DelaySysTick jitter

[romain145]

DelaySysTick has jitter for specific values of 1, 2, 6, 8, 10 (tested up to 10).
DelaySysTick(0) is always 292ns.
DelaySysTick(1) is 292ns or 459ns.
DelaySysTick(2) is 459ns or 626ns.
DelaySysTick(3) is always 793ns.

I'm suspecting compiler optimization at play here. I tried forcing it to be inlined with little success.

Adding volatile changed the behaviour (2 doesn't have jitter anymore) but it still shows jitter for other values. It's also much slower, obviously...

void DelaySysTick( uint32_t n )
{
	volatile uint32_t targend = SysTick->CNT + n;
	while( ((int32_t)( SysTick->CNT - targend )) < 0 );
}

[cnlohr]

I believe this has to do with instruction alignment. Can you see what happens if you surround your code with option norvc?

https://github.com/cnlohr/ch32v003fun/blob/master/ch32v003fun/ch32v003fun.c#L738C20-L741C21