Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}_bit()

Yury Norov
2021-08-26T21:10:00Z

On Thu, Aug 26, 2021 at 03:57:13PM +0200, Petr Mladek wrote:
> On Sat 2021-08-14 14:17:07, Yury Norov wrote:
> > The macros iterate thru all set/clear bits in a bitmap. They search a
> > first bit using find_first_bit(), and the rest bits using find_next_bit().
> >
> > Since find_next_bit() is called shortly after find_first_bit(), we can
> > save few lines of I-cache by not using find_first_bit().
>
> Is this only a speculation or does it fix a real performance problem?
>
> The macro is used like:
>
> for_each_set_bit(bit, addr, size) {
> fn(bit);
> }
>
> IMHO, the micro-opimization does not help when fn() is non-trivial.

The effect is measurable:

Start testing for_each_bit()
for_each_set_bit: 15296 ns, 1000 iterations
for_each_set_bit_from: 15225 ns, 1000 iterations

Start testing for_each_bit() with cash flushing
for_each_set_bit: 547626 ns, 1000 iterations
for_each_set_bit_from: 497899 ns, 1000 iterations

Refer this:

https://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg356151.html

Thanks,
Yury

> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
> > #endif
> >
> > #define for_each_set_bit(bit, addr, size) \
> > - for ((bit) = find_first_bit((addr), (size)); \
> > + for ((bit) = find_next_bit((addr), (size), 0); \
> > (bit) < (size); \
> > (bit) = find_next_bit((addr), (size), (bit) + 1))
> >
>
> It is not a big deal. I just think that the original code is slightly
> more self-explaining.
>
> Best Regards,
> Petr