[Bug tree-optimization/54475] New: -O2 overoptimizes

[bdubbs at linuxfromscratch dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54475

             Bug #: 54475
           Summary: -O2 overoptimizes
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: bdu...@linuxfromscratch.org


Note:  This is *not* my code, but found inside a ./configure file circa 2008.

$ cat t.c
int main()
{
  int j;
  for (j = 1; 0 < j; j *= 2) printf( "%i\n", j );
}

$ gcc -o t t.c

Works fine.

$ gcc -o t -O2 t.c

Loops forever and prints 0 after j reaches 1073741824.

Looking a the asm for the optimized code, there is no test for j<0.
It appears to have been optimized away.

-----------  with -O2
main:
.LFB0:
        .cfi_startproc
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $1, %ebx
        .p2align 4,,10
        .p2align 3
.L2:
        movl    %ebx, %esi
        movl    $.LC0, %edi
        xorl    %eax, %eax
        call    printf
        addl    %ebx, %ebx
        jmp     .L2
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (GNU) 4.7.1"
        .section        .note.GNU-stack,"",@progbits