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