[sdcc-devel] [ sdcc-Bugs-1004921 ] PIC14: loop conditionals sometimes inverted

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Bugs item #1004921, was opened at 2004-08-07 01:08
Message generated for change (Settings changed) made by maartenbrock
You can respond by visiting: 
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Category: pic14 target
>Group: non bugs
>Status: Closed
Resolution: Works For Me
Priority: 5
Submitted By: George Caswell (tetsujin)
Assigned to: Nobody/Anonymous (nobody)
Summary: PIC14: loop conditionals sometimes inverted

Initial Comment:
I've been writing code with FOR and WHILE loops for the
pic16f84a - however, the generated code for the loop
conditionals is incorrect, which causes the loops to
terminate immediately.  For example:


test.c:
#define __16F84A
#include "16f84a.h"


void main(void)
{
    static unsigned char i;

    TRISB = 0;

    i = 0;
    while (1) {
        for (i = 0; i < 42; i++) {
            PORTB = i;
        }
    }
}


> sdcc -mpic14 -p16f84 test.c

Generates this assembly (snippet relevant to the inner
loop)

;#CSRC  test.c 13
;  for (i = 0; i < 42; i++) {
        CLRF    _main_i_1_1
_00108_DS_
        MOVLW   0x2a
        SUBWF   _main_i_1_1,W
        BTFSC   STATUS,0
        GOTO    _00106_DS_
;#CSRC  test.c 14
;  PORTB = i;
        MOVF    _main_i_1_1,W
        BCF     STATUS,5
        BCF     STATUS,6
        MOVWF   _PORTB
;#CSRC  test.c 13
;  for (i = 0; i < 42; i++) {
        BANKSEL _main_i_1_1
        INCF    _main_i_1_1,F
        GOTO    _00108_DS_
        RETURN

The conditional test, (i < 42), where (i) is an
unsigned 8-bit integer compared with a literal, occurs
in the first block of code after the _00108_DS_ label:

        MOVLW   0x2a                - set W to 0x2a (42)
        SUBWF   _main_i_1_1,W  - subtract W from i,
store result in W
        BTFSC   STATUS,0           - skip next
instruction if the carry bit is clear
        GOTO    _00106_DS_       - Restart the inner loop

The carry bit will be set on the subtraction operation
if (i < 42) - so on the first step of the loop, the
carry bit is set, so the GOTO is not skipped, so the
inner loop effectively never runs.

I am attempting to fix the problem, but my familiarity
with the compiler is still very limited.  I am able to
fix the problem by commenting out line 4040 of
src/pic/gen.c:

//rFalseIfx.condition ^= 1;

However, given the code that surrounds this line, I
suspect that, while my change apparently fixes unsigned
8-bit comparisons with literals, it may break
comparisons of larger sizes.

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

Comment By: George Caswell (tetsujin)
Date: 2004-08-09 06:09

Message:
Logged In: YES 
user_id=771

Ugh, it appears I was wrong.

I was testing my code on nitpic, which seems to have this
logic wrong.  On gpsim and actual hardware it appears the
generated code is correct.

Sorry for the erroneous report.  I am trying.  :)

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

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