From 873d395c2976a8321cec03f21d77e11f746da7c0 Mon Sep 17 00:00:00 2001 From: Jason Merrill Date: Tue, 6 Sep 2022 10:35:21 -0400 Subject: [PATCH] libstdc++: small dynamic_cast optimization This change speeds up the simple benchmark below by about 40%. struct A { virtual ~A() {} }; struct B: A { } b; A* ap = &b; void *sink; int main() { for (long i = 0; i < 4000000000L; ++i) sink = dynamic_cast(ap); } libstdc++-v3/ChangeLog: * libsupc++/dyncast.cc (__dynamic_cast): Avoid virtual function call in simple success case. --- libstdc++-v3/libsupc++/dyncast.cc | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/libstdc++-v3/libsupc++/dyncast.cc b/libstdc++-v3/libsupc++/dyncast.cc index 853c911a4cf9..616e4c05766e 100644 --- a/libstdc++-v3/libsupc++/dyncast.cc +++ b/libstdc++-v3/libsupc++/dyncast.cc @@ -71,6 +71,12 @@ __dynamic_cast (const void *src_ptr, // object started from if (whole_prefix->whole_type != whole_type) return NULL; + // Avoid virtual function call in the simple success case. + if (src2dst >= 0 + && src2dst == -prefix->whole_object + && *whole_type == *dst_type) + return const_cast (whole_ptr); + whole_type->__do_dyncast (src2dst, __class_type_info::__contained_public, dst_type, whole_ptr, src_type, src_ptr, result); if (!result.dst_ptr) -- 2.47.2