Commit 658716d1 authored by Brian Behlendorf's avatar Brian Behlendorf Committed by Linus Torvalds
Browse files

div64_u64(): improve precision on 32bit platforms 

The current implementation of div64_u64 for 32bit systems returns an
approximately correct result when the divisor exceeds 32bits.  Since doing
64bit division using 32bit hardware is a long since solved problem we just
use one of the existing proven methods.

Additionally, add a div64_s64 function to correctly handle doing signed
64bit division.

Addresses https://bugzilla.redhat.com/show_bug.cgi?id=616105



Signed-off-by: default avatarBrian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Ben Woodard <bwoodard@llnl.gov>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Mark Grondona <mgrondona@llnl.gov>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5d051dec

include/linux/kernel.h

+5 −0
Original line number Diff line number Diff line
@@ -173,6 +173,11 @@ extern int _cond_resched(void); 
		(__x < 0) ? -__x : __x;		\

	})



#define abs64(x) ({				\

		s64 __x = (x);			\

		(__x < 0) ? -__x : __x;		\

	})



#ifdef CONFIG_PROVE_LOCKING

void might_fault(void);

#else

include/linux/math64.h

+12 −0
Original line number Diff line number Diff line
@@ -35,6 +35,14 @@ static inline u64 div64_u64(u64 dividend, u64 divisor) 
	return dividend / divisor;

}



/**

 * div64_s64 - signed 64bit divide with 64bit divisor

 */

static inline s64 div64_s64(s64 dividend, s64 divisor)

{

	return dividend / divisor;

}



#elif BITS_PER_LONG == 32



#ifndef div_u64_rem
@@ -53,6 +61,10 @@ extern s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder); 
extern u64 div64_u64(u64 dividend, u64 divisor);

#endif



#ifndef div64_s64

extern s64 div64_s64(s64 dividend, s64 divisor);

#endif



#endif /* BITS_PER_LONG */



/**

lib/div64.c

+42 −10
Original line number Diff line number Diff line
@@ -77,26 +77,58 @@ s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder) 
EXPORT_SYMBOL(div_s64_rem);

#endif



/* 64bit divisor, dividend and result. dynamic precision */

/**

 * div64_u64 - unsigned 64bit divide with 64bit divisor

 * @dividend:	64bit dividend

 * @divisor:	64bit divisor

 *

 * This implementation is a modified version of the algorithm proposed

 * by the book 'Hacker's Delight'.  The original source and full proof

 * can be found here and is available for use without restriction.

 *

 * 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c'

 */

#ifndef div64_u64

u64 div64_u64(u64 dividend, u64 divisor)

{

	u32 high, d;

	u32 high = divisor >> 32;

	u64 quot;



	high = divisor >> 32;

	if (high) {

		unsigned int shift = fls(high);

	if (high == 0) {

		quot = div_u64(dividend, divisor);

	} else {

		int n = 1 + fls(high);

		quot = div_u64(dividend >> n, divisor >> n);



		d = divisor >> shift;

		dividend >>= shift;

	} else

		d = divisor;

		if (quot != 0)

			quot--;

		if ((dividend - quot * divisor) >= divisor)

			quot++;

	}



	return div_u64(dividend, d);

	return quot;

}

EXPORT_SYMBOL(div64_u64);

#endif



/**

 * div64_s64 - signed 64bit divide with 64bit divisor

 * @dividend:	64bit dividend

 * @divisor:	64bit divisor

 */

#ifndef div64_s64

s64 div64_s64(s64 dividend, s64 divisor)

{

	s64 quot, t;



	quot = div64_u64(abs64(dividend), abs64(divisor));

	t = (dividend ^ divisor) >> 63;



	return (quot ^ t) - t;

}

EXPORT_SYMBOL(div64_s64);

#endif



#endif /* BITS_PER_LONG == 32 */



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