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

Ganesh Ajjanagadde gajjanag at mit.edu
Thu Jun 4 15:08:20 CEST 2015

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.

> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
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