[Bug middle-end/36578] New: cast to long double not taken into account when result stored to a double

[vincent at vinc17 dot org]

The following program shows that the casts to long double are not taken into
account when the result to stored to a variable of type double. This bug occurs
with gcc from 3.4 to 4.3.1, but didn't occur with gcc 3.3.

The bug can be reproduced on a Linux/x86_64 machine (i.e. where the "double"
type corresponds to the IEEE-754 double precision and the "long double" type
corresponds to the traditional x86 extended precision) with the arguments:
  4294967219 4294967429

(these arguments allow the double rounding effect to be visible).

#include <stdio.h>
#include <stdlib.h>

int main (int argc, char **argv)
{
  double a, b, c, d, e;
  long double al, bl, dl, el;

  if (argc != 3)
    exit (1);

  a = atof (argv[1]);
  b = atof (argv[2]);
  al = a;
  bl = b;

  c = a * b;
  d = (long double) a * (long double) b;
  e = al * bl;
  dl = (long double) a * (long double) b;
  el = al * bl;

  printf ("a  =  %.0f\n", a);
  printf ("b  =  %.0f\n", b);
  printf ("c  =  %.0f\n", c);
  printf ("d  =  %.0f\n", d);
  printf ("e  =  %.0f\n", e);
  printf ("dl =  %.0Lf\n", dl);
  printf ("el =  %.0Lf\n", el);

  return 0;
}

Incorrect result (with gcc 3.4 to gcc 4.3.1):
a  =  4294967219
b  =  4294967429
c  =  18446744314227707904
d  =  18446744314227707904
e  =  18446744314227712000
dl =  18446744314227709952
el =  18446744314227709952

Correct result (as given by gcc 3.3) is the same except:
d  =  18446744314227712000

Note: I compiled with the options -std=c99 -Wall -pedantic.


-- 
           Summary: cast to long double not taken into account when result
                    stored to a double
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vincent at vinc17 dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578