Contributing to Thunder

We appreciate your feedback and contributions. If you have feature requests, questions, or want to contribute code or config files, please don't hesitate to use the GitHub Issue tracker.

We welcome all individual contributors, regardless of their level of experience or hardware. Your contributions are valuable, and we are excited to see what you can accomplish in this collaborative and supportive environment.

For a simple general overview of Thunder, we recommend reading inside Thunder first.

Adding operators

Adding operators might be one of the easiest and fun ways to get involved in contributing to Thunder. The operator GitHub Issue tracker provides a great starting point in deciding which operation to work on first.

The subsections below are structured as follows

Primitives, The Core Language, The Torch Language, Language Context describe the hierarchy of operations and abstractions around them in Thunder in general terms.
Adding operations to the Torch executor moves from theory to practice where we inspect real contributors' pull requests.

We recommend reading the document sequentially!

Primitives

The lowest level is the primitive operations, defined in thunder/core/prims.py. Primitive operations, as seen in the Representing operations section, describe all the computation performed, and they are intended to be as simple as possible so that executors like nvFuser find it easy to manipulate them. Thunder's primitives are similar to PyTorch’s primTorch primitives, and are based on JAX’s jax.lax operations.

Primitives have several parts, as defined by the make_prim function. Most importantly they have an id, a name, and a meta function. The meta function performs error checking and maps the metadata (like dtype, shape, device) of inputs to the primitive metadata of outputs. For operations that are part of a class, like the elementwise unary or reduction operations, they often share a common meta function. More unique operations, like slice, define their own meta functions.

The actual execution of primitive operations is handled by executors like nvFuser or PyTorch – more on that in a moment.

Before adding a primitive, check with the team on its design. It might be appropriate to add primitives when necessary to describe the semantics of an operation or to improve the numerical accuracy or speed of operations.

There is a tradeoff with the design of primitive operations one has to keep in mind. On one hand, fewer primitive operations can make program transformation, and execution easier. Fewer primitives means fewer transformation rules – since transformation rules are defined on primitives – and a smaller interface with executors. On the other hand, too few primitive operations may make it hard, or impossible, to express all the operations that users are interested in. Too few primitive operations may also make it difficult to execute programs quickly and numerically accurately.

For example, the expm1 operation can mathematically be defined in terms of the exp and subtraction operations, and so it does not need to be a primitive to enable any functionality. Many libraries, including C++’s standard library, still define an expm1 operation for numerical accuracy, and so expm1 is a primitive in Thunder.

The Core Language

Above the primitives is the core language, or clang. Clang operations are mostly written like any other Python operation. They ultimately call the primitive operations, although they may call other operations before doing so (for example, clang.foo might call clang.bar which calls prims.bar).

Core language operations are intended to be common functionality that’s useful when defining user-facing languages like torch or numpy. Many of these operations are just wrappers around primitive operations. For example, the elementwise binary primitives are as simple as possible, so they don’t perform broadcasting or type promotion. The core language elementwise binary operations, however, do perform broadcasting and type promotion. For example, take a look at the following implementation of add from clang

def _elementwise_binary_wrapper(a, b, *, prim, type_promotion_kind=utils.ELEMENTWISE_TYPE_PROMOTION_KIND.DEFAULT):
    computation_dtype, result_dtype = utils.elementwise_type_promotion(a, b, type_promotion_kind=type_promotion_kind)

    a, b = maybe_broadcast(a, b)
    a, b = maybe_convert_to_dtype(a, computation_dtype), maybe_convert_to_dtype(b, computation_dtype)

    result = prim(a, b)
    result = maybe_convert_to_dtype(result, result_dtype)

    return result

@clangop(method_name="add")
def add(a, b):
    return _elementwise_binary_wrapper(a, b, prim=prims.add)

Before adding a core language operation consider if the functionality expressed is universal enough.

As a style note, operations in Thunder should defer as much error checking as possible. For example, if a primitive’s meta function will perform an error check for X, then the core language operation that calls it should generally not also check for X.

The Torch Language

To translate torch operations into something that Thunder understands we define a torch language. Operations in the torch should reflect the behavior of their corresponding torch operations (small deviations are sometimes OK).

When a program is interpreted, torch operations are remapped into these operations, which ultimately call primitive operations.

Language Context

In the core and torch languages functions are decorated to set a language context and – for torch operations – to describe how to map operations like torch.foo into thunder.torch.foo.

The language context determines what properties and methods tensor objects have. For example, when a + b is written and the first argument is an array or tensor (so, TensorProxy.add is invoked), the language context decides what that addition means. Or when a.size is used, the language context determines what that means (and it’s different in PyTorch and NumPy).

Adding operations to the Torch executor

Now that we are familiar with the hierarchy of operations and the underlying language contexts, let's see some examples of adding operations.

For simplicity, we only cover adding operations to the torch executor. The sections below are meant to be read sequentially.

Adding a primitive

A good example of adding a primitive operation to the torch executor is the PR #136 which adds support for torch.Tensor.unfold.

Let's outline some of its key parts.

Consider the following update to thunder/core/prims.py

SLICE = auto()
SQUEEZE = auto()
TRANSPOSE = auto()
UNFOLD = auto()
VIEW = auto()
# Memory layout prims (Experimental)
STRIDE_ORDER = auto()

def unfold_meta(a: TensorProxy, /, dim: int, size: int, step: int) -> TensorProxy:
    dim = utils.canonicalize_dim(a.ndim, dim)
    max_size = 1 if a.ndim == 0 else a.shape[dim]

    utils.check(
    size <= max_size, lambda: f"Maximum size for tensor at dimension {dim} is {max_size} but size is {size}"
    )
    utils.check(size >= 0, lambda: f"Size is {size} but must be >= 0")
    utils.check(step > 0, lambda: f"Step is {step} but must be > 0")

    shape = list(a.shape)
    shape.append(size)
    shape[dim] = (shape[dim] - size) // step + 1

    return TensorProxy(like=a, shape=shape)

 unfold = make_prim(PrimIDs.UNFOLD, "unfold", meta=unfold_meta, tags=(OpTags.SHAPE_OP,))

The above registers a primitive symbol unfold using make_prim with id=PrimIDs.UNFOLD, name=unfold, and meta=unfold_meta. One can see that unfold_meta follows the signature of the underlying torch.Tensor.unfold operation (so that the primitive is directly modeled after the PyTorch operation) with the only exception of expecting a TensorProxy and not the torch.Tensor as its input. The rest of the function checks the inputs and returns a TensorProxy of the appropriate shape. like=a means that the output will inherit the meta-data like device and dtype from a. The primitive is also tagged with tags=(OpTags.SHAPE_OP,), and, therefore, is associated with shape-based operations. We use tags to additionally group operations for group-specific operation optimizations inside Thunder.

Once the symbol is created, we need to tell Thunder how to execute it. Since we are updating the torch executor, the following lines are added to the executors/torchex.py file

unbind = _register_torch_operation("unbind")
unfold = _register_torch_operation("unfold", module=torch.Tensor)
unsqueeze = _register_torch_operation("unsqueeze")

_register_implementation(prims.transpose, checker=_always_executable, execution_transform=_transpose_prim_transform)
_register_implementation(prims.unfold, unfold, checker=_always_executable)
_register_implementation(prims.view, view, checker=_always_executable)

the first one registers a new symbol that is directly tied to the torch.Tensor.unfold, and the second associates this symbol with prims.unfold upon execution unless the checker fails. Having checker=_always_executable always greenlights this association, and, hence, whenever the torch executor tries to execute prims.unfold, it executes torch.Tensor.unfold. Note, however, that although the checker does have access to the symbol's inputs, it is different from the meta-function. Meta-functions are supposed to only validate inputs and to be executor-agnostic. Checkers, on the other hand, are not meant to check inputs' validity and they are agnosit to executors. As such, they are useful for checking and enabling symbols for specific versions of executors like PyTorch, for example.

The mapping of the prims.unfold symbol to torch.Tensor.unfold is very simple since the inputs to prims.unfold can directly be passed to torch.Tensor.unfold without any additional pre-preprocessing (association between TensorProxy and torch.Tensor is handled automatically by the torch executor). This is not the case with any operation, however, and sometimes the symbol's interface has to undergo a transformation to be compatible with the registered implementation provided by the executor. For example, the following lines from executors/torchex.py

def _full_transform(
    shape: Sequence[int], fill_value: Number, *, device: None | devices.Device, dtype: None | dtypes.dtype
) -> TensorProxy:
    torch_device: None | torch.device = to_torch_device(device)
    torch_dtype: None | torch.dtype = to_torch_dtype(dtype)

    return full(shape, fill_value, device=torch_device, dtype=torch_dtype)

_register_implementation(prims.full, checker=_always_executable, execution_transform=_full_transform)

show us how to accomplish that with the execution_transform argument of _register_implementation where the Thunder meta-data like device, dtype is converted to the corresponding PyTorch meta-data.

Testing the Operation

In the previous section we saw an example of adding a primitive operation. However, it is not guaranteed that the operation performs as expected. We need to test it!

Operators are typically tested by adding an OpInfo for them. See here to better understand how OpInfos work. OpInfo contains metadata describing an operator, a sample input generator, a sample generator for erroneous inputs that is used for testing handling exceptions/meta function correctness, and test directives. It’s used to automatically generate a variety of tests, most importantly tests that verify the operator’s behavior is consistent with its reference implementations.

It is important to determine whether you need to add test_directives in order to skip tests or expect failures of tests.

Skip (pytest.mark.skip): Skips are needed when something is not implemented by an executor or for a device.
Expected Failures (pytest.mark.xfail): Expected failures indicate that an executor has implemented some aspect of an operation but its behavior is incorrect.

An example of OpInfo for prims.unfold from the PR #136 added to thunder/tests/opinfos.py

def unfold_sample_generator(op, device, dtype, requires_grad, **kwargs):
    make = partial(make_tensor, device=device, dtype=dtype, requires_grad=requires_grad)

    cases = (
        ((), 0, 1, 3),
        ((), -1, 0, 5),
        ((0,), 0, 0, 1),
        ((8,), 0, 2, 1),
        ((6, 2), 0, 2, 2),
    )

    for shape, dim, size, step in cases:
        yield SampleInput(make(shape), dim, size, step)

 def unfold_error_generator(op, device, dtype=torch.float32, **kwargs):
     make = partial(make_tensor, device=device, dtype=dtype)

     cases = (
         ((), 0, 2, 1, RuntimeError, "Maximum size for tensor at dimension 0 is 1 but size is 2"),
         ((0,), 0, 0, -1, RuntimeError, "Step is -1 but must be > 0"),
         ((8,), 1, 2, 1, IndexError, r"Dimension out of range \(expected to be in range of \[-1, 0\], but got 1\)"),
         ((8,), 0, -5, 1, RuntimeError, "Size is -5 but must be >= 0"),
         ((8,), 0, 10, 1, RuntimeError, "Maximum size for tensor at dimension 0 is 8 but size is 10"),
     )

     for shape, dim, size, step, err_type, err_msg in cases:
         yield SampleInput(make(shape), dim, size, step), err_type, err_msg

 unfold_opinfo = OpInfo(
     clang.unfold,
     sample_input_generator=unfold_sample_generator,
     error_input_generator=unfold_error_generator,
     torch_reference=torch.Tensor.unfold,
 )

 shape_ops.append(unfold_opinfo)

Note how comprehensive unfold_sample_generator and unfold_error_generator are. unfold_sample_generator does not shy away from testing scalar inputs (shape=()) and empty inputs (shape=(0,), i.e. shapes containing zeros). And unfold_error_generator tests about every aspect of the underlying meta-function.

To run the tests for a particular operator, use pytest’s -k option. This will run tests for Thunder’s different executors, supported dtypes, and supported device types. For example, to run the tests for unfold the command would be

$ pytest thunder/tests/test_ops.py -k unfold

Another example of an OpInfo with specified test_directives

acos_opinfo = OpInfo(
    ltorch.acos,
    domain=(-1, 1),
    sample_input_generator=elementwise_unary_generator,
    torch_reference=_elementwise_unary_torch(torch.acos),
    test_directives=(
        # Torch doesn't support CPU float16 or complex32 acos
        DecorateInfo(
            pytest.mark.xfail,
            "test_core_vs_torch_consistency",
            dtypes=(datatypes.float16, datatypes.complex32),
            devicetypes=(devices.DeviceType.CPU,),
        ),
    ),
)
elementwise_unary_ops.append(acos_opinfo)

We strive for Thunder to be of the highest quality possible, so it is always a good idea to be very thorough when it comes to testing.

Adding grad support

Operations are not differentiable by default, unless they are implemented as compositions of differentiable operations (related to updating the torch language. More on that later). When an operation is a composition of other operations, we say that this operation is decomposable. Primitive operations, by definition, are not decomposable, and, as such, require an explicit backward/grad/VJP (for simplicity, we use them interchangeably) rule implemented for them. These rules, or grad transforms, are implemented in thunder/core/transforms.py. Note, however, these rules are not exclusively restricted to primitive operations, see Defining custom forward and backward for existing operators, for example, and can be implemented even for decomposable operations for performance reasons.

For now, for simplicity, let's assume that a new primitive is being added and we would like to make it differentiable. Consider the PR #118 which adds a backward support for a primitive operation prims.topk (added in the PR #88) that is modeled after torch.topk. The added to thunder/core/transforms.py lines

@torchctx
def _topk_prim_grad(
    a: TensorProxy, /, k: int, dim: None | int = None, largest: bool = True, sorted: bool = True
):
    fwd = prims.topk(a, k, dim, largest, sorted, out=out)
    val, idx = fwd

    val_grad = get_grad(val)

    a_grad = ltorch.zeros_like(a)
    # TODO: replace with scatter once we have it.
    # scatter_add is a prim and it relies on atomic ops.
    a_grad = ltorch.scatter_add(a_grad, dim, idx, val_grad)
    put_grad(a, a_grad)

    return fwd

register_grad(pids.TOPK, _topk_prim_grad)

define a grad transform for prims.topk. This operation returns a 2-tuple in forward fwd = (val, idx) with only the first element being differentiable. Note that Thunder interleaves forward and backward computations in grad transforms. Take a look at the lines val_grad = get_grad(val), which extracts the in-flowing backward gradient for val, and put_grad(a, a_grad) which sets the backward gradient for the input a.

Do you see that comment about the missing scatter? You could be the one who implements it! :)

Updating the Torch Language

The Torch Language operations are the "highest"-level operations and, as such, are decomposable. If the missing operation can be decomposed into already existing operations, then thunder/torch/init.py is where its implementation is to be placed.

For example, consider the PR #100, that adds support for the Hardswish activation function. The function is implemented in thunder/torch/init.py

@torchsymbol(torch.nn.functional.hardswish, id="torch.hardswish", is_method=False)
def hardswish(a: TensorProxy, /, inplace: bool = False) -> TensorLike:
    utils.check(not inplace, lambda: f"hardswish only supports inplace=False", exception_type=NotImplementedError)
    utils.check(
        dtypes.is_float_dtype(a.dtype),
        lambda: f"hardswish only supports floating point dtypes, got {a.dtype}",
        exception_type=ValueError,
    )
    return a * relu6(a + 3) / 6

Note the checks (Thunder does not support in-place operations yet) and that hardswish is a composition of the relu6 operation (defined in the torch language) and the language context-specific binary operations over the objects that TensorProxy represent. All these basic operations are differentiable (for the Torch/NVFuser executors), and so is hardswish implicitly differentiable (for the Torch/NVFuser executors).

Afterword

We hope that you find information provided here useful and we look forward to your contributions!

We also recommend checking out Defining new Thunder operations and Defining custom forward and backward for existing operators that cover very similar topics related to extending Thunder out of the tree.

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