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* [gcc r13-2521] libstdc++: small dynamic_cast optimization
@ 2022-09-07 14:12 Jason Merrill
0 siblings, 0 replies; only message in thread
From: Jason Merrill @ 2022-09-07 14:12 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
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)
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