[Bug optimization/11477] New: writing 0 to memory-location leads to incorrect optimization

[gcc-bugzilla at jh42 dot de]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11477

           Summary: writing 0 to memory-location leads to incorrect
                    optimization
           Product: gcc
           Version: 3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gcc-bugzilla at jh42 dot de
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: avr-unknown-none

after writing 0 to a memory-location, the next variable written 0 to is loaded
with the value of the memory-location instead of an immediate load.

compile-options:  -g -O1 -Wall -Wstrict-prototypes -mmcu=atmega8515

void main(void) {
        unsigned char i;
        unsigned char *p = (unsigned char *)0x100;
        *p = 0;
        i = 0;
        while (1) {
        *p = i;
        }
}

void main(void) {
  58:   cf e5           ldi     r28, 0x5F       ; 95
  5a:   d2 e0           ldi     r29, 0x02       ; 2
  5c:   de bf           out     0x3e, r29       ; 62
  5e:   cd bf           out     0x3d, r28       ; 61
        unsigned char i;
        unsigned char *p = (unsigned char *)0x100;
  60:   e0 e0           ldi     r30, 0x00       ; 0
  62:   f1 e0           ldi     r31, 0x01       ; 1
        *p = 0;
  64:   10 92 00 01     sts     0x0100, r1
        i = 0;
  68:   80 91 00 01     lds     r24, 0x0100
        while (1) {
        *p = i;
  6c:   80 83           st      Z, r24
  6e:   fe cf           rjmp    .-4             ; 0x6c