[Bug tree-optimization/41898] GCC ignores restrict on array

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41898

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Same for

int * __restrict__ *a;
int * __restrict__ *b;

extern void link_error (void);

int main()
{
  a[0][0] = 0;
  b[0][0] = 1;
  if (a[0][0] != 0)
    link_error ();
  return 0;
}

the restrict-ness is attached to specific variable infos in PTA but here
the restrict pointers fall out from a dereference op which might be present
multiple times but would all need to pick up the same artificial restrict
varinfo.  That's currently not supported in the data structures and/or
the constraint system and/or the graph solving.

For the int * __restrict__ a[1] case we fail to recognize it in

      /* Mark global restrict qualified pointers.  */
      if ((POINTER_TYPE_P (TREE_TYPE (decl))
           && TYPE_RESTRICT (TREE_TYPE (decl)))
          || vi->only_restrict_pointers)
        {

and vi->only_restrict_pointers is only ever set for aggregates which
we track field-wise (which arrays are not).

Possible fix below - it wouldn't handle int * __restrict a[2] optimally
since a[0] and a[1] would still alias.

diff --git a/gcc/tree-ssa-structalias.c b/gcc/tree-ssa-structalias.c
index 416a26c996c..dcc7fafc4b9 100644
--- a/gcc/tree-ssa-structalias.c
+++ b/gcc/tree-ssa-structalias.c
@@ -6128,8 +6128,11 @@ create_variable_info_for_1 (tree decl, const char *name,
bool add_id,
       vi->fullsize = tree_to_uhwi (declsize);
       vi->size = vi->fullsize;
       vi->is_full_var = true;
-      if (POINTER_TYPE_P (decl_type)
-         && (TYPE_RESTRICT (decl_type) || add_restrict))
+      if ((POINTER_TYPE_P (decl_type)
+          && (TYPE_RESTRICT (decl_type) || add_restrict))
+         || (TREE_CODE (decl_type) == ARRAY_TYPE
+             && POINTER_TYPE_P (TREE_TYPE (decl_type))
+             && TYPE_RESTRICT (TREE_TYPE (decl_type))))
        vi->only_restrict_pointers = 1;
       if (vi->only_restrict_pointers
          && !type_contains_placeholder_p (TREE_TYPE (decl_type))