Path: utzoo!utgpu!water!watmath!clyde!feg
From: feg@clyde.ATT.COM (Forrest Gehrke)
Newsgroups: comp.sys.ibm.pc
Subject: MSC v5 Codeview
Keywords: Warning--Optimizer
Message-ID: <20028@clyde.ATT.COM>
Date: 11 Jan 88 17:48:11 GMT
Organization: AT&T Bell Labs, Whippany, NJ
Lines: 25



When compiling with the -Ox flag on, the optimzer ALWAYS tries to registerize 
loop variables. i.e.

   program(ptr)
   char *ptr;
   {
        while(*ptr){
            ......
            ......
            ptr++;
        }
    }

Will load ptr into the SI register and go merrily on its way using SI to
access the data. However, the actual memory variable for ptr will not be
updated until the loop terminates. This is great for speed, but it outsmarts
Codeview. i.e. if you put a watch statement on *ptr and try to single step
through the loop, Codeview doesn't  know enough to watch the register
instead of the memory variable and ptr, never changes. The moral here is don't
use -Ox if you want to trace with Codeview or the QC in-memory debugger.

Forrest Gehrke