[Bug rtl-optimization/67449] New: Branch elimination problem on x86

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67449

            Bug ID: 67449
           Summary: Branch elimination problem on x86
           Product: gcc
           Version: 4.9.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

The following `do ... while` statement creates a loop with two branches:

C code:
<code>
    int a[10];

    void next(int **pp){
        (++*pp == a + 10) && (*pp = 0);
    }

    int main(){
        for(int i = 0; i < 10; ++i){
            a[i] = i;
        }

        int *p = a;
        do {
            __builtin_printf("element = %d\n", *p);
            next(&p);
    //      __asm__("");
        } while(p);
    }
</code>

Assembly output:
<code>
    L13:
        testl   %ebx, %ebx
        je  L6                      ; one here.
    L4:
        movl    (%ebx), %eax
        movl    $LC1, (%esp)
        addl    $4, %ebx
        movl    %eax, 4(%esp)
        call    _printf
        cmpl    $_a+40, %ebx
        jne L13                     ; another one here.
    L6:
</code>

But if we uncomment that empty __asm__ statement, the first branch vanishes:

Assembly output:
<code>
    L5:
        movl    (%ebx), %eax
        movl    $LC1, (%esp)
        addl    $4, %ebx
        movl    %eax, 4(%esp)
        call    _printf
        cmpl    $_a+40, %ebx
        cmove   %esi, %ebx
        testl   %ebx, %ebx          ; the only one branch in this loop.
        jne L5
</code>