[PATCH RFA] libstdc++: small dynamic_cast optimization

[Jason Merrill <jason@redhat.com>]

An email discussion of optimizing EH led me to wonder why we weren't doing
this already.  Not that this change affects EH at all.

Tested x86_64-pc-linux-gnu.  Does this seem worthwhile?

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 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)

base-commit: 0a2fba3697411c07a8330abfe7460ce62bce5e7f
-- 
2.31.1