On Wed, Apr 23, 2014 at 9:43 AM, Nathaniel Smith <n...@pobox.com> wrote:
> On Wed, Apr 23, 2014 at 6:22 AM, Matthew Brett <matthew.br...@gmail.com> > wrote: > > Hi, > > > > I'm exploring Mingw-w64 for numpy building, and I've found it gives a > > slightly different answer for 'exp' than - say - gcc on OSX. > > > > The difference is of the order of the eps value for the output number > > (2 * eps for a result of ~2.0). > > > > Is accuracy somewhere specified for C functions like exp? Or is > > accuracy left as an implementation detail for the C library author? > > C99 says (sec 5.2.4.2.2) that "The accuracy of the floating point > operations ... and of the library functions in <math.h> and > <complex.h> that return floating point results is implemenetation > defined. The implementation may state that the accuracy is unknown." > (This last sentence is basically saying that with regard to some > higher up clauses that required all conforming implementations to > document this stuff, saying "eh, who knows" counts as documenting it. > Hooray for standards!) > > Presumably the accuracy in this case is a function of the C library > anyway, not the compiler? Numpy has its own implementations for a > bunch of the math functions, and it's been unclear in the past whether > numpy or the libc implementations were better in any particular case. > In the case of MS runtime, at least 9 (as shipped in VS 2008), our implementation is likely to be better (most of the code was taken from the sun math library when the license allowed it). David > > -n > > -- > Nathaniel J. Smith > Postdoctoral researcher - Informatics - University of Edinburgh > http://vorpus.org > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion >
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