#491 Wrong code optimization

[Kharitonov Dmitry]

cat bug.c

__sbit __at(0xA0) P2_0;

void main(void) {

  // toggle pin

  for(;;) {
    if(P2_0) {
        P2_0=0;
    } else {
        P2_0=1;
    }
  }
}

compile:
sdcc -mmcs51 --opt-code-size --model-small --Werror -I. --std-sdcc99 bug.c

cat bug.rst

                                      1 ;--------------------------------------------------------
                                      2 ; File Created by SDCC : free open source ANSI-C Compiler
                                      3 ; Version 3.5.0 #9253 (Jun 20 2015) (MINGW64)
                                      4 ; This file was generated Sat Feb 20 16:35:42 2016
                                      5 ;--------------------------------------------------------
                                      6     .module bug
                                      7     .optsdcc -mmcs51 --model-small
----8<---8<---
      000062                        123 _main:
                           000007   124     ar7 = 0x07
                           000006   125     ar6 = 0x06
                           000005   126     ar5 = 0x05
                           000004   127     ar4 = 0x04
                           000003   128     ar3 = 0x03
                           000002   129     ar2 = 0x02
                           000001   130     ar1 = 0x01
                           000000   131     ar0 = 0x00
      000062                        132 00105$:
                                    133 ;   bug.c:8: if(P2_0) {
                                    134 ;   bug.c:9: P2_0=0;
      000062 10 A0 02         [24]  135     jbc _P2_0,00115$
      000065 80 02            [24]  136     sjmp    00102$
      000067                        137 00115$:
      000067 80 F9            [24]  138     sjmp    00105$
      000069                        139 00102$:
                                    140 ;   bug.c:11: P2_0=1;
      000069 D2 A0            [12]  141     setb    _P2_0
      00006B 80 F5            [24]  142     sjmp    00105$

Dmitriy
kharpost@altlinux.org

[Ben Shi]

The generated assembly is right though not quite efficient.

;       a.c:3: void main(void)
;       -----------------------------------------
;        function main
;       -----------------------------------------
_main:
        ar7 = 0x07
        ar6 = 0x06
        ar5 = 0x05
        ar4 = 0x04
        ar3 = 0x03
        ar2 = 0x02
        ar1 = 0x01
        ar0 = 0x00
00105$:
;       a.c:6: if (P2_0) P2_0 = 0;
        jbc     _P2_0,00115$
        sjmp    00102$
00115$:
        sjmp    00105$
00102$:
;       a.c:7: else P2_0 = 1;
        setb    _P2_0
        sjmp    00105$

A better optimized one could be

;       a.c:3: void main(void)
;       -----------------------------------------
;        function main
;       -----------------------------------------
_main:
        ar7 = 0x07
        ar6 = 0x06
        ar5 = 0x05
        ar4 = 0x04
        ar3 = 0x03
        ar2 = 0x02
        ar1 = 0x01
        ar0 = 0x00
00105$:
        jbc     _P2_0,00105$
        setb    _P2_0
        sjmp    00105$

[Kharitonov Dmitry]

[Kharitonov Dmitry]

[Maarten Brock]

This should be fixed in the peephole rules. This is where the jbc is inserted. It needs a jump-jump optimization afterwards. Maybe reorder the rules or add a restart.

[Kharitonov Dmitry]

The compiler generated code is a non-working
A better optimized one could be

00105$:
        cpl     _P2_0
        sjmp    00105$

[Maarten Brock]

The new peephole rules do not go as far to use CPL, but stop at JBC, SETB.

If you want a real complement, you should write:

P2_0 = !P2_0;

[Kharitonov Dmitry]

... or

00105$:
        jbc     _P2_0,00115$
        sjmp    00102$
00115$:
        **clrb    _P2_0**
        sjmp    00105$
00102$:
        setb    _P2_0
        sjmp    00105$

[Maarten Brock]

Look up what JBC stands for and you'll know why the generated code is ok.

[Kharitonov Dmitry]

I'm sorry, I'm confused JBC with JNB.

[Maarten Brock]

Implemented by peephole rules 257.x and 259.x in [r9519].