Question

以原子方式更新单个位（来自std::bitset）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 是最快的解决方案，我很乐意接受内联的x86-64汇编答案。

Answer 1

默认情况下，BTS / BTR不是原子的，但它们可以使用LOCK前缀。

这是我的原子位设置和原子位复位的实现，适用于Intel 64位和32位平台（适用于MSVC，gcc和可能的clang）。

ARM的实施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;
}

原子地更新一点的最便宜/最少侵入性的方法？

1 个答案: