| // Copyright (c) 2011 The LevelDB Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. See the AUTHORS file for names of contributors. |
| |
| // AtomicPointer provides storage for a lock-free pointer. |
| // Platform-dependent implementation of AtomicPointer: |
| // - If the platform provides a cheap barrier, we use it with raw pointers |
| // - If cstdatomic is present (on newer versions of gcc, it is), we use |
| // a cstdatomic-based AtomicPointer. However we prefer the memory |
| // barrier based version, because at least on a gcc 4.4 32-bit build |
| // on linux, we have encountered a buggy <cstdatomic> |
| // implementation. Also, some <cstdatomic> implementations are much |
| // slower than a memory-barrier based implementation (~16ns for |
| // <cstdatomic> based acquire-load vs. ~1ns for a barrier based |
| // acquire-load). |
| // This code is based on atomicops-internals-* in Google's perftools: |
| // http://code.google.com/p/google-perftools/source/browse/#svn%2Ftrunk%2Fsrc%2Fbase |
| |
| #ifndef PORT_ATOMIC_POINTER_H_ |
| #define PORT_ATOMIC_POINTER_H_ |
| |
| #include <stdint.h> |
| #ifdef LEVELDB_CSTDATOMIC_PRESENT |
| #include <cstdatomic> |
| #endif |
| #ifdef OS_WIN |
| #include <windows.h> |
| #endif |
| #ifdef OS_MACOSX |
| #include <libkern/OSAtomic.h> |
| #endif |
| |
| #if defined(_M_X64) || defined(__x86_64__) |
| #define ARCH_CPU_X86_FAMILY 1 |
| #elif defined(_M_IX86) || defined(__i386__) || defined(__i386) |
| #define ARCH_CPU_X86_FAMILY 1 |
| #elif defined(__ARMEL__) |
| #define ARCH_CPU_ARM_FAMILY 1 |
| #endif |
| |
| namespace leveldb { |
| namespace port { |
| |
| // Define MemoryBarrier() if available |
| // Windows on x86 |
| #if defined(OS_WIN) && defined(COMPILER_MSVC) && defined(ARCH_CPU_X86_FAMILY) |
| // windows.h already provides a MemoryBarrier(void) macro |
| // http://msdn.microsoft.com/en-us/library/ms684208(v=vs.85).aspx |
| #define LEVELDB_HAVE_MEMORY_BARRIER |
| |
| // Gcc on x86 |
| #elif defined(__GNUC__) && defined(ARCH_CPU_X86_FAMILY) |
| inline void MemoryBarrier() { |
| // See http://gcc.gnu.org/ml/gcc/2003-04/msg01180.html for a discussion on |
| // this idiom. Also see http://en.wikipedia.org/wiki/Memory_ordering. |
| __asm__ __volatile__("" : : : "memory"); |
| } |
| #define LEVELDB_HAVE_MEMORY_BARRIER |
| |
| // Mac OS |
| #elif defined(OS_MACOSX) |
| inline void MemoryBarrier() { |
| OSMemoryBarrier(); |
| } |
| #define LEVELDB_HAVE_MEMORY_BARRIER |
| |
| // ARM |
| #elif defined(ARCH_CPU_ARM_FAMILY) |
| typedef void (*LinuxKernelMemoryBarrierFunc)(void); |
| LinuxKernelMemoryBarrierFunc pLinuxKernelMemoryBarrier __attribute__((weak)) = |
| (LinuxKernelMemoryBarrierFunc) 0xffff0fa0; |
| inline void MemoryBarrier() { |
| pLinuxKernelMemoryBarrier(); |
| } |
| #define LEVELDB_HAVE_MEMORY_BARRIER |
| |
| #endif |
| |
| // AtomicPointer built using platform-specific MemoryBarrier() |
| #if defined(LEVELDB_HAVE_MEMORY_BARRIER) |
| class AtomicPointer { |
| private: |
| void* rep_; |
| public: |
| AtomicPointer() { } |
| explicit AtomicPointer(void* p) : rep_(p) {} |
| inline void* NoBarrier_Load() const { return rep_; } |
| inline void NoBarrier_Store(void* v) { rep_ = v; } |
| inline void* Acquire_Load() const { |
| void* result = rep_; |
| MemoryBarrier(); |
| return result; |
| } |
| inline void Release_Store(void* v) { |
| MemoryBarrier(); |
| rep_ = v; |
| } |
| }; |
| |
| // AtomicPointer based on <cstdatomic> |
| #elif defined(LEVELDB_CSTDATOMIC_PRESENT) |
| class AtomicPointer { |
| private: |
| std::atomic<void*> rep_; |
| public: |
| AtomicPointer() { } |
| explicit AtomicPointer(void* v) : rep_(v) { } |
| inline void* Acquire_Load() const { |
| return rep_.load(std::memory_order_acquire); |
| } |
| inline void Release_Store(void* v) { |
| rep_.store(v, std::memory_order_release); |
| } |
| inline void* NoBarrier_Load() const { |
| return rep_.load(std::memory_order_relaxed); |
| } |
| inline void NoBarrier_Store(void* v) { |
| rep_.store(v, std::memory_order_relaxed); |
| } |
| }; |
| |
| // We have neither MemoryBarrier(), nor <cstdatomic> |
| #else |
| #error Please implement AtomicPointer for this platform. |
| |
| #endif |
| |
| #undef LEVELDB_HAVE_MEMORY_BARRIER |
| #undef ARCH_CPU_X86_FAMILY |
| #undef ARCH_CPU_ARM_FAMILY |
| |
| } // namespace leveldb::port |
| } // namespace leveldb |
| |
| #endif // PORT_ATOMIC_POINTER_H_ |