ATen "native" functions are the modern mechanism for adding operators and
functions to ATen. Native functions
are declared in native_functions.yaml
and have implementations defined
in one of the cpp
files in this directory.
Like all ATen methods/functions, native functions are made available
from both ATen's C++ and Python APIs. In C++, they are made available
either as methods on Tensor
(t.mymeth()
) and functions in the ATen
namespace (at::myfunc()
). In PyTorch, they are made available as
methods on Variable
or as functions on torch._C._FunctionBase
(it is the user's responsibility to re-exporting these functions in
a more user-facing module.)
The rest of this document describes how to implement an ATen function.
Every native function must have an entry in
native_functions.yaml
. The format can be summarized as:
- func: func_name(ArgType arg0[=default], ArgType arg1[=default], ...) -> Return
variants: function, method
dispatch:
CPU: func_cpu
CUDA: func_cuda
Each component is described in more detail below:
- func: func_name[.overload_name](ArgType arg0[=default], ArgType arg1[=default], ...) -> Return
The func
entry is a string describing the name of the function and its type
signature.
Argument types. These types are permissible as ArgType:
Tensor
. ATensor
argument translates into a C++ argument of typeconst Tensor&
(except when the argument is "inplace"; in this case, it is simplyTensor&
). A trailing?
, as inTensor?
, indicates that the tensor argument is optional and may be omitted by passing c10::nullopt. When a function takes multipleTensor
arguments, these tensors are assumed to be the same type (e.g., if one argument is aFloatTensor
, all other arguments are checked to beFloatTensor
s).Tensor
orTensor?
must sometimes be annotated to indicate aliasing and mutability. In general annotations can be defined via the following four situations:Tensor(a)
-a
is a set of Tensors that may alias to the same data.Tensor(a!)
-a
members of a may be written to thus mutating the underlying data.Tensor!
- shorthand for Tensor(fresh_identifier!)Tensor(a! -> a|b)
- Tensor is in seta
, written to, and after the write is in seta
ANDb
. For more details on when and why this needs to happen, please see the section on annotations.
Tensor[]
. ATensor[]
argument translates into a C++ argument of typeArrayRef<Tensor>
(a.k.a.TensorList
)int[]
.int[]
accepts an optional length specifier, e.g.,int[2]
, which has no effect in C++ but extends our Python bindings to accept a bare number, which will be expanded into an appropriately sized list by repeating the number.int
. Think about this like a Python int. This is translated into a C++ argument of typeint64_t
.float
. Think about this like a Pythonfloat
. It is translated into a C++ argument of typedouble
.bool
str
. It is translated into a C++ argument of typestd::string
(but we should fix this, see pytorch#53546)Scalar
.Scalar
supports binding to any numerical types from Python, including integral types, floating point types, and zero dimensional tensors.int
andfloat
bind to the corresponding Python numerical types. However, you probably don't want to useScalar
;float
andint
argument types should suffice for most algorithms (you should only useScalar
if the operator truly may accept either type).Generator?
, the state for a random number generator,bool[N]
(where N is1-4
).*
is a special sentinel argument, which doesn't translate into an actual argument, but indicates that in the Python bindings, any subsequent arguments must be specified as keyword arguments (and cannot be provided positionally).?
is trailing question mark that annotates an argument to be an optional type. Grep foroptional
to find some example usages. In general, most functions will not need to use this, but there are some cases that we want to use optional for the different types:- You want to pass a
None
to an ATen function/method from Python and handle the None type on the C++ side. For example,clamp(Tensor self, Scalar? min=None, Scalar? max=None)
can takeNone
for itsmin
andmax
parameter, but does not dispatch to different backends if one of the parameters isNone
. Optional type can accept aNone
type (nullopt
in C++) from Python and use the C++ Optional class to interact with the parameters. - You want a default value, which is fine in Python, but would cause ambiguity in C++.
For example,
norm(Tensor self, Scalar p=2, int dim, bool keepdim=False)
would cause ambiguity in C++ since its default args must be adjacent (p
could not have a default value whendim
does not). Therefore, we need to makep
as a optional Scalar, and makep=2
whenp
is not passed in (nullopt). - You want a value to default to the same value as another argument (this cannot be expressed in C++ default arguments).
- You want to pass a
Functions with no tensor inputs are called factory functions, and
are handled specially by code generation. If your function is behaving
differently than another example, check first and see if one is a
factory while another is not. In some rare cases, factory function might have a
tensor argument. In this case mark it with category_override: factory
explicitly.
Argument names. Argument names are meaningful; downstream binding code may make use of the specific
argument name you provide, and a rename of an argument name is considered a BC-breaking
change (e.g., you will probably need to update tools/autograd/derivatives.yaml
at
least, and it may affect Python keyword arguments). For more details please see the section on variants
.
As a convention we use 'out' to indicate an output argument. This aligns with the Python bindings. Even if a function might not be used in the Python bindings, we still advise to follow this convention. Check the generated code when making a change to make sure you're not breaking the API when renaming an argument name of an existing function.
Defaults. Any suffix of arguments can have a default value defined; these default values translate into C++/Python default values which are applied when those positional arguments are not specified.
Here are the supported default values:
- Numbers (e.g.,
0
or5.0
forint
,float
andint[]
with an explicit length (e.g.,int[2]
)--in the case ofint[]
a number is replicated to fill the length (e.g.,int[2] x=2
is equivalent toint[2] x=[2,2]
). - Lists of numbers (e.g.,
[0, 0]
) forIntList
. - Booleans (e.g.,
True
) forbool
. - Empty initializer lists (e.g.,
[]
) forTensor
(this implicitly changes aTensor
argument to accept undefined tensors). None
for pointer types (e.g.,Generator?
)
Returns. The following are permissible on Return:
Non-tuple return:
ReturnType [retarg0]
Tuple return:
(ReturnType [retarg0], ReturnType [retarg1], ...)
The following are permissible on ReturnType:
Tensor
andTensor[]
, which translate into the C++ typesTensor
andstd::vector<Tensor>
, respectively (unless the operation is in-place, in which case the return type isTensor&
.- A tuple of any number of
Tensor
, e.g.,(Tensor, Tensor)
, translating into the C++std::tuple<Tensor, Tensor>
.
If you need a type that is not listed in this list, it may be possible to extend ATen's
code generation to support it. ATen's philosophy on types to support is that it supports
only simple, universal types, as well as a handful of fundamental Tensor structures
(e.g., Tensor
and Generator?
), because these types can be easily ported to any language
bound to ATen (in practice, C++ and Python.)
Return also supports specifying (optional) return argument names. These serve two functions:
-
They let you easily write derivatives in terms of return arguments in
tools/autograd/derivatives.yaml
-
They correspond to the named field the output can be referred to from Python. (This means that changing a return argument name is BC-breaking, be careful!)
Note that argument type modifiers such as defaults and optional are not currently supported on Return.
Overloads. You can register multiple functions with the same name and different function signatures if you give them unique overload names. An overload name is specified after the function name, separated by a dot.
Overload names do not have to be globally unique, but must be unique in the set of all overloads for the same function. Overload names cannot be changed for backwards compatibility reasons. Please try to make overload names semantically meaningful. An overload name that just enumerates all the argument types isn't helpful. In many cases, a semantic name is clear from what the overload is doing differently. As a fallback, you can use the name or type of the first differing argument as an overload name.
If you add a new overload to an existing function, please leave the existing overload names as they are (for backwards compatibility), but give the new overload a new, unique name. Although overload names are not directly used by the Python or C++ APIs, they are public API surface for external backends (who register to specific overload names) and deployed mobile models (which use overload names as part of the serialization format.)
Not specifying an overload name is equivalent to specifying an empty overload name. If you add a new function with multiple overloads, give them unique overload names, at most one overload is allowed to have an empty overload name.
The declarations also support the following attributes.
variants: function, method
Controls whether Tensor method (t.foo()
) or namespace Function (at::foo()
) is
generated as a result of this declaration. If the declaration is a method,
you must have an argument Tensor self
at some position in the method;
in the method variant this argument will be elided from the argument
list. For example, given the declaration where(BoolTensor cond, Tensor self, Tensor other)
,
this generates the function at::where(cond, self, other)
and the method
self.where(cond, other)
.
By default, ATen generates only the function variant for a native function.
When should you also generate a method variant? Tensor operations as methods
are appropriate for "core" Tensor operations (e.g., add, sub, etc.), but not for
more complicated neural network layers (e.g., conv2d
) and internal functions
designed specifically for binding (e.g., cudnn_convolution
).
As we progress along our schema unification of the func
schema with the JIT
signature schema, we must introduce features that allow us to increase compliance.
One of these features are Tensor annotations. As of now we use naming conventions
to indicate whether an argument of a function is going to be mutated and returned.
There are two typical situations in which we mutate the memory of an argument in the Python
frontend:
a) For an inplace operations such as self.abs_()
b) for a function with an output keyword argument such as torch.abs(input, out=None)
.
In order to provide implementations for these Python functions the legacy schema
requires C++ implementations for three situations abs(Tensor self) -> Tensor
,
abs_(Tensor self) -> Tensor
and abs_out(Tensor out, Tensor self) -> Tensor
.
Now, as we move towards the unification, we start to use a different syntax to represent this by using annotations. In the end we still translate to the legacy schema for the downstream consumers such as the C++ code generation, but this will soon change.
If two Tensors carry the same annotation, they both may represent the same memory. A write annotation, as indicated by an exclamation mark, indicates that they both may also be written to.
Let's revisit the previous native function declarations and see the conventions of adding annotations.
abs(Tensor self) -> Tensor
stays the same as it will always allocate new memory.abs_(Tensor(a!) self) -> Tensor(a!)
self
may be written to and returned. Further, the annotation indicates that the return value may alias the input. This indicates an inplace function and by convention ends in a single '_'.abs(Tensor self, *, Tensor(a!) out) -> Tensor(a!)
In the Python frontendout
can be passed as a keyword argument and may be written to. In this case it indicates the schema for a function that must acceptout
as this does not provide a default argument. The idea behind representing this as a optional argument is to document the intended usage. This maps to the legacyabs_out(Tensor out, Tensor self) -> Tensor
. As with the legacy_out
function you must call the argumentTensor out
orTensor out0
,Tensor out1
in the context of multiple arguments.
There is also another situation in which we use annotations, namely views.
transpose(Tensor(a) self, int dim0, int dim1) -> Tensor(a)
An alias to the memory represented byself
may be also returned, however it is not mutated.
We have some asserts to check whether a developer uses these annotations correctly and throw asserts
if she doesn't. For example, any out function must use the (a!)
annotation as described above.
If this causes a lot of confusion please add @cpuhrsch to your PR.
dispatch:
CPU: func_cpu
CUDA: func_cuda
This specifies the actual name of the function you want to dispatch to, so you can dispatch to different functions depending on which backend the passed tensors belong to. If the dispatch table is omitted, we assume a default dispatch table:
# overload is ignored
func: func.overload(...) -> ...
dispatch:
CompositeImplicitAutograd: func
# overload is ignored, but out functions get suffixed with _out in their name
# (NB: no out functions in PyTorch today actually support autograd, but if they
# did, you could call them here and autograd would be inferred)
func: func.out_overload(...) -> ...
dispatch:
CompositeImplicitAutograd: func_out
If two backends have the same dispatch function, you can write CPU, CUDA: func
to reuse the same function name in both cases.
Available backend options can be found by searching dispatch_keys
in
codegen.
There are also two special "generic" backends:
-
CompositeExplicitAutograd
(previously known asDefaultBackend
): implementations of kernels that work for all backends, but require an explicit definition of backward function inderivatives.yaml
to support autograd. The most typical use of this key are for delegating functions; i.e., functions that do a very small amount of work and then delegate to another operator to do the actual heavy lifting. Under the hood, registering a kernel toCompositeExplicitAutograd
is equivalent to registering that kernel to every backend (e.g.,CPU, CUDA
). Note: kernels which call DispatchStub should NOT be registered as CompositeExplicitAutograd, as DispatchStub only works forCPU, CUDA
) -
CompositeImplicitAutograd
(previously known asMath
): implementations of kernels that work for all backends, and also can implicitly support autograd, because all of the operations it calls support autograd. Direct use of this key should be rare: if you provide no dispatch table, we default to registering your kernel asCompositeImplicitAutograd
. Explicitly adding this key to an existing dispatch table may be useful if you have specialized CPU and CUDA implementations, but you might want to provide a fallback lowering for external backends that may not have a specialized implementation.
Functions registered to composite backends should work for any backend, if the nested functions they call work for those backends.
For example, suppose my_op
can be implemented in the following way:
at::Tensor my_op(const Tensor& self, const Tensor& other) {
return self + 2 * other;
}
If we already know inference kernels and derivative formulas for operators +
and *
in our system,
you can just register my_op
to CompositeImplicitAutograd
and both inference & autograd will just work.
Although it seems we only write down the inference formula here, PyTorch autograd system would correctly
set up the backward for my_op
using the chain formula and derivatives of +
& *
operators.
In other words d_out/d_self = 1; d_out/d_other = 2
can be derived automatically from
the my_op
inference kernel. Of course if we don't have derivative formula defined for either +
or *
,
backward of my_op
can no longer be derived automatically.
Whether to use implicit or explicit autograd for your kernel can be decided by the following steps:
- If you can, always start with a
CompositeImplicitAutograd
kernel that's composable from existing operators. - If you don't want to use the derived gradient formula from
CompositeImplicitAutograd
kernel for autograd, either to get better performance or better numerical stability, you should register the kernel withCompositeExplicitAutograd
so that it's only used in inference. Later for autograd, depending on whether your autograd kernel works for all backends or not, you can put them in aliasAutograd
or specific keys likeAutogradCPU
. - If you prefer to write backend-specific kernels, use reserved dispatch keys for your backend instead,
e.g.
CPU/AutogradCPU
.
Important: because a CompositeImplicitAutograd
kernel is implicitly registered for ops with no dispatch:
section,
when you add a backend-specific kernel (and hence a dispatch:
section) to one of these, you must also
add a CompositeImplicitAutograd:
entry that names the old kernel implementation (it's named after the op, with _
added if applicable), so that it's still available for other backends to use.
If you implemented a native function in C++ and want to find out which dispatch keyword should be used in native_functions.yaml, please follow steps in dispatch keywords
device_guard: False
By default, ATen code generation will generate a DeviceGuard invocation, which will ensure that kernel code will run with the current device set to match the device of the first Tensor argument (or first tensor of the first Tensor[] argument, if the function takes a list of tensors). For the most part, this means kernel authors do not have to worry about setting devices.
However, in some cases, setting the device is unnecessary, because,
e.g., you call a function already manages device guard setting, or
you're a function that simply does not interact with any devices. In
that case, code generation of the device guard can be disabled by adding
device_guard: False
to your function definition.
manual_kernel_registration: True
With this flag set, we will not generate code to automatically register the C++ operator implementation
to TypeDefault (catchAll dispatch key) with the dispatcher.
It doesn't make sense to have both dispatch
section and manual_kernel_registration: True
for the same op.
You can find the manual registrations in torch/csrc/autograd/VariableTypeManual.cpp.
Currently ops have this field set to True should match MANUAL_CATCHALL
in tools/autograd/gen_variable_type.py
(It can be a superset of MANUAL_CATCHALL
but we don't have a use case for it).
This field should only be used rarely.
use_const_ref_for_mutable_tensors: True
With this flag set, we will generate arguments for Tensors whose underlying data may change as
const Tensor&
(or similar), just like we would for other Tensors. Previously, we generated these
as Tensor &
, which 1) allowed changing which TensorImpl
the Tensor
itself referred to and 2)
was not necessary to allow the underlying data to change. (This was like using T * const
when we
wanted const T*
.)
Implementations of native functions go in an appropriate C++ file in the
native/
directory (they are organized roughly by topic, but there is no
semantic meaning to their organization aside for the cuda
directory,
which is the only place the build system knows how to build cu
files.)
To write a native function, you only need to write a C++
implementation (no header necessary) with a matching signature to
the generated header from the ATen metadata. There are many
simple native functions; take a look at some of them to see what to do.
Although writing an ATen function is mostly writing the algorithm you want to implement, there are some less obvious details you should also consider.
If you are writing a pair of functions foo
and foo_backward
, with
the intent that foo_backward
implements the derivative of foo
, then
your implementation of foo
is probably not automatically differentiable:
it might make use of functions like data_ptr()
or it dispatches differently
depending on if it's operating on CPU or CUDA tensors. Once you write these two functions,
you will have to write an entry correlating them together in
tools/autograd/derivatives.yaml
.
However, in some situations, you can write a function in ATen and it
will be automatically differentiated! This can be the case if the function implementation
only calls other operations which are themselves differentiable. In this
case, you don't have to write an entry in tools/autograd/derivatives.yaml
.
After writing a native function in C++, it's important to think about which dispatch keyword to use in native_functions.yaml as it gives the dispatcher information about backend and autograd support of the implementation.
Here're steps to follow to decide the right dispatch keyword:
-
Think about inference: does your kernel work for all backends?
-
No: you're likely providing different kernels for different backends, e.g. backend-dependent logic is used in the implementation or it's implemented through DispatchStub. DispatchStub only support a backend if you explicitly provide a kernel through
REGISTER_DISPATCH
. Typically it only supports a few in-tree backends like CPU, CUDA, QuantizedCPU etc but not out-of-tree backends like XLA. Write a dispatch section, enumerate all supported backends and point them to the implementations.dispatch: CPU: kernel_cpu CUDA: kernel_cuda QuantizedCPU: kernel_quantized_cpu
You're done. Now this op will be called in
CPU/CUDA/QuantizedCPU
backend inference!Note: to support training, you're required to write a formula in derivatives.yaml since your backend implementations don't support autograd.
-
Yes: you're likely calling other
at::
ops in the implemetation. Go to step 2.
-
-
Think about training: does your kernel support autograd? check autograd support
-
Yes: in other words, you're providing a
CompositeImplicitAutograd
kernel which supports both inference and autograd. To use autograd support for training, simply skip adding a dispatch section and you're done. This will allow this op to be correctly registered for both inference and training. -
Yes, but you still want to provide a numerically stable gradient formula instead of using autograd, write
dispatch: CompositeExplicitAutograd: kernel
You're done. This op will be called in inference for all backends.
Note: to support training you're required to add a autograd formula, or it'll error out in backward pass when calling with a Tensor has requires_grad=True.
-
No: ops in this category are mainly using
_out
boilerplate where its out version doesn't have a derivative formula defined. For example:Tensor& sign_out(Tensor& result, const Tensor& self) { return unary_op_impl_out(result, self, sign_stub); } Tensor sign(const Tensor& self) { return unary_op_impl(self, at::sign_out); } Tensor& sign_(Tensor& self) { return unary_op_impl_(self, at::sign_out); }
sign_out
uses DispatchStub so the supported backends are enumerated in its dispatch section. Forsign
andsign_
, writedispatch: CompositeExplicitAutograd: kernel
You're done. This op will be called in inference for all backends.
Note: to support training you're required to add an autograd formula for
sign
, or it'll error out in backward pass when calling with a Tensor has requires_grad=True.Note: current plan on record for ops using this boilerplate is to replace
at::
withat::native
in the implementations and add dispatch section with device keywords instead.
-
-
Validate the computed dispatch table matches what you want. You can use
PythonDispatcher
provided in torch/_python_dispatcher.py. It shows for a certain operator, what the computed dispatch table looks like after your registrations.dispatcher = PythonDispatcher() dispatcher.register(["CPU", "XLA", "AutogradCPU", "CompositeImplicitAutograd"]) print(dispatcher.dispatchTable()) # Tells you exactly which kernel is used for certain backend.
-
TODO: AutogradCPUOrCUDA
Note that in native_functions.yaml you can mix using backend keywords and alias keywords above for one op:
- direct registration to backend always has higher precendence than alias
- DO NOT provide multiple alias keywords to the same op: alias keywords have precedence
CompositeExplicitAutograd > CompositeImplicitAutograd
, e.g. adding bothCompositeImplicitAutograd
andCompositeExplicitAutograd
kernels for one op will completely ignoreCompositeImplicitAutograd
kernel for both inference and training. Thus this will trigger an error when native_functions.yaml is parsed.
We don't generate python bindings for all functions. There're certain patterns in function
name that we skip in python binding generation, e.g. *_backward
. Check
tools/autograd/gen_python_functions.py
for the latest rules.
The generated bindings are either exposed as methods on python_variable or functions on
the torch._C._nn (marked with python_module: nn
),
torch._C._fft (marked with python_module: fft
),
torch._C._linalg (marked with python_module: linalg
) objects,
or torch._C._special (marked with python_module: special
) objects.
By default, Tensor
arguments to ATen functions are always defined, unless
you explicitly specified that an undefined tensor was permissible by writing
Tensor?
or Tensor? x=[]
, the latter one is needed when you have to assign
a default value in C++ (e.g. in the middle of other parameters with default values).
The rules for returning undefined Tensors are a bit more subtle, but there is only one case you have to remember:
-
If the function in question is a backward function which accepts a
std::array<bool,N> output_mask
argument, you MUST return an undefinedTensor
at every tuple positioni
for whichoutput_mask[i]
is false, otherwise -
You MUST NOT return an undefined tensor.
The most common situations where you might be tempted to return undefined tensors are when:
-
You have a forward function that may return a buffer if training is enabled, but does not return the buffer in inference mode. In this case, just return an appropriately typed zero-size tensor.
-
You have a backward function where the gradient for an input is zero. In this case, you are expected to create a zero-filled tensor of appropriate size to return for this input. To get the shape, it may be helpful to take a
TensorGeometry
of the input to use.
If you build ATen and get a linker error, that probably means you copy-pasted
the C++ definition of your function incorrectly. Double check your Tensor
arguments, and make sure you wrote const Tensor&
in your signature.