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

[Joe Perches]

On Sun, 2015-04-05 at 09:23 +0200, Nicholas Mc Guire wrote:
> 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.

At least for gcc 4.9, this doesn't allow the compiler
to optimize / precalculation msecs_to_jiffies calls
with a constant.

This does: (on top of your patch x86-64 defconfig)

$ size vmlinux.o.*
   text	   data	    bss	    dec	    hex	filename
11770523	1505971	1018454	14294948	 da1fa4	vmlinux.o.next-b0a12fb5bc8
11770530	1505971	1018454	14294955	 da1fab	vmlinux.o.next-b0a12fb5bc8-inline
11768734	1505971	1018454	14293159	 da18a7	vmlinux.o.next-b0a12fb5bc8-macro

I think this should still move the if (m) < 0 back into the
original __msecs_to_jiffies function.

---

 include/linux/jiffies.h | 71 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 46 insertions(+), 25 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index a75158e..f8fe9f7 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -291,6 +291,39 @@ static inline u64 jiffies_to_nsecs(const unsigned long j)
 
 extern unsigned long __msecs_to_jiffies(const unsigned int m);
 
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+#define const_msecs_to_jiffies(m)					\
+({									\
+	unsigned long j;						\
+	if ((int)m < 0)							\
+		j = MAX_JIFFY_OFFSET;					\
+	else								\
+		j = (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ); \
+	j;								\
+})
+#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+#define const_msecs_to_jiffies(m)					\
+({									\
+	unsigned long j;						\
+	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))			\
+		j = MAX_JIFFY_OFFSET;					\
+	else								\
+		j = m * (HZ / MSEC_PER_SEC);				\
+	j;								\
+})
+#else
+#define const_msecs_to_jiffies(m)					\
+({									\
+	unsigned long j;						\
+	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))\
+		j = MAX_JIFFY_OFFSET;					\
+	else								\
+		j = (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)		\
+			>> MSEC_TO_HZ_SHR32;				\
+	j;								\
+})
+#endif
+
 /**
  * msecs_to_jiffies: - convert milliseconds to jiffies
  * @m:	time in millisecons 
@@ -313,31 +346,19 @@ extern unsigned long __msecs_to_jiffies(const unsigned int m);
  * allow constant folding and the actual conversion must be done at
  * runtime.
  */
-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);
-}
+#define msecs_to_jiffies(m)						\
+({									\
+	unsigned long j;						\
+	if (__builtin_constant_p(m)) {					\
+		if ((int)m < 0)						\
+			j = MAX_JIFFY_OFFSET;				\
+		else							\
+			j = const_msecs_to_jiffies(m);			\
+	} else {							\
+		j = __msecs_to_jiffies(m);				\
+	}								\
+	j;								\
+})
 
 extern unsigned long usecs_to_jiffies(const unsigned int u);
 extern unsigned long timespec_to_jiffies(const struct timespec *value);


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