GCC error ?

GCC error ?

[Hans-Joachim.Schmidt]

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Hello,

there is a problem when I compute:
   
   double x,y,d,e;
   
   x=112.7; 
   y=120.0;

   d=x/y-1;
   e=x/y;  e--;

   d=0x1.70a3d70a3d711p-6    (output of %a format)
   e=0x1.70a3d70a3d700p-6

   and
   d-e=0x1.1000000000000p-54


Does the output of 'd' and 'e' (in %a format) show that there is more than 1
bit difference ?

We use SuSE Linux 8.0 (i386) - Kernel 2.4.18-64GB-SMP on a Pentium III
(Coppermine).


Best regards

   Achim Schmidt 

 <<comp_diff.c>> 

[-- Attachment #2: comp_diff.c --]
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#include <stdio.h>

main() {

   double d,e,x,y;

   x=122.7;
   y=120.0;

   d=x/y-1;
   printf("d=x/y-1=     %a  (%17.15e)\n",d,d);

   e=x/y;
   e--;
   printf("e=x/y; e=e-1=%a  (%17.15e)\n",e,e);

   printf("d-e=         %a (%17.15e)\n",d-e,d-e);
   printf("x/y-1-e=     %a (%17.15e)\n",x/y-1-e,x/y-1-e);


}

Re: GCC error ?

[Tim Prince]

On Friday 07 June 2002 00:40, Hans-Joachim.Schmidt@bmf.bund.de wrote:
> Hello,
>
> there is a problem when I compute:
>
>    double x,y,d,e;
>
>    x=112.7;
>    y=120.0;
>
>    d=x/y-1;
>    e=x/y;  e--;
>
>    d=0x1.70a3d70a3d711p-6    (output of %a format)
>    e=0x1.70a3d70a3d700p-6
>
>    and
>    d-e=0x1.1000000000000p-54
>
>
> Does the output of 'd' and 'e' (in %a format) show that there is more than
> 1 bit difference ?
>
> We use SuSE Linux 8.0 (i386) - Kernel 2.4.18-64GB-SMP on a Pentium III
> (Coppermine).
>
>
It looks as if the division in the first expression has been rounded only to 
64-bit precision, while the value of e has been rounded again to 53-bit 
precision before being decremented.  This would be an expected behavior, in 
the absence of any instructions by you to the contrary.  There was a thread 
on this earlier this week, how the choice of precision mode mode is the 
responsibility of the library, not gcc.

-- 
Tim Prince