Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!rpi!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.lang.c
Subject: Re: is this broken or what?
Message-ID: <2033@crdos1.crd.ge.COM>
Date: 23 Jan 90 13:36:56 GMT
References: <1482@mdbs.UUCP>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 34

In article <1482@mdbs.UUCP> wsmith@mdbs.UUCP (Bill Smith) writes:

If you didn't see the original post this may be heavily trimmed...

| [ original source ]
|
| 	unsigned u = 0;
| 
| 	if (u-- < 4)
| 		printf("yes\n");

| The Data General Aviion 5000 (an 88000 machine) version of GCC converts this 
| into:
| 
| 	unsigned u = 0;
| 
| 	if(--u < 3) ....

| This obviously will produce counter-intuitive results.

  This will generate faster code, due to not keeping the original value
of u around, but it looks as if there would be a surprise if the value
of u were all ones (~0). To wit:

	unsigned u;
	u = ~0;
	if (u-- < 4) /* this is false, value tested is large # */
	if (--u < 3) /* true, value tested is zero */

  This *looks like* an unsafen optimization unless the compiler is sure
that the value of u is never all ones.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
            "Stupidity, like virtue, is its own reward" -me