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 <[e-mail]>
CC: Rasmus Villemoes <[e-mail]>
Signed-off-by: Kuan-Wei Chiu <[e-mail]>
Signed-off-by: Yury Norov <[e-mail]>

include/linux/bitops.h

···
  */
 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;
 }
 
 /**