Commit 001a30c4 authored by Nicolas Pitre's avatar Nicolas Pitre Committed by Russell King
Browse files

ARM: 8785/1: use compiler built-ins for ffs and fls 



On ARMv5 and above, it is beneficial to use compiler built-ins such as
__builtin_ffs() and __builtin_ctzl() to implement ffs(), __ffs(), fls()
and __fls(). The compiler does inline the clz instruction and even the
rbit instruction when available, or provide a constant value when
possible. On ARMv4 the compiler calls out to helper functions for those
built-ins so it is best to keep the open coded versions in that case.

Signed-off-by: default avatarNicolas Pitre <nico@linaro.org>
Signed-off-by: default avatarRussell King <rmk+kernel@armlinux.org.uk>
parent cbfc5619

arch/arm/include/asm/bitops.h

+9 −83
Original line number Diff line number Diff line
@@ -215,7 +215,6 @@ extern int _find_next_bit_be(const unsigned long *p, int size, int offset); 


#if __LINUX_ARM_ARCH__ < 5



#include <asm-generic/bitops/ffz.h>

#include <asm-generic/bitops/__fls.h>

#include <asm-generic/bitops/__ffs.h>

#include <asm-generic/bitops/fls.h>
@@ -223,93 +222,20 @@ extern int _find_next_bit_be(const unsigned long *p, int size, int offset); 


#else



static inline int constant_fls(int x)

{

	int r = 32;



	if (!x)

		return 0;

	if (!(x & 0xffff0000u)) {

		x <<= 16;

		r -= 16;

	}

	if (!(x & 0xff000000u)) {

		x <<= 8;

		r -= 8;

	}

	if (!(x & 0xf0000000u)) {

		x <<= 4;

		r -= 4;

	}

	if (!(x & 0xc0000000u)) {

		x <<= 2;

		r -= 2;

	}

	if (!(x & 0x80000000u)) {

		x <<= 1;

		r -= 1;

	}

	return r;

}



/*

 * On ARMv5 and above those functions can be implemented around the

 * clz instruction for much better code efficiency.  __clz returns

 * the number of leading zeros, zero input will return 32, and

 * 0x80000000 will return 0.

 */

static inline unsigned int __clz(unsigned int x)

{

	unsigned int ret;



	asm("clz\t%0, %1" : "=r" (ret) : "r" (x));



	return ret;

}



/*

 * fls() returns zero if the input is zero, otherwise returns the bit

 * position of the last set bit, where the LSB is 1 and MSB is 32.

 */

static inline int fls(int x)

{

	if (__builtin_constant_p(x))

	       return constant_fls(x);



	return 32 - __clz(x);

}



/*

 * __fls() returns the bit position of the last bit set, where the

 * LSB is 0 and MSB is 31.  Zero input is undefined.

 * On ARMv5 and above, the gcc built-ins may rely on the clz instruction

 * and produce optimal inlined code in all cases. On ARMv7 it is even

 * better by also using the rbit instruction.

 */

static inline unsigned long __fls(unsigned long x)

{

	return fls(x) - 1;

}



/*

 * ffs() returns zero if the input was zero, otherwise returns the bit

 * position of the first set bit, where the LSB is 1 and MSB is 32.

 */

static inline int ffs(int x)

{

	return fls(x & -x);

}



/*

 * __ffs() returns the bit position of the first bit set, where the

 * LSB is 0 and MSB is 31.  Zero input is undefined.

 */

static inline unsigned long __ffs(unsigned long x)

{

	return ffs(x) - 1;

}



#define ffz(x) __ffs( ~(x) )

#include <asm-generic/bitops/builtin-__fls.h>

#include <asm-generic/bitops/builtin-__ffs.h>

#include <asm-generic/bitops/builtin-fls.h>

#include <asm-generic/bitops/builtin-ffs.h>



#endif



#include <asm-generic/bitops/ffz.h>



#include <asm-generic/bitops/fls64.h>



#include <asm-generic/bitops/sched.h>

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