[avr-gcc-list] Bug in optimization of gcc 3.2 (2002/06/16)?

[Roland Zitzke]

Hi,
I am using the gcc distribution from avrfreaks.net.
I discovered some strange code generation effects regarding the use of sbi
and cbi instruction on lower IO addresses.
It looks like the compiler sometimes correctly generates optimized code
while it sometimes outputs read/modify/write instructions.
The following extract from an .lst file assumes definitions like:
#define DALLAS_PORT PORTD
#define DALLAS_INPORT PIND
#define DALLAS_PIN 7
#define DALLAS_DDR DDRD
I include this rather lengthy listing, because it is important to note how
the compiler translates the same calls to cbi() into different code
fragments - I tagged the important lines with !!!.
Of cause, this could also be a stupid mistake I made, but to my
understanding the compiler should optimize the long form either always or
never.
I use the -Os option.
Thanks and regards
Roland
...
  53:mlan.c        **** u08 MLanTouchReset(void)
  54:mlan.c        **** {
  80                 .stabn 68,0,54,.LM1-MLanTouchReset
  81                .LM1:
  82                /* prologue: frame size=0 */
  83 0000 1F93        push r17
  84                /* prologue end (size=1) */
  55:mlan.c        ****   u08 res;
  56:mlan.c        ****
  57:mlan.c        ****   res = 0;
!!!   58:mlan.c        ****   cbi(DALLAS_PORT,DALLAS_PIN);
  85                 .stabn 68,0,58,.LM2-MLanTouchReset
  86                .LM2:
  87                .LBB2:
  88 0002 1FE7        ldi r17,lo8(127)
  89 0004 82B3        in r24,50-0x20
  90 0006 8123        and r24,r17
!!!  91 0008 82BB        out 50-0x20,r24
!!! First, I would expect this block to be optimized with -Os, it is
probably working though - see further below
  59:mlan.c        ****   sbi(DALLAS_DDR,DALLAS_PIN);
  92                 .stabn 68,0,59,.LM3-MLanTouchReset
  93                .LM3:
!!!  94 000a 8F9A        sbi 49-0x20,7!!! here it works
  60:mlan.c        ****   delayus(480); /* send "0" for 500 us */
  95                 .stabn 68,0,60,.LM4-MLanTouchReset
  96                .LM4:
  97 000c 80EB        ldi r24,lo8(1200)
  98 000e 94E0        ldi r25,hi8(1200)
  99 0010 00D0        rcall delayus4mhz
!!  61:mlan.c        ****   cbi(DALLAS_DDR,DALLAS_PIN);
 100                 .stabn 68,0,61,.LM5-MLanTouchReset
 101                .LM5:
 102 0012 81B3        in r24,49-0x20
 103 0014 8123        and r24,r17
 104 0016 81BB        out 49-0x20,r24
!! and again read/modify/write
  62:mlan.c        ****   delayus(60);
 105                 .stabn 68,0,62,.LM6-MLanTouchReset
 106                .LM6:
 107 0018 86E9        ldi r24,lo8(150)
 108 001a 90E0        ldi r25,hi8(150)
 109 001c 00D0        rcall delayus4mhz
  63:mlan.c        ****
  64:mlan.c        ****   res = bit_is_clear(DALLAS_INPORT,DALLAS_PIN);
 110                 .stabn 68,0,64,.LM7-MLanTouchReset
 111                .LM7:
 112 001e 10B3        in r17,48-0x20
 113 0020 1095        com r17
 114 0022 1078        andi r17,lo8(-128)
!!! this could probably be optimized too
  65:mlan.c        ****
  66:mlan.c        ****   delayus(100);
 115                 .stabn 68,0,66,.LM8-MLanTouchReset
 116                .LM8:
 117 0024 8AEF        ldi r24,lo8(250)
 118 0026 90E0        ldi r25,hi8(250)
 119 0028 00D0        rcall delayus4mhz
  67:mlan.c        ****
  68:mlan.c        ****   return (res);
 120                 .stabn 68,0,68,.LM9-MLanTouchReset
 121                .LM9:
 122 002a 812F        mov r24,r17
 123 002c 9927        clr r25
  69:mlan.c        **** }
 124                 .stabn 68,0,69,.LM10-MLanTouchReset
 125                .LM10:
 126                .LBE2:
 127                /* epilogue: frame size=0 */
 128 002e 1F91        pop r17
 129 0030 0895        ret
 130                /* epilogue end (size=2) */
 131                /* function MLanTouchReset size 25 (22) */
 132                .Lfe1:
 133                 .size MLanTouchReset,.Lfe1-MLanTouchReset
 134                 .stabs "res:r(2,1)",64,0,55,17
 135                 .stabn 192,0,0,.LBB2-MLanTouchReset
 136                 .stabn 224,0,0,.LBE2-MLanTouchReset
 137                .Lscope0:
 138                 .stabs "",36,0,0,.Lscope0-MLanTouchReset
 139                 .stabs "MLanTouchBit:F(2,1)",36,0,84,MLanTouchBit
 140                 .stabs "sendbit:P(2,1)",64,0,83,17
 141                .global MLanTouchBit
 142                 .type MLanTouchBit,@function
 143                MLanTouchBit:
  70:mlan.c        ****
  71:mlan.c        ****
  72:mlan.c        ****
//--------------------------------------------------------------
  73:mlan.c        **** // Send 1 bit of communication to the MicroLAN and
return the
  74:mlan.c        **** // result 1 bit read from the MicroLAN.  The
parameter 'sendbit'
  75:mlan.c        **** // least significant bit is used and the least
significant bit
  76:mlan.c        **** // of the result is the return bit.
  77:mlan.c        **** //
  78:mlan.c        **** // 'sendbit' - the least significant bit is the bit
to send
  79:mlan.c        **** //
  80:mlan.c        **** // Returns: 0:   0 bit read from sendbit
  81:mlan.c        **** //          1:   1 bit read from sendbit
  82:mlan.c        **** //
  83:mlan.c        **** u08 MLanTouchBit(u08 sendbit)
  84:mlan.c        **** {
 144                 .stabn 68,0,84,.LM11-MLanTouchBit

 145                .LM11:
 146                /* prologue: frame size=0 */
 147 0032 1F93        push r17
 148                /* prologue end (size=1) */
 149 0034 182F        mov r17,r24
  85:mlan.c        ****   u08 res;
  86:mlan.c        ****
!!!  87:mlan.c        ****   cbi(DALLAS_PORT,DALLAS_PIN);
 150                 .stabn 68,0,87,.LM12-MLanTouchBit
 151                .LM12:
 152                .LBB3:
!!!  153 0036 9798        cbi 50-0x20,7
!!! this is what I expect for this instruction
  88:mlan.c        ****   sbi(DALLAS_DDR,7);
 154                 .stabn 68,0,88,.LM13-MLanTouchBit
 155                .LM13:
 156 0038 8F9A        sbi 49-0x20,7
!! I used a numeric constant instead of DALLAS_PIN, just to avoid an error
source.
  89:mlan.c        ****   delayus(1);
 157                 .stabn 68,0,89,.LM14-MLanTouchBit
 158                .LM14:
 159 003a 80E0        ldi r24,lo8(0)
 160 003c 90E0        ldi r25,hi8(0)
 161 003e 00D0        rcall delayus4mhz
  90:mlan.c        ****   if(sendbit&1){
 162                 .stabn 68,0,90,.LM15-MLanTouchBit
 163                .LM15:
 164 0040 10FD        sbrc r17,0
  91:mlan.c        ****     cbi(DALLAS_DDR,DALLAS_PIN);
 165                 .stabn 68,0,91,.LM16-MLanTouchBit
 166                .LM16:
 167 0042 8F98        cbi 49-0x20,7
 168                .L3:

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