On Sat, Jun 19, 2021 at 05:24:15PM +0100, Marc Zyngier wrote:
On Fri, 18 Jun 2021 20:57:34 +0100, Yury Norov yury.norov@gmail.com 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().
Really?
Signed-off-by: Yury Norov yury.norov@gmail.com
include/linux/find.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/find.h b/include/linux/find.h index 4500e8ab93e2..ae9ed52b52b8 100644 --- 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); \
On which architecture do you observe a gain? Only 32bit ARM and m68k implement their own version of find_first_bit(), and everyone else uses the canonical implementation:
And those who enable GENERIC_FIND_FIRST_BIT - x86, arm64, arc, mips and s390.
#ifndef find_first_bit #define find_first_bit(addr, size) find_next_bit((addr), (size), 0) #endif
These architectures explicitly have different implementations for find_first_bit() and find_next_bit() because they can do better (whether that is true or not is another debate). I don't think you should remove this optimisation until it has been measured on these two architectures.
This patch is based on a series that enables separate implementation of find_first_bit() for all architectures; according to my tests, find_first* is ~ twice faster than find_next* on arm64 and x86.
https://lore.kernel.org/lkml/20210612123639.329047-1-yury.norov@gmail.com/T/...
After applying the series, I noticed that my small kernel module that calls for_each_set_bit() is now using find_first_bit() to just find one bit, and find_next_bit() for all others. I think it's better to always use find_next_bit() in this case to minimize the chance of cache miss. But if it's not that obvious, I'll try to write some test.