jon@ccwf.cc.utexas.edu (Jon Boede) (12/08/90)
Hopefully I'm posting old news... but I found a rather insidious bug in cc
version 1.6. It seems that if you use the -Oa "ignore aliasing" option with
the -Ol "enable loop optimizations" option, it starts thinking deciding that
more things are loop invariants than really are! The seductively named -Ox
"max. optimization" option implies -Oal.
Try: cc x.c; a.out; cc -Ox x.c; a.out; cc -Oal x.c; a.out
----- cut here -- x.c
main()
{
int value;
value = 1;
printf("1. value is %d\n",value);
do {
printf("2. value is %d\n",value);
puts(value ? "works ok" : "your compiler is broke");
value = 0;
} while (false());
}
false()
{
return(0);
}
----- ereh tuc
It's a little bothersome to turn on "max. optimization" and have your code stop
working! ;-)
Jon
--
Jon Boede jon@bodedo.ucm.org ...!{the-world}!uunet!cs.utexas.edu!bodedo!jon
7117 Wood Hollow #1013, People's Republic of Austin, TX 78731 +1 512 346-3142davidsen@sixhub.UUCP (Wm E. Davidsen Jr) (12/09/90)
In article <40991@ut-emx.uucp> jon@ccwf.cc.utexas.edu (Jon Boede) writes: | It's a little bothersome to turn on "max. optimization" and have your code stop | working! ;-) My manual calls this "unsafe optimization" or some such. Besides, you get the wrong answer, but much faster! ;-) -- bill davidsen - davidsen@sixhub.uucp (uunet!crdgw1!sixhub!davidsen) sysop *IX BBS and Public Access UNIX moderator of comp.binaries.ibm.pc and 80386 mailing list "Stupidity, like virtue, is its own reward" -me
karl@robot.in-berlin.de (Karl-P. Huestegge) (12/14/90)
jon@ccwf.cc.utexas.edu (Jon Boede) writes: >Hopefully I'm posting old news... but I found a rather insidious bug in cc >version 1.6. It seems you're a little bit outdated with your release. ;-) Xenix 386 2.3.2 comes with: Microsoft 386 Compiler ver. 4.85 ( was updated with lng095 to ver 5.1) Microsoft 286 Compiler Ver. 5.1 Unix 5.3 C Preprocessor Unix 5.3 curses >Try: cc x.c; a.out; cc -Ox x.c; a.out; cc -Oal x.c; a.out Always 'works ok' with MSC 386 5.1 on my system. BTW: -Ox is default. And is the same as -Oactl. All three cc-commands produce the same code. -- Karl-Peter Huestegge karl@robot.in-berlin.de Berlin Friedenau ..unido!fub!geminix!robot!karl
dkeisen@Gang-of-Four.Stanford.EDU (Dave Eisen) (12/14/90)
In article <1990Dec13.175120.12187@robot.in-berlin.de> karl@robot.in-berlin.de (Karl-P. Huestegge) writes: >Always 'works ok' with MSC 386 5.1 on my system. > >BTW: -Ox is default. And is the same as -Oactl. All three cc-commands > produce the same code. Nope. The default for -O is -Ot (optimize for time), not a, c, or l. -Ox (which is the same as -Oactl) relaxes alias checking so it can change the semantics of your code if there is any aliasing going on. Use it with great caution. I would have preferred to see their "maximum" optimization mean maximum optimization that doesn't break correct code. -- Dave Eisen dkeisen@Gang-of-Four.Stanford.EDU 1447 N. Shoreline Blvd. Mountain View, CA 94043 (415) 967-5644