[PATCH 3/3 V3] time: allow gcc to fold constants when possible

Nicholas Mc Guire
2015-05-18T12:28:14Z

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.

Patch was build tested for:
arm - imx_v6_v7_defconfig, vexpress_defconfig
cris - etrax-100lx_v2_defconfig
m68k - multi_defconfig
mips32 - ar7_defconfig,
mips64 - loongson3_defconfig
powerpc - 44x/virtex5_defconfig
powerpc64 - powerpc64_defconfig
sh - se7780_defconfig,
sparc32 - sparc32_defconfig
sparc64 - sparc64_defconfig
x86 64 - x86_64_defconfig,
boot and run testing:
x86 64 - x86_64_defconfig i3

Patch is against 4.1-rc3 (localversion-next is -next-20150518)

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
---

Thanks to Joe Perches <joe@perches.com> for suggesting this approach
and for his review comments on the first attempts.

include/linux/jiffies.h | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index b41e026e..e96996a 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);
--
1.7.10.4