A g++ bug ... or not ?

[sisyphus1@optusnet.com.au]

Hi,

The demo program:

/****/
/* try.c */
#include <stdio.h>
#include <math.h>

int main(void) {

  int x = -1074;

  printf("%e\n", pow (2.0, (double)x));
  printf("%e\n", pow (2.0, x));

  return 0;
}

/****/

If I build that program (using g++ version 7.3.0) with "g++ -ansi -o try 
try.c" I get output of:

4.940656e-324
0.000000e+000

whereas I expected output to be:

4.940656e-324
4.940656e-324

I guess that means that either my expectation is buggy, or something else is 
buggy.

Note 1:
If the "-ansi" switch is omitted, then the output matches my expectation.

Note 2:
If cmath is #included instead of math.h, then the output matches my 
expectation.

Note 3:
I believe (unproven) that if I use version 5.x.x (or earlier) of g++ then 
the output matches my expectation - otherwise it produces the unexpected 
output.

In a nutshell, I'm just trying to ascertain whether or not I should submit a 
bugzilla report about this.

Cheers,
Rob