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div64_u64(): improve precision on 32bit platforms

Browse source 
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: Brian Behlendorf <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Ben Woodard <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: Mark Grondona <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Brian Behlendorf 2010-10-26 14:23:10 -07:00 • committed by Linus Torvalds
commit 658716d19f
3 changed files with 59 additions and 10 deletions
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52
lib/div64.c
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@ -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 */
/*