From: "A. Sinan Unur" <asu1 AT cornell DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Strange loop with optimize
Date: Mon, 04 Aug 1997 19:02:21 -0400
Organization: Cornell University http://www.cornell.edu
Lines: 43
Sender: asu1 AT cornell DOT edu (Verified)
Message-ID: <33E65F7D.4B22@cornell.edu>
References: <m0wvUhg-0003GiC AT fwd02 DOT btx DOT dtag DOT de>
Reply-To: asu1 AT cornell DOT edu
NNTP-Posting-Host: cu-dialup-0030.cit.cornell.edu
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

Georg Kolling wrote:

>   /* VESA mode 640*480 already set, virtual width = 800 */
>   ...
>   _farsetsel (LFBsel);       /* sets fs */
>   time = rawclock ();
>   for (co = 0; co < 100000000; co++)
>       ppix ();
>   time = rawclock () - time;
>   ...
> 
> I can compile and run this program, i can move the pixel by moving the 
> mouse, i'm getting good performance (393 clockticks, about 4,6 mio. 
> pixel per second) on a Pentium 100...BUT...
> if i compile with -O1, -O2 or -O3, the loop doesn't work! I can still 
> move the pixel, but the loop never gets to an end (i inserted a little 
> 'break' routine that stops when i press a mouse button; the result: co 
> (which is unsigned long) had always a value of about 400 (random), no 
> matter how many pixels it had actually put or how long the program was 
> already running)

i am no expert on this so i may not be putting this exactly in the right
terms.

the optimizer probably realizes that you do not use the value of co in
the loop, and possibly never in the function (can't tell.) hence it
probably implements the loop in some other way (since it is supposed to
execute a fixed number of times) eliminating the constant
increment/test. did you look at the assembly output with optimization?
did you actually wait for 100,000,000 iterations? at the end of the
loop, co should contain the correct value 100,000,001 if the rest of the
function refers to it.
-- 
   Sinan
 
*******************************************************************
 A. Sinan Unur                                WWWWWW
                                              |--O+O
 mailto:sinan DOT unur AT cornell DOT edu                C   ^ 
 http://www.people.cornell.edu/pages/asu1/     \  ~/ 

Unsolicited e-mail is _not_ welcome, and will be billed for.
*******************************************************************