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common implementation of iterative div/mod

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We have a few instances of the open-coded iterative div/mod loop, used
when we don't expcet the dividend to be much bigger than the divisor.
Unfortunately modern gcc's have the tendency to strength "reduce" this
into a full mod operation, which isn't necessarily any faster, and
even if it were, doesn't exist if gcc implements it in libgcc.

The workaround is to put a dummy asm statement in the loop to prevent
gcc from performing the transformation.

This patch creates a single implementation of this loop, and uses it
to replace the open-coded versions I know about.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Jeremy Fitzhardinge 2008-06-12 10:47:56 +02:00 committed by Ingo Molnar
parent 5e70b7f3c2
commit f595ec964d
4 changed files with 30 additions and 19 deletions
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13
arch/x86/xen/time.c
View File

@ -12,6 +12,7 @@
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/kernel_stat.h>
#include <linux/math64.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>
@ -150,11 +151,7 @@ static void do_stolen_accounting(void)
if (stolen < 0)
stolen = 0;
ticks = 0;
while (stolen >= NS_PER_TICK) {
ticks++;
stolen -= NS_PER_TICK;
}
ticks = iter_div_u64_rem(stolen, NS_PER_TICK, &stolen);
__get_cpu_var(residual_stolen) = stolen;
account_steal_time(NULL, ticks);
@ -166,11 +163,7 @@ static void do_stolen_accounting(void)
if (blocked < 0)
blocked = 0;
ticks = 0;
while (blocked >= NS_PER_TICK) {
ticks++;
blocked -= NS_PER_TICK;
}
ticks = iter_div_u64_rem(blocked, NS_PER_TICK, &blocked);
__get_cpu_var(residual_blocked) = blocked;
account_steal_time(idle_task(smp_processor_id()), ticks);
}

2
include/linux/math64.h
View File

@ -81,4 +81,6 @@ static inline s64 div_s64(s64 dividend, s32 divisor)
}
#endif
u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder);
#endif /* _LINUX_MATH64_H */

11
include/linux/time.h
View File

@ -6,6 +6,7 @@
#ifdef __KERNEL__
# include <linux/cache.h>
# include <linux/seqlock.h>
# include <linux/math64.h>
#endif
#ifndef _STRUCT_TIMESPEC
@ -172,15 +173,7 @@ extern struct timeval ns_to_timeval(const s64 nsec);
*/
static inline void timespec_add_ns(struct timespec *a, u64 ns)
{
ns += a->tv_nsec;
while(unlikely(ns >= NSEC_PER_SEC)) {
/* The following asm() prevents the compiler from
* optimising this loop into a modulo operation. */
asm("" : "+r"(ns));
ns -= NSEC_PER_SEC;
a->tv_sec++;
}
a->tv_sec += iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
a->tv_nsec = ns;
}
#endif /* __KERNEL__ */

23
lib/div64.c
View File

@ -98,3 +98,26 @@ EXPORT_SYMBOL(div64_u64);
#endif
#endif /* BITS_PER_LONG == 32 */
/*
* Iterative div/mod for use when dividend is not expected to be much
* bigger than divisor.
*/
u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
{
u32 ret = 0;
while (dividend >= divisor) {
/* The following asm() prevents the compiler from
optimising this loop into a modulo operation. */
asm("" : "+rm"(dividend));
dividend -= divisor;
ret++;
}
*remainder = dividend;
return ret;
}
EXPORT_SYMBOL(iter_div_u64_rem);