linux-kernel - [PATCH] time: Workaround gcc loop optimization that causes 64bit div errors

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Message-Id: <1281724258-19898-1-git-send-email-johnstul@us.ibm.com>
Date:	Fri, 13 Aug 2010 11:30:58 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	John Stultz <johnstul@...ibm.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Larry Finger <Larry.Finger@...inger.net>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] time: Workaround gcc loop optimization that causes 64bit div errors

Early 4.3 versions of gcc apparently aggressively optimize the raw
time accumulation loop, replacing it with a divide.

On 32bit systems, this causes the following link errors:
	undefined reference to `__umoddi3'
	undefined reference to `__udivdi3'

The gcc issue has been fixed in 4.4 and greater.

This patch replaces the accumulation loop with a do_div, as suggested
by Linus.

Signed-off-by: John Stultz <johnstul@...ibm.com>
CC: Jason Wessel <jason.wessel@...driver.com>
CC: Larry Finger <Larry.Finger@...inger.net>
CC: Ingo Molnar <mingo@...e.hu>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
---
 kernel/time/timekeeping.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e960d82..49010d8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -710,9 +710,10 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift)
 	/* Accumulate raw time */
 	raw_nsecs = timekeeper.raw_interval << shift;
 	raw_nsecs += raw_time.tv_nsec;
-	while (raw_nsecs >= NSEC_PER_SEC) {
-		raw_nsecs -= NSEC_PER_SEC;
-		raw_time.tv_sec++;
+	if (raw_nsecs >= NSEC_PER_SEC) {
+		u64 raw_secs = raw_nsecs;
+		raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);
+		raw_time.tv_sec += raw_secs;
 	}
 	raw_time.tv_nsec = raw_nsecs;
 
-- 
1.6.0.4

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