[SciPy-Dev] Floating point differences in CPython

[Stanley Seibert]

I'm speculating, but what jumped out at me is the **1.5, which will likely
invoke the pow() function in glibc.  Since there is no IEEE spec for pow()
precision, it is possible that a change there could produce this effect.
Stretch uses glibc 2.24 and Buster uses glibc 2.28.  The changelog for
glibc 2.27 (https://sourceware.org/ml/libc-announce/2018/msg00000.html)
mentions this:

* Optimized x86-64 asin, atan2, exp, expf, log, pow, atan, sin, cosf,
  sinf, sincosf and tan with FMA, contributed by Arjan van de Ven and
  H.J. Lu from Intel.

Which seems like a likely culprit, although I haven't done the work to
exactly pinpoint it to this change.

On Mon, Dec 2, 2019 at 1:06 AM Juan Luis Cano <juanlu001 at gmail.com> wrote:

> Hi all,
>
> I admit this is not strictly about SciPy, but there is lots of people in
> this list that care about compilers and floating point math so I still
> wanted to give it a try.
>
> I am observing floating point differences between two different versions
> of CPython 3.7.5, one compiled with GCC 7.4.0 (latest Linux Mint) and other
> compared with GCC 8.3.0 (Debian Buster) that can be reproduced like this:
>
> $ docker run --rm -it python:3.7.5-stretch python -c 'print("{:.10f}".format((7091.17117297555159893818 ** 2 + 7118.70008070942458289210 ** 2 + (-1513.67274389594149397453) ** 2) ** 1.5))'
> 1049171479739.7584228516
> $ docker run --rm -it python:3.7.5-buster  python -c 'print("{:.10f}".format((7091.17117297555159893818 ** 2 + 7118.70008070942458289210 ** 2 + (-1513.67274389594149397453) ** 2) ** 1.5))'
> 1049171479739.7585449219
>
>
> I highlight the GCC version because as far as I understand is the only
> thing that could have changed between the two. Does anybody have a more
> detailed explanation of what's going on here?
>
> Best,
>
> --
> Juan Luis Cano
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