Float problen running i386 inary on amd64

Peter Jeremy peterjeremy at optushome.com.au
Thu Nov 15 10:46:27 PST 2007

I've Cc'd bde@ because this relates to the FPU initialisation - which
he is the expert on.

On Thu, Nov 15, 2007 at 12:54:29PM +0000, Pete French wrote:
>> On Fri, Nov 02, 2007 at 10:04:48PM +0000, Pete French wrote:
>> >	int
>> >	main(int argc, char *argv[])
>> >	{
>> >        	if(atof("3.2") == 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!

OK, sorry for the confusion.

>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.

The underlying problem is that the amd64 FPU is initialised to 64-bit
precision mode, whilst the i386 FPU is initialised to 53-bit precision
mode (__INITIAL_FPUCW__ in amd64/include/fpu.h vs __INITIAL_NPXCW__ in
i386/include/npx.h).  It looks like the FPU is initialised during the
machine-dependent CPU initialisation and then inherited by subsequent
processes as they are fork()d.  The fix is probably to explicitly
initialise the FPU for legacy mode processes on the amd64.

A work-around would be to call fpsetprec(FP_PD) (see <machine/ieeefp.h>)
at the start of main().

Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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