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//===- FunctionExtras.h - Function type erasure utilities -------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// This file provides a collection of function (or more generally, callable)
/// type erasure utilities supplementing those provided by the standard library
/// in `<function>`.
/// It provides `unique_function`, which works like `std::function` but supports
/// move-only callable objects and const-qualification.
/// Future plans:
/// - Add a `function` that provides ref-qualified support, which doesn't work
/// with `std::function`.
/// - Provide support for specifying multiple signatures to type erase callable
/// objects with an overload set, such as those produced by generic lambdas.
/// - Expand to include a copyable utility that directly replaces std::function
/// but brings the above improvements.
/// Note that LLVM's utilities are greatly simplified by not supporting
/// allocators.
/// If the standard library ever begins to provide comparable facilities we can
/// consider switching to those.
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/STLForwardCompat.h"
#include "llvm/Support/MemAlloc.h"
#include "llvm/Support/type_traits.h"
#include <cstring>
#include <memory>
#include <type_traits>
namespace llvm {
/// unique_function is a type-erasing functor similar to std::function.
/// It can hold move-only function objects, like lambdas capturing unique_ptrs.
/// Accordingly, it is movable but not copyable.
/// It supports const-qualification:
/// - unique_function<int() const> has a const operator().
/// It can only hold functions which themselves have a const operator().
/// - unique_function<int()> has a non-const operator().
/// It can hold functions with a non-const operator(), like mutable lambdas.
template <typename FunctionT> class unique_function;
namespace detail {
template <typename T>
using EnableIfTrivial =
std::enable_if_t<llvm::is_trivially_move_constructible<T>::value &&
template <typename CallableT, typename ThisT>
using EnableUnlessSameType =
std::enable_if_t<!std::is_same<remove_cvref_t<CallableT>, ThisT>::value>;
template <typename CallableT, typename Ret, typename... Params>
using EnableIfCallable = std::enable_if_t<std::disjunction<
std::is_same<const decltype(std::declval<CallableT>()(
template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
static constexpr size_t InlineStorageSize = sizeof(void *) * 3;
template <typename T, class = void>
struct IsSizeLessThanThresholdT : std::false_type {};
template <typename T>
struct IsSizeLessThanThresholdT<
T, std::enable_if_t<sizeof(T) <= 2 * sizeof(void *)>> : std::true_type {};
// Provide a type function to map parameters that won't observe extra copies
// or moves and which are small enough to likely pass in register to values
// and all other types to l-value reference types. We use this to compute the
// types used in our erased call utility to minimize copies and moves unless
// doing so would force things unnecessarily into memory.
// The heuristic used is related to common ABI register passing conventions.
// It doesn't have to be exact though, and in one way it is more strict
// because we want to still be able to observe either moves *or* copies.
template <typename T> struct AdjustedParamTBase {
"references should be handled by template specialization");
using type = std::conditional_t<
llvm::is_trivially_copy_constructible<T>::value &&
llvm::is_trivially_move_constructible<T>::value &&
T, T &>;
// This specialization ensures that 'AdjustedParam<V<T>&>' or
// 'AdjustedParam<V<T>&&>' does not trigger a compile-time error when 'T' is
// an incomplete type and V a templated type.
template <typename T> struct AdjustedParamTBase<T &> { using type = T &; };
template <typename T> struct AdjustedParamTBase<T &&> { using type = T &; };
template <typename T>
using AdjustedParamT = typename AdjustedParamTBase<T>::type;
// The type of the erased function pointer we use as a callback to dispatch to
// the stored callable when it is trivial to move and destroy.
using CallPtrT = ReturnT (*)(void *CallableAddr,
AdjustedParamT<ParamTs>... Params);
using MovePtrT = void (*)(void *LHSCallableAddr, void *RHSCallableAddr);
using DestroyPtrT = void (*)(void *CallableAddr);
/// A struct to hold a single trivial callback with sufficient alignment for
/// our bitpacking.
struct alignas(8) TrivialCallback {
CallPtrT CallPtr;
/// A struct we use to aggregate three callbacks when we need full set of
/// operations.
struct alignas(8) NonTrivialCallbacks {
CallPtrT CallPtr;
MovePtrT MovePtr;
DestroyPtrT DestroyPtr;
// Create a pointer union between either a pointer to a static trivial call
// pointer in a struct or a pointer to a static struct of the call, move, and
// destroy pointers.
using CallbackPointerUnionT =
PointerUnion<TrivialCallback *, NonTrivialCallbacks *>;
// The main storage buffer. This will either have a pointer to out-of-line
// storage or an inline buffer storing the callable.
union StorageUnionT {
// For out-of-line storage we keep a pointer to the underlying storage and
// the size. This is enough to deallocate the memory.
struct OutOfLineStorageT {
void *StoragePtr;
size_t Size;
size_t Alignment;
} OutOfLineStorage;
sizeof(OutOfLineStorageT) <= InlineStorageSize,
"Should always use all of the out-of-line storage for inline storage!");
// For in-line storage, we just provide an aligned character buffer. We
// provide three pointers worth of storage here.
// This is mutable as an inlined `const unique_function<void() const>` may
// still modify its own mutable members.
mutable std::aligned_storage_t<InlineStorageSize, alignof(void *)>
} StorageUnion;
// A compressed pointer to either our dispatching callback or our table of
// dispatching callbacks and the flag for whether the callable itself is
// stored inline or not.
PointerIntPair<CallbackPointerUnionT, 1, bool> CallbackAndInlineFlag;
bool isInlineStorage() const { return CallbackAndInlineFlag.getInt(); }
bool isTrivialCallback() const {
return isa<TrivialCallback *>(CallbackAndInlineFlag.getPointer());
CallPtrT getTrivialCallback() const {
return cast<TrivialCallback *>(CallbackAndInlineFlag.getPointer())->CallPtr;
NonTrivialCallbacks *getNonTrivialCallbacks() const {
return cast<NonTrivialCallbacks *>(CallbackAndInlineFlag.getPointer());
CallPtrT getCallPtr() const {
return isTrivialCallback() ? getTrivialCallback()
: getNonTrivialCallbacks()->CallPtr;
// These three functions are only const in the narrow sense. They return
// mutable pointers to function state.
// This allows unique_function<T const>::operator() to be const, even if the
// underlying functor may be internally mutable.
// const callers must ensure they're only used in const-correct ways.
void *getCalleePtr() const {
return isInlineStorage() ? getInlineStorage() : getOutOfLineStorage();
void *getInlineStorage() const { return &StorageUnion.InlineStorage; }
void *getOutOfLineStorage() const {
return StorageUnion.OutOfLineStorage.StoragePtr;
size_t getOutOfLineStorageSize() const {
return StorageUnion.OutOfLineStorage.Size;
size_t getOutOfLineStorageAlignment() const {
return StorageUnion.OutOfLineStorage.Alignment;
void setOutOfLineStorage(void *Ptr, size_t Size, size_t Alignment) {
StorageUnion.OutOfLineStorage = {Ptr, Size, Alignment};
template <typename CalledAsT>
static ReturnT CallImpl(void *CallableAddr,
AdjustedParamT<ParamTs>... Params) {
auto &Func = *reinterpret_cast<CalledAsT *>(CallableAddr);
return Func(std::forward<ParamTs>(Params)...);
template <typename CallableT>
static void MoveImpl(void *LHSCallableAddr, void *RHSCallableAddr) noexcept {
new (LHSCallableAddr)
CallableT(std::move(*reinterpret_cast<CallableT *>(RHSCallableAddr)));
template <typename CallableT>
static void DestroyImpl(void *CallableAddr) noexcept {
reinterpret_cast<CallableT *>(CallableAddr)->~CallableT();
// The pointers to call/move/destroy functions are determined for each
// callable type (and called-as type, which determines the overload chosen).
// (definitions are out-of-line).
// By default, we need an object that contains all the different
// type erased behaviors needed. Create a static instance of the struct type
// here and each instance will contain a pointer to it.
// Wrap in a struct to avoid
template <typename CallableT, typename CalledAs, typename Enable = void>
struct CallbacksHolder {
static NonTrivialCallbacks Callbacks;
// See if we can create a trivial callback. We need the callable to be
// trivially moved and trivially destroyed so that we don't have to store
// type erased callbacks for those operations.
template <typename CallableT, typename CalledAs>
struct CallbacksHolder<CallableT, CalledAs, EnableIfTrivial<CallableT>> {
static TrivialCallback Callbacks;
// A simple tag type so the call-as type to be passed to the constructor.
template <typename T> struct CalledAs {};
// Essentially the "main" unique_function constructor, but subclasses
// provide the qualified type to be used for the call.
// (We always store a T, even if the call will use a pointer to const T).
template <typename CallableT, typename CalledAsT>
UniqueFunctionBase(CallableT Callable, CalledAs<CalledAsT>) {
bool IsInlineStorage = true;
void *CallableAddr = getInlineStorage();
if (sizeof(CallableT) > InlineStorageSize ||
alignof(CallableT) > alignof(decltype(StorageUnion.InlineStorage))) {
IsInlineStorage = false;
// Allocate out-of-line storage. FIXME: Use an explicit alignment
// parameter in C++17 mode.
auto Size = sizeof(CallableT);
auto Alignment = alignof(CallableT);
CallableAddr = allocate_buffer(Size, Alignment);
setOutOfLineStorage(CallableAddr, Size, Alignment);
// Now move into the storage.
new (CallableAddr) CallableT(std::move(Callable));
&CallbacksHolder<CallableT, CalledAsT>::Callbacks, IsInlineStorage);
~UniqueFunctionBase() {
if (!CallbackAndInlineFlag.getPointer())
// Cache this value so we don't re-check it after type-erased operations.
bool IsInlineStorage = isInlineStorage();
if (!isTrivialCallback())
IsInlineStorage ? getInlineStorage() : getOutOfLineStorage());
if (!IsInlineStorage)
deallocate_buffer(getOutOfLineStorage(), getOutOfLineStorageSize(),
UniqueFunctionBase(UniqueFunctionBase &&RHS) noexcept {
// Copy the callback and inline flag.
CallbackAndInlineFlag = RHS.CallbackAndInlineFlag;
// If the RHS is empty, just copying the above is sufficient.
if (!RHS)
if (!isInlineStorage()) {
// The out-of-line case is easiest to move.
StorageUnion.OutOfLineStorage = RHS.StorageUnion.OutOfLineStorage;
} else if (isTrivialCallback()) {
// Move is trivial, just memcpy the bytes across.
memcpy(getInlineStorage(), RHS.getInlineStorage(), InlineStorageSize);
} else {
// Non-trivial move, so dispatch to a type-erased implementation.
// Clear the old callback and inline flag to get back to as-if-null.
RHS.CallbackAndInlineFlag = {};
#ifndef NDEBUG
// In debug builds, we also scribble across the rest of the storage.
memset(RHS.getInlineStorage(), 0xAD, InlineStorageSize);
UniqueFunctionBase &operator=(UniqueFunctionBase &&RHS) noexcept {
if (this == &RHS)
return *this;
// Because we don't try to provide any exception safety guarantees we can
// implement move assignment very simply by first destroying the current
// object and then move-constructing over top of it.
new (this) UniqueFunctionBase(std::move(RHS));
return *this;
UniqueFunctionBase() = default;
explicit operator bool() const {
return (bool)CallbackAndInlineFlag.getPointer();
template <typename R, typename... P>
template <typename CallableT, typename CalledAsT, typename Enable>
typename UniqueFunctionBase<R, P...>::NonTrivialCallbacks UniqueFunctionBase<
R, P...>::CallbacksHolder<CallableT, CalledAsT, Enable>::Callbacks = {
&CallImpl<CalledAsT>, &MoveImpl<CallableT>, &DestroyImpl<CallableT>};
template <typename R, typename... P>
template <typename CallableT, typename CalledAsT>
typename UniqueFunctionBase<R, P...>::TrivialCallback
UniqueFunctionBase<R, P...>::CallbacksHolder<
CallableT, CalledAsT, EnableIfTrivial<CallableT>>::Callbacks{
} // namespace detail
template <typename R, typename... P>
class unique_function<R(P...)> : public detail::UniqueFunctionBase<R, P...> {
using Base = detail::UniqueFunctionBase<R, P...>;
unique_function() = default;
unique_function(std::nullptr_t) {}
unique_function(unique_function &&) = default;
unique_function(const unique_function &) = delete;
unique_function &operator=(unique_function &&) = default;
unique_function &operator=(const unique_function &) = delete;
template <typename CallableT>
CallableT Callable,
detail::EnableUnlessSameType<CallableT, unique_function> * = nullptr,
detail::EnableIfCallable<CallableT, R, P...> * = nullptr)
: Base(std::forward<CallableT>(Callable),
typename Base::template CalledAs<CallableT>{}) {}
R operator()(P... Params) {
return this->getCallPtr()(this->getCalleePtr(), Params...);
template <typename R, typename... P>
class unique_function<R(P...) const>
: public detail::UniqueFunctionBase<R, P...> {
using Base = detail::UniqueFunctionBase<R, P...>;
unique_function() = default;
unique_function(std::nullptr_t) {}
unique_function(unique_function &&) = default;
unique_function(const unique_function &) = delete;
unique_function &operator=(unique_function &&) = default;
unique_function &operator=(const unique_function &) = delete;
template <typename CallableT>
CallableT Callable,
detail::EnableUnlessSameType<CallableT, unique_function> * = nullptr,
detail::EnableIfCallable<const CallableT, R, P...> * = nullptr)
: Base(std::forward<CallableT>(Callable),
typename Base::template CalledAs<const CallableT>{}) {}
R operator()(P... Params) const {
return this->getCallPtr()(this->getCalleePtr(), Params...);
} // end namespace llvm