Re: Log2 and beta test failures

[Jaroslav Hajek]

On Tue, May 6, 2008 at 10:05 AM, Michael Goffioul
<address@hidden> wrote:
> On Tue, May 6, 2008 at 8:35 AM, Jaroslav Hajek <address@hidden> wrote:
>  >  I don't have other g++ versions or compilers to try. Ben, Michael, can
>  >  you try this fragment with your configurations? I don't see any
>  >  obvious bug in the xlog2 mappers, so I guess this may be the cause.
>
>  On MSVC, I get (0,inf) whatever the optimization level. I did the same
>  with cygwin gcc-3.4.4 and it gave me (nan,inf) whatever the optimization
>  level.
>
>  However, the test failures I reported are not related to that, but
>  to the second form of xlog2:
>
>  double xlog2(double x, int& exp)
>
>  when x = Inf. This results in a call to frexp with infinite value.
>  I tested it under MSVC and I got:
>
>  frexp(Inf) => NaN, -1
>
>  but from the test code in src/data.cc, it seems the expected
>  values are
>
>  frexp(Inf) => Inf, 0
>
>  Michael.
>

Yes, that is what is expected and what both gcc and Intel always give.
I was convinced that for f = frexp (x,&e), x = f * 2^e is an
invariant; I'm less convinced now although gcc's and Intel's results
make more sense than Microsoft's. I'll try to check the C standard.

-- 
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz