# Native Ops (and DSLs)

The `torch._native` directory provides a place for PyTorch native ops written in python and DSLs, along with utilities to help facilitate this.

# Creating & Registering a native op

**All native ops must be registered to the dispatcher**

When writing native ops, they are required to interact meaningfully with torch's dispatcher, and thus must be registered correctly.

As a further clarification, ops cannot be labelled as `CompositeImplicitAutograd` in `native_functions.yaml`, as-in the op must have an explicit autograd function registered, or at minimum an explicit implementation registered for the same backend as being overridden/added.

Further, we expect the overriding function to take as the first argument `dispatch_keys` (of type `torch.DispatchKeySet`), which is necessary for fallback implementations.

## A Note on Imports

All registrations will happen at the end of `import torch`. It is expected at that point that **no DSL runtime library is loaded by registration code** - this means that the runtime(s) must only be imported lazily. We can still check the presence of a module, and get it's version without importing, but special care must be taken when writing op kernels to not import DSLs too early. An illustrative example is below, using `triton`:

First, we're going to write the registration function, and a top-level call, being very careful to not pull in the `triton` package early:

```
# torch/_native/ops/test_op/triton_impl.py

# NOTE: no triton import in the file

from ... import triton_utils as tu

def calling_fn(dispatch_key, ...):
    # Lazily import the kernels (and triton) on first call
    from .triton_kernels import outer_fn
    torch.library.wrap_triton(outer_fn)(...)

def register_to_dispatch():
    tu.register_op_override(
        "aten",
        "_scaled_mm_v2",
        "CUDA",
        calling_fn,
    )

```

Note the lazy import of kernels inside `calling_fn` - this function isn't called during registration (it just needs to be defined), and on first call it'll import the `triton_kernels` module, with all its dependencies. This kernels module can import anything it wants internally, our restriction on no DSL imports during registration has been met. Quick example of `triton_kernels.py` below:

```
# torch/_native/ops/test_op/triton_kernels.py
import triton

@triton.jit
def inner_fn(...) -> ...:
    # depends internally on triton, triton.language
    pass

@triton.jit
def outer_fn(...) -> ...:
    # depends internally on triton, triton.language
    inner_fn(...)
```


## Registering Implementations to Existing Operators

There are 2 options when interacting with an existing operator:
1. Replace the operator for **all** cases with a new implementation
2. Replace **some subset of functionality** of a given operator with a new implementation, falling-back to the original implementation otherwise.

Both cases are very similar, with 2) only requiring an extra step to obtain the original implementation, and logic to determine which implementation should be run for a given case.

### Replacing an Operator

This follows a simple and standard path, with a good example being the implementation of [FlashAttention v4 (FAv4)](https://github.com/pytorch/pytorch/blob/1f66f34cda5b5ad02d231b90fa0c0de2cb4e02d1/torch/nn/attention/_fa4.py#L67) in torch.

The following example replaces the implementation of `aten._scaled_grouped_mm_v2` on `CUDA` devices:

```
from ... import cutedsl_utils as cu

def my_impl(dispatch_key, ...) -> ...:
    """
    Replacement implementation
    """
    pass

# Override the symbol `aten._scaled_grouped_mm_v2` in this example with the implementation in `my_impl`,
# noting the function signatures must match.
# Replacing an operator completely (no fallback) requires passing the `unconditional_override=True` flag.
def register_kernel_override():
    tu.register_op_override(
        "aten",
        "_scaled_grouped_mm_v2",
        "CUDA",
        my_impl,
        unconditional_override=True
    )
```

### Replacing a Subset of Calls

This time we only want to override the behavior of a subset of `aten._scaled_grouped_mm_v2` calls, and choose whether to invoke our implementation or the original depending on some input arguments. Note that the core of the example -- creating a `torch.library.Library`, and registering our function using `lib.impl(...)` are the same as in [Replacing an Operator](#Replacing-an-Operator).

```
from ... import cutedsl_utils as cu

def my_impl(...) -> ...:
    """
    Replacement implementation - laxy import actual implementation and run it
    """
    from .my_impl_kernel import my_kernel
    return my_kernel(...)


# Note the dispatch_keys argument here - this must be passed as the first argument
# to the fallback kernel.
# Also note that we need the `fallback_kernel` argument to be specified, as
# we this function gets called every operator invocation - the "fallback" is
# actually the currently registered function, which is... this one.
def enable_my_impl(dispatch_keys, arg1, arg2, *args, fallback_kernel, **kwargs):
    # determine if we want to call our implementation
    if arg1 == ... and arg2 == ...:
        return my_impl(arg1, arg2, *args, **kwargs)
    else:
        # Call the fallback
        return fallback_kernel(dispatch_keys,
                               arg1, arg2, *args, **kwargs)

# Override the symbol `aten._scaled_grouped_mm_v2` in this example with the implementation in `my_impl`,
# only when the check-method `enable_my_impl` returns `True`
def register_kernel_override():

    # Get the original implementation for fallback purposes
    fallback_kernel = torch.library.get_kernel("aten::_scaled_grouped_mm_v2", "CUDA")

    # partially-specialize our function so that we're not grabbing the
    # fallback every invocation.
    fn = functools.partial(enable_my_impl, fallback_kernel=fallback_kernel)

    # Same as before
    cu.register_op_override(
        "aten",
        "_scaled_grouped_mm_v2",
        "CUDA",
        fn,
    )
```

## Registering a New Operator

We currently don't have a use for this functionality, please come talk to us if you want it!

# Adding a new DSL

Adding a new DSL is as simple as adding a single helper utils file, then writing your op.

Some universal utilities are provided in `common_utils.py`:
* `check_native_jit_disabled() -> bool` : returns `True` if native DSL ops have been globally disabled.
* `_available_version(package: str) -> tuple[int,int,int] | None` : Gets the installed version of `package` without importing it
* `_unavailable_reasons(deps: list[tuple[str,str]]) -> str | None` : For a list of `(package_name, module_name)` pairs, check (without importing) if the module is available, returning a string describing the mitigation step if it isn't.

## DSL utils file spec

A DSL utils file, named `$dsl_utils.py` (i.e. `cutedsl_utils.py` for `$dsl=cutedsl`) requires three methods to be implemented.

`runtime_available() -> bool` : tell the user if the runtime is available - note that this needs to be available during init, and must be fork-safe. Packages should also not be imported at this time - rely on `importlib.util.find_spec(package_name)` or similar to get the necessary information without importing.

`runtime_version() -> tuple[int, int, int]` : return the `(major, minor, update)` version of the installed package.


```
register_op_override(
    lib_symbol: str,
    op_symbol: str,
    dispatch_key: str,
    implementation_fn: _OpOverrideFn,
    *,
    allow_multiple_override: bool = False,
    unconditional_override: bool = False,
) -> None
```
Register a given implementation to a library - `lib_symbol = "aten"` for most cases, `op_symbol` refers to the library method you wish to override (ex. `"_scaled_grouped_mm_v2"` from above), and dispatch key will generally be one of `("CPU", "CUDA")` depending on what backend you're overriding. For all arguments, please see the comments for `_register_op_override` in [registry.py](registry.py).

An example of an implementation of this spec can be found in [cutedsl_utils.py](cutedsl_utils.py), but please talk to us if you're planning on adding a new DSL.
