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.
> --- 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
