Trouble with floating point constants in printf and different versions of gcc

[Ian Lance Taylor]

Momchil Ivanov <momchil@xaxo.eu> writes:

>> In gcc 4.5 and up -std=c99 implies -fexcess-precision=standard, q.v.
> 
> Thank you for the tip, but I still don't understand why the following produces different output for both floats when compiled with -fexcess-precision=standard:
>
> #include <stdio.h>
>
> int
> main(int argc, char *argv[])
> {
> 	float a = 268517138.f;
> 	printf(" 268517138.f = %f\t%f\n", 268517138.f, a);
> 	
> 	return 0;
> }
>
> output with -fexcess-precision=standard:
>  268517138.f = 268517138.000000	268517152.000000
> output without -fexcess-precision=standard:
>  268517138.f = 268517152.000000	268517152.000000

Good point.  I think it is a bug that the values differ when using
-fexcess-precision=standard.  There is an implicit cast to double when
calling printf; the problem may be that that cast is not being applied
to the properly rounded value.

Please open a bug report as per http://gcc.gnu.org/bugs .  Thanks.

Ian