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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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