在 2007-10-16二的 10:04 +0200，Denis Oliver Kropp写道：
> Bin Chen wrote:
> > 在 2007-10-16二的 09:23 +0200，Denis Oliver Kropp写道：
> >> Phil Endecott wrote:
> >>> I've also noticed that djpeg runs about 15% faster if compiled with -Os
> >>> rather than -O4.
> >> On x86?
> >>
> >> On embedded architectures, -O2 is often better than -O3, but if you have
> >> a very small instruction cache, -Os could be best.
> >
> > Its interesting, why -O2 is better?
>
> O3 can produce code (loop unrolling etc.) where the cache penalty is
> bigger than the speed improvement.
>
Thanks Phil, so what is loop unrolling, is it to expand a loop to repeat
machine code to reduce the penalty from jumping back?
Such as
for (i = 0;i < 5;i++) {
do sth for i;
}
expand to
do sth for 1
do sth for 2
...
do sth for 5
The expanded code doesn't need to do jump, so can increase the prefetch
efficiency.
Right?
Bin
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