[Bug c/83172] -Wstack-size= doesn't detect the correct stack size with VLA or alloca

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83172

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #3 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> The same result:
> 
> $ gcc-7 -Wvla-larger-than=128 -Wstack-usage=102400 -O0 -c t.c 
> t.c: In function ‘stack_usage_only’:
> t.c:23:5: warning: stack usage might be unbounded [-Wstack-usage=]
>  int stack_usage_only(unsigned x)
>      ^~~~~~~~~~~~~~~~
> t.c: In function ‘alloca_fails_even_with_const’:
> t.c:32:5: warning: stack usage might be unbounded [-Wstack-usage=]
>  int alloca_fails_even_with_const()
>      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> -O1 results the same.

Try -Wvla-larger-than=100 though.

In any case, note that for:

int vla_size_only(unsigned x)
{
 if(x > 128) __builtin_unreachable();
 char buf[x];
 do_something(buf);
 return 0;
}

the warning is expected since the code may allocate more than 128 bytes.

-Wstack-usage is designed to be *conservatively* correct and to yield the same
result at all optimization levels, i.e. it will never say that the stack usage
is bounded if there is a path where it may not be.  So it's very different from 
-Wvla-larger-than or -Walloca-larger-than which say nothing at -O0 or -O1 and
are not conservatively correct.