[Bug libstdc++/13450] std::pow(std::complex<double>(-1.,0.),0.5) yields (NaN,0)

["gdr at integrable-solutions dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From gdr at integrable-solutions dot net  2004-01-11 18:14 -------
Subject: Re:  std::pow(std::complex<double>(-1.,0.),0.5) yields (NaN,0)


  OK, I see what is going wrong.  But the submitter's patch is not right
either. To properly implement the branch cut, we need to test whether
the imaginary part is a positive or negative zero (on IEEE-754
plateform).  Yes that is a crazy notion -- but we got it.
Therefore we need a primitive that tells the sign of a real number.
I just looked at the GCC built-ins and there does not seem to be one.
You can't just test for > or < 0.  And == 0 tells whether it is zero,
it does not tell whether it is negative or positive zero.
Furthermore, this is a template code, so we need to come up with a
design that scales to templates.  This probably means we ought to
compute the log() unconditionally -- but then, I don't recall why I
made the previous change in the first place. 

-- Gaby
 


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13450