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