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>
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@ -127,6 +127,20 @@ extern unsigned long __find_first_bit(const unsigned long *addr,
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static __always_inline unsigned long
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find_first_bit(const unsigned long *addr, unsigned long size)
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{
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/* Avoid a function call if the bitmap size is a constant */
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/* and not bigger than BITS_PER_LONG. */
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/* insert a sentinel so that __ffs returns size if there */
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/* are no set bits in the bitmap */
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if (__builtin_constant_p(size) && (size < BITS_PER_LONG))
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return __ffs((*addr) | (1ul << size));
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/* the result of __ffs(0) is undefined, so it needs to be */
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/* handled separately */
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if (__builtin_constant_p(size) && (size == BITS_PER_LONG))
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return ((*addr) == 0) ? BITS_PER_LONG : __ffs(*addr);
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/* size is not constant or too big */
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return __find_first_bit(addr, size);
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}
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@ -143,6 +157,21 @@ extern unsigned long __find_first_zero_bit(const unsigned long *addr,
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static __always_inline unsigned long
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find_first_zero_bit(const unsigned long *addr, unsigned long size)
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{
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/* Avoid a function call if the bitmap size is a constant */
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/* and not bigger than BITS_PER_LONG. */
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/* insert a sentinel so that __ffs returns size if there */
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/* are no set bits in the bitmap */
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if (__builtin_constant_p(size) && (size < BITS_PER_LONG)) {
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return __ffs(~(*addr) | (1ul << size));
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}
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/* the result of __ffs(0) is undefined, so it needs to be */
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/* handled separately */
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if (__builtin_constant_p(size) && (size == BITS_PER_LONG))
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return (~(*addr) == 0) ? BITS_PER_LONG : __ffs(~(*addr));
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/* size is not constant or too big */
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return __find_first_zero_bit(addr, size);
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}
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#endif /* CONFIG_GENERIC_FIND_FIRST_BIT */
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