Bug#328504: libc6: Wrong computation of tanh

[Rafael Laboissiere <rafael@debian.org>]

Package: libc6
Version: 2.3.2.ds1-22
Severity: wishlist


For the full context in which this bug report originated, please see:

   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=327027
   
Before I summarize the discussion in the URL above, let me say that I am not
fully sure that filing this bug report is appropriate (hence severity:
wishlist).  Feel free to close this bug report, in particular if a new
version of glibc which fixes the bug (see below) is coming soon.

Here is the scoop:

The octave package fails a regression test when compiled with g++ 4.0.
This regression test uses the tanh (hyperbolic tangent) function.  When
octave is compiled compiled with g++ 3.3, everything goes well.  We
investigated this problem further and discovered that g++ now uses the
glibc tanh function instead of using "sinh(x)/cosh(x)" as before.

The following simple program triggers the bug (thanks to John W. Eaton):

  #include <cmath>
  #include <iostream>
  #include <complex>
      
  int
  main (void)
  {
    std::cout << std::tanh (std::complex<double> (0, M_PI/2)) << std::endl;
    std::cout << std::tanh (std::complex<double> (0, 3*M_PI/2)) << std::endl;
   
    return 0;
  }
	  
This should produce (0, inf) in both cases.  With g++ 3.3, it is
still not right, as it produces
	  
  (0,1.63318e+16)
  (0,5.44393e+15)
	      
With g++ 4.0:	      

  (nan, inf)
  (nan, inf)
  
This bug has been reported upstream:

    http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=160759
    
and fixed on 2005-07-10.  Indeed, the libc/ChangeLog at:
    
    http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/ChangeLog?rev=1.9521&content-type=text/x-cvsweb-markup&cvsroot=glibc

has the following entry:

2005-07-07  Ulrich Drepper  <drepper@redhat.com>

	* sysdeps/generic/s_ctanh.c (__ctanh): Handle case of zero den better.
	* sysdeps/generic/s_ctanhf.c (__ctanhf): Likewise.
	* sysdeps/generic/s_ctanhl.c (__ctanhl): Likewise.
	* sysdeps/generic/s_ctan.c (__ctan): Likewise.
	* sysdeps/generic/s_ctanf.c (__ctanf): Likewise.
	* sysdeps/generic/s_ctanl.c (__ctanl): Likewise.
			

Could the patch shown in

    http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/generic/s_ctanh.c.diff?r1=1.2&r2=1.3&cvsroot=glibc&f=h

be applied to the Debian package?

Thanks,

-- 
Rafael


-- System Information:
Debian Release: testing/unstable
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing')
Architecture: i386 (i686)
Shell:  /bin/sh linked to /bin/dash
Kernel: Linux 2.6.8-1-686
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Versions of packages libc6 depends on:
ii  libdb1-compat                 2.1.3-7    The Berkeley database routines [gl

libc6 recommends no packages.

-- no debconf information