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 jos...@codesourcery.com
