[PATCH] Split mantissa calculation loop and add branch prediction to mp multiplication

[Andreas Jaeger]

On 12/31/2012 10:28 AM, Siddhesh Poyarekar wrote:
> Hi,
>
> The loop inside __mul in mpa.c is slow because of the branch
> evaluation within the loop.  Attached patch splits the loop to make
> the loop simpler to evaluate and also adds some branch prediction to
> improve performance.  I have verified that this does not break the
> testsuite.
>
> I checked for performance improvement with this patch using the test
> program here:
>
> http://entropymine.com/imageworsener/slowpow/
>
> with the following commandline:
>
> time ./powtest 1000000 1.0000000000000020 1.5000000000500000
>
> Taking into account the best times with and without the patch (with
> more than 10 runs each), the improvement is about a quarter of a
> second.

What's the percentage? 0.25s can be a lot - or very little depending on 
the total run time.

Andreas

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