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

["François Dumont" <frs.dumont@gmail.com>]

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I can indeed restore _M_initialize_dispatch as it was before. It was not 
fixing my initial problem. I simply kept the code simplification.

     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 all the nested 
duplicated class...
             * include/bits/stl_vector.h (_Guard_alloc): ...here and rename.
             (_M_allocate_and_copy): Use latter.
             (_M_initialize_dispatch): Small code simplification.
             (_M_range_initialize): Likewise and set _M_finish first 
from the result
             of __uninitialize_fill_n_a that can throw.

Tested under Linux x86_64.

Ok to commit ?

François

On 28/05/2024 12:30, Jonathan Wakely wrote:
> On Mon, 27 May 2024 at 05:37, François Dumont <frs.dumont@gmail.com> wrote:
>> Here is a new version working also in C++98.
> Can we use a different solution that doesn't involve an explicit
> template argument list for that __uninitialized_fill_n_a call?
>
> -+        this->_M_impl._M_finish = std::__uninitialized_fill_n_a
> ++        this->_M_impl._M_finish =
> ++          std::__uninitialized_fill_n_a<pointer, size_type, value_type>
> +          (__start, __n, __value, _M_get_Tp_allocator());
>
> Using _M_fill_initialize solves the problem :-)
>
>
>
>> Note that I have this failure:
>>
>> FAIL: 23_containers/vector/types/1.cc  -std=gnu++98 (test for excess errors)
>>
>> but it's already failing on master, my patch do not change anything.
> Yes, that's been failing for ages.
>
>> Tested under Linux x64,
>>
>> still ok to commit ?
>>
>> François
>>
>> On 24/05/2024 16:17, Jonathan Wakely wrote:
>>> On Thu, 23 May 2024 at 18:38, François Dumont <frs.dumont@gmail.com> wrote:
>>>> On 23/05/2024 15:31, Jonathan Wakely wrote:
>>>>> 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?
>>>> Yes ! I indeed forgot to say so :-)
>>>>
>>>>
>>>>> 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
>>>> Note that I also had to move call to __uninitialized_copy_a before
>>>> assigning this->_M_impl._M_start so get rid of the -Wfree-nonheap-object
>>>> warn. But _M_realloc_append is already doing potentially throwing
>>>> operations before assigning this->_M_impl so it must be something else.
>>>>
>>>> Though it made me notice another occurence of _Guard in this method. Now
>>>> replaced too in this new patch.
>>>>
>>>>        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 all the nested
>>>> duplicated class...
>>>>                * include/bits/stl_vector.h (_Guard_alloc): ...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.
>>>>
>>>>>> 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.
>>>> _Guard_alloc chosen.
>>>>>> +      {
>>>>>> +    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.
>>>> I forgot about user fancy pointer, fixed.
>>>>
>>>>
>>>>>> +      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.
>>>> Already done in this initial patch proposal, see below.
>>>>
>>>>>> +      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.
>>>> Yes, this is why I called __uninitialized_fill_n_a instead and also to
>>>> do so *before* assigning _M_impl._M_start.
>>>>
>>>>
>>>>>> -      // Called by the first initialize_dispatch above and by the
>>>>>> -      // vector(n,value,a) constructor.
>>>>>> +      // Called by the vector(n,value,a) constructor.
>>>> See, it's here :-)
>>> Doh! Sorry, I'm not sure how I missed that.
>>>
>>>> Ok to commit ?
>>> OK for trunk, thanks!
>>>

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diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index 31169711a48..182ad41ed94 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_alloc
+      {
+	pointer _M_storage;	    // Storage to deallocate
+	size_type _M_len;
+	_Base& _M_vect;
+
+	_GLIBCXX20_CONSTEXPR
+	_Guard_alloc(pointer __s, size_type __l, _Base& __vect)
+	: _M_storage(__s), _M_len(__l), _M_vect(__vect)
+	{ }
+
+	_GLIBCXX20_CONSTEXPR
+	~_Guard_alloc()
+	{
+	  if (_M_storage)
+	    _M_vect._M_deallocate(_M_storage, _M_len);
+	}
+
+	_GLIBCXX20_CONSTEXPR
+	pointer
+	_M_release()
+	{
+	  pointer __res = _M_storage;
+	  _M_storage = pointer();
+	  return __res;
+	}
+
+      private:
+	_Guard_alloc(const _Guard_alloc&);
+      };
+
     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_alloc __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 +1667,14 @@ _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);
+	  const size_type __n = static_cast<size_type>(__int_n);
+	  pointer __start =
+	    _M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
+	  this->_M_impl._M_start = __start;
+	  this->_M_impl._M_end_of_storage = __start + __n;
+	  _M_fill_initialize(__n, __value);
 	}
 
       // Called by the range constructor to implement [23.1.1]/9
@@ -1690,13 +1716,14 @@ _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());
+	  pointer __start =
+	    this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
+	  _Guard_alloc __guard(__start, __n, *this);
+	  this->_M_impl._M_finish = std::__uninitialized_copy_a
+	    (__first, __last, __start, _M_get_Tp_allocator());
+	  this->_M_impl._M_start = __start;
+	  (void) __guard._M_release();
+	  this->_M_impl._M_end_of_storage = __start + __n;
 	}
 
       // Called by the first initialize_dispatch above and by the
diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 25df060beee..36b27dce7b9 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_alloc __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_alloc __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
@@ -833,32 +785,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 		_M_check_len(__n, "vector::_M_default_append");
 	      pointer __new_start(this->_M_allocate(__len));
 
-	      // 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_alloc __guard(__new_start, __len, *this);
 
 		std::__uninitialized_default_n_a(__new_start + __size, __n,
 						 _M_get_Tp_allocator());