Re: Log2 and beta test failures

[Michael Goffioul]

On Mon, May 5, 2008 at 9:33 PM, Ben Abbott <address@hidden> wrote:
>  I'm using gcc 4.3 as well with -O3 and see the same test failure that John 
> sees with -O.
>
>  I don't see the others, which Michael encountered (later I'll try building 
> with -O2 to see if the log2 feature goes away).
>
>  Regarding the log2 feature, I noticed log gives the expected result
>
>  octave:12> log(complex(0,inf))
>  ans = Inf + 1.571i
>
>  and that
>
>  octave:13> log(complex(0,inf))/log(2)
>  ans = Inf - NaNi
>
>  gives a result consistent with log2. I assume that the "log(2)" is being 
> treated as a complex number?
>
>  I'm not proficient in c/c++, but I suspect the log2 feature originates in 
> lo-mappers.cc
>
>  Complex
>  xlog2 (const Complex& x)
>  {
>  #if defined (M_LN2)
>   static double ln2 = M_LN2;
>  #else
>   static double ln2 = log (2);
>  #endif
>
>   return std::log (x) / ln2;
>  }
>
>  Might the return be modified to something like
>
>  y = log(x);
>  return std:: complex(real(y)/ln2, imag(y)/ln2);

I had more the impression that the problem comes from the
other form of xlog2, with 2 input arguments. That form simply
calls frexp, and I suspect that the Win32 version of frexp
returns something different than what's expected for
infinite input.

Michael.