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Re: [PATCH] Simplify pow with constant


On Fri, 4 Aug 2017, Richard Biener wrote:

> >> Do this only for fast-math as accuracy is reduced.  This is much faster
> >> since pow is more complex than exp - with a current GLIBC the speedup
> >> is more than 7 times for this transformation.
> >
> > Is it bound to be so on future glibc revisions and non-glibc platforms?
> 
> And how is accuracy affected?  I think the transform is only reasonable

For pow to be accurate when the result has large (positive or negative) 
exponent, it needs to compute the log and intermediate multiplication to a 
precision around (number of mantissa bits + number of exponent bits).  
This is inevitably slower than when you omit the extra intermediate 
precision (and if you omit that precision, the error can be around MAX_EXP 
ulps).

-- 
Joseph S. Myers
joseph@codesourcery.com


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