Gitweb: http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=3a4830...
Commit: 3a48305028aa38afba93fc05066c71a6ee668ad8
Parent: 2aba6925fdb96428d1129a61b1233597a03a387b
Author: Alexander van Heukelum <heukelum@mailshack.com>
AuthorDate: Tue Apr 1 17:42:21 2008 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Sat Apr 26 19:21:17 2008 +0200
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: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/bitops.h | 29 +++++++++++++++++++++++++++++
1 files changed, 29 insertions(+), 0 deletions(-)
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 355d67b..48bde60 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -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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