Re: [avr-gcc-list] Endless loop: uchar c; for (c=1; c; c++)

[Dmitry K.]

On Wednesday 07 March 2007 04:10, Eric Weddington wrote:
[...]
> It seems that when compiling to assembler (-S as Dmitry recommends above),
> then you'll note that the epilogue is set to noreturn, which also ends up
> taking out the final breq of the outer loop, as there is nothing to branch
> to anymore.
>
> However, when I compile and assemble, and I look at a requested listing
> from the assembler (basically using the WinAVR Makefile Template), the code
> looks fine. An epilog exists, and the correct end condition for the loop is
> in the code. This is also the case when I do a disassembly on the object
> code (avr-objdump -d <object>); the code looks fine, an epilog exists, and
> the branch is in the code.
>
> Does anyone have an idea why using avr-gcc -S ... would be so different
> then the actual code generated (as compared to the disassembly)?

I try to compile without `-S' option: result is the same.
More, the program below is run under the simulator (I use simulavr)
and exit with code 11 (20.2e6 cloks) which illustrates an error:

Program:
~~~~~~~

/* This program is compiled as endless loop: condition `c1 != 0'
   is not tested.  In original (real) program there was an another
   error: in conditional expression based on c1 and c2 inside the
   loops (here this is removed).

   Compiler: avr-gcc 4.1.1
   Options: -W -Wall -Os        */

void exit (int);

void foo (const char *s1, const char *s2)
{
    static unsigned char n;
    if ((*s1 == 1) && (*s2 == 1)) {
        /* The big number 10 is to enlarge a simulation time, number 1
           is sufficient to demonstrate an error.       */
        if (++n > 10)
            exit (n);
    }
}

int main ()
{
    unsigned char c1, c2;
    char s1[2] = ".";
    char s2[2] = ".";

    for (c1 = 1; c1; c1++) {
        for (c2 = 1; c2; c2++) {
            s1[0] = c1;
            s2[0] = c2;
            foo (s1, s2);
        }
    }
    return 0;
}

Disassemble:
~~~~~~~~~~~

        ...
0000007a <main>:
  7a:   cb e5           ldi     r28, 0x5B       ; 91
  7c:   d2 e0           ldi     r29, 0x02       ; 2
  7e:   de bf           out     0x3e, r29       ; 62
  80:   cd bf           out     0x3d, r28       ; 61
  82:   80 91 60 00     lds     r24, 0x0060
  86:   90 91 61 00     lds     r25, 0x0061
  8a:   9a 83           std     Y+2, r25        ; 0x02
  8c:   89 83           std     Y+1, r24        ; 0x01
  8e:   9c 83           std     Y+4, r25        ; 0x04
  90:   8b 83           std     Y+3, r24        ; 0x03
  92:   01 e0           ldi     r16, 0x01       ; 1
  94:   0d c0           rjmp    .+26            ; 0xb0 <main+0x36>
  96:   09 83           std     Y+1, r16        ; 0x01
  98:   1b 83           std     Y+3, r17        ; 0x03
  9a:   6c 2f           mov     r22, r28
  9c:   7d 2f           mov     r23, r29
  9e:   6d 5f           subi    r22, 0xFD       ; 253
  a0:   7f 4f           sbci    r23, 0xFF       ; 255
  a2:   8c 2f           mov     r24, r28
  a4:   9d 2f           mov     r25, r29
  a6:   01 96           adiw    r24, 0x01       ; 1
  a8:   d4 df           rcall   .-88            ; 0x52 <foo>
  aa:   1f 5f           subi    r17, 0xFF       ; 255
  ac:   a1 f7           brne    .-24            ; 0x96 <main+0x1c>
  ae:   0f 5f           subi    r16, 0xFF       ; 255
  b0:   11 e0           ldi     r17, 0x01       ; 1
  b2:   f1 cf           rjmp    .-30            ; 0x96 <main+0x1c>
        ...

Dmitry.