[PicForth] optimisation issue #2

[David McNab]

Hi,

Thanks to Samuel for earlier enlightening reply.

Now, a more challenging scenario. Here's a function from my 5-button 
keypad driver:

   \ return currently pressed button in W, or 5key.NONE if
   \ no key currently pressed
   code 5key.getc-raw  ( -- W:keycode )
       return-in-w

       5key.pin-MENU btfss
           5key.MENU retlw
       5key.pin-UP btfss
           5key.UP retlw
       5key.pin-DOWN btfss
           5key.DOWN retlw
       5key.pin-OK btfss
           5key.OK retlw
       5key.pin-STARTSTOP btfss
           5key.STARTSTOP retlw
       5key.NONE retlw
   end-code

I tried recoding this in pure PicForth, as:

   : 5key.getc-raw
       forth> return-in-w
       5key.pin-MENU      high? if 5key.MENU      exit then
       5key.pin-UP        high? if 5key.UP        exit then
       5key.pin-DOWN      high? if 5key.DOWN      exit then
       5key.pin-OK        high? if 5key.OK        exit then
       5key.pin-STARTSTOP high? if 5key.STARTSTOP exit then
       5key.NONE
   ;

However, this is generating:

   ; name: 5key.getc-raw
   ; max return-stack depth: 0
   0x045F	1C07	btfss	0x07,0
   0x0460	2C64	goto	0x464	; 5key.getc-raw + 0x005
   0x0461	0384	decf	0x04,f
   0x0462	0180	clrf	0x00
   0x0463	0008	return	
   0x0464	1C87	btfss	0x07,1
   0x0465	2C6A	goto	0x46A	; 5key.getc-raw + 0x00B
   0x0466	3001	movlw	0x01
   0x0467	0384	decf	0x04,f
   0x0468	0080	movwf	0x00
   0x0469	0008	return	
   ...

This is using 5 words per test, instead of 2 as with the assembler example.

As I (mis)understand PicForth, the 'return-in-w' command flags a word, 
such that when another word calls it, the calling word knows to look in 
W instead of TOS for the return value.

So my (hopefully small) feature request is:
- expose 'return-in-w' in 'target' wordspace
- extend the PicForth optimiser to take advantage of 'return-in-w'
   and generate code which makes full use of this.

-- 
Cheers
David