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Commit 64970b68 authored by Alexander van Heukelum's avatar Alexander van Heukelum Committed by Ingo Molnar
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x86, generic: optimize find_next_(zero_)bit for small constant-size bitmaps 



This moves an optimization for searching constant-sized small
bitmaps form x86_64-specific to generic code.

On an i386 defconfig (the x86#testing one), the size of vmlinux hardly
changes with this applied. I have observed only four places where this
optimization avoids a call into find_next_bit:

In the functions return_unused_surplus_pages, alloc_fresh_huge_page,
and adjust_pool_surplus, this patch avoids a call for a 1-bit bitmap.
In __next_cpu a call is avoided for a 32-bit bitmap. That's it.

On x86_64, 52 locations are optimized with a minimal increase in
code size:

Current #testing defconfig:
	146 x bsf, 27 x find_next_*bit
   text    data     bss     dec     hex filename
   5392637  846592  724424 6963653  6a41c5 vmlinux

After removing the x86_64 specific optimization for find_next_*bit:
	94 x bsf, 79 x find_next_*bit
   text    data     bss     dec     hex filename
   5392358  846592  724424 6963374  6a40ae vmlinux

After this patch (making the optimization generic):
	146 x bsf, 27 x find_next_*bit
   text    data     bss     dec     hex filename
   5392396  846592  724424 6963412  6a40d4 vmlinux

[ tglx@linutronix.de: build fixes ]

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 60b6783a

include/asm-generic/bitops/find.h

+2 −0
Original line number Diff line number Diff line 
#ifndef _ASM_GENERIC_BITOPS_FIND_H_

#define _ASM_GENERIC_BITOPS_FIND_H_



#ifndef CONFIG_GENERIC_FIND_NEXT_BIT

extern unsigned long find_next_bit(const unsigned long *addr, unsigned long

		size, unsigned long offset);



extern unsigned long find_next_zero_bit(const unsigned long *addr, unsigned

		long size, unsigned long offset);

#endif



#define find_first_bit(addr, size) find_next_bit((addr), (size), 0)

#define find_first_zero_bit(addr, size) find_next_zero_bit((addr), (size), 0)

include/asm-x86/bitops.h

+0 −6
Original line number Diff line number Diff line
@@ -306,12 +306,6 @@ static int test_bit(int nr, const volatile unsigned long *addr); 
#undef BIT_ADDR

#undef ADDR



unsigned long find_next_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset);

unsigned long find_next_zero_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset);





#ifdef CONFIG_X86_32

# include "bitops_32.h"

#else

include/asm-x86/bitops_64.h

+0 −10
Original line number Diff line number Diff line
@@ -15,16 +15,6 @@ static inline long __scanbit(unsigned long val, unsigned long max) 
	return val;

}



#define find_next_bit(addr,size,off) \

((__builtin_constant_p(size) && (size) <= BITS_PER_LONG ? 	  \

  ((off) + (__scanbit((*(unsigned long *)addr) >> (off),(size)-(off)))) : \

	find_next_bit(addr,size,off)))



#define find_next_zero_bit(addr,size,off) \

((__builtin_constant_p(size) && (size) <= BITS_PER_LONG ? 	  \

  ((off)+(__scanbit(~(((*(unsigned long *)addr)) >> (off)),(size)-(off)))) : \

	find_next_zero_bit(addr,size,off)))



#define find_first_bit(addr, size)					\

	((__builtin_constant_p((size)) && (size) <= BITS_PER_LONG	\

	  ? (__scanbit(*(unsigned long *)(addr), (size)))		\

include/linux/bitops.h

+77 −0
Original line number Diff line number Diff line
@@ -112,4 +112,81 @@ static inline unsigned fls_long(unsigned long l) 
	return fls64(l);

}



#ifdef __KERNEL__

#ifdef CONFIG_GENERIC_FIND_NEXT_BIT

extern unsigned long __find_next_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset);



/**

 * find_next_bit - find the next set bit in a memory region

 * @addr: The address to base the search on

 * @offset: The bitnumber to start searching at

 * @size: The bitmap size in bits

 */

static __always_inline unsigned long

find_next_bit(const unsigned long *addr, unsigned long size,

		unsigned long offset)

{

	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)) {

		value = (*addr) & ((~0ul) << offset);

		return (value == 0) ? BITS_PER_LONG : __ffs(value);

	}



	/* size is not constant or too big */

	return __find_next_bit(addr, size, offset);

}



extern unsigned long __find_next_zero_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset);



/**

 * find_next_zero_bit - find the next cleared bit in a memory region

 * @addr: The address to base the search on

 * @offset: The bitnumber to start searching at

 * @size: The bitmap size in bits

 */

static __always_inline unsigned long

find_next_zero_bit(const unsigned long *addr, unsigned long size,

		unsigned long offset)

{

	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)) {

		value = (~(*addr)) & ((~0ul) << offset);

		return (value == 0) ? BITS_PER_LONG : __ffs(value);

	}



	/* size is not constant or too big */

	return __find_next_zero_bit(addr, size, offset);

}

#endif /* CONFIG_GENERIC_FIND_NEXT_BIT */

#endif /* __KERNEL__ */

#endif

lib/find_next_bit.c

+9 −16
Original line number Diff line number Diff line
@@ -15,17 +15,12 @@ 
#include <asm/byteorder.h>



#define BITOP_WORD(nr)		((nr) / BITS_PER_LONG)

#undef find_next_bit

#undef find_next_zero_bit



/**

 * find_next_bit - find the next set bit in a memory region

 * @addr: The address to base the search on

 * @offset: The bitnumber to start searching at

 * @size: The maximum size to search



/*

 * Find the next set bit in a memory region.

 */

unsigned long find_next_bit(const unsigned long *addr, unsigned long size,

		unsigned long offset)

unsigned long __find_next_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset)

{

	const unsigned long *p = addr + BITOP_WORD(offset);

	unsigned long result = offset & ~(BITS_PER_LONG-1);
@@ -62,15 +57,14 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size, 
found_middle:

	return result + __ffs(tmp);

}



EXPORT_SYMBOL(find_next_bit);

EXPORT_SYMBOL(__find_next_bit);



/*

 * This implementation of find_{first,next}_zero_bit was stolen from

 * Linus' asm-alpha/bitops.h.

 */

unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,

		unsigned long offset)

unsigned long __find_next_zero_bit(const unsigned long *addr,

		unsigned long size, unsigned long offset)

{

	const unsigned long *p = addr + BITOP_WORD(offset);

	unsigned long result = offset & ~(BITS_PER_LONG-1);
@@ -107,8 +101,7 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size, 
found_middle:

	return result + ffz(tmp);

}



EXPORT_SYMBOL(find_next_zero_bit);

EXPORT_SYMBOL(__find_next_zero_bit);



#ifdef __BIG_ENDIAN