[PATCH 2/2] time: allow gcc to fold constants when possible

[Nicholas Mc Guire <hofrat@osadl.org>]

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

Patch was build tested for:
  arm - imx_v6_v7_defconfig, vexpress_defconfig
  cris - etrax-100lx_v2_defconfig
  ia64 - generic_defconfig
  (though with lots of build warnings both with and without patch)
  m68k - multi_defconfig
  mips32 - ar7_defconfig, 
  mips64 - loongson3_defconfig 
  powerpc - 44x/virtex5_defconfig 
  powerpc64 - ppc64_defconfig
  s390 - failed to build for all defconfigs with toolchain from kernel.org
  (cc1: error: unrecognized command line option '-mtune=zEC12')
  sh - se7780_defconfig,
  sparc32 - sparc32_defconfig
  sparc64 - sparc64_defconfig
  x86 64 - x86_64_defconfig,

patch is against 4.1-rc5 (localversion-next is -next-20150527)

Link: http://lkml.org/lkml/2015/5/18/274
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
---

Thanks to Joe Perches <joe@perches.com> for suggesting this approach.
and helping with the initial msecs_to_jiffies cleanup patch.

Verification of const folding:
config: x86_64_defconfig + CONFIG_DRM_GMA500=m, CONFIG_DRM_GMA3600=y
drivers/gpu/drm/gma500/intel_gmbus.c
204         gpio->algo.timeout = usecs_to_jiffies(2200);                        
-> reduced to load instruction

config: x86_64_defconfig + CONFIG_INFINIBAND=m,  CONFIG_INFINIBAND_QIB=m
drivers/infiniband/hw/qib/qib_qp.c
809     qp->timeout = attr->timeout;
810     qp->timeout_jiffies =
811             usecs_to_jiffies((4096UL * (1UL << qp->timeout)) /
812                     1000UL);
-> call __usecs_to_jiffies

 include/linux/jiffies.h |   30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index cf6eaaf..c0aa150 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -383,9 +383,37 @@ static inline unsigned long _usecs_to_jiffies(const unsigned int u)
 }
 #endif
 
+/**
+ * usecs_to_jiffies: - convert microseconds to jiffies
+ * @u:	time in microseconds
+ *
+ * conversion is done as follows:
+ *
+ * - 'too large' values [that would result in larger than
+ *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
+ *
+ * - all other values are converted to jiffies by either multiplying
+ *   the input value by a factor or dividing it with a factor and
+ *   handling any 32-bit overflows as for msecs_to_jiffies.
+ *
+ * usecs_to_jiffies() checks for the passed in value being a constant
+ * via __builtin_constant_p() allowing gcc to eliminate most of the
+ * code, __usecs_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 _usecs_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 usecs_to_jiffies(const unsigned int u)
 {
-	return __usecs_to_jiffies(u);
+	if (__builtin_constant_p(u)) {
+		if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET))
+			return MAX_JIFFY_OFFSET;
+		return _usecs_to_jiffies(u);
+	} else {
+		return __usecs_to_jiffies(u);
+	}
 }
 
 extern unsigned long timespec_to_jiffies(const struct timespec *value);
-- 
1.7.10.4