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

[Patrick Palka <ppalka@redhat.com>]

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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).

> 
> 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
> >
> 
>