1//===--- TrailingObjects.h - Variable-length classes ------------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8///
9/// \file
10/// This header defines support for implementing classes that have
11/// some trailing object (or arrays of objects) appended to them. The
12/// main purpose is to make it obvious where this idiom is being used,
13/// and to make the usage more idiomatic and more difficult to get
14/// wrong.
15///
16/// The TrailingObject template abstracts away the reinterpret_cast,
17/// pointer arithmetic, and size calculations used for the allocation
18/// and access of appended arrays of objects, and takes care that they
19/// are all allocated at their required alignment. Additionally, it
20/// ensures that the base type is final -- deriving from a class that
21/// expects data appended immediately after it is typically not safe.
22///
23/// Users are expected to derive from this template, and provide
24/// numTrailingObjects implementations for each trailing type except
25/// the last, e.g. like this sample:
26///
27/// \code
28/// class VarLengthObj : private TrailingObjects<VarLengthObj, int, double> {
29/// friend TrailingObjects;
30///
31/// unsigned NumInts, NumDoubles;
32/// size_t numTrailingObjects(OverloadToken<int>) const { return NumInts; }
33/// };
34/// \endcode
35///
36/// You can access the appended arrays via 'getTrailingObjects', and
37/// determine the size needed for allocation via
38/// 'additionalSizeToAlloc' and 'totalSizeToAlloc'.
39///
40/// All the methods implemented by this class are are intended for use
41/// by the implementation of the class, not as part of its interface
42/// (thus, private inheritance is suggested).
43///
44//===----------------------------------------------------------------------===//
45
46#ifndef LLVM_SUPPORT_TRAILINGOBJECTS_H
47#define LLVM_SUPPORT_TRAILINGOBJECTS_H
48
49#include "llvm/Support/AlignOf.h"
50#include "llvm/Support/Alignment.h"
51#include "llvm/Support/Compiler.h"
52#include "llvm/Support/MathExtras.h"
53#include "llvm/Support/type_traits.h"
54#include <new>
55#include <type_traits>
56
57namespace llvm {
58
59namespace trailing_objects_internal {
60/// Helper template to calculate the max alignment requirement for a set of
61/// objects.
62template <typename First, typename... Rest> class AlignmentCalcHelper {
63private:
64 enum {
65 FirstAlignment = alignof(First),
66 RestAlignment = AlignmentCalcHelper<Rest...>::Alignment,
67 };
68
69public:
70 enum {
71 Alignment = FirstAlignment > RestAlignment ? FirstAlignment : RestAlignment
72 };
73};
74
75template <typename First> class AlignmentCalcHelper<First> {
76public:
77 enum { Alignment = alignof(First) };
78};
79
80/// The base class for TrailingObjects* classes.
81class TrailingObjectsBase {
82protected:
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
87 /// specialization.)
88 template <typename T> struct OverloadToken {};
89};
90
91// Just a little helper for transforming a type pack into the same
92// number of a different type. e.g.:
93// ExtractSecondType<Foo..., int>::type
94template <typename Ty1, typename Ty2> struct ExtractSecondType {
95 typedef Ty2 type;
96};
97
98// TrailingObjectsImpl is somewhat complicated, because it is a
99// recursively inheriting template, in order to handle the template
100// varargs. Each level of inheritance picks off a single trailing type
101// then recurses on the rest. The "Align", "BaseTy", and
102// "TopTrailingObj" arguments are passed through unchanged through the
103// recursion. "PrevTy" is, at each level, the type handled by the
104// level right above it.
105
106template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy,
107 typename... MoreTys>
108class TrailingObjectsImpl {
109 // The main template definition is never used -- the two
110 // specializations cover all possibilities.
111};
112
113template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy,
114 typename NextTy, typename... MoreTys>
115class TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy, NextTy,
116 MoreTys...>
117 : public TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, NextTy,
118 MoreTys...> {
119
120 typedef TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, NextTy, MoreTys...>
121 ParentType;
122
123 struct RequiresRealignment {
124 static const bool value = alignof(PrevTy) < alignof(NextTy);
125 };
126
127 static constexpr bool requiresRealignment() {
128 return RequiresRealignment::value;
129 }
130
131protected:
132 // Ensure the inherited getTrailingObjectsImpl is not hidden.
133 using ParentType::getTrailingObjectsImpl;
134
135 // These two functions are helper functions for
136 // TrailingObjects::getTrailingObjects. They recurse to the left --
137 // the result for each type in the list of trailing types depends on
138 // the result of calling the function on the type to the
139 // left. However, the function for the type to the left is
140 // implemented by a *subclass* of this class, so we invoke it via
141 // the TopTrailingObj, which is, via the
142 // curiously-recurring-template-pattern, the most-derived type in
143 // this recursion, and thus, contains all the overloads.
144 static const NextTy *
145 getTrailingObjectsImpl(const BaseTy *Obj,
146 TrailingObjectsBase::OverloadToken<NextTy>) {
147 auto *Ptr = TopTrailingObj::getTrailingObjectsImpl(
148 Obj, TrailingObjectsBase::OverloadToken<PrevTy>()) +
149 TopTrailingObj::callNumTrailingObjects(
150 Obj, TrailingObjectsBase::OverloadToken<PrevTy>());
151
152 if (requiresRealignment())
153 return reinterpret_cast<const NextTy *>(
154 alignAddr(Ptr, Align::Of<NextTy>()));
155 else
156 return reinterpret_cast<const NextTy *>(Ptr);
157 }
158
159 static NextTy *
160 getTrailingObjectsImpl(BaseTy *Obj,
161 TrailingObjectsBase::OverloadToken<NextTy>) {
162 auto *Ptr = TopTrailingObj::getTrailingObjectsImpl(
163 Obj, TrailingObjectsBase::OverloadToken<PrevTy>()) +
164 TopTrailingObj::callNumTrailingObjects(
165 Obj, TrailingObjectsBase::OverloadToken<PrevTy>());
166
167 if (requiresRealignment())
168 return reinterpret_cast<NextTy *>(alignAddr(Ptr, Align::Of<NextTy>()));
169 else
170 return reinterpret_cast<NextTy *>(Ptr);
171 }
172
173 // Helper function for TrailingObjects::additionalSizeToAlloc: this
174 // function recurses to superclasses, each of which requires one
175 // fewer size_t argument, and adds its own size.
176 static constexpr size_t additionalSizeToAllocImpl(
177 size_t SizeSoFar, size_t Count1,
178 typename ExtractSecondType<MoreTys, size_t>::type... MoreCounts) {
179 return ParentType::additionalSizeToAllocImpl(
180 (requiresRealignment() ? llvm::alignTo<alignof(NextTy)>(SizeSoFar)
181 : SizeSoFar) +
182 sizeof(NextTy) * Count1,
183 MoreCounts...);
184 }
185};
186
187// The base case of the TrailingObjectsImpl inheritance recursion,
188// when there's no more trailing types.
189template <int Align, typename BaseTy, typename TopTrailingObj, typename PrevTy>
190class alignas(Align) TrailingObjectsImpl<Align, BaseTy, TopTrailingObj, PrevTy>
191 : public TrailingObjectsBase {
192protected:
193 // This is a dummy method, only here so the "using" doesn't fail --
194 // it will never be called, because this function recurses backwards
195 // up the inheritance chain to subclasses.
196 static void getTrailingObjectsImpl();
197
198 static constexpr size_t additionalSizeToAllocImpl(size_t SizeSoFar) {
199 return SizeSoFar;
200 }
201
202 template <bool CheckAlignment> static void verifyTrailingObjectsAlignment() {}
203};
204
205} // end namespace trailing_objects_internal
206
207// Finally, the main type defined in this file, the one intended for users...
208
209/// See the file comment for details on the usage of the
210/// TrailingObjects type.
211template <typename BaseTy, typename... TrailingTys>
212class TrailingObjects : private trailing_objects_internal::TrailingObjectsImpl<
213 trailing_objects_internal::AlignmentCalcHelper<
214 TrailingTys...>::Alignment,
215 BaseTy, TrailingObjects<BaseTy, TrailingTys...>,
216 BaseTy, TrailingTys...> {
217
218 template <int A, typename B, typename T, typename P, typename... M>
219 friend class trailing_objects_internal::TrailingObjectsImpl;
220
221 template <typename... Tys> class Foo {};
222
223 typedef trailing_objects_internal::TrailingObjectsImpl<
224 trailing_objects_internal::AlignmentCalcHelper<TrailingTys...>::Alignment,
225 BaseTy, TrailingObjects<BaseTy, TrailingTys...>, BaseTy, TrailingTys...>
226 ParentType;
227 using TrailingObjectsBase = trailing_objects_internal::TrailingObjectsBase;
228
229 using ParentType::getTrailingObjectsImpl;
230
231 // This function contains only a static_assert BaseTy is final. The
232 // static_assert must be in a function, and not at class-level
233 // because BaseTy isn't complete at class instantiation time, but
234 // will be by the time this function is instantiated.
235 static void verifyTrailingObjectsAssertions() {
236 static_assert(std::is_final<BaseTy>(), "BaseTy must be final.");
237 }
238
239 // These two methods are the base of the recursion for this method.
240 static const BaseTy *
241 getTrailingObjectsImpl(const BaseTy *Obj,
242 TrailingObjectsBase::OverloadToken<BaseTy>) {
243 return Obj;
244 }
245
246 static BaseTy *
247 getTrailingObjectsImpl(BaseTy *Obj,
248 TrailingObjectsBase::OverloadToken<BaseTy>) {
249 return Obj;
250 }
251
252 // callNumTrailingObjects simply calls numTrailingObjects on the
253 // provided Obj -- except when the type being queried is BaseTy
254 // itself. There is always only one of the base object, so that case
255 // is handled here. (An additional benefit of indirecting through
256 // this function is that consumers only say "friend
257 // TrailingObjects", and thus, only this class itself can call the
258 // numTrailingObjects function.)
259 static size_t
260 callNumTrailingObjects(const BaseTy *Obj,
261 TrailingObjectsBase::OverloadToken<BaseTy>) {
262 return 1;
263 }
264
265 template <typename T>
266 static size_t callNumTrailingObjects(const BaseTy *Obj,
267 TrailingObjectsBase::OverloadToken<T>) {
268 return Obj->numTrailingObjects(TrailingObjectsBase::OverloadToken<T>());
269 }
270
271public:
272 // Make this (privately inherited) member public.
273#ifndef _MSC_VER
274 using ParentType::OverloadToken;
275#else
276 // An MSVC bug prevents the above from working, (last tested at CL version
277 // 19.28). "Class5" in TrailingObjectsTest.cpp tests the problematic case.
278 template <typename T>
279 using OverloadToken = typename ParentType::template OverloadToken<T>;
280#endif
281
282 /// Returns a pointer to the trailing object array of the given type
283 /// (which must be one of those specified in the class template). The
284 /// array may have zero or more elements in it.
285 template <typename T> const T *getTrailingObjects() const {
286 verifyTrailingObjectsAssertions();
287 // Forwards to an impl function with overloads, since member
288 // function templates can't be specialized.
289 return this->getTrailingObjectsImpl(
290 static_cast<const BaseTy *>(this),
291 TrailingObjectsBase::OverloadToken<T>());
292 }
293
294 /// Returns a pointer to the trailing object array of the given type
295 /// (which must be one of those specified in the class template). The
296 /// array may have zero or more elements in it.
297 template <typename T> T *getTrailingObjects() {
298 verifyTrailingObjectsAssertions();
299 // Forwards to an impl function with overloads, since member
300 // function templates can't be specialized.
301 return this->getTrailingObjectsImpl(
302 static_cast<BaseTy *>(this), TrailingObjectsBase::OverloadToken<T>());
303 }
304
305 /// Returns the size of the trailing data, if an object were
306 /// allocated with the given counts (The counts are in the same order
307 /// as the template arguments). This does not include the size of the
308 /// base object. The template arguments must be the same as those
309 /// used in the class; they are supplied here redundantly only so
310 /// that it's clear what the counts are counting in callers.
311 template <typename... Tys>
312 static constexpr std::enable_if_t<
313 std::is_same<Foo<TrailingTys...>, Foo<Tys...>>::value, size_t>
314 additionalSizeToAlloc(typename trailing_objects_internal::ExtractSecondType<
315 TrailingTys, size_t>::type... Counts) {
316 return ParentType::additionalSizeToAllocImpl(0, Counts...);
317 }
318
319 /// Returns the total size of an object if it were allocated with the
320 /// given trailing object counts. This is the same as
321 /// additionalSizeToAlloc, except it *does* include the size of the base
322 /// object.
323 template <typename... Tys>
324 static constexpr std::enable_if_t<
325 std::is_same<Foo<TrailingTys...>, Foo<Tys...>>::value, size_t>
326 totalSizeToAlloc(typename trailing_objects_internal::ExtractSecondType<
327 TrailingTys, size_t>::type... Counts) {
328 return sizeof(BaseTy) + ParentType::additionalSizeToAllocImpl(0, Counts...);
329 }
330
331 TrailingObjects() = default;
332 TrailingObjects(const TrailingObjects &) = delete;
333 TrailingObjects(TrailingObjects &&) = delete;
334 TrailingObjects &operator=(const TrailingObjects &) = delete;
335 TrailingObjects &operator=(TrailingObjects &&) = delete;
336
337 /// A type where its ::with_counts template member has a ::type member
338 /// suitable for use as uninitialized storage for an object with the given
339 /// trailing object counts. The template arguments are similar to those
340 /// of additionalSizeToAlloc.
341 ///
342 /// Use with FixedSizeStorageOwner, e.g.:
343 ///
344 /// \code{.cpp}
345 ///
346 /// MyObj::FixedSizeStorage<void *>::with_counts<1u>::type myStackObjStorage;
347 /// MyObj::FixedSizeStorageOwner
348 /// myStackObjOwner(new ((void *)&myStackObjStorage) MyObj);
349 /// MyObj *const myStackObjPtr = myStackObjOwner.get();
350 ///
351 /// \endcode
352 template <typename... Tys> struct FixedSizeStorage {
353 template <size_t... Counts> struct with_counts {
354 enum { Size = totalSizeToAlloc<Tys...>(Counts...) };
355 struct type {
356 alignas(BaseTy) char buffer[Size];
357 };
358 };
359 };
360
361 /// A type that acts as the owner for an object placed into fixed storage.
362 class FixedSizeStorageOwner {
363 public:
364 FixedSizeStorageOwner(BaseTy *p) : p(p) {}
365 ~FixedSizeStorageOwner() {
366 assert(p && "FixedSizeStorageOwner owns null?");
367 p->~BaseTy();
368 }
369
370 BaseTy *get() { return p; }
371 const BaseTy *get() const { return p; }
372
373 private:
374 FixedSizeStorageOwner(const FixedSizeStorageOwner &) = delete;
375 FixedSizeStorageOwner(FixedSizeStorageOwner &&) = delete;
376 FixedSizeStorageOwner &operator=(const FixedSizeStorageOwner &) = delete;
377 FixedSizeStorageOwner &operator=(FixedSizeStorageOwner &&) = delete;
378
379 BaseTy *const p;
380 };
381};
382
383} // end namespace llvm
384
385#endif
386