[Cfp-interest 1477] Re: complex multiplication and division

[paul zimmermann]

       Dear Damian,

on the gcc10 machine from the GCC compile farm, with gcc 6.3.0, I get:

x=(-1.0689346237980500e+252,7.8846096489686452e-145)
y=(5.1152592576527620e-318,6.7327111521288819e+78)
x/y=(1.1710898434244477e-223,1.5876733750267203e+173)

Same on the machine "devel03" from the Plafrim platform with gcc 4.8.5:

x=(-1.0689346237980500e+252,7.8846096489686452e-145)
y=(5.1152592576527620e-318,6.7327111521288819e+78)
x/y=(1.1710898434244477e-223,1.5876733750267203e+173)

This is with -O0 (or no -O option), with -O1 or higher I get the MPC result
(since then GCC uses constant folding, calling MPC).

Paul

> Date: Sun, 9 Feb 2020 22:22:35 +1100 (AEDT)
> From: Damian McGuckin <damianm at esi.com.au>
> 
> I tried your program on a GCC 9.2.1 on Power9, and GCC 4.8.1 and 8.? on 
> Xeon. I was specifically looking at division because something is very 
> obviously wrong. You mentioned:
> 
> >  /* MPC gives       (-3.5164330596706266e-225,1.5876733750267203e+173) */
> >  /* GCC 9.2.1 gives (1.1710898434244477e-223,1.5876733750267203e+173) */
> >  /* ICC 19.0.4.243: (-3.5164330596706266e-225,1.5876733750267203e+173) */
> 
> I get the same answer as MPC. Not your answer. On an ancient Xeon with GCC 
> 4.4.7, I get your 1.171.....e-223 answer. A quick guess is that something 
> is probably not quite right with your installation of GCC. But I could be
> wrong.
> 
> Regards - Damian
> 
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