Re: [PATCH] libstdc++: Implement ranges::join_with_view from P2441R2

[Patrick Palka <ppalka@redhat.com>]

On Tue, 4 Oct 2022, Jonathan Wakely wrote:

> On Tue, 4 Oct 2022 at 02:11, Patrick Palka via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?  FWIW using
> 
> OK, thanks.

Thanks a lot, patch committed.

> 
> > variant<_PatternIter, _InnerIter> in the implementation means we need to
> > include <variant> from <ranges>, which increases the preprocessed size
> > of <ranges> by 3% (51.5k vs 53k).  I suppose that's an acceptable cost?
> 
> Yeah, I don't think we want to reimplement a lightweight std::variant,
> because that would just add even more code.

Sounds good.

> 
> As I mentioned on IRC, maybe we could optimize the compilation time
> for some of the visitation using P2637R0, but that can be done later.

Ah, I didn't consider the compile time impact of using std::visit.
Since we already use/instantiate std::get elsewhere in the implementation,
what do you think about doing the visitation manually via index() and
std::get like so?  Seems to reduce compile time/memory usage for
join_with/1.cc by around 6% and doesn't look too messy since we're
dealing with only two alternatives.  (And IIUC this should be equivalent
to std::visit wrt valueless_by_exception handling, since the call to
std::get<1> in each else branch will throw bad_variant_access for us
like std::visit would.)

-- >8 --

Subject: [PATCH] libstdc++: Avoid std::visit in ranges::join_with_view

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_with_view::_Iterator::operator*):
	Replace use of std::visit with manual visitation.
	(join_with_view::_Iterator::operator++): Likewise.
	(join_with_view::_Iterator::operator--): Likewise.
	(join_with_view::_Iterator::iter_move): Likewise.
	(join_with_view::_Iterator::iter_swap): Likewise.
---
 libstdc++-v3/include/std/ranges | 47 +++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index d0d6ce61a87..1f821128d2d 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -7165,18 +7165,23 @@ namespace views::__adaptor
 	_M_inner_it.template emplace<1>(std::get<1>(std::move(__i._M_inner_it)));
     }
 
-    constexpr decltype(auto)
+    constexpr common_reference_t<iter_reference_t<_InnerIter>,
+			         iter_reference_t<_PatternIter>>
     operator*() const
     {
-      using reference = common_reference_t<iter_reference_t<_InnerIter>,
-					   iter_reference_t<_PatternIter>>;
-      return std::visit([](auto& __it) -> reference { return *__it; }, _M_inner_it);
+      if (_M_inner_it.index() == 0)
+	return *std::get<0>(_M_inner_it);
+      else
+	return *std::get<1>(_M_inner_it);
     }
 
     constexpr _Iterator&
     operator++()
     {
-      std::visit([](auto& __it){ ++__it; }, _M_inner_it);
+      if (_M_inner_it.index() == 0)
+	++std::get<0>(_M_inner_it);
+      else
+	++std::get<1>(_M_inner_it);
       _M_satisfy();
       return *this;
     }
@@ -7232,7 +7237,10 @@ namespace views::__adaptor
 	    }
 	}
 
-      std::visit([](auto& __it){ --__it; }, _M_inner_it);
+      if (_M_inner_it.index() == 0)
+	--std::get<0>(_M_inner_it);
+      else
+	--std::get<1>(_M_inner_it);
       return *this;
     }
 
@@ -7253,18 +7261,35 @@ namespace views::__adaptor
 	&& equality_comparable<_OuterIter> && equality_comparable<_InnerIter>
     { return __x._M_outer_it == __y._M_outer_it && __x._M_inner_it ==__y._M_inner_it; }
 
-    friend constexpr decltype(auto)
+    friend constexpr common_reference_t<iter_rvalue_reference_t<_InnerIter>,
+					iter_rvalue_reference_t<_PatternIter>>
     iter_move(const _Iterator& __x)
     {
-      using __rval_ref = common_reference_t<iter_rvalue_reference_t<_InnerIter>,
-					    iter_rvalue_reference_t<_PatternIter>>;
-      return std::visit<__rval_ref>(ranges::iter_move, __x._M_inner_it);
+      if (__x._M_inner_it.index() == 0)
+	return ranges::iter_move(std::get<0>(__x._M_inner_it));
+      else
+	return ranges::iter_move(std::get<1>(__x._M_inner_it));
     }
 
     friend constexpr void
     iter_swap(const _Iterator& __x, const _Iterator& __y)
       requires indirectly_swappable<_InnerIter, _PatternIter>
-    { std::visit(ranges::iter_swap, __x._M_inner_it, __y._M_inner_it); }
+    {
+      if (__x._M_inner_it.index() == 0)
+	{
+	  if (__y._M_inner_it.index() == 0)
+	    ranges::iter_swap(std::get<0>(__x._M_inner_it), std::get<0>(__y._M_inner_it));
+	  else
+	    ranges::iter_swap(std::get<0>(__x._M_inner_it), std::get<1>(__y._M_inner_it));
+	}
+      else
+	{
+	  if (__y._M_inner_it.index() == 0)
+	    ranges::iter_swap(std::get<1>(__x._M_inner_it), std::get<0>(__y._M_inner_it));
+	  else
+	    ranges::iter_swap(std::get<1>(__x._M_inner_it), std::get<1>(__y._M_inner_it));
+	}
+    }
   };
 
   template<input_range _Vp, forward_range _Pattern>
-- 
2.38.0.rc2