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171 righe
5.3 KiB
171 righe
5.3 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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// AtomicPointer provides storage for a lock-free pointer.
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// Platform-dependent implementation of AtomicPointer:
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// - If the platform provides a cheap barrier, we use it with raw pointers
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// - If <atomic> is present (on newer versions of gcc, it is), we use
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// a <atomic>-based AtomicPointer. However we prefer the memory
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// barrier based version, because at least on a gcc 4.4 32-bit build
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// on linux, we have encountered a buggy <atomic> implementation.
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// Also, some <atomic> implementations are much slower than a memory-barrier
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// based implementation (~16ns for <atomic> based acquire-load vs. ~1ns for
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// a barrier based acquire-load).
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// This code is based on atomicops-internals-* in Google's perftools:
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// http://code.google.com/p/google-perftools/source/browse/#svn%2Ftrunk%2Fsrc%2Fbase
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#ifndef PORT_ATOMIC_POINTER_H_
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#define PORT_ATOMIC_POINTER_H_
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#include <stdint.h>
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#include <atomic>
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#if defined(_M_X64) || defined(__x86_64__)
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#define ARCH_CPU_X86_FAMILY 1
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#elif defined(_M_IX86) || defined(__i386__) || defined(__i386)
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#define ARCH_CPU_X86_FAMILY 1
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#elif defined(__ARMEL__)
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#define ARCH_CPU_ARM_FAMILY 1
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#elif defined(__aarch64__)
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#define ARCH_CPU_ARM64_FAMILY 1
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#elif defined(__ppc__) || defined(__powerpc__) || defined(__powerpc64__)
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#define ARCH_CPU_PPC_FAMILY 1
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#elif defined(__mips__)
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#define ARCH_CPU_MIPS_FAMILY 1
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#endif
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namespace leveldb {
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namespace port {
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// Define MemoryBarrier() if available
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// Windows on x86
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#if defined(OS_WIN) && defined(COMPILER_MSVC) && defined(ARCH_CPU_X86_FAMILY)
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// windows.h already provides a MemoryBarrier(void) macro
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// http://msdn.microsoft.com/en-us/library/ms684208(v=vs.85).aspx
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// Mac OS
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#elif defined(__APPLE__)
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inline void MemoryBarrier() {
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std::atomic_thread_fence(std::memory_order_seq_cst);
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// Gcc on x86
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#elif defined(ARCH_CPU_X86_FAMILY) && defined(__GNUC__)
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inline void MemoryBarrier() {
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// See http://gcc.gnu.org/ml/gcc/2003-04/msg01180.html for a discussion on
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// this idiom. Also see http://en.wikipedia.org/wiki/Memory_ordering.
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__asm__ __volatile__("" : : : "memory");
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// Sun Studio
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#elif defined(ARCH_CPU_X86_FAMILY) && defined(__SUNPRO_CC)
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inline void MemoryBarrier() {
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// See http://gcc.gnu.org/ml/gcc/2003-04/msg01180.html for a discussion on
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// this idiom. Also see http://en.wikipedia.org/wiki/Memory_ordering.
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asm volatile("" : : : "memory");
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// ARM Linux
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#elif defined(ARCH_CPU_ARM_FAMILY) && defined(__linux__)
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typedef void (*LinuxKernelMemoryBarrierFunc)(void);
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// The Linux ARM kernel provides a highly optimized device-specific memory
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// barrier function at a fixed memory address that is mapped in every
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// user-level process.
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//
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// This beats using CPU-specific instructions which are, on single-core
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// devices, un-necessary and very costly (e.g. ARMv7-A "dmb" takes more
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// than 180ns on a Cortex-A8 like the one on a Nexus One). Benchmarking
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// shows that the extra function call cost is completely negligible on
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// multi-core devices.
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//
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inline void MemoryBarrier() {
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(*(LinuxKernelMemoryBarrierFunc)0xffff0fa0)();
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// ARM64
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#elif defined(ARCH_CPU_ARM64_FAMILY)
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inline void MemoryBarrier() {
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asm volatile("dmb sy" : : : "memory");
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// PPC
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#elif defined(ARCH_CPU_PPC_FAMILY) && defined(__GNUC__)
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inline void MemoryBarrier() {
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// TODO for some powerpc expert: is there a cheaper suitable variant?
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// Perhaps by having separate barriers for acquire and release ops.
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asm volatile("sync" : : : "memory");
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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// MIPS
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#elif defined(ARCH_CPU_MIPS_FAMILY) && defined(__GNUC__)
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inline void MemoryBarrier() {
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__asm__ __volatile__("sync" : : : "memory");
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}
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#define LEVELDB_HAVE_MEMORY_BARRIER
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#endif
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// AtomicPointer built using platform-specific MemoryBarrier().
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#if defined(LEVELDB_HAVE_MEMORY_BARRIER)
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class AtomicPointer {
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private:
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void* rep_;
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public:
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AtomicPointer() { }
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explicit AtomicPointer(void* p) : rep_(p) {}
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inline void* NoBarrier_Load() const { return rep_; }
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inline void NoBarrier_Store(void* v) { rep_ = v; }
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inline void* Acquire_Load() const {
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void* result = rep_;
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MemoryBarrier();
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return result;
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}
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inline void Release_Store(void* v) {
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MemoryBarrier();
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rep_ = v;
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}
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};
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// AtomicPointer based on C++11 <atomic>.
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#else
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class AtomicPointer {
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private:
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std::atomic<void*> rep_;
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public:
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AtomicPointer() { }
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explicit AtomicPointer(void* v) : rep_(v) { }
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inline void* Acquire_Load() const {
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return rep_.load(std::memory_order_acquire);
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}
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inline void Release_Store(void* v) {
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rep_.store(v, std::memory_order_release);
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}
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inline void* NoBarrier_Load() const {
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return rep_.load(std::memory_order_relaxed);
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}
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inline void NoBarrier_Store(void* v) {
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rep_.store(v, std::memory_order_relaxed);
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}
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};
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#endif
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#undef LEVELDB_HAVE_MEMORY_BARRIER
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#undef ARCH_CPU_X86_FAMILY
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#undef ARCH_CPU_ARM_FAMILY
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#undef ARCH_CPU_ARM64_FAMILY
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#undef ARCH_CPU_PPC_FAMILY
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} // namespace port
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} // namespace leveldb
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#endif // PORT_ATOMIC_POINTER_H_
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