[Octave-bug-tracker] [bug #42583] log2() returns inaccurate result for many integer powers of 2, unlike Matlab

[Rik]

Follow-up Comment #5, bug #42583 (project octave):

This is likely to be a problem with 1) cross-compiling, 2) the availibility of
a log2 function in the C library, or 3) Cygwin's implementation of log2.

For efficiency, Octave tries to map log2 to the local C library function which
is usually a better choice than trying to calculate it ourselves.  The code is
in liboctave/numeric/lo-mappers.cc.


double
xlog2 (double x)
{
#if defined (HAVE_LOG2)
  return log2 (x);
#else
#if defined (M_LN2)
  static double ln2 = M_LN2;
#else
  static double ln2 = log (2);
#endif

  return log (x) / ln2;
#endif
}


If I check my config.h file I see that HAVE_LOG2 is defined to 1 so glibc has
the log2 function.

How did you get Octave for cygwin?  Is this a native build or a cross-compiled
build?  If it is a cross-compiled build using the MXE framework then it may be
that the tools could not determine whether the end system would have log2 in
libc and so undefined HAVE_LOG2.

Another possibility is that the log2 function on cygwin is just broken.  The
gnulib project has replacement functions for ones that are known to have
compatibility problems between platforms.  If I look at log2
(https://www.gnu.org/software/gnulib/manual/html_node/log2.html) I see that
Cygwin is listed.


This function returns slightly wrong values for exact powers of 2 on some
platforms: Cygwin 1.7.9. 



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