Re: [Sdcc-user] Adding PIC support to SDCC

[Scott D. <sc...@da...>]

On Mon, 4 Sep 2000, Raimonds Cicans wrote:

> Hi!
> 
> Scott Dattalo wrote:
> > The question about open-sourced C compilers for the PIC arises frequently.
<snip>
> 
> If you don't need full featured ANSI compatible C compiler, than you can
> try 
> 'Small C'. Somewhere on web I saw port for 16c84 PIC. 

<snip>

Here's my response on the gnupic mailing list to your suggestion:

-----------------------

Over a year ago I took John's C compiler and made it such that it would compile
under Linux.

http://www.dattalo.com/gnupic/pic_cc.tar.gz

I guess you can say that it works. However the code generated is grossly
inefficient. This is because Favata's work is based on a free Small C
implementation that had no optimization.

Consider this example:

GetAD()
{
   char v;
   v=0xff;
   return(v);

}

As an assembly language I'd write:

GetAD:

        retlw  0xff

But look what pic_cc does:

GetAD_
        sub _stackptr, #1
        mov _primary, #0
        add _primary, _stackptr
        call _push_
        mov _primary, #255
        call _pop_
        call _putstk_
        mov _primary, #0
        add _primary, _stackptr
        call _indr_
        jmp _10_

_10_
        add _stackptr, #1
        ret

Sigh.

From this you can see that pic_cc assumes a stack which Favata decided to
emulate in software. Furthermore, you can see how there is no attempt at
optimization. After seeing this code, I decided that there was no hope for
it. This was further confirmed when I examined the source and saw that the
stack assumption is fairly deeply entrenched.

SDCC, OTOH, assumes no stack at all during the optimization phase. It's only
during the 5th or 6th step when code is actually emitted does the use of a
(optional) stack get implemented.

-----------------------

Your comment is true. However, I don't consider it viable. For the Small C port
to be useful, I'm sure that there would have to be a massive over haul in the
code. If I'm going to spend that effort I'd like to leverage off something that
gets me most of the way there.

Regards,
Scott