[PicForth] A program bank switching codegen bug - Christmas special double whammy

[Alex Holden]

Ok, here we have two bugs for the price of one. I suspect they're 
actually two slightly different manifestations of the same bug. 
Basically Picforth sometimes optimises away bank select instructions 
when it shouldn't do. I'm not sure if this is specific to the c" 
construct but it seemed to be the only reliable way to trigger it (this 
has been a particularly difficult one to distill to a minimal example).

The following example causes the PIC to go off into lala land at both 
strings (actually it never reaches the second one but you know what I 
mean) because pclath is set wrong when it does the 'goto' which is 
supposed to skip past the string data.

include libstrings.fs
: type ;
$ffd org
: fred ;
main : test c" hello" type fred c" world" type ;

Changing it to this works around the bug:

include libstrings.fs
: type ;
$ffd org
: fred ;
main : test
         forth> no-cbank
         c" hello" type
         fred
         forth> no-cbank
         c" world" type
;

Note that the banks which 'test' 'type' and 'fred' are located in seem 
to be critical to triggering the bug.

Here's the generated assembly of just the main word from the first 
example (without the workaround). When the goto at 0x1000 is executed, 
pclath contains 0x8 and it jumps to 0x804 instead of 0x1004. When the 
goto at 0x1010 is executed (if the program hadn't already crashed that 
is) pclath contains 0x18 and it jumps to 0x1814 instead of 0x1014.

         ; name: test
         ; max return-stack depth: 1
0x1000  2804    goto    0x004   ; test + 0x004 (0x1004)
0x1001  3465    data    0x65    ; 0x68 [h] 0x65 [e]
0x1002  366C    data    0x6C    ; 0x6C [l] 0x6C [l]
0x1003  3780    data    0x80    ; 0x6F [o] 0x00
0x1004  3010    movlw   0x10
0x1005  1703    bsf     0x03,6
0x1006  008F    movwf   0x0F
0x1007  3001    movlw   0x01
0x1008  008D    movwf   0x0D
0x1009  1303    bcf     0x03,6
0x100A  01A2    clrf    0x22
0x100B  018A    clrf    0x0A
0x100C  203E    call    0x03E   ; type
0x100D  158A    bsf     0x0A,3
0x100E  27FD    call    0x7FD   ; fred (0xFFD)
0x100F  160A    bsf     0x0A,4
0x1010  2814    goto    0x014   ; test + 0x014 (0x1014)
0x1011  3BEF    data    0xEF    ; 0x77 [w] 0x6F [o]
0x1012  396C    data    0x6C    ; 0x72 [r] 0x6C [l]
0x1013  3200    data    0x00    ; 0x64 [d] 0x00
0x1014  3010    movlw   0x10
0x1015  1703    bsf     0x03,6
0x1016  008F    movwf   0x0F
0x1017  3011    movlw   0x11
0x1018  008D    movwf   0x0D
0x1019  1303    bcf     0x03,6
0x101A  01A2    clrf    0x22
0x101B  018A    clrf    0x0A
0x101C  283E    goto    0x03E   ; type

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