Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!decwrl!labrea!rutgers!ukma!husc6!yale!spolsky-joel
From: spolsky-joel@CS.YALE.EDU (Joel Spolsky)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Turbo C 2.0 weirdities
Summary: Optimizing to blame
Message-ID: <45380@yale-celray.yale.UUCP>
Date: 11 Dec 88 22:33:39 GMT
References: <1924@sun.soe.clarkson.edu>
Sender: root@yale.UUCP
Reply-To: spolsky-joel@CS.YALE.EDU (Joel Spolsky)
Organization: Yale University Computer Science Dept, New Haven CT  06520-2158
Lines: 45

In article <1924@sun.soe.clarkson.edu> spam@clutx.clarkson.edu (Spam,,,) writes:
>While debugging what appeared to be a perfectly legit
>piece of code, I noticed that a) some variables were
>ignoring commands that changed their values, and b) that
>some variables changed values in commands that did not
>reference them at all!  

The Turbo-C optimizer often manipulates your code so that it doesn't
resemble the source. In a program like:

main()
{
	int c;
	
	c=0;
	printf("Hello\n");
}

...the statement c=0; won't even be compiled since it cannot possibly
have any effect, well, at least, that's what the optimizer thinks. So
when you execute it in the debugger, lo and behold, the value of 'c'
(which probably wasn't allocated anyway) doesn't change. Or, this code:

	for (;;) {
		c=' ';
		putchar(c);
	}

will in all likelihood be compiled as

	c=' ';
	for (;;) {
		putchar(c);
	}

or worse. Thus, things don't happen exactly as expected. Very often TC
is nice enough to give you warnings like "Unreachable code" or
"variable declared and never used" etc etc when it is doing these
things, but not always.

+----------------+----------------------------------------------------------+
|  Joel Spolsky  | bitnet: spolsky@yalecs.bitnet     uucp: ...!yale!spolsky |
|                | internet: spolsky@cs.yale.edu     voicenet: 203-436-1483 |
+----------------+----------------------------------------------------------+
						      #include <disclaimer.h>