[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.