Re: [ros-dev] gcc problem or not

[Joseph Galbraith]

Hartmut Birr wrote:
>   Hi,
>  
>  I've looked on a map file from an optimized build. I found the following
>  code:
>  
>    /*
>     * If the directory containing the file to open doesn't exist then
>     * fail immediately
>     */
>    if (Status == STATUS_OBJECT_PATH_NOT_FOUND ||
>     12015:    83 c4 0c                 add    $0xc,%esp
>     12018:    89 c3                    mov    %eax,%ebx
>     1201a:    3d 3a 00 00 c0           cmp    $0xc000003a,%eax
>     1201f:    0f 94 c2                 sete   %dl
>     12022:    3d 0d 00 00 c0           cmp    $0xc000000d,%eax
>     12027:    0f 94 c0                 sete   %al
>     1202a:    09 d0                    or     %edx,%eax
>     1202c:    a8 01                    test   $0x1,%al
>     1202e:    0f 85 42 01 00 00        jne    12176 <_VfatCreateFile+0x2e9>
>     12034:    81 fb 56 00 00 c0        cmp    $0xc0000056,%ebx
>     1203a:    0f 84 36 01 00 00        je     12176 <_VfatCreateFile+0x2e9>
>        Status == STATUS_INVALID_PARAMETER ||
>        Status == STATUS_DELETE_PENDING)
>      {
>        if (ParentFcb)
>        {
>           vfatReleaseFCB (DeviceExt, ParentFcb);
>        }
>        return(Status);
>      }
>  
>  I was always the opinion  that the examination of a test condition stops
>  if the result can not change again. A test condition like this:
>  
>  if (pointer == NULL || pointer->member == 0)
>  
>  should never access pointer->member if pointer is zero. Compared with
>  the code above, it is possible that gcc build the result from the right
>  side of the OR statement. This may hit a page fault. Is this a bug in gcc? 
Hmmm... if gcc were to evaluate pointer->member in the
above example, that would definitely be a horrendous gcc
bug.

However, in the example you gave the dissambly for, I believe
gcc can evaluate eax multiple times before 'shortcutting'
the evaluation, because it knows that evaluating eax multiple
times can't have side effects.

Does the if ( ! p || ! p->x ) case generate similar assembler?

Thanks,

Joseph

PS. I disclaim all responsibility for this answer, I have no
idea what I'm talking about.  Everything here is pure speculation.