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Message-ID: <48231959.4050406@goop.org>
Date: Thu, 08 May 2008 16:16:41 +0100
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Segher Boessenkool <segher@...nel.crashing.org>
CC: Robert Hancock <hancockr@...w.ca>,
Christian Kujau <lists@...dbynature.de>,
LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
john stultz <johnstul@...ibm.com>
Subject: [PATCH] common implementation of iterative div/mod
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@...rix.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: john stultz <johnstul@...ibm.com>
Cc: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Christian Kujau <lists@...dbynature.de>
Cc: Robert Hancock <hancockr@...w.ca>
---
arch/x86/xen/time.c | 13 +++----------
include/linux/math64.h | 2 ++
include/linux/time.h | 11 ++---------
lib/div64.c | 23 +++++++++++++++++++++++
4 files changed, 30 insertions(+), 19 deletions(-)
===================================================================
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -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);
}
===================================================================
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -81,4 +81,6 @@ static inline s64 div_s64(s64 dividend,
}
#endif
+unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder);
+
#endif /* _LINUX_MATH64_H */
===================================================================
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -2,6 +2,7 @@
#define _LINUX_TIME_H
#include <linux/types.h>
+#include <linux/math64.h>
#ifdef __KERNEL__
# include <linux/cache.h>
@@ -172,15 +173,7 @@ extern struct timeval ns_to_timeval(cons
*/
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__ */
===================================================================
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -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.
+ */
+unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
+{
+ unsigned 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);
--
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