[LLVMdev] New type of smart pointer for LLVM

[Anton Yartsev]

On 25.09.2014 6:50, Justin Bogner wrote:
> Anton Yartsev <anton.yartsev at gmail.com> writes:
>> Hello everyone,
>>
>> I bring to discussion the necessity/design of a new type of smart pointer.
>> r215176 and r217791 rise the problem, D5443 is devoted to the solution.
>> r215176 applies several temporary ugly fixes of memory leaks in TGParser.cpp
>> which would be great to be refactored using smart pointers. D5443 demonstrates
>> how the solution with a certain type of smart pointer would look like (see
>> changes in TGParser::ParseDef(), TGParser::InstantiateMulticlassDef() and
>> TGParser::ParseSimpleValue()).
>>
>> Briefly:
>> consider a leaky example:
>> {
>>    T* p = new T;
>>    if (condition1) {
>>      f(p); // takes ownership of p
>>    }
>>    p->SomeMethod();
>>
>>    if (condition2) {
>>      return nullptr; // Leak!
>>    }
>>
>>    g(p); // don't take ownership of p
>>    return p;
>> }
>>
>> The preferred solution would look like:
>> {
>>    smart_ptr<T> p(new T);
>>    if (condition1) {
>>      f(p.StopOwn()); // takes ownership of p
> So this takes ownership, but promises not to destroy the pointee in some
> way?
Yes.
>>    }
>>    p->SomeMethod();
>>
>>    if (condition2) {
>>      return nullptr; //
> I guess p is sometimes destroyed here, depending on condition1?
Yes.
>
>>    }
>>
>>    g(p.Get());  // don't take ownership of p
>>    return p.StopOwn();
>> }
> What does it mean to stop owning the pointer twice? Doesn't this leak p
> in the case where condition1 was false?
The above example just illustrate the typical common patterns. By the 
way, this particular example with several insignificant replaces is much 
similar to the logic of TGParser::InstantiateMulticlassDef() (r215176 
<http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/TableGen/TGParser.cpp?r1=215176&r2=215175&pathrev=215176&diff_format=f>, 
memory is allocated at line 2406, at line 2457 Records.addDef(CurRec) 
takes ownership of CurRec). More examples starting from line 2020, 
method TGParser::ParseDef().

>> Neither unique_ptr nor shared_ptr can be used in the place of smart_ptr as
>> unique_ptr sets the raw pointer to nullptr after release() (StopOwn() in the
>> example above) whereas shared_ptr is unable to release.
> I don't understand why shared_ptr wouldn't suffice for the example
> above. There are two cases in your example where you try to release the
> pointer - in one of them it seems you don't actually want to release it,
> because you continue using it after, and in the other the scope ends
> immediately after the release. The shared_ptr releases when it goes out
> of scope, so it seems to be exactly what you want here.
>
> Maybe I'm just misunderstanding the use case here, but I fear that a
> type like this one would just serve to paper over problems where the
> ownership is very unclear, without really helping much.
I agree the solution is not ideal, but it looks much safer to me then 
just adding and switching extra boolean variables indicating whether the 
memory should be freed or not and inserting missing, sometimes 
conditional, 'delete's. With the suggested solution you should only 
notice and call StopOwn() in places, where ownership is transferred.
The ownership logic is quite simple: "the wrapped memory should be freed 
until otherwise stated". To be more clear I also intend to design the 
wrapper for local scope usage only preventing it from copiying/moving.

>> Attached is a scratch that illustrates how the minimal
>> API/implementation of a desired smart pointer sufficient for
>> refactoring would look like. Any ideas and suggestions are
>> appreciated.
>>
>> //===-- CondOwnershipPtr.h - Smart ptr with conditional release -*- C++ -*-===//
>> //
>> //                     The LLVM Compiler Infrastructure
>> //
>> // This file is distributed under the University of Illinois Open Source
>> // License. See LICENSE.TXT for details.
>> //===----------------------------------------------------------------------===//
>>
>> #ifndef LLVM_ADT_CONDOWNERSHIPPTR_H
>> #define LLVM_ADT_CONDOWNERSHIPPTR_H
>>
>> namespace llvm {
>>
>> template<class T>
>> class CondOwnershipPtr {
>>    T* Ptr;
>>    bool Own;
>>
>>    void Delete() {
>>      if (Ptr && !Own)
>>        delete Ptr;
>>    }
> This seems to delete the pointer iff we *don't* own it. I think you have
> that backwards...
Right, thanks!

>
>> public:
>>    CondOwnershipPtr() : Ptr(nullptr), Own(true) {}
>>    explicit CondOwnershipPtr(T* p) : Ptr(p), Own(true) {}
>>
>>    ~CondOwnershipPtr() {
>>      Delete();
>>    }
>>
>>    T* Get() const {
>>      return Ptr;
>>    }
>>
>>    T* StopOwn() const {
>>      Own = false;
>>      return Ptr;
>>    }
>>
>>    void Reset(T* P = nullptr) {
>>      if (P != Ptr) {
>>        Delete();
>>        Ptr = P;
>>        Own = true;
>>      }
>>    }
>>
>>    bool Owns() const {
>>      return Own;
>>    }
>>
>>    operator bool() const {
>>      return Ptr != nullptr;
>>    }
>>
>>    T& operator*() const {
>>      return *Ptr;
>>    }
>>
>>    T* operator->() const {
>>      return Ptr;
>>    }
>> };
>>
>> } // end namespace llvm
>>
>> #endif

-- 
Anton

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