ManagedStatic.h revision 363496 
1//===-- llvm/Support/ManagedStatic.h - Static Global wrapper ----*- 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 the ManagedStatic class and the llvm_shutdown() function.
10//
11//===----------------------------------------------------------------------===//
12
13#ifndef LLVM_SUPPORT_MANAGEDSTATIC_H
14#define LLVM_SUPPORT_MANAGEDSTATIC_H
15
16#include <atomic>
17#include <cstddef>
18
19namespace llvm {
20
21/// object_creator - Helper method for ManagedStatic.
22template <class C> struct object_creator {
23  static void *call() { return new C(); }
24};
25
26/// object_deleter - Helper method for ManagedStatic.
27///
28template <typename T> struct object_deleter {
29  static void call(void *Ptr) { delete (T *)Ptr; }
30};
31template <typename T, size_t N> struct object_deleter<T[N]> {
32  static void call(void *Ptr) { delete[](T *)Ptr; }
33};
34
35// ManagedStatic must be initialized to zero, and it must *not* have a dynamic
36// initializer because managed statics are often created while running other
37// dynamic initializers. In standard C++11, the best way to accomplish this is
38// with a constexpr default constructor. However, different versions of the
39// Visual C++ compiler have had bugs where, even though the constructor may be
40// constexpr, a dynamic initializer may be emitted depending on optimization
41// settings. For the affected versions of MSVC, use the old linker
42// initialization pattern of not providing a constructor and leaving the fields
43// uninitialized. See http://llvm.org/PR41367 for details.
44#if !defined(_MSC_VER) || (_MSC_VER >= 1925) || defined(__clang__)
45#define LLVM_USE_CONSTEXPR_CTOR
46#endif
47
48/// ManagedStaticBase - Common base class for ManagedStatic instances.
49class ManagedStaticBase {
50protected:
51#ifdef LLVM_USE_CONSTEXPR_CTOR
52  mutable std::atomic<void *> Ptr{};
53  mutable void (*DeleterFn)(void *) = nullptr;
54  mutable const ManagedStaticBase *Next = nullptr;
55#else
56  // This should only be used as a static variable, which guarantees that this
57  // will be zero initialized.
58  mutable std::atomic<void *> Ptr;
59  mutable void (*DeleterFn)(void *);
60  mutable const ManagedStaticBase *Next;
61#endif
62
63  void RegisterManagedStatic(void *(*creator)(), void (*deleter)(void*)) const;
64
65public:
66#ifdef LLVM_USE_CONSTEXPR_CTOR
67  constexpr ManagedStaticBase() = default;
68#endif
69
70  /// isConstructed - Return true if this object has not been created yet.
71  bool isConstructed() const { return Ptr != nullptr; }
72
73  void destroy() const;
74};
75
76/// ManagedStatic - This transparently changes the behavior of global statics to
77/// be lazily constructed on demand (good for reducing startup times of dynamic
78/// libraries that link in LLVM components) and for making destruction be
79/// explicit through the llvm_shutdown() function call.
80///
81template <class C, class Creator = object_creator<C>,
82          class Deleter = object_deleter<C>>
83class ManagedStatic : public ManagedStaticBase {
84public:
85  // Accessors.
86  C &operator*() {
87    void *Tmp = Ptr.load(std::memory_order_acquire);
88    if (!Tmp)
89      RegisterManagedStatic(Creator::call, Deleter::call);
90
91    return *static_cast<C *>(Ptr.load(std::memory_order_relaxed));
92  }
93
94  C *operator->() { return &**this; }
95
96  const C &operator*() const {
97    void *Tmp = Ptr.load(std::memory_order_acquire);
98    if (!Tmp)
99      RegisterManagedStatic(Creator::call, Deleter::call);
100
101    return *static_cast<C *>(Ptr.load(std::memory_order_relaxed));
102  }
103
104  const C *operator->() const { return &**this; }
105
106  // Extract the instance, leaving the ManagedStatic uninitialized. The
107  // user is then responsible for the lifetime of the returned instance.
108  C *claim() {
109    return static_cast<C *>(Ptr.exchange(nullptr));
110  }
111};
112
113/// llvm_shutdown - Deallocate and destroy all ManagedStatic variables.
114void llvm_shutdown();
115
116/// llvm_shutdown_obj - This is a simple helper class that calls
117/// llvm_shutdown() when it is destroyed.
118struct llvm_shutdown_obj {
119  llvm_shutdown_obj() = default;
120  ~llvm_shutdown_obj() { llvm_shutdown(); }
121};
122
123} // end namespace llvm
124
125#endif // LLVM_SUPPORT_MANAGEDSTATIC_H
126