[Wien] IEEE_UNDERFLOW_FLAG IEEE_DENORMAL

[Laurence Marks]

Zamt is set by somm, which is a slightly inaccurate Simpson summation (the
accuracy does not matter). Hence it does not need to be set previously.

_____
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu

On Wed, May 9, 2018, 10:42 AM Pavel Ondračka <pavel.ondracka at email.cz>
wrote:

> Laurence Marks píše v St 09. 05. 2018 v 11:51 +0000:
> > This appears to be due to a silly approach in gfortran, and almost
> > certainly is not an error/problem and can be ignored -- see
> https://urldefense.proofpoint.com/v2/url?u=https-3A__s&d=DwIGaQ&c=yHlS04HhBraes5BQ9ueu5zKhE7rtNXt_d012z2PA6ws&r=U_T4PL6jwANfAy4rnxTj8IUxm818jnvqKFdqWLwmqg0&m=R1lxmwh4Y3r4yRx1ZyR4NN9mpSe7RuaT974qRm6Uhfw&s=KDobN5dacXxTk7OUpO1BdJBY45FBX34Hf6Q9hTempg0&e=
> > tackoverflow.com/questions/44308577/ieee-underflow-flag-ieee-
> > denormal-in-fortran-77.
> >
>
> While I do agree that in most cases this is harmless, it can also
> suggest a bug.
>
> I only looked at dstart for the TiC and I have this specific example:
>
> dstart for a TiC case prints a "Note: The following floating-point
> exceptions are signalling: IEEE_DENORMAL" line. If you trap this with
> -ffpe-trap='denormal' flag, to inspect in gdb, the offending line is
> then:
> Program received signal SIGFPE, Arithmetic exception.
> 0x000000000041dc48 in somm (dr=..., dp=..., dpas=0.013585429144994965,
> da=1.1588924125005173e-310, m=0, np=935) at somm.f:10
> 10            D1=DA+MM
>
> so this line looks completely harmless and some prints show why the
> compiler notes about this:
>
> (gdb) print DA
> $1 = 1.1588924125005173e-310
> (gdb) print MM
> $2 = 1
>
> i.e. we just add a really small number (denormal, since DOUBLE_MIN is
> around 1.8e-308) to 1, which is completely OK. But lets take a look
> where this incredibly small value comes from...
>
> (gdb) up
> #1  0x00000000004143c6 in make_spheres (lcore=.FALSE., luse=7) at
> make_spheres.F:81
> 81               call
> somm(rat(1,ia),rhoat(1,ia),dx(ia),zamt,0,nptat(ia))
>
> So this is the zamt variable, surprisingly grepping around for zamt
> finds nothing. As far as I can see it is not declared or initialized
> anywere in dstart. If I just missed something please correct me! The
> same goes for the zamt1 and zamt2.
>
> Note that in this case we get lucky since the random memory value is
> effectively zero, however this might in my opinion lead to problems if
> you hit random memory with another value.
> In fact running the dstart in vagrind shows this as well and the
> terminal is spammed with "Conditional jump or move depends on
> uninitialised value(s)" and "Use of uninitialised value of size 8".
>
> IMO this is a bug, so either the line needs to be changed to
> somm(rat(1,ia),rhoat(1,ia),dx(ia),0,0,nptat(ia))
> or the zamt variable needs to be declared and initialized somewhere.
> But I actually have no idea about the physical meaning of the code so
> please correct me if I just missed something.
>
> Best regards
> Pavel
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