Questions about NCEG

[Paul Eggert]

I wrote:

	There are ways to say the IEEE numbers in ANSI C + IEEE 754:
		#define minus_0 (-0.0)
		#define infinity (1e300*1e300)
		...

Doug Gwyn replied:

	These don't work.  -0.0 is NOT a "minus zero"; it's identical to 0.0.

Surely ANSI C doesn't require this; it is inconsistent with IEEE 754.
For example, given the following strictly conforming ANSI C program

	#include <stdio.h>
	int main() { printf("%g\n", -0.0); return 0; }

an implementation that conforms to both ANSI C and IEEE 754 must print "-0",
not "0".  IEEE 754 doesn't specify the output format, but it does require that
+0 and -0 be output differently, and "-0" is the only real choice here.

This is not purely academic.  Of the two C compilers on my IEEE 754-based
machine, one prints "-0" and the other "0" when given the above program.  I've
sent a bug report to the latter's implementer, and fully expect it to get fixed.

Finally, Gwyn wrote:

	Assuming that 1.0e+300*1.0e+300 is not representable as a
	(floating-point) number, the behavior is explicitly undefined...

(Steve Clamage made a similar point.)

True, if you assume only conformance to ANSI C.
But I explicitly assumed conformance to both ANSI C and IEEE 754.