From: Nate Eldredge <neldredge AT hmc DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: THE -O2 PROBLEM - PART II
Date: 15 Apr 2000 01:54:34 -0700
Organization: InterWorld Communications
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To: djgpp AT delorie DOT com
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"Alexei A. Frounze" <alex DOT fru AT mtu-net DOT ru> writes:

> it eliminates C code but not inline asembly.
> 
> For example:
> 
> if (KeyMap[sTab]) {		// key is pressed
>   do_something();
>   while (KeyMap[sTab]) {};	// this line is replaced with infinite loop
> }
> 
> But I need that line AS-IS.

A C compiler assumes that code is only called by "normal" methods.
Hence, it sees the loop, realizes there is no way for KeyMap[sTab] to
change, and deletes the unnecessary test.  This is legal.

However, you are breaking this rule: I presume that flag is set by an
interrupt handler or some such beast, which is not called by normal
program flow methods.  The compiler doesn't expect this.  However, you
can tell it to expect it.  Declare KeyMap as volatile:

volatile char KeyMap[]; /* or whatever */

The compiler will then ensure that every access to KeyMap is done as
written.  This of course prevents many optimizations, in general, but
here is necessary.

-- 

Nate Eldredge
neldredge AT hmc DOT edu