Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}_bit()
From: Petr Mladek <pmladek@suse.com>
Date: 2021-08-26 13:57:33
Also in:
kvm, linux-arch, linux-kselftest, linux-mm, linux-mmc, lkml
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.
quoted hunk ↗ jump to hunk
--- 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