Re: [avr-gcc-list] Extremly confusing behaviour... (Suspect compiler?, optimization error?)

[Mike Panetta]

Please explain to me why the return value needs to be declared
volatile?  That makes no sense, and even more thats not what the FAQ
suggests.  It suggests to use volatile when you are inspecting (using)
variables that are updated in an ISR.  That is not the case here.  There
are NO ISR's involved in this code.

As for assembly output, see the attached listing:
                do
                {
                        delay = 1000;
     1c4:       88 ee           ldi     r24, 0xE8       ; 232
     1c6:       93 e0           ldi     r25, 0x03       ; 3
     1c8:       a0 e0           ldi     r26, 0x00       ; 0
     1ca:       b0 e0           ldi     r27, 0x00       ; 0
     1cc:       89 83           std     Y+1, r24        ; 0x01
     1ce:       9a 83           std     Y+2, r25        ; 0x02
     1d0:       ab 83           std     Y+3, r26        ; 0x03
     1d2:       bc 83           std     Y+4, r27        ; 0x04
                        while (delay--);
     1d4:       89 81           ldd     r24, Y+1        ; 0x01
     1d6:       9a 81           ldd     r25, Y+2        ; 0x02
     1d8:       ab 81           ldd     r26, Y+3        ; 0x03
     1da:       bc 81           ldd     r27, Y+4        ; 0x04
     1dc:       01 97           sbiw    r24, 0x01       ; 1
     1de:       a1 09           sbc     r26, r1
     1e0:       b1 09           sbc     r27, r1
     1e2:       89 83           std     Y+1, r24        ; 0x01
     1e4:       9a 83           std     Y+2, r25        ; 0x02
     1e6:       ab 83           std     Y+3, r26        ; 0x03
     1e8:       bc 83           std     Y+4, r27        ; 0x04
     1ea:       89 81           ldd     r24, Y+1        ; 0x01
     1ec:       9a 81           ldd     r25, Y+2        ; 0x02
     1ee:       ab 81           ldd     r26, Y+3        ; 0x03
     1f0:       bc 81           ldd     r27, Y+4        ; 0x04
     1f2:       8f 5f           subi    r24, 0xFF       ; 255
     1f4:       9f 4f           sbci    r25, 0xFF       ; 255
     1f6:       af 4f           sbci    r26, 0xFF       ; 255
     1f8:       bf 4f           sbci    r27, 0xFF       ; 255
     1fa:       61 f7           brne    .-40            ; 0x1d4
                        PORTA |= _BV (PA1);
     1fc:       d9 9a           sbi     0x1b, 1 ; 27
                        retval = srf08_read(1, 0, pingdata, 2);
     1fe:       65 e0           ldi     r22, 0x05       ; 5
     200:       c6 2e           mov     r12, r22
     202:       d1 2c           mov     r13, r1
     204:       cc 0e           add     r12, r28
     206:       dd 1e           adc     r13, r29
     208:       02 e0           ldi     r16, 0x02       ; 2
     20a:       10 e0           ldi     r17, 0x00       ; 0
     20c:       20 e0           ldi     r18, 0x00       ; 0
     20e:       30 e0           ldi     r19, 0x00       ; 0
     210:       a6 01           movw    r20, r12
     212:       60 e0           ldi     r22, 0x00       ; 0
     214:       81 e0           ldi     r24, 0x01       ; 1
     216:       0e 94 9c 04     call    0x938
                        PORTA &= ~(_BV(PA1));
     21a:       d9 98           cbi     0x1b, 1 ; 27
                        PORTB = retval;
     21c:       68 bb           out     0x18, r22       ; 24
                } while (retval != 2);
     21e:       60 17           cp      r22, r16
     220:       71 07           cpc     r23, r17
     222:       82 07           cpc     r24, r18
     224:       93 07           cpc     r25, r19
     226:       71 f6           brne    .-100           ; 0x1c4


On Wed, 2003-05-14 at 12:42, Keith Gudger wrote:
> You say you read the FAQ, but I see that your return value is still not
> declared volatile.
> 
> If you would like us to help you, you must send us the assembly language
> produced for this loop.
> 
> Try avr-gcc with the -S option to give you the assembly output.
> 
> Keith
> 
> On 14 May 2003, Mike Panetta wrote:
> 
> > A: I already did that.
> > B: I am not using the variables you mentioned yet, and they are not
> > involved in the code that I am having problems with.
> > 
> > I have read the FAQ (many times, its even where I got the basis for my
> > i2c code for this project from), and I know what volatile is for.
> > 
> > The problem I am having is GCC is seemingly IGNORING the return value of
> > a function I am calling.  Why is it doing that?  That is what I need
> > help with.
> > 
> > Thanks,
> > Mike
> > 
> > Wondering if anyone is actually reading his emails before pushing him at
> > the FAQ...
> > 
> > On Wed, 2003-05-14 at 12:17, Joerg Wunsch wrote:
> > > Mike Panetta <address@hidden> wrote:
> > > 
> > > > If someone could help me understand my error, I would be greatly
> > > > appreciated. 
> > > 
> > > Why didn't you follow Keith's advise, and looked into the FAQ?  You'll
> > > certainly want to declare at least one of your variables to be
> > > "volatile", namely SPIByteCounter, but maybe more -- please try to
> > > read and understand the FAQ.  It would have saved you hours.
> > > 
> > > In case you don't know:
> > > 
> > > http://savannah.nongnu.org/download/avr-libc/doc/avr-libc-user-manual/FAQ.html
> > > 
> > > -- 
> > > J"org Wunsch                                             Unix support 
> > > engineer
> > > address@hidden        http://www.interface-systems.de/~j/
> > > 
> > > _______________________________________________
> > > avr-gcc-list mailing list
> > > address@hidden
> > > http://www.avr1.org/mailman/listinfo/avr-gcc-list
> > 
> > 
> > 
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> > 
> > 
> > 
>