From: "Michael Beck" <beck AT dresearch DOT de>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: -O3 breaks my code: inline assembler & optimization
Date: Sat, 24 Oct 1998 18:19:06 +0200
Organization: DResearch - Digital Media Systems
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Ignacio García Pérez wrote in message <36237ef5 DOT 671209 AT crispin>...
[..]
>I've traced down the problem to the following function:
>
>//------------------------------
>volatile dword Com::TimTicks=0;
>
>dword Com::Ticks(void) {
> dword TRef T;
> asm ("pushf");
> asm ("cli");
> T=TimTicks;
> asm ("popf");
> return T;
> }


At least with 2.7.2 it works for me in all combinations, however the problem
are the asm instructions.
asm did NOT work like inline assembly in typical x86 compilers. GCC did not
recognize any instruction
in the asm statement, it simply add the string you have specified into the
assembly it generates. That's why
you have to tell gcc which registers/memory your inline asm uses/changes.
The way you have specified your
instructions means: Neither changes registers NOR memory. So, the optimizer
is free to remove your code!
The right way is the following:

__asm __volatile__("pushf"
    "cli");

The __volatile__ here tells the compiler, that this code cannot be removed
and further that it cannot be moved to another
place. If you specify two asm and both change no register, gcc is free to
change the execution order of it!

BTW, your code makes no sense in the DJGPP environment, because it normally
runs in RING3. In this protection
level the cli insstruction is emulated, so there may be interrupts even
after the cli.

--
Michael Beck   beck AT dresearch DOT de