[erlang-bugs] math:exp() erlang bug on SPARC

[Jesper Louis Andersen]

On Wed, Apr 13, 2016 at 3:46 PM, Matt Keenan <matt.keenan@REDACTED> wrote:

> Just wanted to report a SPARC specific issue with Erlang.


I'm not entirely convinced it is with Erlang, so consider this inquiry.
Sparc uses a 64bit wide internal representation of IEEE 754 double binary
fp numbers, whereas the internal representation in x86 is 80 bit wide (due
to that being the width of the 387 coprocessor back in the day).

exp(-1162.102134881488)

is 2.02048... x 10^-505 which is smaller than the smallest representable
float. The limit, as x -> -infty is 0 for exp(_), so one would argue that
the correct output in this case is 0.0.

Erlang simply calls into the C math.h exp function for this, but it does FP
exception checks around the point. One way to figure out what happens would
be to get hold of a SPARC machine, write a simple C program doing the same
and verifying that the SPARC compilers agrees on that one with an x86
program. I could imagine some way it provokes an FP exception on the SPARC
architecture, but not on x86 due to the extra bits x86 has in internal
encoding width.

-- 
J.
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