Bug in gmp/mpfr/mpc, never ending ctan computation
rguenther at suse.de
Mon Nov 19 11:49:12 UTC 2018
On Mon, 19 Nov 2018, Torbjörn Granlund wrote:
> Richard Biener <rguenther at suse.de> writes:
> :/ But then with GCC we want deterministic behavior -- ISL
> added some "computation budget" and error reporting if that was
> taken up. Maybe gmp/mpfr/mpc could consider adding such a thing
> (reporting budget overruns via a signal could be acceptable if
> the API churn would be too bad for another way).
> I'll leave this up to the mpc maintainers.
> > I simplified your test case using 1 as the real part and an integer for
> > the imaginary part. With the real part set to 10000, the computation
> > finishes in about 10 seconds, with every doubling of it the runtime
> > almost triples. (If my observation generalises to an extended real part
> > range, the compilation for your test case should terminate in about
> > 10*3^log2(4.045689747817175388336181640625e+8/10000)/3600/24/365 years.)
> Ick. I wonder if the actual ctan result is infinite or we just
> fail to compute it in an efficient manner...
> The result should be extremely small, so returning Inf would be
> quite inaccurate. :-)
I see. So I guess one issue might be that mpfr numbers have a
precision (mantissa bits?) but no restriction on the exponent?
That is, for ctan eventually zero _is_ a validly rounded result
and the computation should simply terminate...
Richard Biener <rguenther at suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)
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