[PicForth] Another possible code generation bug involving 'if' and bank switching

[Alex Holden]

I'm not certain if this one was present before the recent fix for a 
similar bug or not. Basically I test if all the bytes in an array are 
zero, and do one thing if they are and another if they are not. The 
rollover from 0x7ff to 0x800 just happens to occur between the test for 
zero and the 'else' block. The generated code causes the bank-select 
instruction to be skipped and a crash to occur if the array is not all zero.

Here's some example code:

7f2 org
create somedata 2 allot
: dosomething ;
: dosomethingelse ;
main : test        somedata @ somedata 1 + @ or
         if dosomething
         else dosomethingelse
         then
;

And the resulting generated code. Note addresses 0x7fd to 0x7ff:

0x0000  018A    clrf    0x0A
0x0001  2FF4    goto    0x7F4   ; (init-picforth)
         ; name: dosomething
         ; max return-stack depth: 0
0x07F2  0008    return
         ; name: dosomethingelse
         ; max return-stack depth: 0
0x07F3  0008    return
         ; name: (init-picforth)
         ; max return-stack depth: 0
0x07F4  3034    movlw   0x34
0x07F5  0084    movwf   0x04
         ; name: test
         ; max return-stack depth: 1
0x07F6  0822    movf    0x22,w
0x07F7  0384    decf    0x04,f
0x07F8  0080    movwf   0x00
0x07F9  0823    movf    0x23,w
0x07FA  0400    iorwf   0x00,w
0x07FB  0A84    incf    0x04,f
0x07FC  3800    iorlw   0x00
0x07FD  1903    btfsc   0x03,2
0x07FE  158A    bsf     0x0A,3
0x07FF  2802    goto    0x002   ; test + 0x00C (0x802)
0x0800  158A    bsf     0x0A,3
0x0801  2804    goto    0x004   ; test + 0x00E (0x804)
0x0802  27F3    call    0x7F3   ; dosomethingelse
0x0803  158A    bsf     0x0A,3
0x0804  0008    return

On a related note, is there any way PicForth could automatically work 
around the dreaded 'Bank switch over test' error? I seem to keep running 
into it every time I change something. It's usually pointing at a string 
defined with 'c"'. Manually inserting a 'dummy word' consisting of 
several nops before the affected word usually seems to work around the 
problem, until I change something elsewhere and the location of the bank 
switch changes.

-- 
------------ Alex Holden - http://www.linuxhacker.org ------------
If it doesn't work, you're not hitting it with a big enough hammer