Update //base to chromium 9659b08ea5a34f889dc4166217f438095ddc10d2

base/atomicops_internals_arm_gcc.h

Index: base/atomicops_internals_arm_gcc.h
diff --git a/base/atomicops_internals_arm_gcc.h b/base/atomicops_internals_arm_gcc.h
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-// Copyright 2013 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// This file is an internal atomic implementation, use base/atomicops.h instead.
-//
-// LinuxKernelCmpxchg and Barrier_AtomicIncrement are from Google Gears.
-
-#ifndef BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
-#define BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
-
-#if defined(OS_QNX)
-#include <sys/cpuinline.h>
-#endif
-
-namespace base {
-namespace subtle {
-
-// Memory barriers on ARM are funky, but the kernel is here to help:
-//
-// * ARMv5 didn't support SMP, there is no memory barrier instruction at
-//   all on this architecture, or when targeting its machine code.
-//
-// * Some ARMv6 CPUs support SMP. A full memory barrier can be produced by
-//   writing a random value to a very specific coprocessor register.
-//
-// * On ARMv7, the "dmb" instruction is used to perform a full memory
-//   barrier (though writing to the co-processor will still work).
-//   However, on single core devices (e.g. Nexus One, or Nexus S),
-//   this instruction will take up to 200 ns, which is huge, even though
-//   it's completely un-needed on these devices.
-//
-// * There is no easy way to determine at runtime if the device is
-//   single or multi-core. However, the kernel provides a useful helper
-//   function at a fixed memory address (0xffff0fa0), which will always
-//   perform a memory barrier in the most efficient way. I.e. on single
-//   core devices, this is an empty function that exits immediately.
-//   On multi-core devices, it implements a full memory barrier.
-//
-// * This source could be compiled to ARMv5 machine code that runs on a
-//   multi-core ARMv6 or ARMv7 device. In this case, memory barriers
-//   are needed for correct execution. Always call the kernel helper, even
-//   when targeting ARMv5TE.
-//
-
-inline void MemoryBarrier() {
-#if defined(OS_LINUX) || defined(OS_ANDROID)
-  // Note: This is a function call, which is also an implicit compiler barrier.
-  typedef void (*KernelMemoryBarrierFunc)();
-  ((KernelMemoryBarrierFunc)0xffff0fa0)();
-#elif defined(OS_QNX)
-  __cpu_membarrier();
-#else
-#error MemoryBarrier() is not implemented on this platform.
-#endif
-}
-
-// An ARM toolchain would only define one of these depending on which
-// variant of the target architecture is being used. This tests against
-// any known ARMv6 or ARMv7 variant, where it is possible to directly
-// use ldrex/strex instructions to implement fast atomic operations.
-#if defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) || \
-    defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || \
-    defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) || \
-    defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6Z__) || \
-    defined(__ARM_ARCH_6ZK__) || defined(__ARM_ARCH_6T2__)
-
-inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
-                                         Atomic32 old_value,
-                                         Atomic32 new_value) {
-  Atomic32 prev_value;
-  int reloop;
-  do {
-    // The following is equivalent to:
-    //
-    //   prev_value = LDREX(ptr)
-    //   reloop = 0
-    //   if (prev_value != old_value)
-    //      reloop = STREX(ptr, new_value)
-    __asm__ __volatile__("    ldrex %0, [%3]\n"
-                         "    mov %1, #0\n"
-                         "    cmp %0, %4\n"
-#ifdef __thumb2__
-                         "    it eq\n"
-#endif
-                         "    strexeq %1, %5, [%3]\n"
-                         : "=&r"(prev_value), "=&r"(reloop), "+m"(*ptr)
-                         : "r"(ptr), "r"(old_value), "r"(new_value)
-                         : "cc", "memory");
-  } while (reloop != 0);
-  return prev_value;
-}
-
-inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
-                                       Atomic32 old_value,
-                                       Atomic32 new_value) {
-  Atomic32 result = NoBarrier_CompareAndSwap(ptr, old_value, new_value);
-  MemoryBarrier();
-  return result;
-}
-
-inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
-                                       Atomic32 old_value,
-                                       Atomic32 new_value) {
-  MemoryBarrier();
-  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
-}
-
-inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
-                                          Atomic32 increment) {
-  Atomic32 value;
-  int reloop;
-  do {
-    // Equivalent to:
-    //
-    //  value = LDREX(ptr)
-    //  value += increment
-    //  reloop = STREX(ptr, value)
-    //
-    __asm__ __volatile__("    ldrex %0, [%3]\n"
-                         "    add %0, %0, %4\n"
-                         "    strex %1, %0, [%3]\n"
-                         : "=&r"(value), "=&r"(reloop), "+m"(*ptr)
-                         : "r"(ptr), "r"(increment)
-                         : "cc", "memory");
-  } while (reloop);
-  return value;
-}
-
-inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
-                                        Atomic32 increment) {
-  // TODO(digit): Investigate if it's possible to implement this with
-  // a single MemoryBarrier() operation between the LDREX and STREX.
-  // See http://crbug.com/246514
-  MemoryBarrier();
-  Atomic32 result = NoBarrier_AtomicIncrement(ptr, increment);
-  MemoryBarrier();
-  return result;
-}
-
-inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr,
-                                         Atomic32 new_value) {
-  Atomic32 old_value;
-  int reloop;
-  do {
-    // old_value = LDREX(ptr)
-    // reloop = STREX(ptr, new_value)
-    __asm__ __volatile__("   ldrex %0, [%3]\n"
-                         "   strex %1, %4, [%3]\n"
-                         : "=&r"(old_value), "=&r"(reloop), "+m"(*ptr)
-                         : "r"(ptr), "r"(new_value)
-                         : "cc", "memory");
-  } while (reloop != 0);
-  return old_value;
-}
-
-// This tests against any known ARMv5 variant.
-#elif defined(__ARM_ARCH_5__) || defined(__ARM_ARCH_5T__) || \
-      defined(__ARM_ARCH_5TE__) || defined(__ARM_ARCH_5TEJ__)
-
-// The kernel also provides a helper function to perform an atomic
-// compare-and-swap operation at the hard-wired address 0xffff0fc0.
-// On ARMv5, this is implemented by a special code path that the kernel
-// detects and treats specially when thread pre-emption happens.
-// On ARMv6 and higher, it uses LDREX/STREX instructions instead.
-//
-// Note that this always perform a full memory barrier, there is no
-// need to add calls MemoryBarrier() before or after it. It also
-// returns 0 on success, and 1 on exit.
-//
-// Available and reliable since Linux 2.6.24. Both Android and ChromeOS
-// use newer kernel revisions, so this should not be a concern.
-namespace {
-
-inline int LinuxKernelCmpxchg(Atomic32 old_value,
-                              Atomic32 new_value,
-                              volatile Atomic32* ptr) {
-  typedef int (*KernelCmpxchgFunc)(Atomic32, Atomic32, volatile Atomic32*);
-  return ((KernelCmpxchgFunc)0xffff0fc0)(old_value, new_value, ptr);
-}
-
-}  // namespace
-
-inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
-                                         Atomic32 old_value,
-                                         Atomic32 new_value) {
-  Atomic32 prev_value;
-  for (;;) {
-    prev_value = *ptr;
-    if (prev_value != old_value)
-      return prev_value;
-    if (!LinuxKernelCmpxchg(old_value, new_value, ptr))
-      return old_value;
-  }
-}
-
-inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr,
-                                         Atomic32 new_value) {
-  Atomic32 old_value;
-  do {
-    old_value = *ptr;
-  } while (LinuxKernelCmpxchg(old_value, new_value, ptr));
-  return old_value;
-}
-
-inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
-                                          Atomic32 increment) {
-  return Barrier_AtomicIncrement(ptr, increment);
-}
-
-inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
-                                        Atomic32 increment) {
-  for (;;) {
-    // Atomic exchange the old value with an incremented one.
-    Atomic32 old_value = *ptr;
-    Atomic32 new_value = old_value + increment;
-    if (!LinuxKernelCmpxchg(old_value, new_value, ptr)) {
-      // The exchange took place as expected.
-      return new_value;
-    }
-    // Otherwise, *ptr changed mid-loop and we need to retry.
-  }
-}
-
-inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
-                                       Atomic32 old_value,
-                                       Atomic32 new_value) {
-  Atomic32 prev_value;
-  for (;;) {
-    prev_value = *ptr;
-    if (prev_value != old_value) {
-      // Always ensure acquire semantics.
-      MemoryBarrier();
-      return prev_value;
-    }
-    if (!LinuxKernelCmpxchg(old_value, new_value, ptr))
-      return old_value;
-  }
-}
-
-inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
-                                       Atomic32 old_value,
-                                       Atomic32 new_value) {
-  // This could be implemented as:
-  //    MemoryBarrier();
-  //    return NoBarrier_CompareAndSwap();
-  //
-  // But would use 3 barriers per succesful CAS. To save performance,
-  // use Acquire_CompareAndSwap(). Its implementation guarantees that:
-  // - A succesful swap uses only 2 barriers (in the kernel helper).
-  // - An early return due to (prev_value != old_value) performs
-  //   a memory barrier with no store, which is equivalent to the
-  //   generic implementation above.
-  return Acquire_CompareAndSwap(ptr, old_value, new_value);
-}
-
-#else
-#  error "Your CPU's ARM architecture is not supported yet"
-#endif
-
-// NOTE: Atomicity of the following load and store operations is only
-// guaranteed in case of 32-bit alignement of |ptr| values.
-
-inline void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value) {
-  *ptr = value;
-}
-
-inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
-  *ptr = value;
-  MemoryBarrier();
-}
-
-inline void Release_Store(volatile Atomic32* ptr, Atomic32 value) {
-  MemoryBarrier();
-  *ptr = value;
-}
-
-inline Atomic32 NoBarrier_Load(volatile const Atomic32* ptr) { return *ptr; }
-
-inline Atomic32 Acquire_Load(volatile const Atomic32* ptr) {
-  Atomic32 value = *ptr;
-  MemoryBarrier();
-  return value;
-}
-
-inline Atomic32 Release_Load(volatile const Atomic32* ptr) {
-  MemoryBarrier();
-  return *ptr;
-}
-
-}  // namespace subtle
-}  // namespace base
-
-#endif  // BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_