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Subject: Re: Add prefetch switch stack hook in scheduler function
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: "Chen, Kenneth W" <kenneth.w.chen@intel.com>,
       Keith Owens <kaos@ocs.com.au>, David.Mosberger@acm.org,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org,
       linux-ia64@vger.kernel.org
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Date: Fri, 29 Jul 2005 11:17:55 +0200
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> ---------
> 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>