Re: [PATCH] Avoid vector -Wfree-nonheap-object warnings

[Jonathan Wakely <jwakely@redhat.com>]

On 23/05/24 06:55 +0200, François Dumont wrote:
>As explained in this email:
>
>https://gcc.gnu.org/pipermail/libstdc++/2024-April/058552.html
>
>I experimented -Wfree-nonheap-object because of my enhancements on algos.
>
>So here is a patch to extend the usage of the _Guard type to other 
>parts of vector.

Nice, that fixes the warning you were seeing?

We recently got a bug report about -Wfree-nonheap-object in
std::vector, but that is coming from _M_realloc_append which already
uses the RAII guard :-(
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115016

>    libstdc++: Use RAII to replace try/catch blocks
>
>    Move _Guard into std::vector declaration and use it to guard all 
>calls to
>    vector _M_allocate.
>
>    Doing so the compiler has more visibility on what is done with the 
>pointers
>    and do not raise anymore the -Wfree-nonheap-object warning.
>
>    libstdc++-v3/ChangeLog:
>
>            * include/bits/vector.tcc (_Guard): Move...
>            * include/bits/stl_vector.h: ...here.
>            (_M_allocate_and_copy): Use latter.
>            (_M_initialize_dispatch): Likewise and set _M_finish first 
>from the result
>            of __uninitialize_fill_n_a that can throw.
>            (_M_range_initialize): Likewise.
>
>Tested under Linux x86_64, ok to commit ?
>
>François
>

>diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
>index 31169711a48..4ea74e3339a 100644
>--- a/libstdc++-v3/include/bits/stl_vector.h
>+++ b/libstdc++-v3/include/bits/stl_vector.h
>@@ -1607,6 +1607,39 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       clear() _GLIBCXX_NOEXCEPT
>       { _M_erase_at_end(this->_M_impl._M_start); }
>
>+    private:
>+      // RAII guard for allocated storage.
>+      struct _Guard

If it's being defined at class scope instead of locally in a member
function, I think a better name would be good. Maybe _Ptr_guard or
_Dealloc_guard or something.

>+      {
>+	pointer _M_storage;	    // Storage to deallocate
>+	size_type _M_len;
>+	_Base& _M_vect;
>+
>+	_GLIBCXX20_CONSTEXPR
>+	_Guard(pointer __s, size_type __l, _Base& __vect)
>+	: _M_storage(__s), _M_len(__l), _M_vect(__vect)
>+	{ }
>+
>+	_GLIBCXX20_CONSTEXPR
>+	~_Guard()
>+	{
>+	  if (_M_storage)
>+	    _M_vect._M_deallocate(_M_storage, _M_len);
>+	}
>+
>+	_GLIBCXX20_CONSTEXPR
>+	pointer
>+	_M_release()
>+	{
>+	  pointer __res = _M_storage;
>+	  _M_storage = 0;

I don't think the NullablePointer requirements include assigning 0,
only from nullptr, which isn't valid in C++98.

https://en.cppreference.com/w/cpp/named_req/NullablePointer

Please use _M_storage = pointer() instead.

>+	  return __res;
>+	}
>+
>+      private:
>+	_Guard(const _Guard&);
>+      };
>+
>     protected:
>       /**
>        *  Memory expansion handler.  Uses the member allocation function to
>@@ -1618,18 +1651,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> 	_M_allocate_and_copy(size_type __n,
> 			     _ForwardIterator __first, _ForwardIterator __last)
> 	{
>-	  pointer __result = this->_M_allocate(__n);
>-	  __try
>-	    {
>-	      std::__uninitialized_copy_a(__first, __last, __result,
>-					  _M_get_Tp_allocator());
>-	      return __result;
>-	    }
>-	  __catch(...)
>-	    {
>-	      _M_deallocate(__result, __n);
>-	      __throw_exception_again;
>-	    }
>+	  _Guard __guard(this->_M_allocate(__n), __n, *this);
>+	  std::__uninitialized_copy_a
>+	    (__first, __last, __guard._M_storage, _M_get_Tp_allocator());
>+	  return __guard._M_release();
> 	}
>
>
>@@ -1642,13 +1667,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       // 438. Ambiguity in the "do the right thing" clause
>       template<typename _Integer>
> 	void
>-	_M_initialize_dispatch(_Integer __n, _Integer __value, __true_type)
>+	_M_initialize_dispatch(_Integer __int_n, _Integer __value, __true_type)
> 	{
>-	  this->_M_impl._M_start = _M_allocate(_S_check_init_len(
>-		static_cast<size_type>(__n), _M_get_Tp_allocator()));
>-	  this->_M_impl._M_end_of_storage =
>-	    this->_M_impl._M_start + static_cast<size_type>(__n);
>-	  _M_fill_initialize(static_cast<size_type>(__n), __value);

Please fix the comment on _M_fill_initialize if you're removing the
use of it here.

>+	  const size_type __n = static_cast<size_type>(__int_n);
>+	  _Guard __guard(_M_allocate(_S_check_init_len(
>+	    __n, _M_get_Tp_allocator())), __n, *this);

I think this would be easier to read if the _S_check_init_len call was
done first, and maybe the allocation too, since we are going to need a
local __start later anyway. So maybe like this:

   template<typename _Integer>
     void
     _M_initialize_dispatch(_Integer __ni, _Integer __value, __true_type)
     {
       const size_type __n = static_cast<size_type>(__ni);
       pointer __start = _M_allocate(_S_check_init_len(__n),
                                     _M_get_Tp_allocator());
       _Guard __guard(__start, __n, *this);
       this->_M_impl._M_start = __start;
       _M_fill_initialize(__n, __value);
       this->_M_impl._M_end_of_storage = __start + __n;
       (void) __guard._M_release();
     }

Or inline the __uninitialized_fill_n_a call if you want to (but then
fix the comment on _M_fill_initialize). Inlining it does make this
function more consistent with the next one, which calls
__uninitialized_copy_a directly.

>+	  this->_M_impl._M_finish = std::__uninitialized_fill_n_a
>+	    (__guard._M_storage, __n, __value, _M_get_Tp_allocator());
>+	  pointer __start = this->_M_impl._M_start = __guard._M_release();
>+	  this->_M_impl._M_end_of_storage = __start + __n;
> 	}
>
>       // Called by the range constructor to implement [23.1.1]/9
>@@ -1690,17 +1717,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> 			    std::forward_iterator_tag)
> 	{
> 	  const size_type __n = std::distance(__first, __last);
>-	  this->_M_impl._M_start
>-	    = this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
>-	  this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
>-	  this->_M_impl._M_finish =
>-	    std::__uninitialized_copy_a(__first, __last,
>-					this->_M_impl._M_start,
>-					_M_get_Tp_allocator());
>+	  _Guard __guard(this->_M_allocate(_S_check_init_len(
>+	    __n, _M_get_Tp_allocator())), __n, *this);

Again, I think this would be easier to read if split up into two
statements, rather than doing the _S_check_init_len call and the
_M_allocate call and the _Guard initialization all at once.

>+	  this->_M_impl._M_finish = std::__uninitialized_copy_a
>+	    (__first, __last, __guard._M_storage, _M_get_Tp_allocator());
>+	  pointer __start = this->_M_impl._M_start = __guard._M_release();
>+	  this->_M_impl._M_end_of_storage = __start + __n;
> 	}
>
>-      // Called by the first initialize_dispatch above and by the
>-      // vector(n,value,a) constructor.
>+      // Called by the vector(n,value,a) constructor.
>       _GLIBCXX20_CONSTEXPR
>       void
>       _M_fill_initialize(size_type __n, const value_type& __value)
>diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
>index 25df060beee..e31da4f6c4c 100644
>--- a/libstdc++-v3/include/bits/vector.tcc
>+++ b/libstdc++-v3/include/bits/vector.tcc
>@@ -467,32 +467,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       pointer __new_start(this->_M_allocate(__len));
>       pointer __new_finish(__new_start);
>
>-      // RAII guard for allocated storage.
>-      struct _Guard
>       {
>-	pointer _M_storage;	    // Storage to deallocate
>-	size_type _M_len;
>-	_Tp_alloc_type& _M_alloc;
>-
>-	_GLIBCXX20_CONSTEXPR
>-	_Guard(pointer __s, size_type __l, _Tp_alloc_type& __a)
>-	: _M_storage(__s), _M_len(__l), _M_alloc(__a)
>-	{ }
>-
>-	_GLIBCXX20_CONSTEXPR
>-	~_Guard()
>-	{
>-	  if (_M_storage)
>-	    __gnu_cxx::__alloc_traits<_Tp_alloc_type>::
>-	      deallocate(_M_alloc, _M_storage, _M_len);
>-	}
>-
>-      private:
>-	_Guard(const _Guard&);
>-      };
>-
>-      {
>-	_Guard __guard(__new_start, __len, _M_impl);
>+	_Guard __guard(__new_start, __len, *this);
>
> 	// The order of the three operations is dictated by the C++11
> 	// case, where the moves could alter a new element belonging
>@@ -596,32 +572,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       pointer __new_start(this->_M_allocate(__len));
>       pointer __new_finish(__new_start);
>
>-      // RAII guard for allocated storage.
>-      struct _Guard
>-      {
>-	pointer _M_storage;	    // Storage to deallocate
>-	size_type _M_len;
>-	_Tp_alloc_type& _M_alloc;
>-
>-	_GLIBCXX20_CONSTEXPR
>-	_Guard(pointer __s, size_type __l, _Tp_alloc_type& __a)
>-	: _M_storage(__s), _M_len(__l), _M_alloc(__a)
>-	{ }
>-
>-	_GLIBCXX20_CONSTEXPR
>-	~_Guard()
>-	{
>-	  if (_M_storage)
>-	    __gnu_cxx::__alloc_traits<_Tp_alloc_type>::
>-	      deallocate(_M_alloc, _M_storage, _M_len);
>-	}
>-
>-      private:
>-	_Guard(const _Guard&);
>-      };
>-
>       {
>-	_Guard __guard(__new_start, __len, _M_impl);
>+	_Guard __guard(__new_start, __len, *this);
>
> 	// The order of the three operations is dictated by the C++11
> 	// case, where the moves could alter a new element belonging