libstdc++: Simplify std::shared_ptr construction from std::weak_ptr

The _M_add_ref_lock() and _M_add_ref_lock_nothrow() members of
_Sp_counted_base are very similar, except that the former throws an
exception when the use count is zero and the latter returns false. The
former (and its callers) can be implemented in terms of the latter.
This results in a small reduction in code size, because throwing an
exception now only happens in one place.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr.h (shared_ptr(const weak_ptr&, nothrow_t)):
	Add noexcept.
	* include/bits/shared_ptr_base.h (_Sp_counted_base::_M_add_ref_lock):
	Remove specializations and just call _M_add_ref_lock_nothrow.
	(__shared_count, __shared_ptr): Use nullptr for null pointer
	constants.
	(__shared_count(const __weak_count&)): Use _M_add_ref_lock_nothrow
	instead of _M_add_ref_lock.
	(__shared_count(const __weak_count&, nothrow_t)): Add noexcept.
	(__shared_ptr::operator bool()): Add noexcept.
	(__shared_ptr(const __weak_ptr&, nothrow_t)): Add noexcept.

2 files changed, 22 insertions, 63 deletions

diff --git a/libstdc++-v3/include/bits/shared_ptr.h b/libstdc++-v3/include/bits/shared_ptr.h
index 0c393e2..0bfb525 100644
--- a/libstdc++-v3/include/bits/shared_ptr.h
+++ b/libstdc++-v3/include/bits/shared_ptr.h
@@ -413,7 +413,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	allocate_shared(const _Alloc& __a, _Args&&... __args);
 
       // This constructor is non-standard, it is used by weak_ptr::lock().
-      shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
+      shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t) noexcept
       : __shared_ptr<_Tp>(__r, std::nothrow) { }
 
       friend class weak_ptr<_Tp>;
diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index ff578e6..ca37f2b 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -142,10 +142,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }
 
       void
-      _M_add_ref_lock();
+      _M_add_ref_lock()
+      {
+	if (!_M_add_ref_lock_nothrow())
+	  __throw_bad_weak_ptr();
+      }
 
       bool
-      _M_add_ref_lock_nothrow();
+      _M_add_ref_lock_nothrow() noexcept;
 
       void
       _M_release() noexcept
@@ -215,48 +219,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
   template<>
-    inline void
-    _Sp_counted_base<_S_single>::
-    _M_add_ref_lock()
-    {
-      if (_M_use_count == 0)
-	__throw_bad_weak_ptr();
-      ++_M_use_count;
-    }
-
-  template<>
-    inline void
-    _Sp_counted_base<_S_mutex>::
-    _M_add_ref_lock()
-    {
-      __gnu_cxx::__scoped_lock sentry(*this);
-      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
-	{
-	  _M_use_count = 0;
-	  __throw_bad_weak_ptr();
-	}
-    }
-
-  template<>
-    inline void
-    _Sp_counted_base<_S_atomic>::
-    _M_add_ref_lock()
-    {
-      // Perform lock-free add-if-not-zero operation.
-      _Atomic_word __count = _M_get_use_count();
-      do
-	{
-	  if (__count == 0)
-	    __throw_bad_weak_ptr();
-	  // Replace the current counter value with the old value + 1, as
-	  // long as it's not changed meanwhile.
-	}
-      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
-					  true, __ATOMIC_ACQ_REL,
-					  __ATOMIC_RELAXED));
-    }
-
-  template<>
     inline bool
     _Sp_counted_base<_S_single>::
     _M_add_ref_lock_nothrow()
@@ -736,7 +698,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __shared_count(const __shared_count& __r) noexcept
       : _M_pi(__r._M_pi)
       {
-	if (_M_pi != 0)
+	if (_M_pi != nullptr)
 	  _M_pi->_M_add_ref_copy();
       }
 
@@ -746,9 +708,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 	if (__tmp != _M_pi)
 	  {
-	    if (__tmp != 0)
+	    if (__tmp != nullptr)
 	      __tmp->_M_add_ref_copy();
-	    if (_M_pi != 0)
+	    if (_M_pi != nullptr)
 	      _M_pi->_M_release();
 	    _M_pi = __tmp;
 	  }
@@ -765,7 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       long
       _M_get_use_count() const noexcept
-      { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }
+      { return _M_pi ? _M_pi->_M_get_use_count() : 0; }
 
       bool
       _M_unique() const noexcept
@@ -897,9 +859,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __shared_count<_Lp>::__shared_count(const __weak_count<_Lp>& __r)
     : _M_pi(__r._M_pi)
     {
-      if (_M_pi != nullptr)
-	_M_pi->_M_add_ref_lock();
-      else
+      if (_M_pi == nullptr || !_M_pi->_M_add_ref_lock_nothrow())
 	__throw_bad_weak_ptr();
     }
 
@@ -907,12 +867,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<_Lock_policy _Lp>
     inline
     __shared_count<_Lp>::
-    __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t)
+    __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept
     : _M_pi(__r._M_pi)
     {
-      if (_M_pi != nullptr)
-	if (!_M_pi->_M_add_ref_lock_nothrow())
-	  _M_pi = nullptr;
+      if (_M_pi && !_M_pi->_M_add_ref_lock_nothrow())
+	_M_pi = nullptr;
     }
 
 #define __cpp_lib_shared_ptr_arrays 201611L
@@ -1175,7 +1134,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_ptr(__p), _M_refcount()
 	{
 	  _M_refcount._M_swap(__r._M_refcount);
-	  __r._M_ptr = 0;
+	  __r._M_ptr = nullptr;
 	}
 
       __shared_ptr(const __shared_ptr&) noexcept = default;
@@ -1191,7 +1150,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_ptr(__r._M_ptr), _M_refcount()
       {
 	_M_refcount._M_swap(__r._M_refcount);
-	__r._M_ptr = 0;
+	__r._M_ptr = nullptr;
       }
 
       template<typename _Yp, typename = _Compatible<_Yp>>
@@ -1199,7 +1158,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_ptr(__r._M_ptr), _M_refcount()
 	{
 	  _M_refcount._M_swap(__r._M_refcount);
-	  __r._M_ptr = 0;
+	  __r._M_ptr = nullptr;
 	}
 
       template<typename _Yp, typename = _Compatible<_Yp>>
@@ -1305,7 +1264,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	reset(_Yp* __p) // _Yp must be complete.
 	{
 	  // Catch self-reset errors.
-	  __glibcxx_assert(__p == 0 || __p != _M_ptr);
+	  __glibcxx_assert(__p == nullptr || __p != _M_ptr);
 	  __shared_ptr(__p).swap(*this);
 	}
 
@@ -1325,8 +1284,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return _M_ptr; }
 
       /// Return true if the stored pointer is not null.
-      explicit operator bool() const // never throws
-      { return _M_ptr == 0 ? false : true; }
+      explicit operator bool() const noexcept
+      { return _M_ptr != nullptr; }
 
       /// Return true if use_count() == 1.
       bool
@@ -1378,7 +1337,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // This constructor is used by __weak_ptr::lock() and
       // shared_ptr::shared_ptr(const weak_ptr&, std::nothrow_t).
-      __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t)
+      __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t) noexcept
       : _M_refcount(__r._M_refcount, std::nothrow)
       {
 	_M_ptr = _M_refcount._M_get_use_count() ? __r._M_ptr : nullptr;