bitops: Optimize fns() for improved performance

The current fns() repeatedly uses __ffs() to find the index of the
least significant bit and then clears the corresponding bit using
__clear_bit(). The method for clearing the least significant bit can be
optimized by using word &= word - 1 instead.

Typically, the execution time of one __ffs() plus one __clear_bit() is
longer than that of a bitwise AND operation and a subtraction. To
improve performance, the loop for clearing the least significant bit
has been replaced with word &= word - 1, followed by a single __ffs()
operation to obtain the answer. This change reduces the number of
__ffs() iterations from n to just one, enhancing overall performance.

This modification significantly accelerates the fns() function in the
test_bitops benchmark, improving its speed by approximately 7.6 times.
Additionally, it enhances the performance of find_nth_bit() in the
find_bit benchmark by approximately 26%.

Before:
test_bitops: fns:             58033164 ns
find_nth_bit:                  4254313 ns,  16525 iterations

After:
test_bitops: fns:              7637268 ns
find_nth_bit:                  3362863 ns,  16501 iterations

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 2ba557e067fe69d4d96b5472721001e87a6535cc..57ecef354f47bbe491da00e50bdcdf7491152468 100644 (file)
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -254,16 +254,10 @@ static inline unsigned long __ffs64(u64 word)
  */
 static inline unsigned long fns(unsigned long word, unsigned int n)
 {
-       unsigned int bit;
+       while (word && n--)
+               word &= word - 1;
 
-       while (word) {
-               bit = __ffs(word);
-               if (n-- == 0)
-                       return bit;
-               __clear_bit(bit, &word);
-       }
-
-       return BITS_PER_LONG;
+       return word ? __ffs(word) : BITS_PER_LONG;
 }
 
 /**