Re: [cc65] Zeropage Regbank

On Mon, Feb 02, 2009 at 10:48:21PM -0800, Shawn Jefferson wrote:
> How difficult, or possible would it be to allow the zeropage regbank size to
> be specified in the linker config script, or via a pragma command?

Why do you need a linker config option or #pragma? Have you tried the
--register-space compiler command line option?

Decreasing the size requires checking the library, because some modules (for
example _printf.s) assume that the register bank is at least 6 bytes large,
but increasing it shouldn't be a problem. I'm not sure however, if this gives
noticeable speed increases. The best use for register variables is using it
for (struct-) pointers and functions with more than three pointers are
candidates for a rewrite anyway :-)

> I know there is some overhead in saving and restoring the zeropage
> variables, and at some point doing this negates the benefit you get.  It
> would be nice if the compiler could tell you when that is the case or not as
> well, but that may be much more difficult.

That's too much for a compiler like cc65. Fortunately, it's not too difficult
to make a rough estimate: Three uses of a pointer means it is usually worth
declaring it as a register variable. If you have three distinct uses, you will
even get a size decrease, if you have a loop that uses the pointer at least
three times, you will get a size increase but also a speed increase. That's
the rule that I'm using.

Regards


        Uz


-- 
Ullrich von Bassewitz                                  uz@musoftware.de
----------------------------------------------------------------------
To unsubscribe from the list send mail to majordomo@musoftware.de with
the string "unsubscribe cc65" in the body(!) of the mail.