Date: Thu, 15 Nov 2007 12:54:29 +0000 From: Pete French <petefrench@ticketswitch.com> To: peterjeremy@optushome.com.au Cc: stable@freebsd.org Subject: Re: Float problen running i386 inary on amd64 Message-ID: <E1IseEn-000315-Uy@dilbert.ticketswitch.com> In-Reply-To: <20071115062002.GK89746@server.vk2pj.dyndns.org>
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> On Fri, Nov 02, 2007 at 10:04:48PM +0000, Pete French wrote:
> > int
> > main(int argc, char *argv[])
> > {
> > if(atof("3.2") =3D=3D atof("3.200"))
> > puts("They are equal");
> > else
> > puts("They are NOT equal!");
> > return 0;
> > }
>
> Since the program as defined above does not include any prototype for
> atof(), its return value is assumed to be int. The i386 code for the
> comparison is therefore:
Sorry, I didn't bother sticking the include lines in when I sent it
to the mailing list as I assumed it would be ovious that you need
to include the prototypes! In the actual tests I did I included <stdio.h>
and <stdlib.h>, so the compiler did know the return type. The result is the
same, different behaviour when running the i386 binary on amd64.
> Note that this is comparing the %eax returned by each atof(). Since
> atof() actually returns a double in %st(0) and %eax is a scratch
> register, the results are completely undefined.
I just tried this with the actual code I used for the test (i.e. with the
header files included) and I get something a lot longer than the
assembler you posted. I don't really understand what it is doing as I don't
read 386 assembler, and it's not exactly self explanatory. But the error
is still there.
Interestingly, if you recode like this:
double x = atof("3.2");
double y = atof("3.200");
if(x == y)
puts("They are equal");
else
puts("They are NOT equal!");
Then the problem goes away! Glancing at the assembly code they both appear to
be doing the same thing as regards the comparison.
> Unfortunately, I can't explain why an i386 would be different to an amd64
> in i386 mode.
me neither :-(
So, this is a bug, yes ? but it is a bug in FreeBSD or not ?
-pete.
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