Re: thoughts on native code

[Andreas Rottmann <a.rottmann@gmx.at>]

Sjoerd van Leent Privé <svanleent@gmail.com> writes:

> Hi Stefan,
>
> Just my idea about an assembler in Scheme. Sounds interesting. If it's
> done properly, it can be very promising to use scheme itself to
> directly emit machine instructions. This would also be interesting for
> meta compilation in the future (think of aiding GCC).
>
> So you are thinking about an assembler for x86? Perhaps I can help out
> on this one. I would like to do this part, as I haven't been able to
> aid on other parts besides voicing my ideas (anyways, I am on embedded
> development these days.)
>
> The only discussion is the syntax I believe, I mean, should it be AT&T
> like, Intel like, or leave this domain and do something new. I would
> go for instructions like this (using macros):
>
> (let ((target :x686))
>   (assemble target
>    ((mov long 100 EAX)
>     (mov long 200 EBX)
>     (add long EBX EAX))))
>
> Giving back the native machine code instructions. Perhaps special
> constructions can be made to return partially complete instructions
> (such as missing labels or calls to guile procedures...)
>
Regarding the assembler: I have a working AVR assembler [0] in my
"avrth" AVR Forth implementation [1]. That assembler also employs this
"partially complete instructions" idea you mentioned, having a symbol
resolution step. It also supports a simple evaluator for Assembly-time
expressions.  Maybe you find it interesting :-).

Here's an example snippet, from the Forth implementation's runtime:

(define-primitive-vocable vocabulary:primitive "1ms" (vm)
  (scheme
   (sleep-seconds 0.001))
  (assembly
   (ldi zl (lo8 (/ cpu-frequency 4000)))
   (ldi zh (hi8 (/ cpu-frequency 4000)))
   (sbiw zl 42) ;internal plus forth kernel overhead
   PFA_1MS1
   (sbiw zl 1)
   (brne PFA_1MS1)))

The assembler code is the `(assembly ...)' part, but above that you can
see the runable documentation, written in Scheme ;-).  You may also have
noticed the expressions used, i.e. `(lo8 ...)' and `(hi8 ...)' -- these
are evaluated at assembly-time by the `assembler-eval' as found in [0].

While I'm probably too time-starved to really help with Guile's
assembler, I'd be interested in having, and maybe working on (if time
permits), a Scheme-based assembler targeting ARM platforms, so I might
chime in this area at some point.

It should be fine to use my code (from a copyright view angle) as a
basis something to be incorporated into Guile (even though that file is
marked GPLv2, not GPLv2+), as long as you take care to eliminate the
actual AVR instruction generation code.  This part (i.e., the section
marked "Code emitters") is mostly originally transcribed from the
assembler [2] written in Forth included in AmForth [3], which is GPLv2
(only, unfortunatly).  However, I'm not even sure if the AmForth
copyright is still applicable to that part of my code, as the code has
been transformed substantially by transcription.  That might well be a
moot point, we probably don't want to target 8-bit microcontrollers as
Guile platforms anyway ;-).

[0] http://rotty.xx.vu/gitweb/?p=scheme/avrth.git;a=blob;f=assembler.sls;hb=HEAD
[1] http://rotty.xx.vu/gitweb/?p=scheme/avrth.git;a=summary
[2] http://amforth.svn.sourceforge.net/viewvc/amforth/trunk/lib/assembler.frt?revision=1301&view=markup
[3] http://amforth.sourceforge.net/

Regards, Rotty
-- 
Andreas Rottmann -- <http://rotty.xx.vu/>