* class.c (finish_struct_1): Add commentary on prevous change.

From-SVN: r22884

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 0ca49d216590fd5ab14bf5cbe766684df41c7b83..fc4ce6230e282a4876dc92ff9ba3b5e48a1a4dbb 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,7 @@
 1998-10-07  Mark Mitchell  <mark@markmitchell.com>
 
+       * class.c (finish_struct_1): Add commentary on prevous change.
+
        * cp-tree.h (vtbl_ptr_type_node): New variable.
        * class.c (build_vtbl_ref): Don't indirect through the vptr; it's
        already of the right type.

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index da331fa251e4f03b60d3f2d5f1f69b0404814d2c..cd16b59c97e399198225f64643a51b89ba9ce674 100644 (file)
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -3777,8 +3777,25 @@ finish_struct_1 (t, warn_anon)
 
   if (vfield == NULL_TREE && has_virtual)
     {
-      /* We build this decl with ptr_type_node, and
-        change the type when we know what it should be.  */
+      /* We build this decl with vtbl_ptr_type_node, which is a
+        `vtable_entry_type*'.  It might seem more precise to use
+        `vtable_entry_type (*)[N]' where N is the number of firtual
+        functions.  However, that would require the vtable pointer in
+        base classes to have a different type than the vtable pointer
+        in derived classes.  We could make that happen, but that
+        still wouldn't solve all the problems.  In particular, the
+        type-based alias analysis code would decide that assignments
+        to the base class vtable pointer can't alias assignments to
+        the derived class vtable pointer, since they have different
+        types.  Thus, in an derived class destructor, where the base
+        class constructor was inlined, we could generate bad code for
+        setting up the vtable pointer.  
+
+         Therefore, we use one type for all vtable pointers.  We still
+        use a type-correct type; it's just doesn't indicate the array
+        bounds.  That's better than using `void*' or some such; it's
+        cleaner, and it let's the alias analysis code know that these
+        stores cannot alias stores to void*!  */
       vfield = build_lang_field_decl (FIELD_DECL, get_vfield_name (t),
                                      vtbl_ptr_type_node);
       /* If you change any of the below, take a look at all the