[tip:timers/core] time: Allow gcc to fold constants when possible

tip-bot for Nicholas Mc Guire
2015-05-19T13:37:48Z

Commit-ID: daa67b4b70568a07fef3cffacb2055891bf42ddb
Gitweb: http://git.kernel.org/tip/daa67b4b70568a07fef3cffacb2055891bf42ddb
Author: Nicholas Mc Guire <hofrat@osadl.org>
AuthorDate: Mon, 18 May 2015 14:19:14 +0200
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 19 May 2015 15:14:16 +0200

time: Allow gcc to fold constants when possible

To allow constant folding in msecs_to_jiffies() conditionally calls
the HZ dependent _msecs_to_jiffies() helpers or, when gcc can not
figure out constant folding, __msecs_to_jiffies which is the renamed
original msecs_to_jiffies() function.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Joe Perches <joe@perches.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Andrew Hunter <ahh@google.com>
Cc: Paul Turner <pjt@google.com>
Cc: Michal Marek <mmarek@suse.cz>
Link: http://lkml.kernel.org/r/1431951554-5563-3-git-send-email-hofrat@osadl.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
include/linux/jiffies.h | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 9527ddb..5e75af6 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -343,12 +343,24 @@ static inline unsigned long _msecs_to_jiffies(const unsigned int m)
* handling any 32-bit overflows.
* for the details see __msecs_to_jiffies()
*
- * the HZ range specific helpers _msecs_to_jiffies() are called from
- * __msecs_to_jiffies().
+ * msecs_to_jiffies() checks for the passed in value being a constant
+ * via __builtin_constant_p() allowing gcc to eliminate most of the
+ * code, __msecs_to_jiffies() is called if the value passed does not
+ * allow constant folding and the actual conversion must be done at
+ * runtime.
+ * the HZ range specific helpers _msecs_to_jiffies() are called both
+ * directly here and from __msecs_to_jiffies() in the case where
+ * constant folding is not possible.
*/
static inline unsigned long msecs_to_jiffies(const unsigned int m)
{
- return __msecs_to_jiffies(m);
+ if (__builtin_constant_p(m)) {
+ if ((int)m < 0)
+ return MAX_JIFFY_OFFSET;
+ return _msecs_to_jiffies(m);
+ } else {
+ return __msecs_to_jiffies(m);
+ }
}

extern unsigned long usecs_to_jiffies(const unsigned int u);