Re: First-time contributor -- speeding up `std::unique_ptr` compilation

[Jonathan Wakely <jwakely@redhat.com>]

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On Sat, 25 Jun 2022 at 13:45, Vittorio Romeo wrote:
>
> Thank you, Jonathan, for the very thorough explanation and for pointing out that I completely forgot to consider ABI stability in this case.
>
> I still think that it might be worthwhile to attempt some work in this area as multiple projects I work on have various TUs using unique_ptr but not tuple. I will look into the refactoring you suggested.

Something like this would avoid instantiating most of std::tuple while
preserving ABI, but it doesn't seem to really make much difference to
compilation time.

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diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index e1ad7721a59..777c7dec15a 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -172,7 +172,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       __uniq_ptr_impl() = default;
       _GLIBCXX23_CONSTEXPR
-      __uniq_ptr_impl(pointer __p) : _M_t() { _M_ptr() = __p; }
+      __uniq_ptr_impl(pointer __p) : _M_t(__p) { }
 
       template<typename _Del>
 	_GLIBCXX23_CONSTEXPR
@@ -193,13 +193,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       _GLIBCXX23_CONSTEXPR
-      pointer&   _M_ptr() noexcept { return std::get<0>(_M_t); }
+      pointer&   _M_ptr() noexcept { return _M_t._M_ptr(); }
       _GLIBCXX23_CONSTEXPR
-      pointer    _M_ptr() const noexcept { return std::get<0>(_M_t); }
+      pointer    _M_ptr() const noexcept { return _M_t._M_ptr(); }
       _GLIBCXX23_CONSTEXPR
-      _Dp&       _M_deleter() noexcept { return std::get<1>(_M_t); }
+      _Dp&       _M_deleter() noexcept { return _M_t._M_deleter(); }
       _GLIBCXX23_CONSTEXPR
-      const _Dp& _M_deleter() const noexcept { return std::get<1>(_M_t); }
+      const _Dp& _M_deleter() const noexcept { return _M_t._M_deleter(); }
 
       _GLIBCXX23_CONSTEXPR
       void reset(pointer __p) noexcept
@@ -226,9 +226,35 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	swap(this->_M_ptr(), __rhs._M_ptr());
 	swap(this->_M_deleter(), __rhs._M_deleter());
       }
-
     private:
-      tuple<pointer, _Dp> _M_t;
+      struct _Tuple : _Head_base<1, _Dp>, _Head_base<0, pointer>
+      {
+	using _Ptr_base = _Head_base<0, pointer>;
+	using _Del_base = _Head_base<1, _Dp>;
+
+	_Tuple() = default;
+	_Tuple(_Tuple&&) = default;
+
+	constexpr
+	_Tuple(pointer __p) : _Ptr_base(__p) { }
+
+	template<typename _Del>
+	  constexpr
+	  _Tuple(pointer __p, _Del&& __d)
+	  : _Del_base(std::forward<_Del>(__d)), _Ptr_base(__p)
+	  { }
+
+	constexpr pointer&
+	_M_ptr() noexcept { return _Ptr_base::_M_head(*this); }
+	constexpr pointer
+	_M_ptr() const noexcept { return _Ptr_base::_M_head(*this); }
+	constexpr _Dp&
+	_M_deleter() noexcept { return _Del_base::_M_head(*this); }
+	constexpr const _Dp&
+	_M_deleter() const noexcept { return _Del_base::_M_head(*this); }
+      };
+
+      _Tuple _M_t;
     };
 
   // Defines move construction + assignment as either defaulted or deleted.