Messages in this thread | | | Date | Wed, 27 Feb 2008 15:43:41 -0800 | From | Andrew Morton <> | Subject | Re: [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.
| |