[Bug fortran/69368] [6 Regression] spec2006 test case 416.gamess fails with the g++ 6.0 compiler starting with r232508

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69368

alalaw01 at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---

--- Comment #33 from alalaw01 at gcc dot gnu.org ---
(In reply to rguenther@suse.de from comment #31)
> 
> Thus a "fix" for the case where treating a[i] as a[0] is the issue
> would be
> 
> Index: gcc/tree-dfa.c
> ===================================================================
> --- gcc/tree-dfa.c      (revision 233172)
> +++ gcc/tree-dfa.c      (working copy)
> @@ -617,7 +617,11 @@ get_ref_base_and_extent (tree exp, HOST_
>        if (maxsize == -1
>           && DECL_SIZE (exp)
>           && TREE_CODE (DECL_SIZE (exp)) == INTEGER_CST)
> -       maxsize = wi::to_offset (DECL_SIZE (exp)) - bit_offset;
> +       {
> +         maxsize = wi::to_offset (DECL_SIZE (exp)) - bit_offset;
> +         if (maxsize == size)
> +           maxsize = -1;
> +       }
>      }
>    else if (CONSTANT_CLASS_P (exp))
>      {

So is there a case where we want this for C ?

If I declare a struct with a VLA, and access it through a pointer - GCC
recognizes the VLA idiom and keeps the accesses. If I access it from a decl,
yes we optimize away the out-of-bounds accesses (in FRE, long before we reach
the tree-ssa-scopedtables changes). So OK, if I access it from a extern or
__attribute__((weak) decl, which I then get the linker to replace with a bigger
decl, then I get "wrong" code (it ignores the extra elements in the bigger
decl) - but I'd say that was invalid code.

So if this is Fortran-only, we probably have to hook off --std=legacy, right?