1 //===- Memory.h -------------------------------------------------*- 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 // This file defines arena allocators.
10 //
11 // Almost all large objects, such as files, sections or symbols, are
12 // used for the entire lifetime of the linker once they are created.
13 // This usage characteristic makes arena allocator an attractive choice
14 // where the entire linker is one arena. With an arena, newly created
15 // objects belong to the arena and freed all at once when everything is done.
16 // Arena allocators are efficient and easy to understand.
17 // Most objects are allocated using the arena allocators defined by this file.
18 //
19 //===----------------------------------------------------------------------===//
20
21 #ifndef LLD_COMMON_MEMORY_H
22 #define LLD_COMMON_MEMORY_H
23
24 #include "llvm/Support/Allocator.h"
25
26 namespace lld {
27 // A base class only used by the CommonLinkerContext to keep track of the
28 // SpecificAlloc<> instances.
29 struct SpecificAllocBase {
30 virtual ~SpecificAllocBase() = default;
31 static SpecificAllocBase *getOrCreate(void *tag, size_t size, size_t align,
32 SpecificAllocBase *(&creator)(void *));
33 };
34
35 // An arena of specific types T, created on-demand.
36 template <class T> struct SpecificAlloc : public SpecificAllocBase {
createSpecificAlloc37 static SpecificAllocBase *create(void *storage) {
38 return new (storage) SpecificAlloc<T>();
39 }
40 llvm::SpecificBumpPtrAllocator<T> alloc;
41 static int tag;
42 };
43
44 // The address of this static member is only used as a key in
45 // CommonLinkerContext::instances. Its value does not matter.
46 template <class T> int SpecificAlloc<T>::tag = 0;
47
48 // Creates the arena on-demand on the first call; or returns it, if it was
49 // already created.
50 template <typename T>
getSpecificAllocSingleton()51 inline llvm::SpecificBumpPtrAllocator<T> &getSpecificAllocSingleton() {
52 SpecificAllocBase *instance = SpecificAllocBase::getOrCreate(
53 &SpecificAlloc<T>::tag, sizeof(SpecificAlloc<T>),
54 alignof(SpecificAlloc<T>), SpecificAlloc<T>::create);
55 return ((SpecificAlloc<T> *)instance)->alloc;
56 }
57
58 // Creates new instances of T off a (almost) contiguous arena/object pool. The
59 // instances are destroyed whenever lldMain() goes out of scope.
make(U &&...args)60 template <typename T, typename... U> T *make(U &&... args) {
61 return new (getSpecificAllocSingleton<T>().Allocate())
62 T(std::forward<U>(args)...);
63 }
64
65 template <typename T>
66 inline llvm::SpecificBumpPtrAllocator<T> &
getSpecificAllocSingletonThreadLocal()67 getSpecificAllocSingletonThreadLocal() {
68 thread_local SpecificAlloc<T> instance;
69 return instance.alloc;
70 }
71
72 // Create a new instance of T off a thread-local SpecificAlloc, used by code
73 // like parallel input section initialization. The use cases assume that the
74 // return value outlives the containing parallelForEach (if exists), which is
75 // currently guaranteed: when parallelForEach returns, the threads allocating
76 // the TLS are not destroyed.
77 //
78 // Note: Some ports (e.g. ELF) have lots of global states which are currently
79 // infeasible to remove, and context() just adds overhead with no benefit. The
80 // allocation performance is of higher importance, so we simply use thread_local
81 // allocators instead of doing context indirection and pthread_getspecific.
makeThreadLocal(U &&...args)82 template <typename T, typename... U> T *makeThreadLocal(U &&...args) {
83 return new (getSpecificAllocSingletonThreadLocal<T>().Allocate())
84 T(std::forward<U>(args)...);
85 }
86
makeThreadLocalN(size_t n)87 template <typename T> T *makeThreadLocalN(size_t n) {
88 return new (getSpecificAllocSingletonThreadLocal<T>().Allocate(n)) T[n];
89 }
90
91 } // namespace lld
92
93 #endif
94