[PATCH 2/3] time: allow gcc to fold constants when using msecs_to_jiffies

[Nicholas Mc Guire]

The majority of the msecs_to_jiffies() users in the kernel are passing in
constants which would allow gcc to do constant folding by checking with
__builtin_constant_p() in msecs_to_jiffies().

The original msecs_to_jiffies is renamed to __msecs_to_jiffies and aside
from the removal of the check for negative values being moved out, is
unaltered.

Signed-off-by: Nicholas Mc Guire <hofrat@xxxxxxxxx>
---

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

Patch was compile and boot tested (x86_64) and a few msecs_to_jiffies() 
locations verified by inspection of the .lst files.

Patch is against 4.0-rc6 (localversion-next is -next-20150402)

 include/linux/jiffies.h |   29 ++++++++++++++++++++++++++++-
 kernel/time/time.c      |   13 +++++--------
 2 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index c367cbd..dcd8ba5 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -7,6 +7,7 @@
 #include <linux/time.h>
 #include <linux/timex.h>
 #include <asm/param.h>			/* for HZ */
+#include <generated/timeconst.h>
 
 /*
  * The following defines establish the engineering parameters of the PLL
@@ -288,7 +289,33 @@ static inline u64 jiffies_to_nsecs(const unsigned long j)
 	return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC;
 }
 
-extern unsigned long msecs_to_jiffies(const unsigned int m);
+extern unsigned long __msecs_to_jiffies(const unsigned int m);
+static inline unsigned long msecs_to_jiffies(const unsigned int m)
+{
+	/*
+	 * Negative value, means infinite timeout:
+	 */
+	if ((int)m < 0)
+		return MAX_JIFFY_OFFSET;
+
+	if (__builtin_constant_p(m)) {
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+		return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
+#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+		if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+			return MAX_JIFFY_OFFSET;
+		return m * (HZ / MSEC_PER_SEC);
+#else
+		if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+			return MAX_JIFFY_OFFSET;
+
+		return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
+			>> MSEC_TO_HZ_SHR32;
+#endif
+	} else
+		return __msecs_to_jiffies(m);
+}
+
 extern unsigned long usecs_to_jiffies(const unsigned int u);
 extern unsigned long timespec_to_jiffies(const struct timespec *value);
 extern void jiffies_to_timespec(const unsigned long jiffies,
diff --git a/kernel/time/time.c b/kernel/time/time.c
index 4fa1d26..3797540 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -488,6 +488,8 @@ EXPORT_SYMBOL(ns_to_timespec64);
  * the following way:
  *
  * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
+ *   negative values are handled in msecs_to_jiffies in 
+ *   include/linux/jiffies.h
  *
  * - 'too large' values [that would result in larger than
  *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
@@ -496,15 +498,10 @@ EXPORT_SYMBOL(ns_to_timespec64);
  *   the input value by a factor or dividing it with a factor
  *
  * We must also be careful about 32-bit overflows.
+ *
  */
-unsigned long msecs_to_jiffies(const unsigned int m)
+unsigned long __msecs_to_jiffies(const unsigned int m)
 {
-	/*
-	 * Negative value, means infinite timeout:
-	 */
-	if ((int)m < 0)
-		return MAX_JIFFY_OFFSET;
-
 #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
 	/*
 	 * HZ is equal to or smaller than 1000, and 1000 is a nice
@@ -537,7 +534,7 @@ unsigned long msecs_to_jiffies(const unsigned int m)
 		>> MSEC_TO_HZ_SHR32;
 #endif
 }
-EXPORT_SYMBOL(msecs_to_jiffies);
+EXPORT_SYMBOL(__msecs_to_jiffies);
 
 unsigned long usecs_to_jiffies(const unsigned int u)
 {
-- 
1.7.10.4

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