// 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_6KZ__) || 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 base::subtle
} // namespace base
#endif // BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_