[FFmpeg-devel] GCC 5.1 warning: -Warray-bounds

[James Almer]

On 04/06/15 10:08 AM, Ganesh Ajjanagadde wrote:
> On Thu, Jun 4, 2015 at 7:00 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> On Wed, Jun 03, 2015 at 10:05:54PM -0400, Ganesh Ajjanagadde wrote:
>>> While compiling ffmpeg, I noticed a bunch of -Warray-bounds warnings.
>>> I think it would be great if ffmpeg could be built warning-free, and
>>> -Warray-bounds seems to be one of the biggest culprits. Nevertheless,
>>
>>> -Warray-bounds is quite useful in most cases.
>>
>> how many of the warnings shown are real and how many are false
>> positives (in some version not neccessarily the latest of FFmpeg)
>> ?
> 
> So I checked out n2.5 release. There are too many warnings to quickly
> audit all of them.
> Nevertheless, I went through about 6 instances (randomly selected),
> and found all of them to be false positives.
> Basically, there is a basic pattern to all of these warnings:
> struct foo {
>     int bar[MAX_BAR];
>     int num_bar;
> }; // could have other declarations as well
> 
> // in usage
> int set_foo(foo* my_foo) {
>     my_foo->num_bar = get_num_bar();
>     if (num_bar > MAX_BAR)
>         return -1;
> }
> 
> int loop_over(const foo* my_foo) {
>     int i;
>     for(i = 0; i < foo->num_bar; i++) {
>         // do something with foo->bar[i]
>         // this triggers the warning: GCC tries to do a constant
> propagation to verify array in bounds, since it knows MAX_BAR, i.e
> foo->bar is an array.
>         // However, it can't do deep enough analysis to verify that in
> set_foo, num_bar is sanitized to ensure proper in bounds access.
>    }
> }

Could you make a bug report about this on https://gcc.gnu.org/bugzilla/?