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Extensions (#962)

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* start to fix extensions

* mostly fixed extensions

* fix extension build

* couple more nits
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Awni Hannun
2024-04-09 08:50:36 -07:00
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@@ -1,24 +1,16 @@
Developer Documentation
=======================
MLX provides a open and flexible backend to which users may add operations
and specialized implementations without much hassle. While the library supplies
efficient operations that can be used and composed for any number of
applications, there may arise cases where new functionalities or highly
optimized implementations are needed. For such cases, you may design and
implement your own operations that link to and build on top of :mod:`mlx.core`.
We will introduce the inner-workings of MLX and go over a simple example to
learn the steps involved in adding new operations to MLX with your own CPU
and GPU implementations.
You can extend MLX with custom operations on the CPU or GPU. This guide
explains how to do that with a simple example.
Introducing the Example
-----------------------
Let's say that you would like an operation that takes in two arrays,
``x`` and ``y``, scales them both by some coefficients ``alpha`` and ``beta``
respectively, and then adds them together to get the result
``z = alpha * x + beta * y``. Well, you can very easily do that by just
writing out a function as follows:
Let's say you would like an operation that takes in two arrays, ``x`` and
``y``, scales them both by coefficients ``alpha`` and ``beta`` respectively,
and then adds them together to get the result ``z = alpha * x + beta * y``.
You can do that in MLX directly:
.. code-block:: python
@@ -27,44 +19,35 @@ writing out a function as follows:
def simple_axpby(x: mx.array, y: mx.array, alpha: float, beta: float) -> mx.array:
return alpha * x + beta * y
This function performs that operation while leaving the implementations and
differentiation to MLX.
This function performs that operation while leaving the implementation and
function transformations to MLX.
However, you work with vector math libraries often and realize that the
``axpby`` routine defines the same operation ``Y = (alpha * X) + (beta * Y)``.
You would really like the part of your applications that does this operation
on the CPU to be very fast - so you decide that you want it to rely on the
``axpby`` routine provided by the Accelerate_ framework. Continuing to impose
our assumptions on to you, let's also assume that you want to learn how to add
your own implementation for the gradients of your new operation while going
over the ins-and-outs of the MLX framework.
However you may need to customize the underlying implementation, perhaps to
make it faster or for custom differentiation. In this tutorial we will go
through adding custom extensions. It will cover:
Well, what a coincidence! You are in the right place. Over the course of this
example, we will learn:
* The structure of the MLX library from the frontend API to the backend implementations.
* How to implement your own CPU backend that redirects to Accelerate_ when appropriate (and a fallback if needed).
* How to implement your own GPU implementation using metal.
* How to add your own ``vjp`` and ``jvp``.
* How to build your implementations, link them to MLX, and bind them to python.
* The structure of the MLX library.
* Implementing a CPU operation that redirects to Accelerate_ when appropriate.
* Implementing a GPU operation using metal.
* Adding the ``vjp`` and ``jvp`` function transformation.
* Building a custom extension and binding it to python.
Operations and Primitives
-------------------------
In one sentence, operations in MLX build the computation graph, and primitives
provide the rules for evaluation and transformations of said graph. Let's start
by discussing operations in more detail.
Operations in MLX build the computation graph. Primitives provide the rules for
evaluating and transforming the graph. Let's start by discussing operations in
more detail.
Operations
^^^^^^^^^^^
Operations are the frontend functions that operate on arrays. They are defined
in the C++ API (:ref:`cpp_ops`) and then we provide bindings to these
operations in the Python API (:ref:`ops`).
Operations are the front-end functions that operate on arrays. They are defined
in the C++ API (:ref:`cpp_ops`), and the Python API (:ref:`ops`) binds them.
We would like an operation, :meth:`axpby` that takes in two arrays ``x`` and ``y``,
and two scalars, ``alpha`` and ``beta``. This is how we would define it in the
C++ API:
We would like an operation, :meth:`axpby` that takes in two arrays ``x`` and
``y``, and two scalars, ``alpha`` and ``beta``. This is how to define it in
C++:
.. code-block:: C++
@@ -83,10 +66,7 @@ C++ API:
StreamOrDevice s = {} // Stream on which to schedule the operation
);
This operation itself can call other operations within it if needed. So, the
simplest way to go about implementing this operation would be do so in terms
of existing operations.
The simplest way to this operation is in terms of existing operations:
.. code-block:: C++
@@ -100,25 +80,23 @@ of existing operations.
// Scale x and y on the provided stream
auto ax = multiply(array(alpha), x, s);
auto by = multiply(array(beta), y, s);
// Add and return
return add(ax, by, s);
}
However, as we discussed earlier, this is not our goal. The operations themselves
do not contain the implementations that act on the data, nor do they contain the
rules of transformations. Rather, they are an easy to use interface that build
on top of the building blocks we call :class:`Primitive`.
The operations themselves do not contain the implementations that act on the
data, nor do they contain the rules of transformations. Rather, they are an
easy to use interface that use :class:`Primitive` building blocks.
Primitives
^^^^^^^^^^^
A :class:`Primitive` is part of the computation graph of an :class:`array`. It
defines how to create an output given a set of input :class:`array` . Further,
a :class:`Primitive` is a class that contains rules on how it is evaluated
on the CPU or GPU, and how it acts under transformations such as ``vjp`` and
``jvp``. These words on their own can be a bit abstract, so lets take a step
back and go to our example to give ourselves a more concrete image.
A :class:`Primitive` is part of the computation graph of an :class:`array`. It
defines how to create outputs arrays given a input arrays. Further, a
:class:`Primitive` has methods to run on the CPU or GPU and for function
transformations such as ``vjp`` and ``jvp``. Lets go back to our example to be
more concrete:
.. code-block:: C++
@@ -134,11 +112,15 @@ back and go to our example to give ourselves a more concrete image.
* To avoid unnecessary allocations, the evaluation function
* is responsible for allocating space for the array.
*/
void eval_cpu(const std::vector<array>& inputs, array& out) override;
void eval_gpu(const std::vector<array>& inputs, array& out) override;
void eval_cpu(
const std::vector<array>& inputs,
std::vector<array>& outputs) override;
void eval_gpu(
const std::vector<array>& inputs,
std::vector<array>& outputs) override;
/** The Jacobian-vector product. */
array jvp(
std::vector<array> jvp(
const std::vector<array>& primals,
const std::vector<array>& tangents,
const std::vector<int>& argnums) override;
@@ -147,7 +129,8 @@ back and go to our example to give ourselves a more concrete image.
std::vector<array> vjp(
const std::vector<array>& primals,
const array& cotan,
const std::vector<int>& argnums) override;
const std::vector<int>& argnums,
const std::vector<array>& outputs) override;
/**
* The primitive must know how to vectorize itself across
@@ -155,7 +138,7 @@ back and go to our example to give ourselves a more concrete image.
* representing the vectorized computation and the axis which
* corresponds to the output vectorized dimension.
*/
std::pair<array, int> vmap(
virtual std::pair<std::vector<array>, std::vector<int>> vmap(
const std::vector<array>& inputs,
const std::vector<int>& axes) override;
@@ -175,22 +158,22 @@ back and go to our example to give ourselves a more concrete image.
void eval(const std::vector<array>& inputs, array& out);
};
The :class:`Axpby` class derives from the base :class:`Primitive` class and
follows the above demonstrated interface. :class:`Axpby` treats ``alpha`` and
``beta`` as parameters. It then provides implementations of how the array ``out``
is produced given ``inputs`` through :meth:`Axpby::eval_cpu` and
:meth:`Axpby::eval_gpu`. Further, it provides rules of transformations in
:meth:`Axpby::jvp`, :meth:`Axpby::vjp`, and :meth:`Axpby::vmap`.
The :class:`Axpby` class derives from the base :class:`Primitive` class. The
:class:`Axpby` treats ``alpha`` and ``beta`` as parameters. It then provides
implementations of how the output array is produced given the inputs through
:meth:`Axpby::eval_cpu` and :meth:`Axpby::eval_gpu`. It also provides rules
of transformations in :meth:`Axpby::jvp`, :meth:`Axpby::vjp`, and
:meth:`Axpby::vmap`.
Using the Primitives
^^^^^^^^^^^^^^^^^^^^^
Using the Primitive
^^^^^^^^^^^^^^^^^^^
Operations can use this :class:`Primitive` to add a new :class:`array` to
the computation graph. An :class:`array` can be constructed by providing its
data type, shape, the :class:`Primitive` that computes it, and the
:class:`array` inputs that are passed to the primitive.
Operations can use this :class:`Primitive` to add a new :class:`array` to the
computation graph. An :class:`array` can be constructed by providing its data
type, shape, the :class:`Primitive` that computes it, and the :class:`array`
inputs that are passed to the primitive.
Let's re-implement our operation now in terms of our :class:`Axpby` primitive.
Let's reimplement our operation now in terms of our :class:`Axpby` primitive.
.. code-block:: C++
@@ -238,27 +221,26 @@ This operation now handles the following:
Implementing the Primitive
--------------------------
No computation happens when we call the operation alone. In effect, the
operation only builds the computation graph. When we evaluate the output
array, MLX schedules the execution of the computation graph, and calls
:meth:`Axpby::eval_cpu` or :meth:`Axpby::eval_gpu` depending on the
stream/device specified by the user.
No computation happens when we call the operation alone. The operation only
builds the computation graph. When we evaluate the output array, MLX schedules
the execution of the computation graph, and calls :meth:`Axpby::eval_cpu` or
:meth:`Axpby::eval_gpu` depending on the stream/device specified by the user.
.. warning::
When :meth:`Primitive::eval_cpu` or :meth:`Primitive::eval_gpu` are called,
no memory has been allocated for the output array. It falls on the implementation
of these functions to allocate memory as needed
of these functions to allocate memory as needed.
Implementing the CPU Backend
Implementing the CPU Back-end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Let's start by trying to implement a naive and generic version of
:meth:`Axpby::eval_cpu`. We declared this as a private member function of
:class:`Axpby` earlier called :meth:`Axpby::eval`.
Let's start by implementing a naive and generic version of
:meth:`Axpby::eval_cpu`. We declared this as a private member function of
:class:`Axpby` earlier called :meth:`Axpby::eval`.
Our naive method will go over each element of the output array, find the
corresponding input elements of ``x`` and ``y`` and perform the operation
pointwise. This is captured in the templated function :meth:`axpby_impl`.
Our naive method will go over each element of the output array, find the
corresponding input elements of ``x`` and ``y`` and perform the operation
point-wise. This is captured in the templated function :meth:`axpby_impl`.
.. code-block:: C++
@@ -296,19 +278,19 @@ pointwise. This is captured in the templated function :meth:`axpby_impl`.
}
}
Now, we would like our implementation to be able to do this pointwise operation
for all incoming floating point arrays. Accordingly, we add dispatches for
``float32``, ``float16``, ``bfloat16`` and ``complex64``. We throw an error
if we encounter an unexpected type.
Our implementation should work for all incoming floating point arrays.
Accordingly, we add dispatches for ``float32``, ``float16``, ``bfloat16`` and
``complex64``. We throw an error if we encounter an unexpected type.
.. code-block:: C++
/** Fall back implementation for evaluation on CPU */
void Axpby::eval(const std::vector<array>& inputs, array& out) {
// Check the inputs (registered in the op while constructing the out array)
assert(inputs.size() == 2);
void Axpby::eval(
const std::vector<array>& inputs,
const std::vector<array>& outputs) {
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Dispatch to the correct dtype
if (out.dtype() == float32) {
@@ -321,28 +303,26 @@ if we encounter an unexpected type.
return axpby_impl<complex64_t>(x, y, out, alpha_, beta_);
} else {
throw std::runtime_error(
"Axpby is only supported for floating point types.");
"[Axpby] Only supports floating point types.");
}
}
We have a fallback implementation! Now, to do what we are really here to do.
Remember we wanted to use the ``axpby`` routine provided by the Accelerate_
framework? Well, there are 3 complications to keep in mind:
This is good as a fallback implementation. We can use the ``axpby`` routine
provided by the Accelerate_ framework for a faster implementation in certain
cases:
#. Accelerate does not provide implementations of ``axpby`` for half precision
floats. We can only direct to it for ``float32`` types
#. Accelerate assumes the inputs ``x`` and ``y`` are contiguous and all elements
have fixed strides between them. Possibly due to broadcasts and transposes,
we aren't guaranteed that the inputs fit this requirement. We can
only direct to Accelerate if both ``x`` and ``y`` are row contiguous or
column contiguous.
#. Accelerate performs the routine ``Y = (alpha * X) + (beta * Y)`` inplace.
MLX expects to write out the answer to a new array. We must copy the elements
of ``y`` into the output array and use that as an input to ``axpby``
floats. We can only use it for ``float32`` types.
#. Accelerate assumes the inputs ``x`` and ``y`` are contiguous and all
elements have fixed strides between them. We only direct to Accelerate
if both ``x`` and ``y`` are row contiguous or column contiguous.
#. Accelerate performs the routine ``Y = (alpha * X) + (beta * Y)`` in-place.
MLX expects to write the output to a new array. We must copy the elements
of ``y`` into the output and use that as an input to ``axpby``.
Let's write out an implementation that uses Accelerate in the right conditions.
It must simply allocate data for the output, copy elements of ``y`` into it,
and then call the :meth:`catlas_saxpby` from accelerate.
Let's write an implementation that uses Accelerate in the right conditions.
It allocates data for the output, copies ``y`` into it, and then calls the
:func:`catlas_saxpby` from accelerate.
.. code-block:: C++
@@ -356,17 +336,7 @@ and then call the :meth:`catlas_saxpby` from accelerate.
// Accelerate library provides catlas_saxpby which does
// Y = (alpha * X) + (beta * Y) in place
// To use it, we first copy the data in y over to the output array
// This specialization requires both x and y be contiguous in the same mode
// i.e: corresponding linear indices in both point to corresponding elements
// The data in the output array is allocated to match the strides in y
// such that x, y, and out are contiguous in the same mode and
// no transposition is needed
out.set_data(
allocator::malloc_or_wait(y.data_size() * out.itemsize()),
y.data_size(),
y.strides(),
y.flags());
out.set_data(allocator::malloc_or_wait(out.nbytes()));
// We then copy over the elements using the contiguous vector specialization
copy_inplace(y, out, CopyType::Vector);
@@ -389,18 +359,20 @@ and then call the :meth:`catlas_saxpby` from accelerate.
/* INCY = */ 1);
}
Great! But what about the inputs that do not fit the criteria for accelerate?
Luckily, we can always just direct back to :meth:`Axpby::eval`.
With this in mind, lets finally implement our :meth:`Axpby::eval_cpu`.
For inputs that do not fit the criteria for accelerate, we fall back to
:meth:`Axpby::eval`. With this in mind, let's finish our
:meth:`Axpby::eval_cpu`.
.. code-block:: C++
/** Evaluate primitive on CPU using accelerate specializations */
void Axpby::eval_cpu(const std::vector<array>& inputs, array& out) {
void Axpby::eval_cpu(
const std::vector<array>& inputs,
const std::vector<array>& outputs) {
assert(inputs.size() == 2);
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Accelerate specialization for contiguous single precision float arrays
if (out.dtype() == float32 &&
@@ -410,35 +382,33 @@ With this in mind, lets finally implement our :meth:`Axpby::eval_cpu`.
return;
}
// Fall back to common backend if specializations are not available
eval(inputs, out);
// Fall back to common back-end if specializations are not available
eval(inputs, outputs);
}
We have now hit a milestone! Just this much is enough to run the operation
:meth:`axpby` on a CPU stream!
Just this much is enough to run the operation :meth:`axpby` on a CPU stream! If
you do not plan on running the operation on the GPU or using transforms on
computation graphs that contain :class:`Axpby`, you can stop implementing the
primitive here and enjoy the speed-ups you get from the Accelerate library.
If you do not plan on running the operation on the GPU or using transforms on
computation graphs that contain :class:`Axpby`, you can stop implementing the
primitive here and enjoy the speed-ups you get from the Accelerate library.
Implementing the GPU Backend
Implementing the GPU Back-end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Apple silicon devices address their GPUs using the Metal_ shading language, and
all GPU kernels in MLX are written using metal.
Apple silicon devices address their GPUs using the Metal_ shading language, and
GPU kernels in MLX are written using Metal.
.. note::
Here are some helpful resources if you are new to metal!
Here are some helpful resources if you are new to Metal:
* A walkthrough of the metal compute pipeline: `Metal Example`_
* Documentation for metal shading language: `Metal Specification`_
* Using metal from C++: `Metal-cpp`_
Let's keep the GPU algorithm simple. We will launch exactly as many threads
as there are elements in the output. Each thread will pick the element it needs
from ``x`` and ``y``, do the pointwise operation, and then update its assigned
element in the output.
Let's keep the GPU kernel simple. We will launch exactly as many threads as
there are elements in the output. Each thread will pick the element it needs
from ``x`` and ``y``, do the point-wise operation, and update its assigned
element in the output.
.. code-block:: C++
@@ -457,15 +427,14 @@ element in the output.
// Convert linear indices to offsets in array
auto x_offset = elem_to_loc(index, shape, x_strides, ndim);
auto y_offset = elem_to_loc(index, shape, y_strides, ndim);
// Do the operation and update the output
out[index] =
out[index] =
static_cast<T>(alpha) * x[x_offset] + static_cast<T>(beta) * y[y_offset];
}
We then need to instantiate this template for all floating point types and give
each instantiation a unique host name so we can identify the right kernel for
each data type.
each instantiation a unique host name so we can identify it.
.. code-block:: C++
@@ -488,29 +457,21 @@ each data type.
instantiate_axpby(bfloat16, bfloat16_t);
instantiate_axpby(complex64, complex64_t);
This kernel will be compiled into a metal library ``mlx_ext.metallib`` as we
will see later in :ref:`Building with CMake`. In the following example, we
assume that the library ``mlx_ext.metallib`` will always be co-located with
the executable/ shared-library calling the :meth:`register_library` function.
The :meth:`register_library` function takes the library's name and potential
path (or in this case, a function that can produce the path of the metal
library) and tries to load that library if it hasn't already been registered
by the relevant static :class:`mlx::core::metal::Device` object. This is why,
it is important to package your C++ library with the metal library. We will
go over this process in more detail later.
The logic to determine the kernel, set the inputs, resolve the grid dimensions
and dispatch it to the GPU are contained in :meth:`Axpby::eval_gpu` as shown
The logic to determine the kernel, set the inputs, resolve the grid dimensions,
and dispatch to the GPU are contained in :meth:`Axpby::eval_gpu` as shown
below.
.. code-block:: C++
/** Evaluate primitive on GPU */
void Axpby::eval_gpu(const std::vector<array>& inputs, array& out) {
void Axpby::eval_gpu(
const std::vector<array>& inputs,
std::vector<array>& outputs) {
// Prepare inputs
assert(inputs.size() == 2);
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Each primitive carries the stream it should execute on
// and each stream carries its device identifiers
@@ -518,10 +479,10 @@ below.
// We get the needed metal device using the stream
auto& d = metal::device(s.device);
// Allocate output memory
// Allocate output memory
out.set_data(allocator::malloc_or_wait(out.nbytes()));
// Resolve name of kernel (corresponds to axpby.metal)
// Resolve name of kernel
std::ostringstream kname;
kname << "axpby_" << "general_" << type_to_name(out);
@@ -552,7 +513,7 @@ below.
compute_encoder->setBytes(&alpha_, sizeof(float), 3);
compute_encoder->setBytes(&beta_, sizeof(float), 4);
// Encode shape, strides and ndim
// Encode shape, strides and ndim
compute_encoder->setBytes(x.shape().data(), ndim * sizeof(int), 5);
compute_encoder->setBytes(x.strides().data(), ndim * sizeof(size_t), 6);
compute_encoder->setBytes(y.strides().data(), ndim * sizeof(size_t), 7);
@@ -575,28 +536,25 @@ below.
We can now call the :meth:`axpby` operation on both the CPU and the GPU!
A few things to note about MLX and metal before moving on. MLX keeps track
of the active ``compute_encoder``. We rely on :meth:`d.get_command_encoder`
to give us the active metal compute command encoder instead of building a
new one and calling :meth:`compute_encoder->end_encoding` at the end.
MLX keeps adding kernels (compute pipelines) to the active command encoder
until some specified limit is hit or the compute encoder needs to be flushed
for synchronization. MLX also handles enqueuing and committing the associated
command buffers as needed. We suggest taking a deeper dive into
:class:`metal::Device` if you would like to study this routine further.
A few things to note about MLX and Metal before moving on. MLX keeps track of
the active ``command_buffer`` and the ``MTLCommandBuffer`` to which it is
associated. We rely on :meth:`d.get_command_encoder` to give us the active
metal compute command encoder instead of building a new one and calling
:meth:`compute_encoder->end_encoding` at the end. MLX adds kernels (compute
pipelines) to the active command buffer until some specified limit is hit or
the command buffer needs to be flushed for synchronization.
Primitive Transforms
^^^^^^^^^^^^^^^^^^^^^
Now that we have come this far, let's also learn how to add implementations to
transformations in a :class:`Primitive`. These transformations can be built on
top of our operations, including the one we just defined now. Which then gives
us the following :meth:`Axpby::jvp` and :meth:`Axpby::vjp` implementations.
Next, let's add implementations for transformations in a :class:`Primitive`.
These transformations can be built on top of other operations, including the
one we just defined:
.. code-block:: C++
/** The Jacobian-vector product. */
array Axpby::jvp(
std::vector<array> Axpby::jvp(
const std::vector<array>& primals,
const std::vector<array>& tangents,
const std::vector<int>& argnums) {
@@ -611,12 +569,12 @@ us the following :meth:`Axpby::jvp` and :meth:`Axpby::vjp` implementations.
if (argnums.size() > 1) {
auto scale = argnums[0] == 0 ? alpha_ : beta_;
auto scale_arr = array(scale, tangents[0].dtype());
return multiply(scale_arr, tangents[0], stream());
return {multiply(scale_arr, tangents[0], stream())};
}
// If, argnums = {0, 1}, we take contributions from both
// which gives us jvp = tangent_x * alpha + tangent_y * beta
else {
return axpby(tangents[0], tangents[1], alpha_, beta_, stream());
return {axpby(tangents[0], tangents[1], alpha_, beta_, stream())};
}
}
@@ -625,34 +583,35 @@ us the following :meth:`Axpby::jvp` and :meth:`Axpby::vjp` implementations.
/** The vector-Jacobian product. */
std::vector<array> Axpby::vjp(
const std::vector<array>& primals,
const array& cotan,
const std::vector<int>& argnums) {
const std::vector<array>& cotangents,
const std::vector<int>& argnums,
const std::vector<int>& /* unused */) {
// Reverse mode diff
std::vector<array> vjps;
for (auto arg : argnums) {
auto scale = arg == 0 ? alpha_ : beta_;
auto scale_arr = array(scale, cotan.dtype());
vjps.push_back(multiply(scale_arr, cotan, stream()));
auto scale_arr = array(scale, cotangents[0].dtype());
vjps.push_back(multiply(scale_arr, cotangents[0], stream()));
}
return vjps;
}
Finally, you need not have a transformation fully defined to start using your
own :class:`Primitive`.
Note, a transformation does not need to be fully defined to start using
the :class:`Primitive`.
.. code-block:: C++
/** Vectorize primitive along given axis */
std::pair<array, int> Axpby::vmap(
std::pair<std::vector<array>, std::vector<int>> Axpby::vmap(
const std::vector<array>& inputs,
const std::vector<int>& axes) {
throw std::runtime_error("Axpby has no vmap implementation.");
throw std::runtime_error("[Axpby] vmap not implemented.");
}
Building and Binding
--------------------
Let's look at the overall directory structure first.
Let's look at the overall directory structure first.
| extensions
| ├── axpby
@@ -666,40 +625,39 @@ Let's look at the overall directory structure first.
| └── setup.py
* ``extensions/axpby/`` defines the C++ extension library
* ``extensions/mlx_sample_extensions`` sets out the structure for the
associated python package
* ``extensions/bindings.cpp`` provides python bindings for our operation
* ``extensions/CMakeLists.txt`` holds CMake rules to build the library and
python bindings
* ``extensions/mlx_sample_extensions`` sets out the structure for the
associated Python package
* ``extensions/bindings.cpp`` provides Python bindings for our operation
* ``extensions/CMakeLists.txt`` holds CMake rules to build the library and
Python bindings
* ``extensions/setup.py`` holds the ``setuptools`` rules to build and install
the python package
the Python package
Binding to Python
^^^^^^^^^^^^^^^^^^
We use PyBind11_ to build a Python API for the C++ library. Since bindings for
We use nanobind_ to build a Python API for the C++ library. Since bindings for
components such as :class:`mlx.core.array`, :class:`mlx.core.stream`, etc. are
already provided, adding our :meth:`axpby` is simple!
already provided, adding our :meth:`axpby` is simple.
.. code-block:: C++
PYBIND11_MODULE(mlx_sample_extensions, m) {
m.doc() = "Sample C++ and metal extensions for MLX";
NB_MODULE(_ext, m) {
m.doc() = "Sample extension for MLX";
m.def(
"axpby",
&axpby,
"x"_a,
"y"_a,
py::pos_only(),
"alpha"_a,
"beta"_a,
py::kw_only(),
"stream"_a = py::none(),
R"pbdoc(
nb::kw_only(),
"stream"_a = nb::none(),
R"(
Scale and sum two vectors element-wise
``z = alpha * x + beta * y``
Follows numpy style broadcasting between ``x`` and ``y``
Inputs are upcasted to floats if needed
@@ -711,17 +669,17 @@ already provided, adding our :meth:`axpby` is simple!
Returns:
array: ``alpha * x + beta * y``
)pbdoc");
)");
}
Most of the complexity in the above example comes from additional bells and
Most of the complexity in the above example comes from additional bells and
whistles such as the literal names and doc-strings.
.. warning::
:mod:`mlx.core` needs to be imported before importing
:mod:`mlx_sample_extensions` as defined by the pybind11 module above to
ensure that the casters for :mod:`mlx.core` components like
:mod:`mlx.core` must be imported before importing
:mod:`mlx_sample_extensions` as defined by the nanobind module above to
ensure that the casters for :mod:`mlx.core` components like
:class:`mlx.core.array` are available.
.. _Building with CMake:
@@ -729,8 +687,8 @@ whistles such as the literal names and doc-strings.
Building with CMake
^^^^^^^^^^^^^^^^^^^^
Building the C++ extension library itself is simple, it only requires that you
``find_package(MLX CONFIG)`` and then link it to your library.
Building the C++ extension library only requires that you ``find_package(MLX
CONFIG)`` and then link it to your library.
.. code-block:: cmake
@@ -752,12 +710,12 @@ Building the C++ extension library itself is simple, it only requires that you
# Link to mlx
target_link_libraries(mlx_ext PUBLIC mlx)
We also need to build the attached metal library. For convenience, we provide a
:meth:`mlx_build_metallib` function that builds a ``.metallib`` target given
sources, headers, destinations, etc. (defined in ``cmake/extension.cmake`` and
automatically imported with MLX package).
We also need to build the attached Metal library. For convenience, we provide a
:meth:`mlx_build_metallib` function that builds a ``.metallib`` target given
sources, headers, destinations, etc. (defined in ``cmake/extension.cmake`` and
automatically imported with MLX package).
Here is what that looks like in practice!
Here is what that looks like in practice:
.. code-block:: cmake
@@ -779,27 +737,29 @@ Here is what that looks like in practice!
endif()
Finally, we build the Pybind11_ bindings
Finally, we build the nanobind_ bindings
.. code-block:: cmake
pybind11_add_module(
mlx_sample_extensions
${CMAKE_CURRENT_LIST_DIR}/bindings.cpp
nanobind_add_module(
_ext
NB_STATIC STABLE_ABI LTO NOMINSIZE
NB_DOMAIN mlx
${CMAKE_CURRENT_LIST_DIR}/bindings.cpp
)
target_link_libraries(mlx_sample_extensions PRIVATE mlx_ext)
target_link_libraries(_ext PRIVATE mlx_ext)
if(BUILD_SHARED_LIBS)
target_link_options(mlx_sample_extensions PRIVATE -Wl,-rpath,@loader_path)
target_link_options(_ext PRIVATE -Wl,-rpath,@loader_path)
endif()
Building with ``setuptools``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Once we have set out the CMake build rules as described above, we can use the
build utilities defined in :mod:`mlx.extension` for a simple build process.
build utilities defined in :mod:`mlx.extension`:
.. code-block:: python
.. code-block:: python
from mlx import extension
from setuptools import setup
@@ -809,48 +769,50 @@ build utilities defined in :mod:`mlx.extension` for a simple build process.
name="mlx_sample_extensions",
version="0.0.0",
description="Sample C++ and Metal extensions for MLX primitives.",
ext_modules=[extension.CMakeExtension("mlx_sample_extensions")],
ext_modules=[extension.CMakeExtension("mlx_sample_extensions._ext")],
cmdclass={"build_ext": extension.CMakeBuild},
packages = ["mlx_sample_extensions"],
package_dir = {"": "mlx_sample_extensions"},
package_data = {"mlx_sample_extensions" : ["*.so", "*.dylib", "*.metallib"]},
packages=["mlx_sample_extensions"],
package_data={"mlx_sample_extensions": ["*.so", "*.dylib", "*.metallib"]},
extras_require={"dev":[]},
zip_safe=False,
python_requires=">=3.7",
python_requires=">=3.8",
)
.. note::
We treat ``extensions/mlx_sample_extensions`` as the package directory
even though it only contains a ``__init__.py`` to ensure the following:
* :mod:`mlx.core` is always imported before importing :mod:`mlx_sample_extensions`
* The C++ extension library and the metal library are co-located with the python
bindings and copied together if the package is installed
You can build inplace for development using
* :mod:`mlx.core` must be imported before importing :mod:`_ext`
* The C++ extension library and the metal library are co-located with the python
bindings and copied together if the package is installed
To build the package, first install the build dependencies with ``pip install
-r requirements.txt``. You can then build inplace for development using
``python setup.py build_ext -j8 --inplace`` (in ``extensions/``)
This will result in a directory structure as follows:
This results in the directory structure:
| extensions
| ├── mlx_sample_extensions
| │ ├── __init__.py
| │ ├── libmlx_ext.dylib # C++ extension library
| │ ├── mlx_ext.metallib # Metal library
| │ └── mlx_sample_extensions.cpython-3x-darwin.so # Python Binding
| │ └── _ext.cpython-3x-darwin.so # Python Binding
| ...
When you try to install using the command ``python -m pip install .``
(in ``extensions/``), the package will be installed with the same structure as
``extensions/mlx_sample_extensions`` and the C++ and metal library will be
copied along with the python binding since they are specified as ``package_data``.
When you try to install using the command ``python -m pip install .`` (in
``extensions/``), the package will be installed with the same structure as
``extensions/mlx_sample_extensions`` and the C++ and Metal library will be
copied along with the Python binding since they are specified as
``package_data``.
Usage
-----
After installing the extension as described above, you should be able to simply
import the python package and play with it as you would any other MLX operation!
After installing the extension as described above, you should be able to simply
import the Python package and play with it as you would any other MLX operation.
Let's looks at a simple script and it's results!
Let's look at a simple script and its results:
.. code-block:: python
@@ -874,12 +836,12 @@ Output:
c correctness: True
Results
^^^^^^^^^^^^^^^^
^^^^^^^
Let's run a quick benchmark and see how our new ``axpby`` operation compares
with the naive :meth:`simple_axpby` we defined at first on the CPU.
Let's run a quick benchmark and see how our new ``axpby`` operation compares
with the naive :meth:`simple_axpby` we first defined on the CPU.
.. code-block:: python
.. code-block:: python
import mlx.core as mx
from mlx_sample_extensions import axpby
@@ -898,7 +860,7 @@ with the naive :meth:`simple_axpby` we defined at first on the CPU.
alpha = 4.0
beta = 2.0
mx.eval((x, y))
mx.eval(x, y)
def bench(f):
# Warm up
@@ -919,30 +881,23 @@ with the naive :meth:`simple_axpby` we defined at first on the CPU.
print(f"Simple axpby: {simple_time:.3f} s | Custom axpby: {custom_time:.3f} s")
Results:
.. code-block::
Simple axpby: 0.114 s | Custom axpby: 0.109 s
We see some modest improvements right away!
The results are ``Simple axpby: 0.114 s | Custom axpby: 0.109 s``. We see
modest improvements right away!
This operation is now good to be used to build other operations, in
:class:`mlx.nn.Module` calls, and also as a part of graph transformations like
:meth:`grad`!
:meth:`grad`.
Scripts
-------
.. admonition:: Download the code
The full example code is available in `mlx <code>`_.
.. code: `https://github.com/ml-explore/mlx/tree/main/examples/extensions/`_
The full example code is available in `mlx <https://github.com/ml-explore/mlx/tree/main/examples/extensions/>`_.
.. _Accelerate: https://developer.apple.com/documentation/accelerate/blas?language=objc
.. _Metal: https://developer.apple.com/documentation/metal?language=objc
.. _Metal-cpp: https://developer.apple.com/metal/cpp/
.. _`Metal Specification`: https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf
.. _`Metal Example`: https://developer.apple.com/documentation/metal/performing_calculations_on_a_gpu?language=objc
.. _PyBind11: https://pybind11.readthedocs.io/en/stable/
.. _nanobind: https://nanobind.readthedocs.io/en/latest/

23
examples/extensions/CMakeLists.txt
View File

@@ -1,6 +1,6 @@
cmake_minimum_required(VERSION 3.27)
project(mlx_sample_extensions LANGUAGES CXX)
project(_ext LANGUAGES CXX)
# ----------------------------- Setup -----------------------------
set(CMAKE_CXX_STANDARD 17)
@@ -11,8 +11,12 @@ option(BUILD_SHARED_LIBS "Build extensions as a shared library" ON)
# ----------------------------- Dependencies -----------------------------
find_package(MLX CONFIG REQUIRED)
find_package(Python COMPONENTS Interpreter Development)
find_package(pybind11 CONFIG REQUIRED)
find_package(Python 3.8 COMPONENTS Interpreter Development.Module REQUIRED)
execute_process(
COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE NB_DIR)
list(APPEND CMAKE_PREFIX_PATH "${NB_DIR}")
find_package(nanobind CONFIG REQUIRED)
# ----------------------------- Extensions -----------------------------
@@ -38,7 +42,6 @@ target_link_libraries(mlx_ext PUBLIC mlx)
# Build metallib
if(MLX_BUILD_METAL)
mlx_build_metallib(
TARGET mlx_ext_metallib
TITLE mlx_ext
@@ -54,13 +57,15 @@ if(MLX_BUILD_METAL)
endif()
# ----------------------------- Pybind -----------------------------
pybind11_add_module(
mlx_sample_extensions
# ----------------------------- Python Bindings -----------------------------
nanobind_add_module(
_ext
NB_STATIC STABLE_ABI LTO NOMINSIZE
NB_DOMAIN mlx
${CMAKE_CURRENT_LIST_DIR}/bindings.cpp
)
target_link_libraries(mlx_sample_extensions PRIVATE mlx_ext)
target_link_libraries(_ext PRIVATE mlx_ext)
if(BUILD_SHARED_LIBS)
target_link_options(mlx_sample_extensions PRIVATE -Wl,-rpath,@loader_path)
target_link_options(_ext PRIVATE -Wl,-rpath,@loader_path)
endif()

18
examples/extensions/README.md Normal file
View File

@@ -0,0 +1,18 @@
## Build the extensions
```
pip install -e .
```
For faster builds during development, you can also pre-install the requirements:
```
pip install -r requirements.txt
```
And then run:
```
python setup.py build_ext -j8 --inplace
```

30
examples/extensions/axpby/axpby.cpp
View File

@@ -1,4 +1,4 @@
// Copyright © 2023 Apple Inc.
// Copyright © 2023-2024 Apple Inc.
#include <cassert>
#include <iostream>
@@ -43,7 +43,7 @@ array axpby(
auto promoted_dtype = promote_types(x.dtype(), y.dtype());
// Upcast to float32 for non-floating point inputs x and y
auto out_dtype = is_floating_point(promoted_dtype)
auto out_dtype = issubdtype(promoted_dtype, float32)
? promoted_dtype
: promote_types(promoted_dtype, float32);
@@ -106,12 +106,12 @@ void axpby_impl(
/** Fall back implementation for evaluation on CPU */
void Axpby::eval(
const std::vector<array>& inputs,
std::vector<array>& out_arr) {
auto out = out_arr[0];
std::vector<array>& outputs) {
// Check the inputs (registered in the op while constructing the out array)
assert(inputs.size() == 2);
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Dispatch to the correct dtype
if (out.dtype() == float32) {
@@ -150,11 +150,7 @@ void axpby_impl_accelerate(
// The data in the output array is allocated to match the strides in y
// such that x, y, and out are contiguous in the same mode and
// no transposition is needed
out.set_data(
allocator::malloc_or_wait(y.data_size() * out.itemsize()),
y.data_size(),
y.strides(),
y.flags());
out.set_data(allocator::malloc_or_wait(out.nbytes()));
// We then copy over the elements using the contiguous vector specialization
copy_inplace(y, out, CopyType::Vector);
@@ -180,11 +176,11 @@ void axpby_impl_accelerate(
/** Evaluate primitive on CPU using accelerate specializations */
void Axpby::eval_cpu(
const std::vector<array>& inputs,
std::vector<array>& outarr) {
auto out = outarr[0];
std::vector<array>& outputs) {
assert(inputs.size() == 2);
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Accelerate specialization for contiguous single precision float arrays
if (out.dtype() == float32 &&
@@ -195,7 +191,7 @@ void Axpby::eval_cpu(
}
// Fall back to common backend if specializations are not available
eval(inputs, outarr);
eval(inputs, outputs);
}
#else // Accelerate not available
@@ -203,8 +199,8 @@ void Axpby::eval_cpu(
/** Evaluate primitive on CPU falling back to common backend */
void Axpby::eval_cpu(
const std::vector<array>& inputs,
std::vector<array>& out) {
eval(inputs, out);
const std::vector<array>& outputs) {
eval(inputs, outputs);
}
#endif
@@ -218,12 +214,12 @@ void Axpby::eval_cpu(
/** Evaluate primitive on GPU */
void Axpby::eval_gpu(
const std::vector<array>& inputs,
std::vector<array>& outarr) {
std::vector<array>& outputs) {
// Prepare inputs
auto out = outarr[0];
assert(inputs.size() == 2);
auto& x = inputs[0];
auto& y = inputs[1];
auto& out = outputs[0];
// Each primitive carries the stream it should execute on
// and each stream carries its device identifiers
@@ -372,4 +368,4 @@ bool Axpby::is_equivalent(const Primitive& other) const {
return alpha_ == r_other.alpha_ && beta_ == r_other.beta_;
}
} // namespace mlx::core
} // namespace mlx::core

8
examples/extensions/axpby/axpby.h
View File

@@ -42,9 +42,9 @@ class Axpby : public Primitive {
* To avoid unnecessary allocations, the evaluation function
* is responsible for allocating space for the array.
*/
void eval_cpu(const std::vector<array>& inputs, std::vector<array>& out)
void eval_cpu(const std::vector<array>& inputs, std::vector<array>& outputs)
override;
void eval_gpu(const std::vector<array>& inputs, std::vector<array>& out)
void eval_gpu(const std::vector<array>& inputs, std::vector<array>& outputs)
override;
/** The Jacobian-vector product. */
@@ -83,7 +83,7 @@ class Axpby : public Primitive {
float beta_;
/** Fall back implementation for evaluation on CPU */
void eval(const std::vector<array>& inputs, std::vector<array>& out);
void eval(const std::vector<array>& inputs, std::vector<array>& outputs);
};
} // namespace mlx::core
} // namespace mlx::core

28
examples/extensions/bindings.cpp
View File

@@ -1,31 +1,31 @@
// Copyright © 2023 Apple Inc.
// Copyright © 2023-2024 Apple Inc.
#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <nanobind/nanobind.h>
#include <nanobind/stl/variant.h>
#include "axpby/axpby.h"
namespace py = pybind11;
using namespace py::literals;
namespace nb = nanobind;
using namespace nb::literals;
using namespace mlx::core;
PYBIND11_MODULE(mlx_sample_extensions, m) {
m.doc() = "Sample C++ and metal extensions for MLX";
NB_MODULE(_ext, m) {
m.doc() = "Sample extension for MLX";
m.def(
"axpby",
&axpby,
"x"_a,
"y"_a,
py::pos_only(),
"alpha"_a,
"beta"_a,
py::kw_only(),
"stream"_a = py::none(),
R"pbdoc(
nb::kw_only(),
"stream"_a = nb::none(),
R"(
Scale and sum two vectors element-wise
``z = alpha * x + beta * y``
Follows numpy style broadcasting between ``x`` and ``y``
Inputs are upcasted to floats if needed
@@ -37,5 +37,5 @@ PYBIND11_MODULE(mlx_sample_extensions, m) {
Returns:
array: ``alpha * x + beta * y``
)pbdoc");
}
)");
}

9
examples/extensions/pyproject.toml
View File

@@ -1,3 +1,8 @@
[build-system]
requires = ["setuptools>=42", "pybind11>=2.10", "cmake>=3.24", "mlx @ git+https://github.com/mlx-explore/mlx@main"]
build-backend = "setuptools.build_meta"
requires = [
"setuptools>=42",
"cmake>=3.24",
"mlx>=0.9.0",
"nanobind@git+https://github.com/wjakob/nanobind.git#egg=4148debcf91f5ccab0c3b8d67b5c3cabd61f407f",
]
build-backend = "setuptools.build_meta"

4
examples/extensions/requirements.txt Normal file
View File

@@ -0,0 +1,4 @@
setuptools>=42
cmake>=3.24
mlx>=0.9.0
nanobind@git+https://github.com/wjakob/nanobind.git#egg=4148debcf91f5ccab0c3b8d67b5c3cabd61f407f

6
examples/extensions/setup.py
View File

@@ -1,4 +1,4 @@
# Copyright © 2023 Apple Inc.
# Copyright © 2023-2024 Apple Inc.
from setuptools import setup
@@ -9,11 +9,11 @@ if __name__ == "__main__":
name="mlx_sample_extensions",
version="0.0.0",
description="Sample C++ and Metal extensions for MLX primitives.",
ext_modules=[extension.CMakeExtension("mlx_sample_extensions")],
ext_modules=[extension.CMakeExtension("mlx_sample_extensions._ext")],
cmdclass={"build_ext": extension.CMakeBuild},
packages=["mlx_sample_extensions"],
package_dir={"": "."},
package_data={"mlx_sample_extensions": ["*.so", "*.dylib", "*.metallib"]},
extras_require={"dev": []},
zip_safe=False,
python_requires=">=3.8",
)