提交 3a483050 编写于 作者: A Alexander van Heukelum 提交者: Ingo Molnar

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: NAlexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
上级 2aba6925
......@@ -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 */
......
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