[Bug c/50773] float values are printed with greater precision than the float data type has when given as an argument to printf()

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50773

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2011-10-18
                 CC|                            |jsm28 at gcc dot gnu.org
         Resolution|INVALID                     |
     Ever Confirmed|0                           |1
      Known to fail|                            |4.7.0

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-18 10:43:08 UTC ---
(In reply to comment #2)
> This is definitely a C front-end bug.  If you look at the gimple dump, you can
> see that the literal 268517138.f isn't being correctly truncated, but instead
> the constant 2.68517138e+8 is being used instead:
> 
> main (int argc, char * * argv)
> {
>   double D.2549;
>   const char * restrict D.2550;
>   double D.2551;
>   const char * restrict D.2552;
>   int D.2553;
>   float a;
> 
>   a = 2.68517152e+8;
>   D.2549 = (double) a;
>   D.2550 = (const char * restrict) &" 268517138.f = %f\t%f\n"[0];
>   printf (D.2550, 2.68517138e+8, D.2549);
> 
> C++ FE does it correctly:
> 
> int main(int, char**) (int argc, char * * argv)
> {
>   double D.2226;
>   double D.2227;
>   double D.2228;
>   double D.2229;
>   int D.2230;
> 
>   {
>     float a;
> 
>     a = 2.68517152e+8;
>     D.2226 = (double) a;
>     D.2227 = (double) 2.68517152e+8;
>     printf (&" 268517138.f = %f\t%f\n"[0], D.2227, D.2226);

Needs -fexcess-precision=standard -m32 to trigger.  libcpp does the
parsing of FP constants IIRC, and the C++ frontend does not implement
-fexcess-precision.

CCing Joseph.

Testcase that fails with -fexcess-precision=standard -m32:

extern void abort (void);
int
main()
{
      float a = 268517138.f;
      if (a != 268517138.f)
        abort ();
      return 0;
}