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Re: [PATCH] Minor tweak to mp multiplication


On 01/11/2013 06:28 AM, Siddhesh Poyarekar wrote:
> Hi,
> 
> Attached is a minor tweak to the multiplication function (__mul) to
> help the compiler a bit to generate slightly faster code.  It adds a
> local variable zk that acts as an accumulator across the loops (and
> within them) to reduce the reads to/writes from the array z.d.  I took
> this idea from the powerpc code.  This results in an improvement of
> about 5% on x86_64 for the pow function and no regressions in the
> testsuite due to this.
> 
> For 100000 iterations of pow with input as (1.0000000000000020, 1.5):
> 
> Without the patch:
> 
> Total:43300983220, Fastest:421664, Slowest:1347553, Avg:433009.832200
> 
> With the patch:
> 
> Total:41006921512, Fastest:402743, Slowest:1326084, Avg:410069.215120
> 
> The fastest time improved by about 4.5% and the average improved by
> about 5.3%.
> 
> OK to commit?
> 
> Siddhesh
> 
> 	* sysdeps/ieee754/dbl-64/mpa.c (__mul): Add a local variable
> 	to optimize copies.

Looks good to me.

Could you please document this briefly here:
http://sourceware.org/glibc/wiki/benchmarking/results_2_18

That way we record the criteria for acceptance as +5.4% for x86_64
when we go back to look at 2.18.

Cheers,
Carlos.


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