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Commit 3a483050 authored by Alexander van Heukelum's avatar Alexander van Heukelum Committed by Ingo Molnar
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x86: optimize find_first_bit for small bitmaps 



Avoid a call to find_first_bit if the bitmap size is know at
compile time and small enough to fit in a single long integer.
Modeled after an optimization in the original x86_64-specific
code.

Signed-off-by: default avatarAlexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 2aba6925

include/linux/bitops.h

+29 −0
Original line number Diff line number Diff line
@@ -127,6 +127,20 @@ extern unsigned long __find_first_bit(const unsigned long *addr, 
static __always_inline unsigned long

find_first_bit(const unsigned long *addr, unsigned long size)

{

	/* Avoid a function call if the bitmap size is a constant */

	/* and not bigger than BITS_PER_LONG. */



	/* insert a sentinel so that __ffs returns size if there */

	/* are no set bits in the bitmap */

	if (__builtin_constant_p(size) && (size < BITS_PER_LONG))

		return __ffs((*addr) | (1ul << size));



	/* the result of __ffs(0) is undefined, so it needs to be */

	/* handled separately */

	if (__builtin_constant_p(size) && (size == BITS_PER_LONG))

		return ((*addr) == 0) ? BITS_PER_LONG : __ffs(*addr);



	/* size is not constant or too big */

	return __find_first_bit(addr, size);

}
@@ -143,6 +157,21 @@ extern unsigned long __find_first_zero_bit(const unsigned long *addr, 
static __always_inline unsigned long

find_first_zero_bit(const unsigned long *addr, unsigned long size)

{

	/* Avoid a function call if the bitmap size is a constant */

	/* and not bigger than BITS_PER_LONG. */



	/* insert a sentinel so that __ffs returns size if there */

	/* are no set bits in the bitmap */

	if (__builtin_constant_p(size) && (size < BITS_PER_LONG)) {

		return __ffs(~(*addr) | (1ul << size));

	}



	/* the result of __ffs(0) is undefined, so it needs to be */

	/* handled separately */

	if (__builtin_constant_p(size) && (size == BITS_PER_LONG))

		return (~(*addr) == 0) ? BITS_PER_LONG : __ffs(~(*addr));



	/* size is not constant or too big */

	return __find_first_zero_bit(addr, size);

}

#endif /* CONFIG_GENERIC_FIND_FIRST_BIT */