Re: [PATCH, libgcc]: Avoid warning: array subscript is above array bounds when compiling crtstuff.c

[Uros Bizjak <ubizjak at gmail dot com>]

On Sun, Mar 9, 2014 at 6:31 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Sun, Mar 09, 2014 at 09:41:59AM -0700, Ian Lance Taylor wrote:
>> >>> Attached patch avoids a bunch of:
>> >>>
>> >>> ../../../gcc-svn/trunk/libgcc/crtstuff.c: In function 'frame_dummy':
>> >>> ../../../gcc-svn/trunk/libgcc/crtstuff.c:463:19: warning: array
>> >>> subscript is above array bounds [-Warray-bounds]
>> >>>    if (__JCR_LIST__[0])
>> >>>                    ^
>> >>>
>> >>> when compiling libgcc.
>> >>>
>> >>> 2014-03-08  Uros Bizjak  <ubizjak@gmail.com>
>> >>>
>> >>>     * crtstuff.c (__JCR_LIST__): Declare as zero-length array.
>
> I guess the only thing to avoid the warning (and potential miscompilation)
> is to hide the access from the optimizers through something like:
>   void *jcr_list;
>   __asm ("" : "=g" (jcr_list) : "0" (__JCR_LIST__));
> and then use jcr_list instead of __JCR_LIST__.

Attached patch builds on your idea, but jcr_list temporary has to be
declared as void ** to allow proper dereference of pointer to void
array.

The resulting code is also a bit better, as shown by following test:

--cut here--
void register_classes (void *);

static void *__JCR_LIST__[]  __attribute__ ((used)) = { };

void frame_dummy (void)
{
  void **jcr_list;
  __asm ("" : "=g" (jcr_list) : "0" (__JCR_LIST__));
  if (*jcr_list)
    register_classes (jcr_list);
}

void _frame_dummy (void)
{
  if (__JCR_LIST__[0])
    register_classes (__JCR_LIST__);
}
--cut here--

gcc -O2 -Wall:

frame_dummy:
        movl    $__JCR_LIST__, %edi
        cmpq    $0, (%rdi)
        je      .L1
        jmp     register_classes
        .p2align 4,,10
        .p2align 3
.L1:
        rep ret

_frame_dummy:
        cmpq    $0, __JCR_LIST__(%rip)
        je      .L4
        movl    $__JCR_LIST__, %edi
        jmp     register_classes
        .p2align 4,,10
        .p2align 3
.L4:
        rep ret

The new code preloads __JCR_LIST__ into a temporary register that is
reused in the call to register_classes.

2014-03-09  Uros Bizjak  <ubizjak@gmail.com>

    * crtstuff.c (frame_dummy): Use void **jcr_list temporary variable to
    avoid array subscript is above array bounds warnings.

Patch was bootstrapped and regression tested on x86_64-pc-linux-gnu
{,-m32} with all default languages + go.

OK for mainline?

Uros.

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