Bug in gmp/mpfr/mpc, never ending ctan computation

[Richard Biener]

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.

-- 
Richard Biener <rguenther at suse.de>
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