backward inputs.
3. When hardware memory permits, execution of an active offset beyond INT32_MAX, compared with an equivalent compact layout. Singleton-stride tests prove ABI routing but not wide device pointer arithmetic.

Also assert ordinary layouts select int32 and measure both variants before claiming no regression; int64 address arithmetic can increase instructions and registers. Use test/test_kda_int64_offsets.py as the project reference.

Validate registration with opcheck

torch.library.opcheck validates operator registration. Its default utilities are:

• test_schema: runtime mutation and aliasing agree with the schema;
• test_autograd_registration: autograd is registered correctly;
• test_faketensor: fake execution matches real output metadata;
• test_aot_dispatch_dynamic: AOT dispatch works with dynamic shapes.

Run it through pytest after importing the registration module:

import pytest
import torch


OPCHECK_UTILITIES = (
    "test_schema",
    "test_autograd_registration",
    "test_faketensor",
    "test_aot_dispatch_dynamic",
)


@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16])
@pytest.mark.parametrize("requires_grad", [False, True])
def test_example_forward_registration(dtype, requires_grad):
    query, key, value = make_inputs(dtype=dtype, requires_grad=requires_grad)
    torch.library.opcheck(
        example_forward,
        (query, key, value),
        test_utils=OPCHECK_UTILITIES,
    )

Pass keyword arguments as the third argument:

torch.library.opcheck(example_forward, args, {"causal": True})

Create separate cases for materially different registration paths:

• each supported device and dtype;
• inference and gradient-requiring inputs;
• optional arguments and outputs;
• contiguous and intentionally supported non-contiguous layouts;
• boundary, non-aligned, and zero-size shapes when supported;
• mutable inputs;
• dynamic dimensions.

Keep raise_exception=True, the default, in committed tests. Use raise_exception=False only while diagnosing multiple failures:

results = torch.library.opcheck(example_forward, args, raise_exception=False)
for utility, result in results.items():
    print(utility, result)

opcheck does not validate numerical correctness. A kernel can pass every utility and still compute wrong values. Pair it with reference forward tests and gradient tests.

For unordered index outputs, AOT opcheck's exact tensor comparison can reject a valid permutation. Keep all utilities on a stable-output case (for example, selecting every candidate), then test the selective production path under fullgraph/dynamic compilation and replay with range, uniqueness, valid-count and reference-score checks. Do not widen integer tolerances or require deterministic ordering merely to satisfy opcheck when ordering and boundary tie choices are unspecified.

Numerical correctness

Every optimized backend needs a trusted eager/reference oracle. Compare:

• outputs;
• final recurrent state;
• gradients for every differentiable input and initial state;
• segmented state-carrying execution against full-sequence execution.

Use torch.autograd.gradcheck when double precision is supported:

def test_example_forward_gradcheck():
    inputs = make_inputs(dtype=torch.double, requires_grad=True)
    assert torch.autograd.gradcheck(example_forward, inputs)

For lower-precision-only GPU kernels, compare gradients against the reference with explicit atol/rtol values justified by dtype and reduction order.

Compile the public operation

Compile the documented function with strict graph capture:

def test_compiled_forward_and_backward():
    eager_inputs = make_inputs(requires_grad=True)
    compiled_inputs = clone_inputs(eager_inputs)

    expected = operation(*eager_inputs, backend="triton")
    compiled_operation = torch.compile(operation, fullgraph=True)
    actual = compiled_operation(*compiled_inputs, backend="triton")

    torch.testing.assert_close(actual.output, expected.output, atol=ATOL, rtol=RTOL)
    torch.testing.assert_close(
        actual.final_state,
        expected.final_state,
        atol=ATOL,
        rtol=RTOL,
    )

    expected_gradients = torch.autograd.grad(loss(expected), eager_inputs)
    actual_gradients = torch.autograd.grad(loss(actual), compiled_inputs)
    for actual_gradient, expected_gradient in zip(actual_gradients, expected_gradients):
        torch.testing.assert_close(
            actual_gradient,
            expected_gradient,
            atol=GRAD_ATOL,
            rtol=GRAD_RTOL,
        )

Include every mode that selects a materially different implementation, initial state present and absent, and final state requested and omitted. A graph break under fullgraph=True fails the advertised compilation contract.

Dynamic shapes

If dynamic shapes are supported, compile once and reuse the callable across multiple sizes:

compiled_operation = torch.compile(operation, fullgraph=True, dynamic=True)
for sequence_length in (127, 193):
    inputs = make_inputs(sequence_length)
    expected = operation(*inputs)
    actual = compiled_operation(*clone_inputs(inputs))
    assert_outputs_close(actual, expected)

Use TORCH_LOGS="recompiles" to verify dimensions advertised as dynamic are reused rather than silently specialized.

CUDA Graphs

When CUDA Graph support is claimed:

1. Compile, autotune, and warm up before capture.
2. Allocate static inputs and outputs before capture.
3. Ensure capture performs no lazy compilation, allocation, CPU synchronization, or data-dependent host work.
4. Capture and replay representative static-shape calls.
5. Compare replayed outputs and final state against eager execution.

Do not infer CUDA Graph compatibility from a successful torch.compile call.

Failure triage

• Graph break: run with TORCH_LOGS="graph_breaks,recompiles"; locate whether validation, dispatch, registration, or the backend launcher caused it.
• FakeTensor mismatch: compare real and fake metadata field by field; remove data reads and data-dependent Python from fake code.
• Schema failure: inspect mutation and aliasing before changing the schema.
• Autograd registration failure: ensure inputs require gradients, then separately validate numerical gradients.
• AOT dynamic failure: remove Python specialization on symbolic dimensions or narrow the supported dynamic-shape contract.
• Compile-only mismatch: compare eager and compiled calls to the same selected backend before debugging the low-level kernel.

Required completion report

Report these independently:

• reference forward and gradient tests;
• opcheck cases and utilities exercised;
• public torch.compile(fullgraph=True) forward/backward results;
• dynamic sizes tested and recompilation observations;
• CUDA Graph capture/replay results when claimed;
• unsupported contracts or unavailable hardware with exact reasons.

Do not present lint, imports, registration-only checks, or opcheck alone as behavioral correctness.

References

• PyTorch custom Python operators: https://docs.pytorch.org/tutorials/advanced/python_custom_ops.html
• torch.library APIs, including custom_op, register_fake, register_autograd, and opcheck: https://docs.pytorch.org/docs/stable/library.html
• User-defined Triton kernels with torch.compile: https://docs.pytorch.org/tutorials/recipes/torch_compile_user_defined_triton_kernel_tutorial.html