This patch optimizes out calls to __dynamic_cast when the type being cast to is
final (or its destructor), replacing them with a simple comparison of the
types' vtable addresses. This is already implemented and done by default in
clang (https://reviews.llvm.org/D154658), but can be turned off with
-fno-assume-unique-vtables, due to the problems it can cause with unmerged
vtables when using shared libraries. With the optimization not really fitting
well under any existing flag (that I know of), I believe an
-fassume-unique-vtables flag (or something similar) would be appropriate to add
to gcc, which besides enabling this optimization, should probably also define
__GXX_MERGED_TYPEINFO_NAMES as 1.
diff --git a/gcc/cp/rtti.cc b/gcc/cp/rtti.cc
index c06a18b3ff1..a4259253f8d 100644
--- a/gcc/cp/rtti.cc
+++ b/gcc/cp/rtti.cc
@@ -761,6 +761,51 @@ build_dynamic_cast_1 (location_t loc, tree type, tree expr,
if (tc == REFERENCE_TYPE)
expr1 = cp_build_addr_expr (expr1, complain);
+
+
+ /* If type is final, don't call dynamic_cast.
+ * Instead just check vtable equivalence at runtime.
+ * TYPE_FINAL_P does not return true for non-final class with
+ * final destructor overriding virtual though,
+ * so look through virtual functions for final destructor */
+
+ bool can_inherit = !TYPE_FINAL_P (target_type);
+ tree vchain;
+ for (vchain = BINFO_VIRTUALS (TYPE_BINFO (target_type));
+ vchain && can_inherit;
+ vchain = TREE_CHAIN (vchain))
+ {
+ if (!DECL_DESTRUCTOR_P (BV_FN (vchain)))
+ continue;
+ if (!DECL_FINAL_P (BV_FN (vchain)))
+ continue;
+ can_inherit = false;
+ }
+
+ if (!can_inherit)
+ {
+ /* Retrieve vtable declaration and address. */
+ tree trgt_vtbl_addr = build_address (get_vtable_decl (target_type, 0));
+ /* The offset-to-top field is at index -2 from the vptr.
+ * Offsetting -2 on build_vtbl_ref (at low optimization) results in
register being
+ * subtracted from at runtime, done on vtbl address to prevent */
+ tree trgt_vptr_addr = build2 (POINTER_PLUS_EXPR,
TREE_TYPE(trgt_vtbl_addr),
+ trgt_vtbl_addr, size_int(2 * TARGET_VTABLE_DATA_ENTRY_DISTANCE *
+ tree_to_shwi (TYPE_SIZE(ptr_type_node))/8));
+
+ tree src_obj = cp_build_fold_indirect_ref (expr);
+ tree src_vptr = build_address (build_vtbl_ref (src_obj, size_int(0)));
+
+ /* Check vtable equivalence by vptr address */
+ tree cond = build2 (NE_EXPR, boolean_type_node, trgt_vptr_addr,
src_vptr);
+ tree result = build3 (COND_EXPR, type, cond, nullptr_node,
+ build_static_cast (loc, type, expr, complain));
+ SET_EXPR_LOCATION (result, loc);
+ result = cp_convert (type, result, complain);
+
+ return build_if_nonnull (expr, result, complain);
+ }
+
elems[0] = expr1;
elems[1] = td3;
elems[2] = td2;
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