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Date: Mon, 28 Apr 2008 16:32:20 +0200 From: "Alexander van Heukelum" <heukelum@...tmail.fm> To: "Harvey Harrison" <harvey.harrison@...il.com>, "Ingo Molnar" <mingo@...e.hu>, "Linus Torvalds" <torvalds@...ux-foundation.org> Cc: "Andrew Morton" <akpm@...ux-foundation.org>, "LKML" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] bitops: simplify generic bit finding functions Hi, Ingo just pointed me to this thread. On Sun, 27 Apr 2008 13:19:51 -0700, "Harvey Harrison" <harvey.harrison@...il.com> said: > No need for a sentinal if we explicitly catch the no bits/all bits set > cases, make it clear they are special cases returning size/BITS_PER_LONG. > > Signed-off-by: Harvey Harrison <harvey.harrison@...il.com> > --- > include/linux/bitops.h | 93 > ++++++++++++++++++++---------------------------- > 1 files changed, 39 insertions(+), 54 deletions(-) > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 48bde60..d9eb58a 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -127,18 +127,17 @@ 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); > + /* > + * Avoid a function call if the bitmap size is a constant and not > + * bigger than BITS_PER_LONG. Ensure we return size if there are > + * no set bits. > + */ > + if (__builtin_constant_p(size) && (size <= BITS_PER_LONG)) { > + if (*addr == 0) > + return (size < BITS_PER_LONG) ? size : BITS_PER_LONG; > + else > + return __ffs(*addr); > + } Ehm... assume size=16 and *addr=0x80000000 then __ffs(*addr) = 31. Not 16. > /* size is not constant or too big */ > return __find_first_bit(addr, size); > @@ -157,20 +156,17 @@ 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)); > + /* > + * Avoid a function call if the bitmap size is a constant and not > + * bigger than BITS_PER_LONG. Ensure we return size if all bits set. > + */ > + if (__builtin_constant_p(size) && (size <= BITS_PER_LONG)) { > + if ((~(*addr)) == 0) > + return (size < BITS_PER_LONG) ? size : BITS_PER_LONG; > + else > + return __ffs(~(*addr)); > } > > - /* 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); > } > @@ -192,22 +188,17 @@ find_next_bit(const unsigned long *addr, unsigned > long size, > { > unsigned long value; > > - /* 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)) { > + /* > + * Avoid a function call if the bitmap size is a constant and not > + * bigger than BITS_PER_LONG. Ensure we return size if there are > + * no set bits. > + */ > + if (__builtin_constant_p(size) && (size <= BITS_PER_LONG)) { > value = (*addr) & ((~0ul) << offset); > - value |= (1ul << size); > - return __ffs(value); > - } > - > - /* the result of __ffs(0) is undefined, so it needs to be */ > - /* handled separately */ > - if (__builtin_constant_p(size) && (size == BITS_PER_LONG)) { > - value = (*addr) & ((~0ul) << offset); > - return (value == 0) ? BITS_PER_LONG : __ffs(value); > + if (value == 0) > + return (size < BITS_PER_LONG) ? size : BITS_PER_LONG; > + else > + return __ffs(value); > } > > /* size is not constant or too big */ > @@ -229,22 +220,16 @@ find_next_zero_bit(const unsigned long *addr, > unsigned long size, > { > unsigned long value; > > - /* 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)) { > - value = (~(*addr)) & ((~0ul) << offset); > - value |= (1ul << size); > - return __ffs(value); > - } > - > - /* the result of __ffs(0) is undefined, so it needs to be */ > - /* handled separately */ > - if (__builtin_constant_p(size) && (size == BITS_PER_LONG)) { > + /* > + * Avoid a function call if the bitmap size is a constant and not > + * bigger than BITS_PER_LONG. Ensure we return size if all bits set. > + */ > + if (__builtin_constant_p(size) && (size <= BITS_PER_LONG)) { > value = (~(*addr)) & ((~0ul) << offset); > - return (value == 0) ? BITS_PER_LONG : __ffs(value); > + if (value == 0) > + return (size < BITS_PER_LONG) ? size : BITS_PER_LONG; > + else > + return __ffs(value); > } > > /* size is not constant or too big */ > -- > 1.5.5.1.270.g89765 -- Alexander van Heukelum heukelum@...tmail.fm -- http://www.fastmail.fm - mmm... Fastmail... -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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