Perry Smith writes:
> On Mar 22, 2006, at 3:05 PM, dups41@xxxxxxxxx wrote:
>
> > On x86 the following code intended to calculate log2(4096) gives an
> > unexpected result of 11 when the value is cast directly to an int. If
> > the result is stored in a double and then cast to an int the expected
> > value (12) is given.
> >
> >
> > double r0;
> > double d_msb;
> > int i_msb;
> >
> > r0=4096;
> >
> > i_msb = (int)(log10(r0)/log10(2));
> > printf("%d\n",i_msb);
> >
> > d_msb = (log10(r0)/log10(2));
> > i_msb = (int)d_msb;
> > printf("%d\n",i_msb);
> >
> >
> > # uname -srm
> > Linux 2.4.9-34smp i686
> > # g++ -o log log.c
> > # ./log
> > 11
> > 12
> >
> >
> > I have tried several versions of gcc on x86 and all give the same
> > behaviour. (3.0, 3.0.1, 3.0.4, 3.2, 3.2.2, 3.3.1)
> >
> > On Solaris 8 this code gives the expected output (12 12). AMD64 with
> > -m64 gives the expected results but with -m32 the results differ (11
> > 12).
> >
> > Why do the two values differ? If it is the case that a
> > rounding/precision error is causing an off by one result, should it
> > not be consistent between the two forms of the code?
>
> This seems odd to me too. I realize that floating point is not exact
> but that inexactness should be the same in both methods of your
> algorithms.
>
> My suggestion is to dump out the listing of the code and look at it.
> The instruction sequence must be different and that may clue you in
> on why the results are different. I'm curious what you find.
This is because of excess precision in the x87.
This is http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323.
Andrew.
