ia32 instructions and micro-arch for locks

back to index

PAUSE instruction

Spin Loop Hint, give hint to processor that improves performance of spin-wait loops. Specifically, to transfer yield CPU to another hyperthread.

The PAUSE instruction provides a hint to the processor that the code sequence is a spin-wait loop. The processor uses this hint to avoid the memory order violation in most situations, which greatly improves processor performance.

An additional function of the PAUSE instruction is to reduce the power consumed by a processor while executing a spin loop. A processor can execute a spin-wait loop extremely quickly, causing the processor to consume a lot of power while it waits for the resource it is spinning on to become available. Inserting a pause instruction in a spin-wait loop greatly reduces the processor’s power consumption.

GCC _mm_pause() expands to __builtin_ia32_pause() then the PAUSE instruction.

This can be used to improve the spinlock performance (instead of a NOP in the busy loop).

while (__stomic_sub_fetch(&lock, 1, __ATOMIC_ACQUIRE) < 0) {
    do
        _mm_pause();
    while (__atomic_load_n(&lock, __ATOMIC_ACQUIRE) != 1);
}