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SubjectRe: [PATCH] Prevent the loop in timespec_add_ns() to be optimised away
On Fri, 22 Feb 2008 22:40:45 +0100
Segher Boessenkool <segher@kernel.crashing.org> wrote:

> ...since some architectures don't support __udivdi3() (and
> we don't want to use that, anyway).
>
> Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
> ---
> include/linux/time.h | 4 ++++
> 1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/time.h b/include/linux/time.h
> index 2091a19..d32ef0a 100644
> --- a/include/linux/time.h
> +++ b/include/linux/time.h
> @@ -174,6 +174,10 @@ static inline void timespec_add_ns(struct timespec *a, u64 ns)
> {
> ns += a->tv_nsec;
> while(unlikely(ns >= NSEC_PER_SEC)) {
> + /* The following asm() prevents the compiler from
> + * optimising this loop into a modulo operation. */
> + asm("" : "+r"(ns));
> +
> ns -= NSEC_PER_SEC;
> a->tv_sec++;
> }

It's pretty sad that we need to turn this into a loop just because of the
__udivdi3() thing.

otoh, it's rarely occurring, and it could be that the number of times it
loops is usually 1 (if it wasn't zero), so perhaps a loop is faster than a
divide anyway.

This code is probably too large to be inlined.

I queued this patch as needed-in-2.6.25, to-be-merged-via-Thomas.


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