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 //===- Memory.h -------------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines arena allocators.
 //
 // Almost all large objects, such as files, sections or symbols, are
 // used for the entire lifetime of the linker once they are created.
 // This usage characteristic makes arena allocator an attractive choice
 // where the entire linker is one arena. With an arena, newly created
 // objects belong to the arena and freed all at once when everything is done.
 // Arena allocators are efficient and easy to understand.
 // Most objects are allocated using the arena allocators defined by this file.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_COMMON_MEMORY_H
 #define LLD_COMMON_MEMORY_H
 
 #include "llvm/Support/Allocator.h"
 
 namespace lld {
 // A base class only used by the CommonLinkerContext to keep track of the
 // SpecificAlloc<> instances.
 struct SpecificAllocBase {
   virtual ~SpecificAllocBase() = default;
   static SpecificAllocBase *getOrCreate(void *tag, size_t size, size_t align,
                                         SpecificAllocBase *(&creator)(void *));
 };
 
 // An arena of specific types T, created on-demand.
 template <class T> struct SpecificAlloc : public SpecificAllocBase {
   static SpecificAllocBase *create(void *storage) {
     return new (storage) SpecificAlloc<T>();
   }
   llvm::SpecificBumpPtrAllocator<T> alloc;
   static int tag;
 };
 
 // The address of this static member is only used as a key in
 // CommonLinkerContext::instances. Its value does not matter.
 template <class T> int SpecificAlloc<T>::tag = 0;
 
 // Creates the arena on-demand on the first call; or returns it, if it was
 // already created.
 template <typename T>
 inline llvm::SpecificBumpPtrAllocator<T> &getSpecificAllocSingleton() {
   SpecificAllocBase *instance = SpecificAllocBase::getOrCreate(
       &SpecificAlloc<T>::tag, sizeof(SpecificAlloc<T>),
       alignof(SpecificAlloc<T>), SpecificAlloc<T>::create);
   return ((SpecificAlloc<T> *)instance)->alloc;
 }
 
 // Creates new instances of T off a (almost) contiguous arena/object pool. The
 // instances are destroyed whenever lldMain() goes out of scope.
 template <typename T, typename... U> T *make(U &&... args) {
   return new (getSpecificAllocSingleton<T>().Allocate())
       T(std::forward<U>(args)...);
 }
 
 template <typename T>
 inline llvm::SpecificBumpPtrAllocator<T> &
 getSpecificAllocSingletonThreadLocal() {
   thread_local SpecificAlloc<T> instance;
   return instance.alloc;
 }
 
 // Create a new instance of T off a thread-local SpecificAlloc, used by code
 // like parallel input section initialization. The use cases assume that the
 // return value outlives the containing parallelForEach (if exists), which is
 // currently guaranteed: when parallelForEach returns, the threads allocating
 // the TLS are not destroyed.
 //
 // Note: Some ports (e.g. ELF) have lots of global states which are currently
 // infeasible to remove, and context() just adds overhead with no benefit. The
 // allocation performance is of higher importance, so we simply use thread_local
 // allocators instead of doing context indirection and pthread_getspecific.
 template <typename T, typename... U> T *makeThreadLocal(U &&...args) {
   return new (getSpecificAllocSingletonThreadLocal<T>().Allocate())
       T(std::forward<U>(args)...);
 }
 
 template <typename T> T *makeThreadLocalN(size_t n) {
   return new (getSpecificAllocSingletonThreadLocal<T>().Allocate(n)) T[n];
 }
 
 } // namespace lld
 
 #endif

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