Re: Add prefetch switch stack hook in scheduler function

Peter Zijlstra
2005-07-29T09:17:55Z

> ---------
> unroll prefetch_range() loops manually.
>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> include/linux/prefetch.h | 31 +++++++++++++++++++++++++++++--
> 1 files changed, 29 insertions(+), 2 deletions(-)
>
> Index: linux/include/linux/prefetch.h
> =================================> --- linux.orig/include/linux/prefetch.h
> +++ linux/include/linux/prefetch.h
> @@ -58,11 +58,38 @@ static inline void prefetchw(const void
> static inline void prefetch_range(void *addr, size_t len)
> {
> #ifdef ARCH_HAS_PREFETCH
> - char *cp;
> + char *cp = addr;
> char *end = addr + len;
>
> - for (cp = addr; cp < end; cp += PREFETCH_STRIDE)
> + /*
> + * Unroll agressively:
> + */
> + if (len <= PREFETCH_STRIDE)
> prefetch(cp);
> + else if (len <= 2*PREFETCH_STRIDE) {
> + prefetch(cp);
> + prefetch(cp + PREFETCH_STRIDE);
> + }
> + else if (len <= 3*PREFETCH_STRIDE) {
> + prefetch(cp);
> + prefetch(cp + PREFETCH_STRIDE);
> + prefetch(cp + 2*PREFETCH_STRIDE);
> + }
> + else if (len <= 4*PREFETCH_STRIDE) {
> + prefetch(cp);
> + prefetch(cp + PREFETCH_STRIDE);
> + prefetch(cp + 2*PREFETCH_STRIDE);
> + prefetch(cp + 3*PREFETCH_STRIDE);
> + }
> + else if (len <= 5*PREFETCH_STRIDE) {
> + prefetch(cp);
> + prefetch(cp + PREFETCH_STRIDE);
> + prefetch(cp + 2*PREFETCH_STRIDE);
> + prefetch(cp + 3*PREFETCH_STRIDE);
> + prefetch(cp + 4*PREFETCH_STRIDE);
> + } else
> + for (; cp < end; cp += PREFETCH_STRIDE)
> + prefetch(cp);
> #endif
> }
>

code like that always makes me think of duffs-device
http://www.lysator.liu.se/c/duffs-device.html

although it might be that the compiler generates better code from the
current incarnation; just my .02 ;-)

regards,

--
Peter Zijlstra <a.p.zijlstra@chello.nl>