From: Yunsheng Lin
> Sent: 20 July 2021 03:22
>
> As x86 and arm64 is the two available systems that I can build
> and test the cpu_relax() implementation, so only add cpu_relax()
> implementation for x86 and arm64, other arches can be added easily
> when needed.
>
...
> +#if defined(__i386__) || defined(__x86_64__)
> +/* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
> +static __always_inline void rep_nop(void)
> +{
> + asm volatile("rep; nop" ::: "memory");
> +}
Beware, Intel increased the stall for 'rep nop' in some recent
cpu to IIRC about 200 cycles.
They even document that this might have a detrimental effect.
It is basically far too long for the sort of thing it makes
sense to busy-wait for.
David
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