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# Backends & GPU Support

`opt_einsum` is largely agnostic to the type of n-dimensional arrays (tensors) it uses, since finding the contraction path only relies on getting the shape attribute of each array supplied. It can perform the underlying tensor contractions with various libraries. In fact, any library that provides a `numpy.tensordot` and `numpy.transpose` implementation can perform most normal contractions. However, certain special functionalities such as axes reduction are reliant on a `numpy.einsum` implementation. The following is a brief overview of libraries which have been tested with `opt_einsum`:

- tensorflow: compiled tensor expressions that can run on GPU.
- theano: compiled tensor expressions that can run on GPU.
- cupy: numpy-like api for GPU tensors.
- dask: larger-than-memory tensor computations, distributed scheduling, and potential reuse of intermediaries.
- sparse: sparse tensors.
- pytorch: numpy-like api for GPU tensors.
- autograd: automatic derivative computation for tensor expressions
- jax: compiled GPU tensor expressions including `autograd`-like functionality

Note

For a contraction to be possible without using a backend einsum, it must satisfy the following rule: in the full expression (including output indices) each index must appear twice. In other words, each dimension must be either contracted with one other dimension or left alone.

## Backend agnostic contractions

The automatic backend detection will be detected based on the first supplied array (default), this can be overridden by specifying the correct `backend` argument for the type of arrays supplied when calling `opt_einsum.contract`. For example, if you had a library installed called `'foo'` which provided an `numpy.ndarray` like object with a `.shape` attribute as well as `foo.tensordot` and `foo.transpose` then you could contract them with something like:

    contract(einsum_str, *foo_arrays, backend='foo')

Behind the scenes `opt_einsum` will find the contraction path, perform pairwise contractions using e.g. `foo.tensordot` and finally return the canonical type those functions return.

### Dask

dask is an example of a library which satisfies these requirements. For example:

    import opt_einsum as oe
    import dask.array as da
    shapes = (3, 200), (200, 300), (300, 4)
    dxs = [da.random.normal(0, 1, shp, chunks=(100, 100)) for shp in shapes]
    dxs
    #> [dask.array<da.random.normal, shape=(3, 200), dtype=float64, chunksize=(3, 100)>,
    #>  dask.array<da.random.normal, shape=(200, 300), dtype=float64, chunksize=(100, 100)>,
    #>  dask.array<da.random.normal, shape=(300, 4), dtype=float64, chunksize=(100, 4)>]

    dy = oe.contract("ab,bc,cd", *dxs)  # will infer backend='dask'
    dy
    #> dask.array<transpose, shape=(3, 4), dtype=float64, chunksize=(3, 4)>

    dy.compute()
    #> array([[ 470.71404665,    2.44931372,  -28.47577265,  424.37716615],
    #>        [  64.38328345, -287.40753131,  144.46515642,  324.88169821],
    #>        [-142.07153553, -180.41739259,  125.0973783 , -239.16754541]])

In this case, dask arrays in = dask array out, since dask arrays have a shape attribute, and `opt_einsum` can find `dask.array.tensordot` and `dask.array.transpose`.

### Sparse

The sparse library also fits the requirements and is supported. An example:

    import sparse as sp
    shapes = (3, 200), (200, 300), (300, 4)
    sxs = [sp.random(shp) for shp in shapes]
    sxs
    #> [<COO: shape=(3, 200), dtype=float64, nnz=6, sorted=False, duplicates=True>,
    #>  <COO: shape=(200, 300), dtype=float64, nnz=600, sorted=False, duplicates=True>,
    #>  <COO: shape=(300, 4), dtype=float64, nnz=12, sorted=False, duplicates=True>]

    oe.contract("ab,bc,cd", *sxs)
    #> <COO: shape=(3, 4), dtype=float64, nnz=0, sorted=False, duplicates=False>

### Autograd

The autograd library is a drop-in for `numpy` that can automatically compute the gradients of array expressions. `opt_einsum` automatically dispatches the `autograd` arrays correctly, enabling a simple way to compute gradients of tensor contractions:

    import numpy as np
    import autograd
    shapes = [(2, 3), (3, 4), (4, 2)]
    x, y, z = [np.random.rand(*s) for s in shapes]

    # make single arg function as autograd takes derivative of first arg
    def foo(xyz):
       return oe.contract('ij,jk,ki->', *xyz)

    foo([x, y, z])
    #> array(4.90422159)

    # wrap foo with autograd to compute gradients instead
    dfoo = autograd.grad(foo)
    dx, dy, dz = dfoo(arrays)
    dx, dy, dz
    #> (array([[1.10056194, 1.25078356, 1.48211494],
    #>         [1.38945961, 1.5572077 , 1.65234003]]),
    #>  array([[0.41710717, 0.63202881, 0.84573502, 0.95069975],
    #>         [0.42706777, 0.73630994, 0.99328938, 0.77415267],
    #>         [0.40773334, 0.61693475, 0.82545726, 0.93132302]]),
    #>  array([[0.78747828, 1.28979012],
    #>         [1.26051133, 1.48835538],
    #>         [0.46896666, 0.55003072],
    #>         [1.10840828, 1.16722494]]))

### Jax

jax is itself a drop-in for `autograd`, that additionally uses XLA to compile the expressions, particularly for the GPU. Using it with `opt_einsum` is very simple:

    import jax
    # generate a compiled version of the above function
    jit_foo = jax.jit(foo)
    jit_foo([x, y, z])
    #> DeviceArray(4.9042215, dtype=float32)

    # generate a compiled version of the gradient function
    jit_dfoo = jax.jit(jax.grad(foo))
    jit_dfoo([x, y, z])
    #> [DeviceArray([[1.10056198, 1.25078356, 1.48211491],
    #>               [1.38945973, 1.5572077, 1.65234005]], dtype=float32),
    #>  DeviceArray([[0.41710716, 0.63202882, 0.84573501, 0.95069975],
    #>               [0.42706776, 0.73630995, 0.99328935, 0.7741527 ],
    #>               [0.40773335, 0.61693472, 0.82545722, 0.93132305]],
    #>              dtype=float32),
    #>  DeviceArray([[0.78747827, 1.28979015],
    #>               [1.2605114 , 1.4883554 ],
    #>               [0.46896666, 0.55003077],
    #>               [1.10840821, 1.16722488]], dtype=float32)]

Note

`jax` defaults to converting all arrays to single precision. This behaviour can be changed by running `from jax.config import config; config.update("jax_enable_x64", True)` before it has been imported and used at all.

## Special (GPU) backends for numpy arrays

A particular case is if numpy arrays are required for the input and output, however, a more performant backend is required such as performing the contraction on a GPU. Unless the specified backend works on numpy arrays, this requires converting to and from the backend array type. Currently `opt_einsum` can handle this automatically for:

- tensorflow
- theano
- cupy
- pytorch
- jax

all of which offer GPU support. Since `tensorflow` and `theano` both require compiling the expression, this functionality is encapsulated in generating a `opt_einsum.ContractExpression` using `opt_einsum.contract_expression`, which can then be called using numpy arrays whilst specifying `backend='tensorflow'` etc. Additionally, if arrays are marked as `constant` (see `constants-section`), then these arrays will be kept on the device for optimal performance.

### Theano

If `theano` is installed, using it as backend is as simple as specifying `backend='theano'`:

    shapes = (3, 200), (200, 300), (300, 4)
    expr = oe.contract_expression("ab,bc,cd", *shapes)
    expr
    #> <ContractExpression('ab,bc,cd')>

    import numpy as np
    # GPU advantage mainly for low precision numbers
    xs = [np.random.randn(*shp).astype(np.float32) for shp in shapes]
    expr(*xs, backend='theano')  # might see some fluff on first run
    #> array([[ 129.28352  , -128.00702  , -164.62917  , -335.11682  ],
    #>        [-462.52344  , -121.12657  ,  -67.847626 ,  624.5457   ],
    #>        [   5.2838974,   36.441578 ,   81.62851  ,  703.1576   ]],
    #>       dtype=float32)

Note that you can still supply `theano.tensor.TensorType` directly to `opt_einsum` (with `backend='theano'`), and it will return the relevant `theano` type.

### Tensorflow

To run the expression with tensorflow, you need to register a default session:

    import tensorflow as tf
    sess = tf.Session()

    with sess.as_default():
        out = expr(*xs, backend='tensorflow')

    out
    #> array([[ 129.28357  , -128.00684  , -164.62903  , -335.1167   ],
    #>        [-462.52362  , -121.12659  ,  -67.84769  ,  624.5455   ],
    #>        [   5.2839584,   36.44155  ,   81.62852  ,  703.15784  ]],
    #>       dtype=float32)

Note that you can still supply this expression with, for example, a `tensorflow.placeholder` using `backend='tensorflow'`, and then no conversion would take place, instead you'd get a `tensorflow.Tensor` back.

Version 1.9 of tensorflow also added support for eager execution of computations. If compilation of the contraction expression tensorflow graph is taking a substantial amount of time up then it can be advantageous to use this, especially since tensor contractions are quite compute-bound. This is achieved by running the following snippet:

    import tensorflow as tf
    tf.enable_eager_execution()

After which `opt_einsum` will automatically detect eager mode if `backend='tensorflow'` is supplied to a `opt_einsum.ContractExpression`.

### Pytorch & Cupy

Both pytorch and cupy offer numpy-like, GPU-enabled arrays which execute eagerly rather than requiring any compilation. If they are installed, no steps are required to utilize them other than specifying the `backend` keyword:

    expr(*xs, backend='torch')
    #> array([[ 129.28357  , -128.00684  , -164.62903  , -335.1167   ],
    #>        [-462.52362  , -121.12659  ,  -67.84769  ,  624.5455   ],
    #>        [   5.2839584,   36.44155  ,   81.62852  ,  703.15784  ]],
    #>       dtype=float32)

    expr(*xs, backend='cupy')
    #> array([[ 129.28357  , -128.00684  , -164.62903  , -335.1167   ],
    #>        [-462.52362  , -121.12659  ,  -67.84769  ,  624.5455   ],
    #>        [   5.2839584,   36.44155  ,   81.62852  ,  703.15784  ]],
    #>       dtype=float32)

And as with the other GPU backends, if raw `cupy` or `pytorch` arrays are supplied the returned array will be of the same type, with no conversion to or from `numpy` arrays.

### Jax

jax, as introduced above, can compile tensor functions, in doing so often achieving better performance. `opt_einsum` expressions can handle this behind the scenes, so again just the `backend` keyword needs to be supplied:

    expr(*xs, backend='jax')
    #> array([[ 129.28357  , -128.00684  , -164.62903  , -335.1167   ],
    #>        [-462.52362  , -121.12659  ,  -67.84769  ,  624.5455   ],
    #>        [   5.2839584,   36.44155  ,   81.62852  ,  703.15784  ]],
    #>       dtype=float32)

## Contracting arbitrary objects

There is one more explicit backend that can handle arbitrary arrays of objects, so long the objects themselves just support multiplication and addition ( `__mul__` and `__add__` dunder methods respectively). Use it by supplying `backend='object'`.

For example, imagine we want to perform a contraction of arrays made up of sympy symbols:

    import opt_einsum as oe
    import numpy as np
    import sympy

    # define the symbols
    a, b, c, d, e, f, g, h, i, j, k, l = [sympy.symbols(oe.get_symbol(i)) for i in range(12)]
    a * b + c * d
    𝑑

    # define the tensors (you might explicitly specify `dtype=object`)
    X = np.array([[a, b], [c, d]])
    Y = np.array([[e, f], [g, h]])
    Z = np.array([[i, j], [k, l]])

    # contract the tensors!
    oe.contract('uv,vw,wu->u', X, Y, Z, backend='object')
    # array([i*(a*e + b*g) + k*(a*f + b*h), j*(c*e + d*g) + l*(c*f + d*h)],
    #       dtype=object)

There are a few things to note here:

- The returned array is a `numpy.ndarray` but since it has `dtype=object` it can really hold any python objects
- We had to explicitly use `backend='object'`, since `numpy.einsum` would have otherwise been dispatched to, which can't handle `dtype=object` (though `numpy.tensordot` in fact can)
- Although an optimized pairwise contraction order is used, the looping in each single contraction is performed in python so performance will be drastically lower than for numeric dtypes!