原子地更新一点的最便宜/最少侵入性的方法？

Question

以std::bitset原子方式更新单个位(来自a )的最便宜的技术是什么？我不认为x86 BTR是原子的.

我想知道我是否必须读取最近的字节,然后使用CAS？

如果LOCK+ BTR 是最快速的解决方案,我很乐意接受内联的x86-64汇编答案.

Answer 1

BTS/BTR 默认情况下不是原子的，但它们可以带有 LOCK 前缀。

这是我对原子位设置和原子位重置的实现，它适用于 Intel 64 位和 32 位平台（适用于 MSVC、gcc 和可能的 clang）。

ARM 的实现也可以从以下位置获得： http ://alice.loria.fr/software/geogram/doc/html/atomics_8h_source.html

/**
 * \brief Atomically tests and sets a bit (INTEL only)
 * \details Sets bit \p bit of *\p ptr and returns its previous value.
 * The function is atomic and acts as a read-write memory barrier.
 * \param[in] ptr a pointer to an unsigned integer
 * \param[in] bit index of the bit to set in *\p ptr
 * \return the previous value of bit \p bit
 */
inline char atomic_bittestandset_x86(volatile unsigned int* ptr, unsigned int bit) {
    char out;
#if defined(__x86_64)
    __asm__ __volatile__ (
        "lock; bts %2,%1\n"  // set carry flag if bit %2 (bit) of %1 (ptr) is set
                             //   then set bit %2 of %1
        "sbb %0,%0\n"        // set %0 (out) if carry flag is set
        : "=r" (out), "=m" (*ptr)
        : "Ir" (bit)
        : "memory"
    );
#else
    __asm__ __volatile__ (
        "lock; bts %2,%1\n"  // set carry flag if bit %2 (bit) of %1 (ptr) is set
                             //   then set bit %2 of %1
        "sbb %0,%0\n"        // set %0 (out) if carry flag is set
        : "=q" (out), "=m" (*ptr)
        : "Ir" (bit)
        : "memory"
    );
#endif
    return out;
}

/**
 * \brief Atomically tests and resets a bit (INTEL only)
 * \details Resets bit \p bit of *\p ptr and returns its previous value.
 * The function is atomic and acts as a read-write memory barrier
 * \param[in] ptr a pointer to an unsigned integer
 * \param[in] bit index of the bit to reset in \p ptr
 * \return the previous value of bit \p bit
 */
inline char atomic_bittestandreset_x86(volatile unsigned int* ptr, unsigned int bit) {
    char out;
#if defined(__x86_64)
    __asm__ __volatile__ (
        "lock; btr %2,%1\n"  // set carry flag if bit %2 (bit) of %1 (ptr) is set
                             //   then reset bit %2 of %1
        "sbb %0,%0\n"        // set %0 (out) if carry flag is set
        : "=r" (out), "=m" (*ptr)
        : "Ir" (bit)
        : "memory"
    );
#else
    __asm__ __volatile__ (
        "lock; btr %2,%1\n"  // set carry flag if bit %2 (bit) of %1 (ptr) is set
                             //   then reset bit %2 of %1
        "sbb %0,%0\n"        // set %0 (out) if carry flag is set
        : "=q" (out), "=m" (*ptr)
        : "Ir" (bit)
        : "memory"
    );
#endif
    return out;
}