Re: Potentially false-positive -Wstringop-overflow= warning with gcc >= 11.1

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Jan 31, 2022 at 02:49:43PM -0700, Martin Sebor wrote:
> Some statements are also isolated from the source code even when they
> don't appear there, or appear with different arguments.  That's also
> what happens in this case: GCC replaces the memcpy call and
> the subsequent store with two: one pair is valid  and another pair
> of invalid statements.  Here's the problem in the IL the warning
> sees and points out:
> 
>   <bb 4> [count: 0]:
>   memcpy (4B, &somedata[0], 6);        <<< -Warray-bounds
>   MEM[(struct hdr2 *)0B].h21 ={v} 0;   <<< causes path isolation
>   __builtin_trap ();                       and trap to be added

Yup.  And GCC does not (yet) consider that memcpy to be UB itself, or
the trap would be moved there, removing the warning.  This may be the
best we can get :-)

(This of course requires the analysis to be always correct :-) )

> More to your point, it's also an example where warnings issued for
> code that's reachable only under some conditions make it seem like
> they point out unconditional bugs.

Yes, this is hard to understand often.  It makes it easy to think GCC
is wrong and the user code is correct (esp. for the user who wrote it).


Segher