runtime/atomic.h

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef ART_RUNTIME_ATOMIC_H_
#define ART_RUNTIME_ATOMIC_H_

#include <stdint.h>
#include <vector>

#include "base/macros.h"

namespace art {

class Mutex;

// NOTE: Two "quasiatomic" operations on the exact same memory address
// are guaranteed to operate atomically with respect to each other,
// but no guarantees are made about quasiatomic operations mixed with
// non-quasiatomic operations on the same address, nor about
// quasiatomic operations that are performed on partially-overlapping
// memory.
class QuasiAtomic {
#if !defined(__arm__) && !defined(__i386__)
  static constexpr bool kNeedSwapMutexes = true;
#else
  static constexpr bool kNeedSwapMutexes = false;
#endif

 public:
  static void Startup();

  static void Shutdown();

  // Reads the 64-bit value at "addr" without tearing.
  static int64_t Read64(volatile const int64_t* addr) {
    if (!kNeedSwapMutexes) {
      return *addr;
    } else {
      return SwapMutexRead64(addr);
    }
  }

  // Writes to the 64-bit value at "addr" without tearing.
  static void Write64(volatile int64_t* addr, int64_t val) {
    if (!kNeedSwapMutexes) {
      *addr = val;
    } else {
      SwapMutexWrite64(addr, val);
    }
  }

  // Atomically compare the value at "addr" to "old_value", if equal replace it with "new_value"
  // and return true. Otherwise, don't swap, and return false.
  static bool Cas64(int64_t old_value, int64_t new_value, volatile int64_t* addr) {
    if (!kNeedSwapMutexes) {
      return __sync_bool_compare_and_swap(addr, old_value, new_value);
    } else {
      return SwapMutexCas64(old_value, new_value, addr);
    }
  }

  // Does the architecture provide reasonable atomic long operations or do we fall back on mutexes?
  static bool LongAtomicsUseMutexes() {
    return !kNeedSwapMutexes;
  }

  static void MembarLoadStore() {
  #if defined(__arm__)
    __asm__ __volatile__("dmb ish" : : : "memory");
  #elif defined(__i386__)
    __asm__ __volatile__("" : : : "memory");
  #elif defined(__mips__)
    __asm__ __volatile__("sync" : : : "memory");
  #else
  #error Unexpected architecture
  #endif
  }

  static void MembarLoadLoad() {
  #if defined(__arm__)
    __asm__ __volatile__("dmb ish" : : : "memory");
  #elif defined(__i386__)
    __asm__ __volatile__("" : : : "memory");
  #elif defined(__mips__)
    __asm__ __volatile__("sync" : : : "memory");
  #else
  #error Unexpected architecture
  #endif
  }

  static void MembarStoreStore() {
  #if defined(__arm__)
    __asm__ __volatile__("dmb ishst" : : : "memory");
  #elif defined(__i386__)
    __asm__ __volatile__("" : : : "memory");
  #elif defined(__mips__)
    __asm__ __volatile__("sync" : : : "memory");
  #else
  #error Unexpected architecture
  #endif
  }

  static void MembarStoreLoad() {
  #if defined(__arm__)
    __asm__ __volatile__("dmb ish" : : : "memory");
  #elif defined(__i386__)
    __asm__ __volatile__("mfence" : : : "memory");
  #elif defined(__mips__)
    __asm__ __volatile__("sync" : : : "memory");
  #else
  #error Unexpected architecture
  #endif
  }

 private:
  static Mutex* GetSwapMutex(const volatile int64_t* addr);
  static int64_t SwapMutexRead64(volatile const int64_t* addr);
  static void SwapMutexWrite64(volatile int64_t* addr, int64_t val);
  static bool SwapMutexCas64(int64_t old_value, int64_t new_value, volatile int64_t* addr);

  // We stripe across a bunch of different mutexes to reduce contention.
  static constexpr size_t kSwapMutexCount = 32;
  static std::vector<Mutex*>* gSwapMutexes;

  DISALLOW_COPY_AND_ASSIGN(QuasiAtomic);
};

}  // namespace art

#endif  // ART_RUNTIME_ATOMIC_H_