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x86: optimize find_first_bit for small bitmaps

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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: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Alexander van Heukelum 2008-04-01 17:42:21 +02:00 committed by Ingo Molnar
parent 2aba6925fd
commit 3a48305028
1 changed files with 29 additions and 0 deletions
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29
include/linux/bitops.h
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@ -127,6 +127,20 @@ extern unsigned long __find_first_bit(const unsigned long *addr,
static __always_inline unsigned long static __always_inline unsigned long
find_first_bit(const unsigned long *addr, unsigned long size) 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); 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 static __always_inline unsigned long
find_first_zero_bit(const unsigned long *addr, unsigned long size) 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); return __find_first_zero_bit(addr, size);
} }
#endif /* CONFIG_GENERIC_FIND_FIRST_BIT */ #endif /* CONFIG_GENERIC_FIND_FIRST_BIT */