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

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

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Here is a new version working also in C++98.

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.

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..81fe3825064 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 +1667,18 @@ _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()));
+	  _Guard_alloc __guard(__start, __n, *this);
+	  this->_M_impl._M_finish =
+	    std::__uninitialized_fill_n_a<pointer, size_type, value_type>
+	    (__start, __n, __value, _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 range constructor to implement [23.1.1]/9
@@ -1690,17 +1720,17 @@ _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
-      // 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..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());
diff --git a/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_1_neg.cc b/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_1_neg.cc
index 6b3d48f0c92..d732ca8bed8 100644
--- a/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_1_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_1_neg.cc
@@ -19,7 +19,7 @@
 
 // { dg-do compile }
 // { dg-prune-output "cannot convert" }
-// { dg-prune-output "no matching function .*_M_fill_initialize" }
+// { dg-prune-output "no matching function .*__uninitialized_fill_n_a" }
 
 #include <vector>
 
diff --git a/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_2_neg.cc b/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_2_neg.cc
index 65ce2fd30e5..58c4c8f9a73 100644
--- a/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_2_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/requirements/dr438/constructor_2_neg.cc
@@ -19,7 +19,7 @@
 
 // { dg-do compile }
 // { dg-prune-output "cannot convert" }
-// { dg-prune-output "no matching function .*_M_fill_initialize" }
+// { dg-prune-output "no matching function .*__uninitialized_fill_n_a" }
 
 #include <vector>
 #include <utility>