1 //===--- TrailingObjects.h - Variable-length classes ------------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
11 /// This header defines support for implementing classes that have
12 /// some trailing object (or arrays of objects) appended to them. The
13 /// main purpose is to make it obvious where this idiom is being used,
14 /// and to make the usage more idiomatic and more difficult to get
17 /// The TrailingObject template abstracts away the reinterpret_cast,
18 /// pointer arithmetic, and size calculations used for the allocation
19 /// and access of appended arrays of objects, and takes care that they
20 /// are all allocated at their required alignment. Additionally, it
21 /// ensures that the base type is final -- deriving from a class that
22 /// expects data appended immediately after it is typically not safe.
24 /// Users are expected to derive from this template, and provide
25 /// numTrailingObjects implementations for each trailing type except
26 /// the last, e.g. like this sample:
29 /// class VarLengthObj : private TrailingObjects<VarLengthObj, int, double> {
30 /// friend TrailingObjects;
32 /// unsigned NumInts, NumDoubles;
33 /// size_t numTrailingObjects(OverloadToken<int>) const { return NumInts; }
37 /// You can access the appended arrays via 'getTrailingObjects', and
38 /// determine the size needed for allocation via
39 /// 'additionalSizeToAlloc' and 'totalSizeToAlloc'.
41 /// All the methods implemented by this class are are intended for use
42 /// by the implementation of the class, not as part of its interface
43 /// (thus, private inheritance is suggested).
45 //===----------------------------------------------------------------------===//
47 #ifndef LLVM_SUPPORT_TRAILINGOBJECTS_H
48 #define LLVM_SUPPORT_TRAILINGOBJECTS_H
50 #include "llvm/Support/AlignOf.h"
51 #include "llvm/Support/Compiler.h"
52 #include "llvm/Support/MathExtras.h"
53 #include "llvm/Support/type_traits.h"
55 #include <type_traits>
59 namespace trailing_objects_internal {
60 /// Helper template to calculate the max alignment requirement for a set of
62 template <typename First, typename... Rest> class AlignmentCalcHelper {
65 FirstAlignment = alignof(First),
66 RestAlignment = AlignmentCalcHelper<Rest...>::Alignment,
71 Alignment = FirstAlignment > RestAlignment ? FirstAlignment : RestAlignment
75 template <typename First> class AlignmentCalcHelper<First> {
77 enum { Alignment = alignof(First) };
80 /// The base class for TrailingObjects* classes.
81 class TrailingObjectsBase {
83 /// OverloadToken's purpose is to allow specifying function overloads
84 /// for different types, without actually taking the types as
85 /// parameters. (Necessary because member function templates cannot
86 /// be specialized, so overloads must be used instead of
88 template <typename T> struct OverloadToken {};
91 /// This helper template works-around MSVC 2013's lack of useful
92 /// alignas() support. The argument to alignas(), in MSVC, is
93 /// required to be a literal integer. But, you *can* use template
94 /// specialization to select between a bunch of different alignas()
97 class TrailingObjectsAligner : public TrailingObjectsBase {};
99 class alignas(1) TrailingObjectsAligner<1> : public TrailingObjectsBase {};
101 class alignas(2) TrailingObjectsAligner<2> : public TrailingObjectsBase {};
103 class alignas(4) TrailingObjectsAligner<4> : public TrailingObjectsBase {};
105 class alignas(8) TrailingObjectsAligner<8> : public TrailingObjectsBase {};
107 class alignas(16) TrailingObjectsAligner<16> : public TrailingObjectsBase {
110 class alignas(32) TrailingObjectsAligner<32> : public TrailingObjectsBase {
113 // Just a little helper for transforming a type pack into the same
114 // number of a different type. e.g.:
115 // ExtractSecondType<Foo..., int>::type
116 template <typename Ty1, typename Ty2> struct ExtractSecondType {
120 // TrailingObjectsImpl is somewhat complicated, because it is a
121 // recursively inheriting template, in order to handle the template
122 // varargs. Each level of inheritance picks off a single trailing type
123 // then recurses on the rest. The "Align", "BaseTy", and
124 // "TopTrailingObj" arguments are passed through unchanged through the
125 // recursion. "PrevTy" is, at each level, the type handled by the
126 // level right above it.
128 template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy,
130 class TrailingObjectsImpl {
131 // The main template definition is never used -- the two
132 // specializations cover all possibilities.
135 template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy,
136 typename NextTy, typename... MoreTys>
137 class TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy, NextTy,
139 : public TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, NextTy,
142 typedef TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, NextTy, MoreTys...>
145 struct RequiresRealignment {
146 static const bool value = alignof(PrevTy) < alignof(NextTy);
149 static constexpr bool requiresRealignment() {
150 return RequiresRealignment::value;
154 // Ensure the inherited getTrailingObjectsImpl is not hidden.
155 using ParentType::getTrailingObjectsImpl;
157 // These two functions are helper functions for
158 // TrailingObjects::getTrailingObjects. They recurse to the left --
159 // the result for each type in the list of trailing types depends on
160 // the result of calling the function on the type to the
161 // left. However, the function for the type to the left is
162 // implemented by a *subclass* of this class, so we invoke it via
163 // the TopTrailingObj, which is, via the
164 // curiously-recurring-template-pattern, the most-derived type in
165 // this recursion, and thus, contains all the overloads.
166 static const NextTy *
167 getTrailingObjectsImpl(const BaseTy *Obj,
168 TrailingObjectsBase::OverloadToken<NextTy>) {
169 auto *Ptr = TopTrailingObj::getTrailingObjectsImpl(
170 Obj, TrailingObjectsBase::OverloadToken<PrevTy>()) +
171 TopTrailingObj::callNumTrailingObjects(
172 Obj, TrailingObjectsBase::OverloadToken<PrevTy>());
174 if (requiresRealignment())
175 return reinterpret_cast<const NextTy *>(
176 llvm::alignAddr(Ptr, alignof(NextTy)));
178 return reinterpret_cast<const NextTy *>(Ptr);
182 getTrailingObjectsImpl(BaseTy *Obj,
183 TrailingObjectsBase::OverloadToken<NextTy>) {
184 auto *Ptr = TopTrailingObj::getTrailingObjectsImpl(
185 Obj, TrailingObjectsBase::OverloadToken<PrevTy>()) +
186 TopTrailingObj::callNumTrailingObjects(
187 Obj, TrailingObjectsBase::OverloadToken<PrevTy>());
189 if (requiresRealignment())
190 return reinterpret_cast<NextTy *>(llvm::alignAddr(Ptr, alignof(NextTy)));
192 return reinterpret_cast<NextTy *>(Ptr);
195 // Helper function for TrailingObjects::additionalSizeToAlloc: this
196 // function recurses to superclasses, each of which requires one
197 // fewer size_t argument, and adds its own size.
198 static constexpr size_t additionalSizeToAllocImpl(
199 size_t SizeSoFar, size_t Count1,
200 typename ExtractSecondType<MoreTys, size_t>::type... MoreCounts) {
201 return ParentType::additionalSizeToAllocImpl(
202 (requiresRealignment() ? llvm::alignTo<alignof(NextTy)>(SizeSoFar)
204 sizeof(NextTy) * Count1,
209 // The base case of the TrailingObjectsImpl inheritance recursion,
210 // when there's no more trailing types.
211 template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy>
212 class TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy>
213 : public TrailingObjectsAligner<Align> {
215 // This is a dummy method, only here so the "using" doesn't fail --
216 // it will never be called, because this function recurses backwards
217 // up the inheritance chain to subclasses.
218 static void getTrailingObjectsImpl();
220 static constexpr size_t additionalSizeToAllocImpl(size_t SizeSoFar) {
224 template <bool CheckAlignment> static void verifyTrailingObjectsAlignment() {}
227 } // end namespace trailing_objects_internal
229 // Finally, the main type defined in this file, the one intended for users...
231 /// See the file comment for details on the usage of the
232 /// TrailingObjects type.
233 template <typename BaseTy, typename... TrailingTys>
234 class TrailingObjects : private trailing_objects_internal::TrailingObjectsImpl<
235 trailing_objects_internal::AlignmentCalcHelper<
236 TrailingTys...>::Alignment,
237 BaseTy, TrailingObjects<BaseTy, TrailingTys...>,
238 BaseTy, TrailingTys...> {
240 template <int A, typename B, typename T, typename P, typename... M>
241 friend class trailing_objects_internal::TrailingObjectsImpl;
243 template <typename... Tys> class Foo {};
245 typedef trailing_objects_internal::TrailingObjectsImpl<
246 trailing_objects_internal::AlignmentCalcHelper<TrailingTys...>::Alignment,
247 BaseTy, TrailingObjects<BaseTy, TrailingTys...>, BaseTy, TrailingTys...>
249 using TrailingObjectsBase = trailing_objects_internal::TrailingObjectsBase;
251 using ParentType::getTrailingObjectsImpl;
253 // This function contains only a static_assert BaseTy is final. The
254 // static_assert must be in a function, and not at class-level
255 // because BaseTy isn't complete at class instantiation time, but
256 // will be by the time this function is instantiated.
257 static void verifyTrailingObjectsAssertions() {
259 static_assert(LLVM_IS_FINAL(BaseTy), "BaseTy must be final.");
263 // These two methods are the base of the recursion for this method.
264 static const BaseTy *
265 getTrailingObjectsImpl(const BaseTy *Obj,
266 TrailingObjectsBase::OverloadToken<BaseTy>) {
271 getTrailingObjectsImpl(BaseTy *Obj,
272 TrailingObjectsBase::OverloadToken<BaseTy>) {
276 // callNumTrailingObjects simply calls numTrailingObjects on the
277 // provided Obj -- except when the type being queried is BaseTy
278 // itself. There is always only one of the base object, so that case
279 // is handled here. (An additional benefit of indirecting through
280 // this function is that consumers only say "friend
281 // TrailingObjects", and thus, only this class itself can call the
282 // numTrailingObjects function.)
284 callNumTrailingObjects(const BaseTy *Obj,
285 TrailingObjectsBase::OverloadToken<BaseTy>) {
289 template <typename T>
290 static size_t callNumTrailingObjects(const BaseTy *Obj,
291 TrailingObjectsBase::OverloadToken<T>) {
292 return Obj->numTrailingObjects(TrailingObjectsBase::OverloadToken<T>());
296 // Make this (privately inherited) member public.
298 using ParentType::OverloadToken;
300 // MSVC bug prevents the above from working, at least up through CL
302 template <typename T>
303 using OverloadToken = typename ParentType::template OverloadToken<T>;
306 /// Returns a pointer to the trailing object array of the given type
307 /// (which must be one of those specified in the class template). The
308 /// array may have zero or more elements in it.
309 template <typename T> const T *getTrailingObjects() const {
310 verifyTrailingObjectsAssertions();
311 // Forwards to an impl function with overloads, since member
312 // function templates can't be specialized.
313 return this->getTrailingObjectsImpl(
314 static_cast<const BaseTy *>(this),
315 TrailingObjectsBase::OverloadToken<T>());
318 /// Returns a pointer to the trailing object array of the given type
319 /// (which must be one of those specified in the class template). The
320 /// array may have zero or more elements in it.
321 template <typename T> T *getTrailingObjects() {
322 verifyTrailingObjectsAssertions();
323 // Forwards to an impl function with overloads, since member
324 // function templates can't be specialized.
325 return this->getTrailingObjectsImpl(
326 static_cast<BaseTy *>(this), TrailingObjectsBase::OverloadToken<T>());
329 /// Returns the size of the trailing data, if an object were
330 /// allocated with the given counts (The counts are in the same order
331 /// as the template arguments). This does not include the size of the
332 /// base object. The template arguments must be the same as those
333 /// used in the class; they are supplied here redundantly only so
334 /// that it's clear what the counts are counting in callers.
335 template <typename... Tys>
336 static constexpr typename std::enable_if<
337 std::is_same<Foo<TrailingTys...>, Foo<Tys...>>::value, size_t>::type
338 additionalSizeToAlloc(typename trailing_objects_internal::ExtractSecondType<
339 TrailingTys, size_t>::type... Counts) {
340 return ParentType::additionalSizeToAllocImpl(0, Counts...);
343 /// Returns the total size of an object if it were allocated with the
344 /// given trailing object counts. This is the same as
345 /// additionalSizeToAlloc, except it *does* include the size of the base
347 template <typename... Tys>
348 static constexpr typename std::enable_if<
349 std::is_same<Foo<TrailingTys...>, Foo<Tys...>>::value, size_t>::type
350 totalSizeToAlloc(typename trailing_objects_internal::ExtractSecondType<
351 TrailingTys, size_t>::type... Counts) {
352 return sizeof(BaseTy) + ParentType::additionalSizeToAllocImpl(0, Counts...);
355 /// A type where its ::with_counts template member has a ::type member
356 /// suitable for use as uninitialized storage for an object with the given
357 /// trailing object counts. The template arguments are similar to those
358 /// of additionalSizeToAlloc.
360 /// Use with FixedSizeStorageOwner, e.g.:
364 /// MyObj::FixedSizeStorage<void *>::with_counts<1u>::type myStackObjStorage;
365 /// MyObj::FixedSizeStorageOwner
366 /// myStackObjOwner(new ((void *)&myStackObjStorage) MyObj);
367 /// MyObj *const myStackObjPtr = myStackObjOwner.get();
370 template <typename... Tys> struct FixedSizeStorage {
371 template <size_t... Counts> struct with_counts {
372 enum { Size = totalSizeToAlloc<Tys...>(Counts...) };
373 typedef llvm::AlignedCharArray<alignof(BaseTy), Size> type;
377 /// A type that acts as the owner for an object placed into fixed storage.
378 class FixedSizeStorageOwner {
380 FixedSizeStorageOwner(BaseTy *p) : p(p) {}
381 ~FixedSizeStorageOwner() {
382 assert(p && "FixedSizeStorageOwner owns null?");
386 BaseTy *get() { return p; }
387 const BaseTy *get() const { return p; }
390 FixedSizeStorageOwner(const FixedSizeStorageOwner &) = delete;
391 FixedSizeStorageOwner(FixedSizeStorageOwner &&) = delete;
392 FixedSizeStorageOwner &operator=(const FixedSizeStorageOwner &) = delete;
393 FixedSizeStorageOwner &operator=(FixedSizeStorageOwner &&) = delete;
399 } // end namespace llvm