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[ADT] Support const-qualified unique_functions

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Summary:
This technique should extend to rvalue-qualified etc, but I didn't add any.
I removed "volatile" from the future plans, which seems... speculative at best.

While here I moved the callbacks object out of the constructor into a
variable template, which I believe addresses the fixme there about unused
objects.

(I'm not a template guru, so it's always possible the old version was designed
for compile-time performance in a way I'm missing)

Reviewers: kadircet

Subscribers: dexonsmith, llvm-commits, chandlerc

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82581
This commit is contained in:
Sam McCall 2020-06-25 18:25:53 +02:00
parent 8b6c832604
commit 8618b3d880
2 changed files with 194 additions and 72 deletions
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228
include/llvm/ADT/FunctionExtras.h
View File

@ -11,11 +11,11 @@
/// in `<function>`.
///
/// It provides `unique_function`, which works like `std::function` but supports
/// move-only callable objects.
/// move-only callable objects and const-qualification.
///
/// Future plans:
/// - Add a `function` that provides const, volatile, and ref-qualified support,
/// which doesn't work with `std::function`.
/// - 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
@ -37,13 +37,31 @@
#include "llvm/Support/MemAlloc.h"
#include "llvm/Support/type_traits.h"
#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;
template <typename ReturnT, typename... ParamTs>
class unique_function<ReturnT(ParamTs...)> {
namespace detail {
template <typename T>
using EnableIfTrivial =
std::enable_if_t<llvm::is_trivially_move_constructible<T>::value &&
std::is_trivially_destructible<T>::value>;
template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
protected:
static constexpr size_t InlineStorageSize = sizeof(void *) * 3;
// MSVC has a bug and ICEs if we give it a particular dependent value
@ -113,8 +131,11 @@ class unique_function<ReturnT(ParamTs...)> {
// 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
InlineStorage;
// This is mutable as an inlined `const unique_function<void() const>` may
// still modify its own mutable members.
mutable
typename std::aligned_storage<InlineStorageSize, alignof(void *)>::type
InlineStorage;
} StorageUnion;
// A compressed pointer to either our dispatching callback or our table of
@ -137,11 +158,25 @@ class unique_function<ReturnT(ParamTs...)> {
.template get<NonTrivialCallbacks *>();
}
void *getInlineStorage() { return &StorageUnion.InlineStorage; }
CallPtrT getCallPtr() const {
return isTrivialCallback() ? getTrivialCallback()
: getNonTrivialCallbacks()->CallPtr;
}
void *getOutOfLineStorage() {
// 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;
}
@ -153,10 +188,11 @@ class unique_function<ReturnT(ParamTs...)> {
StorageUnion.OutOfLineStorage = {Ptr, Size, Alignment};
}
template <typename CallableT>
static ReturnT CallImpl(void *CallableAddr, AdjustedParamT<ParamTs>... Params) {
return (*reinterpret_cast<CallableT *>(CallableAddr))(
std::forward<ParamTs>(Params)...);
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>
@ -170,11 +206,49 @@ class unique_function<ReturnT(ParamTs...)> {
reinterpret_cast<CallableT *>(CallableAddr)->~CallableT();
}
public:
unique_function() = default;
unique_function(std::nullptr_t /*null_callable*/) {}
// 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).
~unique_function() {
// 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.
template <typename CallableT, typename CalledAs, typename Enable = void>
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>
static TrivialCallback
Callbacks<CallableT, CalledAs, EnableIfTrivial<CallableT>>;
// 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));
CallbackAndInlineFlag = {&Callbacks<CallableT, CalledAsT>, IsInlineStorage};
}
~UniqueFunctionBase() {
if (!CallbackAndInlineFlag.getPointer())
return;
@ -190,7 +264,7 @@ public:
getOutOfLineStorageAlignment());
}
unique_function(unique_function &&RHS) noexcept {
UniqueFunctionBase(UniqueFunctionBase &&RHS) noexcept {
// Copy the callback and inline flag.
CallbackAndInlineFlag = RHS.CallbackAndInlineFlag;
@ -219,75 +293,85 @@ public:
#endif
}
unique_function &operator=(unique_function &&RHS) noexcept {
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.
this->~unique_function();
new (this) unique_function(std::move(RHS));
this->~UniqueFunctionBase();
new (this) UniqueFunctionBase(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};
return;
}
// 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...);
}
UniqueFunctionBase() = default;
public:
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...>::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...>::Callbacks<CallableT, CalledAsT, EnableIfTrivial<CallableT>>{
&CallImpl<CalledAsT>};
} // namespace detail
template <typename R, typename... P>
class unique_function<R(P...)> : public detail::UniqueFunctionBase<R, P...> {
using Base = detail::UniqueFunctionBase<R, P...>;
public:
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>
unique_function(CallableT Callable)
: 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...>;
public:
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>
unique_function(CallableT Callable)
: 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
#endif // LLVM_ADT_FUNCTION_H

38
unittests/ADT/FunctionExtrasTest.cpp
View File

@ -10,6 +10,7 @@
#include "gtest/gtest.h"
#include <memory>
#include <type_traits>
using namespace llvm;
@ -224,4 +225,41 @@ TEST(UniqueFunctionTest, CountForwardingMoves) {
UnmovableF(X);
}
TEST(UniqueFunctionTest, Const) {
// Can assign from const lambda.
unique_function<int(int) const> Plus2 = [X(std::make_unique<int>(2))](int Y) {
return *X + Y;
};
EXPECT_EQ(5, Plus2(3));
// Can call through a const ref.
const auto &Plus2Ref = Plus2;
EXPECT_EQ(5, Plus2Ref(3));
// Can move-construct and assign.
unique_function<int(int) const> Plus2A = std::move(Plus2);
EXPECT_EQ(5, Plus2A(3));
unique_function<int(int) const> Plus2B;
Plus2B = std::move(Plus2A);
EXPECT_EQ(5, Plus2B(3));
// Can convert to non-const function type, but not back.
unique_function<int(int)> Plus2C = std::move(Plus2B);
EXPECT_EQ(5, Plus2C(3));
// Overloaded call operator correctly resolved.
struct ChooseCorrectOverload {
StringRef operator()() { return "non-const"; }
StringRef operator()() const { return "const"; }
};
unique_function<StringRef()> ChooseMutable = ChooseCorrectOverload();
ChooseCorrectOverload A;
EXPECT_EQ("non-const", ChooseMutable());
EXPECT_EQ("non-const", A());
unique_function<StringRef() const> ChooseConst = ChooseCorrectOverload();
const ChooseCorrectOverload &X = A;
EXPECT_EQ("const", ChooseConst());
EXPECT_EQ("const", X());
}
} // anonymous namespace