| Message ID | 20220907014444.1469136-1-jason@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | [RFA] libstdc++: small dynamic_cast optimization | expand |
On Wed, 7 Sept 2022 at 02:45, Jason Merrill via Libstdc++ <libstdc++@gcc.gnu.org> wrote: > > 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? Yes, I think so. If there's a fast path, we should take it. > > 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 >
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)