Re: False positive from -Warray-bounds?

[Ian Lance Taylor <iant at google dot com>]

Vincent Lefevre <vincent+gcc@vinc17.org> writes:

> On 2011-12-29 16:20:48 -0800, Ian Lance Taylor wrote:
>> To me this only proves that the compiler is not smart enough to see that
>> (s >> 1 == 0) implies that ((s & 0xffff) == 0xffff) can not be true.
>> 
>> Are you suggesting that the compiler should never warn if there is a
>> conditional guarding the array access?  Would that in practice be better
>> or worse than the current behaviour?
>
> I think there should be two different options:
>   * one that would trigger the warning if the compiler can prove
>     that there will always be an out-of-bound access when the
>     function is executed (unless the compiler can prove that the
>     function will never be executed);
>   * one that would trigger the warning if there may be an out-of-bound
>     access.

I wonder how often the first one would actually trigger.  And I wonder
how much correct code the second one triggers on today.  I personally
think it would be reasonable to rewrite the original example to avoid
the warning, since the code certainly looks like it can generate an out
of bounds access.


> BTW, can the user inform the compiler that some condition holds?
> i.e. some kind of assert() but specifically for the compiler.

As far as I know there is no way to do this directly.  The closest you
can come is something like

  if (!condition_which_must_be_true ())
    __builtin_unreachable ();

This will help the compiler in some cases but it won't track a complex
condition in any useful way.

Ian