On Thursday, August 11, 2016 at 12:15:32 PM UTC+1, leif wrote: > > Erik Bray wrote: > > On Wed, Aug 10, 2016 at 2:45 PM, Thierry Dumont > > <tdu...@math.univ-lyon1.fr <javascript:>> wrote: > >> Le 10/08/2016 à 13:38, Erik Bray a écrit : > >>> Hi all, > >>> > >>> Sorry if this has been discussed ad-infinitum before--I looked around > >>> a bit but didn't find a definitive answer. > >>> > >>> I have one (well at least one) test that's failing on Cygwin due to > >>> tiny difference in the last digit of the result of log(3). > >>> > >>> This leads to to several questions: > >>> > >>> 1) Is it worth investigating the reason for the difference? > >>> 2) Is it worth trying to provide a workaround for the difference? > >>> 3) Or should the test just be updated to ignore the last couple digits > >>> of the result, and if so how (ellipses?) > >>> > >>> Thanks, > >>> Erik > >>> > >> Hello, > >> > >> What do you mean by log(3) ? > >> > >> Is it log(3.) or log(RDF(3)) ? > > > > Really what I meant was `float(log(3))` which I guess in Sage ends up > > being same as `log(RDF(3))` > > > >> With log(RDF(3)), this is classical; changing the library version, or > >> the compiler version often changes the way some expressions and > >> functions are computed. As floating point arithmetic is not > associative, > >> this is classical. With log(3.) (RealField()), I don't know. > >> When we wrote the book "Calcul Mathématique avec Sage", we had this > >> problem: after some release (some months) some doctests of the chapter > >> on numerical algebra and floating point numbers failed. The only work > >> around we found was to truncate the last digit in the output, to try to > >> test what should remain constant. > > > > I have: > > > > sage: R = RealField(100) > > sage: log(R(3)) > > 1.0986122886681096913952452369 > > > > Okay. With Cygwin's libm: > > > > sage: float(log(3)) > > 1.0986122886681096 > > > > While on Linux I get: > > > > sage: float(log(3)) > > 1.0986122886681098 > > > > It looks like the latter result is properly rounded up (with an error > > of 1ulp) while my result on Cygwin is just truncated. > > > FWIW, on Cygwin we used to use the Cephes library for a while. (No idea > whether it now replaced libm or parts of it got merged into libm > upstream, or none of that.) > > But the result you get presumably also depends on whether libm was > compiled with -mfpmath=sse or -mfpmath=387 etc. > > not sure about this; cygwin uses libm from something called newlib, which is mostly developed by Sun some 20+ years ago. So it's pretty much non-optimised for particular hardware. Although they keep adding more stuff to it even now, cf. e.g. https://sourceware.org/ml/newlib-cvs/2016-q1/msg00046.html
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