[avr-gcc-list] WinAVR 20050214 (gcc 3.4.3) and optimizer bug.

[Vesa Jääskeläinen]

Hi,

I have been debugging why Ethernut brakes in certain situations with
gcc. And found an optimizer bug.

Here is smallest piece of code that I was able to reproduce it:

--- begin of code snippet ---
#define u_long unsigned long
#define u_char unsigned char

void test(int req, void *conf)
{
    void **ppv = (void **) conf;
    u_long *lvp = (u_long *) conf;
    u_long lv = *lvp;
    u_char bv = (u_char) lv;

    switch (req) {
    case 1:
        if (bv)
        {
            asm ( "nop ; bv needs to be defined and used");
        }
        break;

    case 2:
        asm ( "nop ; entry to case 2");

        if (ppv && (*ppv != 0)) {
            asm ( "nop ; ppv and *ppv are not zeros");
        } else {
            asm ( "nop ; either ppv or *ppv is zero");
        }

        break;
    }
}
--- end of code snippet ---

Now if we look at generated code here:
--- begin of assembly snippet ---
  51                    /* #APP */
  52 001e 0000                  nop ; entry to case 2
  53                    .LM9:
  54                    /* #NOAPP */
  55 0020 8081                  ld r24,Z
  56 0022 9181                  ldd r25,Z+1
  57 0024 892B                  or r24,r25
  58 0026 11F0                  breq .L6
  59                    .LM10:
  60                    /* #APP */
  61 0028 0000                  nop ; ppv and *ppv are not zeros
  62                    /* #NOAPP */
  63 002a 0895                  ret
  64                    .L6:
  65                    .LM11:
  66                    /* #APP */
  67 002c 0000                  nop ; either ppv or *ppv is zero
  68                    /* #NOAPP */
  69                    .L1:
  70 002e 0895                  ret
--- end of assembly snippet ---

We can see that one compare is missing and this causes functional change
to code.

Could someone verify this bug and possibly fix it ;)

I used following command line:
avr-gcc -c -mmcu=atmega128 -Os -Wall -Wstrict-prototypes 
-Wa,-ahlms=test.lst test.c -o test.o

Thanks,
Vesa Jääskeläinen