Re: [PATCH v7 0/6] c++, libstdc++: get std::is_object to dispatch to new built-in traits

[Ken Matsui <kmatsui@cs.washington.edu>]

On Tue, Jun 20, 2023 at 8:32 AM Patrick Palka <ppalka@redhat.com> wrote:
>
> On Thu, 15 Jun 2023, Ken Matsui via Libstdc++ wrote:
>
> > Hi,
> >
> > For those curious about the performance improvements of this patch, I
> > conducted a benchmark that instantiates 256k specializations of
> > is_object_v based on Patrick's code. You can find the benchmark code
> > at this link:
> >
> > https://github.com/ken-matsui/gcc-benches/blob/main/is_object_benchmark.cc
> >
> > On my computer, using the gcc HEAD of this patch for a release build,
> > the patch with -DUSE_BUILTIN took 64% less time and used 44-47% less
> > memory compared to not using it.
>
> That's more like it :D  Though the benchmark should also invoke the
> trait on non-object types too, e.g. Instantiator& or Instantiator(int).

Here is the updated benchmark:

https://github.com/ken-matsui/gcc-benches/blob/main/is_object.md#sat-jun-24-080110-am-pdt-2023

Time: -74.7544%
Peak Memory Usage: -62.5913%
Total Memory Usage: -64.2708%

> >
> > Sincerely,
> > Ken Matsui
> >
> > On Mon, Jun 12, 2023 at 3:49 PM Ken Matsui <kmatsui@cs.washington.edu> wrote:
> > >
> > > Hi,
> > >
> > > This patch series gets std::is_object to dispatch to built-in traits and
> > > implements the following built-in traits, on which std::object depends.
> > >
> > >         * __is_reference
> > >         * __is_function
> > >         * __is_void
> > >
> > > std::is_object was depending on them with disjunction and negation.
> > >
> > > __not_<__or_<is_function<_Tp>, is_reference<_Tp>, is_void<_Tp>>>::type
> > >
> > > Therefore, this patch uses them directly instead of implementing an additional
> > > built-in trait __is_object, which makes the compiler slightly bigger and
> > > slower.
> > >
> > > __bool_constant<!(__is_function(_Tp) || __is_reference(_Tp) || __is_void(_Tp))>
> > >
> > > This would instantiate only __bool_constant<true> and __bool_constant<false>,
> > > which can be mostly shared. That is, the purpose of built-in traits is
> > > considered as achieved.
> > >
> > > Changes in v7
> > >
> > >         * Removed an unnecessary new line.
> > >
> > > Ken Matsui (6):
> > >   c++: implement __is_reference built-in trait
> > >   libstdc++: use new built-in trait __is_reference for std::is_reference
> > >   c++: implement __is_function built-in trait
> > >   libstdc++: use new built-in trait __is_function for std::is_function
> > >   c++, libstdc++: implement __is_void built-in trait
> > >   libstdc++: make std::is_object dispatch to new built-in traits
> > >
> > >  gcc/cp/constraint.cc                          |  9 +++
> > >  gcc/cp/cp-trait.def                           |  3 +
> > >  gcc/cp/semantics.cc                           | 12 ++++
> > >  gcc/testsuite/g++.dg/ext/has-builtin-1.C      |  9 +++
> > >  gcc/testsuite/g++.dg/ext/is_function.C        | 58 +++++++++++++++++++
> > >  gcc/testsuite/g++.dg/ext/is_reference.C       | 34 +++++++++++
> > >  gcc/testsuite/g++.dg/ext/is_void.C            | 35 +++++++++++
> > >  gcc/testsuite/g++.dg/tm/pr46567.C             |  6 +-
> > >  libstdc++-v3/include/bits/cpp_type_traits.h   | 15 -----
> > >  libstdc++-v3/include/debug/helper_functions.h |  5 +-
> > >  libstdc++-v3/include/std/type_traits          | 51 ++++++++++++++++
> > >  11 files changed, 216 insertions(+), 21 deletions(-)
> > >  create mode 100644 gcc/testsuite/g++.dg/ext/is_function.C
> > >  create mode 100644 gcc/testsuite/g++.dg/ext/is_reference.C
> > >  create mode 100644 gcc/testsuite/g++.dg/ext/is_void.C
> > >
> > > --
> > > 2.41.0
> > >
> >
> >