Re: [cc65] Optimization possible?

Hi!

On Mon, Oct 19, 2009 at 07:40:54PM +0200, Thomas Giesel wrote:
> Accidently I found the piece of code below. Is it possible to optimize
> the ldx L000F out?

Technically: yes. But as with all optimization steps I have to consider if the
sequence in question is used often enough to warrant code in the compiler to
replace it. Which in this special case is probably not true. But I've checked
similar sequences and I think I can come up with something that improves them
all.

If you notice other sequences that are apparently stupid and could be improved
easily, please let me know. I will not be able to fix all of them, but I will
at least give it a thought.

> I don't know if the compiler knows that the value
> isn't used later.

The backend does register tracking, even for a few zeropage locations. It does
not only know if a register is used later, it does also track contents along
branches. If you look at the code generated for the following sequence:

-----------------------------------------------------------------------------
int b = 3;
char c = 0;
int main (void)
{
    char a;
    if (b != 3) {
        if (c == 0) {
            c = 3;
        } else if (c != 3) {
            b = 3;
        }
    }
    a = 3;
    return 0;
}
-----------------------------------------------------------------------------

you will notice that for the final assignment, the accumulator is not
reloaded, because the compiler knows that it contains the correct value for
each possible execution path.

Regards


        Uz


-- 
Ullrich von Bassewitz                                  uz@musoftware.de
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