[Bug c/68764] C frontend does not fold away trivial expressions that refer to const variables

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68764

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=66618,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=69960
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed for GCC 8 by r8-4755-gf9c59f7e9511 (aka PR 66618 and PR 69960).
Explicitly this part of c_fully_fold_internal:
   /* Constants, declarations, statements, errors, and anything else not
      counted as an expression cannot usefully be folded further at this
      point.  */
-  if (!IS_EXPR_CODE_CLASS (kind)
-      || kind == tcc_statement)
-    return expr;
+  if (!IS_EXPR_CODE_CLASS (kind) || kind == tcc_statement)
+    {
+      /* Except for variables which we can optimize to its initializer.  */
+      if (VAR_P (expr) && !lval && (optimize || in_init))
+       {
+         ret = decl_constant_value (expr);
+         /* Avoid unwanted tree sharing between the initializer and current
+            function's body where the tree can be modified e.g. by the
+            gimplifier.  */
+         if (ret != expr && TREE_STATIC (expr))
+           ret = unshare_expr (ret);
+         return ret;
+       }
+      return expr;
+    }