Re: [PATCH] Fix up ICEs with TREE_CONSTANT references (PR c++/78373)

[Jason Merrill]

On Wed, Nov 16, 2016 at 4:00 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> Jason's recent patch to turn reference vars initialized with invariant
> addresses broke the first testcase below, because &self->singleton
> is considered TREE_CONSTANT (because self is TREE_CONSTANT VAR_DECL and
> singleton field has constant offset), but after going into SSA form
> it is not supposed to be TREE_CONSTANT anymore (&self_2->singleton),
> because SSA_NAMEs don't have TREE_CONSTANT set on them.
>
> The following patch fixes it by gimplifying such vars into their
> DECL_INITIAL unless in OpenMP regions, where such folding is deferred
> until omplower pass finishes.

Hmm, this seems like a workaround; why don't we see the same problem
with constant pointer variables?

A simpler workaround would be to not set TREE_CONSTANT on references
in the first place, since the constexpr code doesn't need it.  What do
you think?

commit 6cdd28bb152fcb07a7eb6c9f053cd435cf719a20
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Nov 16 16:13:25 2016 -0500

    ref

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index c54a2de..87db589 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -6839,7 +6839,8 @@ cp_finish_decl (tree decl, tree init, bool 
init_const_expr_p,
          /* Set these flags now for templates.  We'll update the flags in
             store_init_value for instantiations.  */
          DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl) = 1;
-         if (decl_maybe_constant_var_p (decl))
+         if (decl_maybe_constant_var_p (decl)
+             && TREE_CODE (type) != REFERENCE_TYPE)
            TREE_CONSTANT (decl) = 1;
        }
     }
diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 022a478..dcdb710 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -824,7 +824,8 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** 
cleanups, int flags)
       const_init = (reduced_constant_expression_p (value)
                    || error_operand_p (value));
       DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl) = const_init;
-      TREE_CONSTANT (decl) = const_init && decl_maybe_constant_var_p (decl);
+      if (TREE_CODE (type) != REFERENCE_TYPE)
+       TREE_CONSTANT (decl) = const_init && decl_maybe_constant_var_p (decl);
     }
   value = cp_fully_fold (value);