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 
19 namespace llvm {
20 
21 /// object_creator - Helper method for ManagedStatic.
22 template <class C> struct object_creator {
callobject_creator23   static void *call() { return new C(); }
24 };
25 
26 /// object_deleter - Helper method for ManagedStatic.
27 ///
28 template <typename T> struct object_deleter {
callobject_deleter29   static void call(void *Ptr) { delete (T *)Ptr; }
30 };
31 template <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.
49 class ManagedStaticBase {
50 protected:
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 
65 public:
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 ///
81 template <class C, class Creator = object_creator<C>,
82           class Deleter = object_deleter<C>>
83 class ManagedStatic : public ManagedStaticBase {
84 public:
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.
114 void llvm_shutdown();
115 
116 /// llvm_shutdown_obj - This is a simple helper class that calls
117 /// llvm_shutdown() when it is destroyed.
118 struct 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