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

[tip-bot for Nicholas Mc Guire]

Commit-ID:  daa67b4b70568a07fef3cffacb2055891bf42ddb
Gitweb:     http://git.kernel.org/tip/daa67b4b70568a07fef3cffacb2055891bf42ddb
Author:     Nicholas Mc Guire <hofrat@xxxxxxxxx>
AuthorDate: Mon, 18 May 2015 14:19:14 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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@xxxxxxxxx>
Cc: Masahiro Yamada <yamada.m@xxxxxxxxxxxxxxxx>
Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Andrew Hunter <ahh@xxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Michal Marek <mmarek@xxxxxxx>
Link: http://lkml.kernel.org/r/1431951554-5563-3-git-send-email-hofrat@xxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 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);
--
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