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/* Copyright 2012 10gen Inc.
*
* 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.
*/
/**
* Implementation of the AtomicIntrinsics<T>::* operations for IA-32 and AMD64 systems using a
* GCC-compatible compiler toolchain.
*/
#pragma once
#include <boost/utility.hpp>
namespace mongo {
/**
* Instantiation of AtomicIntrinsics<> for all word types T where sizeof<T> <= sizeof(void *).
*
* On 32-bit systems, this handles 8-, 16- and 32-bit word types. On 64-bit systems,
* it handles 8-, 16, 32- and 64-bit types.
*/
template <typename T, typename IsTLarge=void>
class AtomicIntrinsics {
public:
static T compareAndSwap(volatile T* dest, T expected, T newValue) {
T result;
asm volatile ("lock cmpxchg %[src], %[dest]"
: [dest] "+m" (*dest),
"=a" (result)
: [src] "r" (newValue),
"a" (expected)
: "memory", "cc");
return result;
}
static T swap(volatile T* dest, T newValue) {
T result = newValue;
// No need for "lock" prefix on "xchg".
asm volatile ("xchg %[r], %[dest]"
: [dest] "+m" (*dest),
[r] "+r" (result)
:
: "memory");
return result;
}
static T load(volatile const T* value) {
asm volatile ("mfence" ::: "memory");
T result = *value;
asm volatile ("mfence" ::: "memory");
return result;
}
static T loadRelaxed(volatile const T* value) {
return *value;
}
static void store(volatile T* dest, T newValue) {
asm volatile ("mfence" ::: "memory");
*dest = newValue;
asm volatile ("mfence" ::: "memory");
}
static T fetchAndAdd(volatile T* dest, T increment) {
T result = increment;
asm volatile ("lock xadd %[src], %[dest]"
: [dest] "+m" (*dest),
[src] "+r" (result)
:
: "memory", "cc");
return result;
}
private:
AtomicIntrinsics();
~AtomicIntrinsics();
};
/**
* Instantiation of AtomicIntrinsics<T> where sizeof<T> exceeds sizeof(void*).
*
* On 32-bit systems, this handles the 64-bit word type. Not used on 64-bit systems.
*
* Note that the implementations of swap, store and fetchAndAdd spin until they succeed. This
* implementation is thread-safe, but may have poor performance in high-contention environments.
* However, no superior solution exists for IA-32 (32-bit x86) systems.
*/
template <typename T>
class AtomicIntrinsics<T, typename boost::disable_if_c<sizeof(T) <= sizeof(void*)>::type> {
public:
static T compareAndSwap(volatile T* dest, T expected, T newValue) {
T result = expected;
asm volatile ("push %%eax\n"
"push %%ebx\n"
"push %%ecx\n"
"push %%edx\n"
"mov (%%edx), %%ebx\n"
"mov 4(%%edx), %%ecx\n"
"mov (%%edi), %%eax\n"
"mov 4(%%edi), %%edx\n"
"lock cmpxchg8b (%%esi)\n"
"mov %%eax, (%%edi)\n"
"mov %%edx, 4(%%edi)\n"
"pop %%edx\n"
"pop %%ecx\n"
"pop %%ebx\n"
"pop %%eax\n"
:
: "S" (dest),
"D" (&result),
"d" (&newValue)
: "memory", "cc");
return result;
}
static T swap(volatile T* dest, T newValue) {
T expected;
T actual;
do {
expected = *dest;
actual = compareAndSwap(dest, expected, newValue);
} while (actual != expected);
return actual;
}
static T load(volatile const T* value) {
return compareAndSwap(const_cast<volatile T*>(value), T(0), T(0));
}
static void store(volatile T* dest, T newValue) {
swap(dest, newValue);
}
static T fetchAndAdd(volatile T* dest, T increment) {
T expected;
T actual;
do {
expected = load(dest);
actual = compareAndSwap(dest, expected, expected + increment);
} while (actual != expected);
return actual;
}
private:
AtomicIntrinsics();
~AtomicIntrinsics();
};
} // namespace mongo
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