[gcc r13-2521] libstdc++: small dynamic_cast optimization

[Jason Merrill]

https://gcc.gnu.org/g:873d395c2976a8321cec03f21d77e11f746da7c0

commit r13-2521-g873d395c2976a8321cec03f21d77e11f746da7c0
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Sep 6 10:35:21 2022 -0400

    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<B*>(ap);
    }
    
    libstdc++-v3/ChangeLog:
    
            * libsupc++/dyncast.cc (__dynamic_cast): Avoid virtual function
            call in simple success case.

Diff:
---
 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 853c911a4cf..616e4c05766 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 <void *> (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)