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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.
/// Future plans:
/// - Add a `function` that provides const, volatile, and 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/Support/type_traits.h"
#include <memory>
namespace llvm {
template <typename FunctionT> class unique_function;
template <typename ReturnT, typename... ParamTs>
class unique_function<ReturnT(ParamTs...)> {
static constexpr size_t InlineStorageSize = sizeof(void *) * 3;
// MSVC has a bug and ICEs if we give it a particular dependent value
// expression as part of the `std::conditional` below. To work around this,
// we build that into a template struct's constexpr bool.
template <typename T> struct IsSizeLessThanThresholdT {
static constexpr bool value = sizeof(T) <= (2 * sizeof(void *));
// 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>
using AdjustedParamT = typename std::conditional<
!std::is_reference<T>::value &&
llvm::is_trivially_copy_constructible<T>::value &&
llvm::is_trivially_move_constructible<T>::value &&
T, 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.
typename std::aligned_storage<InlineStorageSize, alignof(void *)>::type
} 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 CallbackAndInlineFlag.getPointer().template is<TrivialCallback *>();
CallPtrT getTrivialCallback() const {
return CallbackAndInlineFlag.getPointer().template get<TrivialCallback *>()->CallPtr;
NonTrivialCallbacks *getNonTrivialCallbacks() const {
return CallbackAndInlineFlag.getPointer()
.template get<NonTrivialCallbacks *>();
void *getInlineStorage() { return &StorageUnion.InlineStorage; }
void *getOutOfLineStorage() {
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 CallableT>
static ReturnT CallImpl(void *CallableAddr, AdjustedParamT<ParamTs>... Params) {
return (*reinterpret_cast<CallableT *>(CallableAddr))(
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();
unique_function() = default;
unique_function(std::nullptr_t /*null_callable*/) {}
~unique_function() {
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(),
unique_function(unique_function &&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);
unique_function &operator=(unique_function &&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) unique_function(std::move(RHS));
return *this;
template <typename CallableT> unique_function(CallableT Callable) {
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));
// 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.
// FIXME: We should use constexpr if here and below to avoid instantiating
// the non-trivial static objects when unnecessary. While the linker should
// remove them, it is still wasteful.
if (llvm::is_trivially_move_constructible<CallableT>::value &&
std::is_trivially_destructible<CallableT>::value) {
// We need to create a nicely aligned object. We use a static variable
// for this because it is a trivial struct.
static TrivialCallback Callback = { &CallImpl<CallableT> };
CallbackAndInlineFlag = {&Callback, IsInlineStorage};
// Otherwise, we need to point at an object that contains all the different
// type erased behaviors needed. Create a static instance of the struct type
// here and then use a pointer to that.
static NonTrivialCallbacks Callbacks = {
&CallImpl<CallableT>, &MoveImpl<CallableT>, &DestroyImpl<CallableT>};
CallbackAndInlineFlag = {&Callbacks, IsInlineStorage};
ReturnT operator()(ParamTs... Params) {
void *CallableAddr =
isInlineStorage() ? getInlineStorage() : getOutOfLineStorage();
return (isTrivialCallback()
? getTrivialCallback()
: getNonTrivialCallbacks()->CallPtr)(CallableAddr, Params...);
explicit operator bool() const {
return (bool)CallbackAndInlineFlag.getPointer();
} // end namespace llvm