DispatchKey.h

#pragma once

#include <c10/core/DeviceType.h>
#include <c10/macros/Export.h>
#include <cstddef>
#include <cstdint>
#include <functional>
#include <ostream>
#include <string>

namespace c10 {

// Semantically, each value of BackendComponent identifies a "backend" for our
// dispatch. Some functionalities that we may dispatch to are allowed to
// register different handlers for each backend. The BackendComponent is then
// used to figure out which backend implementation to dispatch to.

// In implementation terms, the backend component identifies a specific "bit" in
// a DispatchKeySet. The bits in the DispatchKeySet are split between the bottom
// ~12 "BackendComponent" bits, while the remaining upper bits are assigned to
// functionalities. When we encounter a functionality bit that is known to be
// customizable per-backend, then we also look at the lower BackendComponent
// bits and take the highest bit to determine which backend's implementation to
// use.

// WARNING!  If you add a new backend component to the end of this list,
// make sure you register it before Meta.
// Meta must be at the end so that meta key in tls triggers meta kernels.
// (But you shouldn't: private use keys should have higher precedence than all
// built-in keys)

// If you add a new (non-privateuse) backend here,
// make sure to add an Autograd<Backend> fallthrough kernel
// in aten/src/ATen/core/VariableFallbackKernel.cpp

#define C10_FORALL_BACKEND_COMPONENTS(_, extra) \
  _(CPU, extra)                                 \
  _(CUDA, extra)                                \
  _(HIP, extra)                                 \
  _(XLA, extra)                                 \
  _(MPS, extra)                                 \
  _(IPU, extra)                                 \
  _(XPU, extra)                                 \
  _(HPU, extra)                                 \
  _(VE, extra)                                  \
  _(Lazy, extra)                                \
  _(MTIA, extra)                                \
  _(PrivateUse1, extra)                         \
  _(PrivateUse2, extra)                         \
  _(PrivateUse3, extra)                         \
  _(Meta, extra)

// WARNING!  If we add a new per-backend functionality key that has higher
// priority than Autograd, then make sure you update EndOfRuntimeBackendKeys

#define C10_FORALL_FUNCTIONALITY_KEYS(_) \
  _(Dense, )                             \
  _(Quantized, Quantized)                \
  _(Sparse, Sparse)                      \
  _(SparseCsr, SparseCsr)                \
  _(NestedTensor, NestedTensor)          \
  _(AutogradFunctionality, Autograd)

enum class BackendComponent : uint8_t {

  // A "backend" is colloquially used to refer to handlers for dispatch
  // which actually implement the numerics of an operation in question.
  //
  // Due to the nature of the enum, these backends are specified in
  // an ordered way, but for most backends this order is not semantically
  // meaningful (e.g., it's valid to reorder these backends without changing
  // semantics).  The only situation when backend ordering is meaningful
  // is when the backend participates in multiple dispatch with another
  // backend; e.g., CPU and CUDA (cuda must have higher priority).

  // These keys don't correspond to individual kernels.
  // Instead, they represent the backends that are allowed to override specific
  // pieces of functionality:
  // - dense kernels (e.g. DispatchKey::CPU)
  // - sparse kernels (e.g. DispatchKey::SparseCPU)
  // - quantized kernels (e.g. DispatchKey::QuantizedCPU)
  // - autograd kernels (e.g. DispatchKey::AutogradCPU)
  // We reserve space in the runtime operator table for this full cross product
  // of
  // [backends in this enum] x [keys below that are explicitly marked as having
  // per-backend functionality]
  //
  // A meta tensor is a tensor without any data associated with it.  (They
  // have also colloquially been referred to as tensors on the "null" device).
  // A meta tensor can be used to dry run operators without actually doing any
  // computation, e.g., add on two meta tensors would give you another meta
  // tensor with the output shape and dtype, but wouldn't actually add anything.

  InvalidBit = 0,
#define DEFINE_BACKEND_COMPONENT(n, _) n##Bit,
  C10_FORALL_BACKEND_COMPONENTS(DEFINE_BACKEND_COMPONENT, unused)
#undef DEFINE_BACKEND_COMPONENT

  // Define an alias to represent end of backend dispatch keys.
  // If you add new backend keys after PrivateUse3, please also update it here.
  EndOfBackendKeys = MetaBit,
};

// Semantically, a dispatch key identifies a possible "level" in our
// dispatch, for which a handler may be registered. Each handler corresponds
// to a type of functionality.
//
// In implementation terms, the dispatch key identifies a specific "bit" in a
// DispatchKeySet.  Higher bit indexes get handled by dispatching first (because
// we "count leading zeros" when we extract the highest priority dispatch
// key.)
//
// Note [DispatchKey Classification]
// This enum actually contains several types of keys, which are explained
// in more detail further down:
// (1) non-customizable backends (e.g. FPGA)
// (2) non-customizable functionalities (e.g. Functionalize)
// (3) functionalized that are customizable per backend (e.g. Dense, Sparse,
// AutogradFunctionality) (4) per-backend instances of customizable
// functionalities (e.g. CPU, SparseCPU, AutogradCPU) (5) alias keys (e.g.
// CompositeImplicitAutograd)
//
// Of the categories above, it's important to note:
// (a) which keys are assigned individual bits in a DispatchKeySet
// (b) which keys are assigned individual slots in the runtime operator table
// ("Runtime keys")
//
// (1), (2) and (3) all get their own dedicated bits in the DispatchKeySet.
// (1), (2) and (4) all get their own dedicated slots in the runtime operator
// table.

// See Note [DispatchKeySet Internal Representation] for more details.
//
// NOTE: Keep the list in sync with `DispatchKey` in torchgen/model.py
enum class DispatchKey : uint16_t {

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~ UNDEFINED ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //
  // This is not a "real" functionality, but it exists to give us a "nullopt"
  // element we can return for cases when a DispatchKeySet contains no elements.
  // You can think a more semantically accurate definition of DispatchKey is:
  //
  //    using DispatchKey = optional<RealDispatchKey>
  //
  // and Undefined == nullopt.  We didn't actually represent
  // it this way because optional<RealDispatchKey> would take two
  // words, when DispatchKey fits in eight bits.

  Undefined = 0,

  // Define an alias for Undefined to represent CatchAll (long term
  // this will get eliminated, but for now it's convenient)
  CatchAll = Undefined,

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~ Functionality Keys ~~~~~~~~~~~~~~~~~~~~~~ //
  // Every value in the enum (up to EndOfFunctionalityKeys)
  // corresponds to an individual "functionality" that can be dispatched to.
  // This is represented in the DispatchKeySet by assigning each of these enum
  // values
  // to each of the remaining (64 - len(BackendComponent)) bits.
  //
  // Most of these functionalities have a single handler assigned to them,
  // making them "runtime keys".
  // That map to a single slot in the runtime operator table.
  //
  // A few functionalities are allowed to be customizable per backend.
  // See [Note: Per-Backend Functionality Dispatch Keys] for details.

  // See [Note: Per-Backend Functionality Dispatch Keys]
  Dense,

  // Below are non-extensible backends.
  // These are backends that currently don't have their own overrides for
  // Autograd/Sparse/Quantized kernels,
  // and we therefore don't waste space in the runtime operator table allocating
  // space for them.
  // If any of these backends ever need to customize, e.g., Autograd, then we'll
  // need to add a DispatchKey::*Bit for them.

  // TODO: put this in BackendComponents
  FPGA, // Xilinx support lives out of tree at
  // https://gitlab.com/pytorch-complex/vitis_kernels

  // TODO: put this in BackendComponents
  // MAIA backend lives out of tree
  // - test/cpp_extensions/maia_extension.cpp
  // - test/test_torch.py
  // - aten/src/ATen/test/extension_backend_test.cpp
  MAIA,

  Vulkan, // TODO: put this in BackendComponents
  Metal, // TODO: put this in BackendComponents

  // See [Note: Per-Backend Functionality Dispatch Keys]
  Quantized,

  // This backend is to support custom RNGs; it lets you go
  // to a different kernel if you pass in a generator that is not a
  // traditional CPUGeneratorImpl/CUDAGeneratorImpl.  To make use of this
  // key:
  //  1) set it as a second parameter of at::Generator constructor call in
  //     the user-defined PRNG class.
  //  2) use it as a dispatch key while registering custom kernels
  //     (templatized kernels specialized for user-defined PRNG class)
  // intended for out of tree use; tested by aten/src/ATen/test/rng_test.cpp
  CustomRNGKeyId,

  // TODO: Make Mkldnn a functionality key, so we can give it Meta
  // support
  // Here are backends which specify more specialized operators
  // based on the layout of the tensor.  Note that the sparse backends
  // are one case where ordering matters: sparse multi-dispatches with
  // the corresponding dense tensors, and must be handled before them.
  MkldnnCPU, // registered at build/aten/src/ATen/RegisterMkldnnCPU.cpp
  // NB: not to be confused with MKLDNN, which is Caffe2 only

  // See [Note: Per-Backend Functionality Dispatch Keys]
  Sparse,

  SparseCsr,

  NestedTensor,

  // In some situations, it is not immediately obvious what the correct
  // backend for function is, because the function in question doesn't
  // have any "tensor" arguments.  In this case, a BackendSelect function
  // can be registered to implement the custom determination of the
  // correct backend.
  BackendSelect,

  Python,

  // Out-of-core key for Fake Tensor in torchdistx.
  // See https://pytorch.org/torchdistx/latest/fake_tensor.html
  // TODO: delete this in favor of Python-implemented fake tensor
  Fake,
  // See Note [Out-of-tree vmap+grad prototype]. The purpose of this key
  // is to insert code after the "autograd subsystem" runs, so this key should
  // be directly after ADInplaceOrView and all of the autograd keys.
  FuncTorchDynamicLayerBackMode,

  // Alias and mutation removal.
  // If some backends want to opt into only alias removal or only mutation
  // removal,
  // we can consider adding separate keys dedicated to those individual passes.
  // See Note [Functionalization Pass In Core] for details.
  Functionalize,

  // The named dispatch key is set for any tensors with named dimensions.
  // Although we have a dispatch key for named tensors, for historical reasons,
  // this dispatch key doesn't do any of the substantive functionality for named
  // tensor (though, hypothetically, it could!)  At the moment, it's just
  // responsible for letting us give good error messages when operations
  // don't support named tensors.
  //
  // NB: If you ever consider moving named tensor functionality into
  // this dispatch key, note that it might be necessary add another dispatch
  // key that triggers before composite operators, in case a composite operator
  // has named dimension propagation that doesn't match that of its
  // constituent parts.
  // TODO: delete this once torchdim lands in functorch
  Named,

  // The Conjugate dispatch key is set for any tensors that need to perform
  // conjugation
  // This is implemented at a dispatch level right before any backends run
  Conjugate,

  // The Negative dispatch key is set for any tensors that need to perform
  // negation
  // This is implemented at a dispatch level right before any backends run
  Negative,

  ZeroTensor, // registered at build/aten/src/ATen/RegisterZeroTensor.cpp

  // Note [ADInplaceOrView key]
  // ADInplaceOrView key is used by inplace or view ops to register a kernel
  // that does additional setup for future autograd computation.
  //
  // 1. For inplace ops this kernel does version bump
  // 2. For view ops this kernel does `as_view` setup where we properly setup
  //    DifferentiableViewMeta on the view tensors.
  //
  // For other ops it's fallthrough kernel since there's no extra
  // work to do.
  //
  // Note [Dream: skip VariableType kernel when requires_grad=false]
  //
  // In an ideal world where we can skip VariableType kernel for inputs
  // with requires_grad=false, instead of a fallthrough kernel, we'll
  // register a kernel shown below to all functional ops as well:
  // torch::Tensor my_functional_op(...) {
  //   {
  //     // Note for every op in VariableType, you need to go through
  //     // `AutoDispatchBelowADInplaceOrView` guard exactly once to add the
  //     // key to TLS excluded set. If you don't go through it at all,
  //     // inplace/view ops called through `at::` inside your backend
  //     // kernel will dispatch to ADInplaceOrView kernels and do a lot
  //     // of extra work.
  //     at::AutoDispatchBelowADInplaceOrView guard;
  //     at::redispatch::my_functional_op(...);
  //   }
  // }
  // But this work is currently blocked since it adds an extra dispatch
  // for all ops and it's non-trivial overhead at model level(a few percents).
  // Thus our current approach takes advantage of the fact every kernel go
  // through VariableType kernel first and pulls the
  // `at::AutoDispatchBelowADInplaceOrView` guard of functional ops
  // up to the `VariableType` kernel. Thus we only add the extra dispatch
  // to view/inplace ops to minimize its perf impact to real models.
  ADInplaceOrView,
  // Note [Alias Dispatch Key : Autograd]
  // All backends are oblivious to autograd; autograd is handled as a
  // layer which happens on top of all backends. It inspects the autograd
  // metadata of all inputs, determines what autograd metadata should be
  // constructed by the output, and otherwise defers to the backend to
  // actually do the numeric computation.  Autograd contains
  // the bulk of this logic.

  // Autograd is now an alias dispatch key which by default maps to all
  // backend-specific autograd keys.
  // Backend-specific allow backends to override the default kernel registered
  // to Autograd key as needed.
  // For example, XLA wants to define autograd for einsum directly.
  // Registering a custom autograd implementation at the XLA key won't work
  // because we process Autograd before XLA.  This key has higher priority and
  // gets processed first.  You generally should NOT redispatch after handling
  // autograd here (since that would result in execution of the Autograd
  // operator, which you're trying to skip).  In AutogradXLA implementations,
  // you are responsible for handling autograd yourself, or deferring to other
  // operators which support autograd.

  // Currently we only have backend-specific autograd keys for CPU/CUDA/XLA and
  // reserved user-defined backends. All other in-tree backends share the
  // AutogradOther key. We can add specific autograd key for those backends
  // upon request.
  AutogradOther,

  // See [Note: Per-Backend Functionality Dispatch Keys]
  AutogradFunctionality,

  // NestedTensor is an example of something that isn't a "real backend"
  // (because it mostly consists of redispatching kernels)
  // but it would like to override autograd functionality in C++.
  // We can handle cases like this by adding an extra functionality key
  // exclusively for handling autograd for NestedTensor.
  // lives out of tree at
  // https://github.com/pytorch/nestedtensor
  AutogradNestedTensor,

  Tracer,

  // TODO: make Autocast a functionality key
  // Autocasting precedes VariableTypeId, to ensure casts are autograd-exposed
  // and inputs are saved for backward in the post-autocast type.
  AutocastCPU,
  AutocastXPU,
  AutocastIPU,
  AutocastHPU,
  AutocastXLA,
  // AutocastXLA is only being used for TPUs. XLA GPUs continue to use
  // AutocastCUDA.
  AutocastCUDA,
  AutocastPrivateUse1,

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~ WRAPPERS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //
  // There are a number of alternative modes which may want to handle before
  // autograd; for example, error checking, tracing, profiling or vmap.  They
  // go here.

  FuncTorchBatched, // See Note [Out-of-tree vmap+grad prototype]

  // Dispatch key for BatchedTensorImpl wrapping a nested tensor.
  BatchedNestedTensor,

  FuncTorchVmapMode, // See Note [Out-of-tree vmap+grad prototype]

  // This is the dispatch key for BatchedTensorImpl, which is used to implement
  // batching rules for vmap.
  Batched,

  // When we are inside a vmap, all tensors dispatch on this key.
  // See Note: [DispatchKey::VmapMode usage] for more details.
  VmapMode,

  FuncTorchGradWrapper, // See Note [Out-of-tree vmap+grad prototype]

  // Out-of-core key for Deferred Module Initialization in torchdistx.
  // See https://pytorch.org/torchdistx/latest/deferred_init.html
  DeferredInit,

  // Used by Python key logic to know the set of tls on entry to the dispatcher
  // This kernel assumes it is the top-most non-functorch-related DispatchKey.
  // If you add a key above, make sure to update the fallback implementation for
  // this.
  PythonTLSSnapshot,

  // This key should be at the very top of the dispatcher
  FuncTorchDynamicLayerFrontMode, // See Note [Out-of-tree vmap+grad prototype]

  // TESTING: This is intended to be a generic testing tensor type id.
  // Don't use it for anything real; its only acceptable use is within a single
  // process test.  Use it by creating a TensorImpl with this DispatchKey, and
  // then registering operators to operate on this type id.  See
  // aten/src/ATen/core/dispatch/backend_fallback_test.cpp for a usage example.
  TESTING_ONLY_GenericWrapper,

  // TESTING: This is intended to be a generic testing tensor type id.
  // Don't use it for anything real; its only acceptable use is within a ingle
  // process test.  Use it by toggling the mode on and off via
  // TESTING_ONLY_tls_generic_mode_set_enabled and then registering operators
  // to operate on this type id.  See
  // aten/src/ATen/core/dispatch/backend_fallback_test.cpp
  // for a usage example
  TESTING_ONLY_GenericMode,

  // This key is used for pre-dispatch tracing in make_fx.
  // It has lower priority than the PythonDispatcher key
  // because we use the PythonDispatcher to intercept the key from python,
  // and avoid having to implement it in C++.
  PreDispatch,

  // This is a bypass that allows you to skip running the C++ dispatcher
  // entirely
  PythonDispatcher,

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ FIN ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //
  EndOfFunctionalityKeys, // End of functionality keys.

// ~~~~~~~~~~~~~~ "Dense" Per-Backend Dispatch keys ~~~~~~~~~~~~~~~~~~~~ //
// Here are backends which you think of as traditionally specifying
// how to implement operations on some device.

#define DEFINE_PER_BACKEND_KEYS_FOR_BACKEND(n, prefix) prefix##n,

#define DEFINE_PER_BACKEND_KEYS(fullname, prefix)      \
  StartOf##fullname##Backends,                         \
      C10_FORALL_BACKEND_COMPONENTS(                   \
          DEFINE_PER_BACKEND_KEYS_FOR_BACKEND, prefix) \
          EndOf##fullname##Backends = prefix##Meta,

  C10_FORALL_FUNCTIONALITY_KEYS(DEFINE_PER_BACKEND_KEYS)

#undef DEFINE_PER_BACKEND_KEYS
#undef DEFINE_PER_BACKEND_KEYS_FOR_BACKEND

      EndOfRuntimeBackendKeys = EndOfAutogradFunctionalityBackends,

  // ~~~~~~~~~~~~~~~~~~~~~~ Alias Dispatch Keys ~~~~~~~~~~~~~~~~~~~~~~~~~~ //
  // Note [Alias Dispatch Keys]
  // Alias dispatch keys are synthetic dispatch keys which map to multiple
  // runtime dispatch keys. Alisa keys have precedence, but they are always
  // lower precedence than runtime keys. You can register a kernel to an
  // alias key, the kernel might be populated to the mapped runtime keys
  // during dispatch table computation.
  // If a runtime dispatch key has multiple kernels from alias keys, which
  // kernel wins is done based on the precedence of alias keys (but runtime
  // keys always have precedence over alias keys).
  // Alias keys won't be directly called during runtime.

  // See Note [Alias Dispatch Key : Autograd]
  Autograd,
  CompositeImplicitAutograd, // registered at
  // build/aten/src/ATen/RegisterCompositeImplicitAutograd.cpp

  // Note: The alias keyset for FuncTorchBatchedDecomposition is disjoint from
  // all
  // other alias keysets
  // and so precedence order doesn't matter
  FuncTorchBatchedDecomposition, // registered at
  // build/aten/src/ATen/RegisterFuncTorchBatchedDecomposition.cpp
  // Note: The alias keyset for CompositeImplicitAutogradNestedTensor is
  // disjoint from all other alias keysets
  CompositeImplicitAutogradNestedTensor, // registered at
  // build/aten/src/ATen/RegisterCompositeImplicitAutogradNestedTensor.cpp
  CompositeExplicitAutograd, // registered at
  // build/aten/src/ATen/RegisterCompositeExplicitAutograd.cpp
  // See Note [CompositeExplicitAutogradNonFunctional Key]
  CompositeExplicitAutogradNonFunctional, // registered at
  // build/aten/src/ATen/RegisterCompositeExplicitAutograd.cpp

  // Define an alias key to represent end of alias dispatch keys.
  // If you add new alias keys after Autograd, please also update it here.
  StartOfAliasKeys = Autograd,
  EndOfAliasKeys = CompositeExplicitAutogradNonFunctional, //

  // ~~~~~~~~~~~~~~~~~~~~~~~~~ BC ALIASES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //
  // The aliases exist for backwards compatibility reasons, they shouldn't
  // be used
  CPUTensorId = CPU,
  CUDATensorId = CUDA,
  DefaultBackend = CompositeExplicitAutograd,
  PrivateUse1_PreAutograd = AutogradPrivateUse1,
  PrivateUse2_PreAutograd = AutogradPrivateUse2,
  PrivateUse3_PreAutograd = AutogradPrivateUse3,
  Autocast = AutocastCUDA,
};

// Note [Private use DispatchKey]
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Private use tensor IDs are preallocated tensor type IDs for use in user
// applications.  Similar to private use fields in HTTP, they can be used
// by end users for experimental or private applications, without needing
// to "standardize" the tensor ID (which would be done by submitting a PR
// to PyTorch to add your type ID).
//
// Private use tensor IDs are appropriate to use if you want to experiment
// with adding a new tensor type (without having to patch PyTorch first) or
// have a private, non-distributed application that needs to make use of a
// new tensor type.  Private use tensor IDs are NOT appropriate to use for
// libraries intended to be distributed to further users: please contact
// the PyTorch developers to get a type ID registered in this case.
//
// We provide two classes of private user tensor id: regular DispatchKeys
// and Autograd DispatchKeys.  DispatchKeys serve the role of ordinary "backend"
// DispatchKeys; if you were adding support for a new type of accelerator, you
// would use a backend DispatchKey, and ideally automatically reuse
// AutogradOther definitions already defined in PyTorch.  AutogradPrivateUse
// DispatchKeys serve as "wrapper" DispatchKeys: they are only necessary for
// tensors that compose multiple internal tensors, and for cases when the
// built-in autograd formulas for operators are not appropriate.

static_assert(
    (static_cast<uint8_t>(BackendComponent::EndOfBackendKeys) +
     static_cast<uint8_t>(DispatchKey::EndOfFunctionalityKeys)) <= 64,
    "The BackendComponent and DispatchKey enums (below EndOfFunctionalityKeys)"
    " both map to backend and functionality bits"
    " into a 64-bit bitmask; you must have less than 64 total entries between them");

// Check if a DispatchKey is an alias mapping to other runtime keys.
constexpr bool isAliasDispatchKey(DispatchKey k) {
  return k >= DispatchKey::StartOfAliasKeys && k <= DispatchKey::EndOfAliasKeys;
}

// [Note: Per-Backend Functionality Dispatch Keys]
// Check if a DispatchKey is a per-backend functionality key
// Any functionalities that can be customized per-backend should be added here.
// These keys correspond to functionalities that can be customized individually
// per backend. While they only take up one bit in the `DispatchKeySet` bitset,
// they map to (# backends) slots in the operator table.
// Each of these keys also has a separate set of "runtime keys" in the dispatch
// key enum, per backend, which *do* map to the individual operator table slots.
// For example, the "Sparse" key maps to an individual bit in the
// DispatchKeySet, while `SparseCPU`, `SparseCUDA`, etc all map to individual
// slots in the runtime operator table.

constexpr bool isPerBackendFunctionalityKey(DispatchKey k) {
  if (k == DispatchKey::Dense || k == DispatchKey::Quantized ||
      k == DispatchKey::Sparse || k == DispatchKey::SparseCsr ||
      k == DispatchKey::AutogradFunctionality ||
      k == DispatchKey::NestedTensor) {
    return true;
  } else {
    return false;
  }
}

// Note that this includes Undefined in the total count.
// BUT EndOfFunctionalityKeys is its own (placeholder) key.
// e.g. Undefined=0, Dense=1, Sparse=2, EndOfFunctionalityKeys=3.
// In the above example, there are 3 total functionality keys.
constexpr uint8_t num_functionality_keys =
    static_cast<uint8_t>(DispatchKey::EndOfFunctionalityKeys);

constexpr uint8_t num_backends =
    static_cast<uint8_t>(BackendComponent::EndOfBackendKeys);

// Note [No More Than 16 Backends]
// Search for this note to find places in the code where the "no more than 16
// backends" invariant is baked in.
static_assert(
    static_cast<uint8_t>(BackendComponent::EndOfBackendKeys) <= 16,
    "BackendComponent currently only supports <= 16 backends. If we really need to extend this, \
there are a few places where this invariant is baked in");

constexpr uint8_t numPerBackendFunctionalityKeys() {
  uint8_t count = 0;
  for (uint8_t k = 0; k <= num_functionality_keys; ++k) {
    if (isPerBackendFunctionalityKey(static_cast<DispatchKey>(k)))
      ++count;
  }
  return count;
}

#if defined(C10_MOBILE_TRIM_DISPATCH_KEYS)
// See [Note: Trimmed Mobile Dispatch Keys]
constexpr uint16_t num_runtime_entries = 8;
#else
constexpr uint16_t num_runtime_entries = num_functionality_keys +
    (numPerBackendFunctionalityKeys() * (num_backends - 1));
#endif

// See Note [No More Than 16 Backends]
constexpr uint16_t full_backend_mask =
    (static_cast<uint16_t>(1) << num_backends) - 1;

C10_API const char* toString(DispatchKey);
C10_API const char* toString(BackendComponent);
C10_API std::ostream& operator<<(std::ostream&, DispatchKey);
C10_API std::ostream& operator<<(std::ostream&, BackendComponent);

C10_API DispatchKey getAutogradKeyFromBackend(BackendComponent k);

// Parses a string into a dispatch key.
// If the string cannot be correctly parsed, throws an exception.
C10_API c10::DispatchKey parseDispatchKey(const std::string& k);

// These are some convenience identifiers for dispatch keys which are
// shorter to type than their long counterparts.  Note that some of these
// dispatch keys directly correspond to DeviceType; and most APIs that
// accept DispatchKey also accept DeviceType; e.g.,
// torch::dispatch(torch::kCPU, ...) is also valid.
constexpr DispatchKey kAutograd = DispatchKey::Autograd;

// See Note [The Ordering of Per-Backend Dispatch Keys Matters!]
// This function relies on the invariant that the dispatch keys between
// StartOfDenseBackends and EndOfRuntimeBackendKeys are ordered by backend
// in the same order as `BackendComponent`.
constexpr BackendComponent toBackendComponent(DispatchKey k) {
  if (k >= DispatchKey::StartOfDenseBackends &&
      k <= DispatchKey::EndOfDenseBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(DispatchKey::StartOfDenseBackends));
  } else if (
      k >= DispatchKey::StartOfQuantizedBackends &&
      k <= DispatchKey::EndOfQuantizedBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(DispatchKey::StartOfQuantizedBackends));
  } else if (
      k >= DispatchKey::StartOfSparseBackends &&
      k <= DispatchKey::EndOfSparseBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(DispatchKey::StartOfSparseBackends));
  } else if (
      k >= DispatchKey::StartOfSparseCsrBackends &&
      k <= DispatchKey::EndOfSparseCsrBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(DispatchKey::StartOfSparseCsrBackends));
  } else if (
      k >= DispatchKey::StartOfNestedTensorBackends &&
      k <= DispatchKey::EndOfNestedTensorBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(DispatchKey::StartOfNestedTensorBackends));
  } else if (
      k >= DispatchKey::StartOfAutogradFunctionalityBackends &&
      k <= DispatchKey::EndOfAutogradFunctionalityBackends) {
    return static_cast<BackendComponent>(
        static_cast<uint8_t>(k) -
        static_cast<uint8_t>(
            DispatchKey::StartOfAutogradFunctionalityBackends));
  } else {
    return BackendComponent::InvalidBit;
  }
}

constexpr DispatchKey toFunctionalityKey(DispatchKey k) {
  if (k <= DispatchKey::EndOfFunctionalityKeys) {
    return k;
  } else if (k <= DispatchKey::EndOfDenseBackends) {
    return DispatchKey::Dense;
  } else if (k <= DispatchKey::EndOfQuantizedBackends) {
    return DispatchKey::Quantized;
  } else if (k <= DispatchKey::EndOfSparseBackends) {
    return DispatchKey::Sparse;
  } else if (k <= DispatchKey::EndOfSparseCsrBackends) {
    return DispatchKey::SparseCsr;
  } else if (k <= DispatchKey::EndOfNestedTensorBackends) {
    return DispatchKey::NestedTensor;
  } else if (k <= DispatchKey::EndOfAutogradFunctionalityBackends) {
    return DispatchKey::AutogradFunctionality;
  } else {
    return DispatchKey::Undefined;
  }
}

BackendComponent toBackendComponent(DeviceType device_type);

// Given (DispatchKey::Dense, BackendComponent::CUDABit), returns
// DispatchKey::CUDA.
// See Note [The Ordering of Per-Backend Dispatch Keys Matters!]
// This function relies on the invariant that the dispatch keys between
// StartOfDenseBackends and EndOfRuntimeBackendKeys are ordered by backend
// in the same order as `BackendComponent`.
constexpr DispatchKey toRuntimePerBackendFunctionalityKey(
    DispatchKey functionality_k,
    BackendComponent backend_k) {
  if (functionality_k == DispatchKey::Dense) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(DispatchKey::StartOfDenseBackends) +
        static_cast<uint8_t>(backend_k));
  }
  if (functionality_k == DispatchKey::Sparse) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(DispatchKey::StartOfSparseBackends) +
        static_cast<uint8_t>(backend_k));
  }
  if (functionality_k == DispatchKey::SparseCsr) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(DispatchKey::StartOfSparseCsrBackends) +
        static_cast<uint8_t>(backend_k));
  }
  if (functionality_k == DispatchKey::Quantized) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(DispatchKey::StartOfQuantizedBackends) +
        static_cast<uint8_t>(backend_k));
  }
  if (functionality_k == DispatchKey::NestedTensor) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(DispatchKey::StartOfNestedTensorBackends) +
        static_cast<uint8_t>(backend_k));
  }
  if (functionality_k == DispatchKey::AutogradFunctionality) {
    return static_cast<DispatchKey>(
        static_cast<uint8_t>(
            DispatchKey::StartOfAutogradFunctionalityBackends) +
        static_cast<uint8_t>(backend_k));
  }
  return DispatchKey::Undefined;
}

} // namespace c10

namespace torch {
// Expose the constant, but not the TYPE (DispatchKey is an implementation
// detail!)
// NOLINTNEXTLINE(misc-unused-using-decls)
using c10::kAutograd;
} // namespace torch

// NB: You really shouldn't use this instance; this enum is guaranteed
// to be pretty small so a regular array should be acceptable.
namespace std {
template <>
struct hash<c10::DispatchKey> {
  typedef size_t result_type;
  typedef c10::DispatchKey argument_type;

  size_t operator()(c10::DispatchKey x) const {
    return static_cast<size_t>(x);
  }
};
} // namespace std