[LLVMdev] [cfe-dev] New type of smart pointer for LLVM

[David Blaikie]

On Thu, Nov 13, 2014 at 10:59 AM, Argyrios Kyrtzidis <akyrtzi at gmail.com>
wrote:

> Could we consider moving the things you listed to shared pointer semantics
> ? It will be a simple and clear model; unless someone justifies that it
> will be a performance concern to use shared pointers there I don’t think we
> need a new and more complex to reason about smart pointer.
>

I'd generally prefer conditional ownership over shared ownership if
possible - it's a narrower contract & I can still think about where the
single owner is.

I know in at least some of these uses, shared pointer semantics would not
be applicable - TextDiagnosticPrinter::OwnsOutputStream, for example either
owns its own newly allocated stream or uses std::cout (or cerr, or
something) - it can never share the ownership of that stream, so it really
must be "own something or own nothing". (I suppose we could use a custom
no-op deleter on a shared_ptr in that case, though)

But I'm open to the idea that that's the simpler/better answer than
introducing a new construct - just a bit hesitant. Thanks for bringing it
up as a possibility.

- David


>
> On Nov 13, 2014, at 9:42 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
> Ping - we've hit another of these (propagating Diagnostic::OwnsDiagClient
> into other places) in
>
> http://llvm.org/viewvc/llvm-project?view=revision&revision=221884
>
> Any ideas how we should be tackling this overall? I'm not entirely
> convinced these are fixable by design and I think we might honestly want a
> conditional-ownership smart pointer...
>
> But I'm happy to hold off on that a while longer if we're going to make a
> concerted effort to avoid propagating these half-baked solutions and
> actually try to remove them when we come up against problems with/around
> them.
>
> On Wed, Oct 8, 2014 at 1:01 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> & repeating for the benefit of cfe-dev, here's the list of known cases of
>> conditional ownership (I'm sure there are others that don't fit the exact
>> naming convention I grep'd for):
>>
>> In Clang, simply grepping for "Owns" comes up with the following boolean
>> members:
>>
>> CodeGenAction::OwnsVMContext
>> ASTReader::OwnsDeserializationListener
>> Diagnostic::OwnsDiagClient
>> Preprocessor::OwnsHeaderSearch
>> TokenLexer::OwnsTokens
>> Action::OwnsInputs (this ones trickier - it's a boolean that indicates
>> whether all the elements of a vector<T*> are owned or unowned)
>> ASTUnit::OwnsRemappedFileBuffers
>> VerifyDiagnosticConsumer::OwnsPrimaryClient
>> TextDiagnosticPrinter::OwnsOutputStream
>> FixItRewriter::OwnsClient
>> Tooling::OwnsAction
>>
>> Some in LLVM:
>>
>> circular_raw_ostream::OwnsStream
>> Arg::OwnsValues (another tricky one with a bool flag and a vector of raw
>> pointers, if I recall correctly)
>>
>>
>> And a couple that I changed {T*, bool} to {T*, unique_ptr<T>}:
>>
>> LogDiagnosticPrinter::StreamOwner
>> ASTUnit::ComputedPreamble::Owner
>>
>> On Wed, Oct 8, 2014 at 1:00 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>> [+cfe-dev]
>>>
>>> This conversation has already been happening on llvm-dev so there's no
>>> good way for me to capture the entire existing discussion (so I'm jumping
>>> you in part-way) & the subject line could be more descriptive, but I wanted
>>> to add Clang developers since many of the interesting cases of conditional
>>> ownership I've seen were in Clang.
>>>
>>> I know some of you are also on llvm-dev but not active readers, so it
>>> might be worth using this as a jumping off point to go & find the full
>>> llvm-dev thread, read that and, when replying, add cfe-dev.
>>>
>>> If anyone not on llvm-dev wants some more context there's the email
>>> archive here (
>>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/thread.html#77136
>>> ) and/or I'm happy to provide more context/summary myself.
>>>
>>>
>>> On Thu, Oct 2, 2014 at 9:44 AM, David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Thu, Oct 2, 2014 at 2:43 AM, Anton Yartsev <anton.yartsev at gmail.com>
>>>> wrote:
>>>>
>>>>>  Thanks for the feedback!
>>>>>
>>>>>
>>>>> On Wed, Oct 1, 2014 at 3:36 PM, David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> On Wed, Oct 1, 2014 at 3:14 PM, Anton Yartsev <
>>>>>> anton.yartsev at gmail.com> wrote:
>>>>>>
>>>>>>>  Ping!
>>>>>>>
>>>>>>> Suggested is a wrapper over a raw pointer that is intended for
>>>>>>> freeing wrapped memory at the end of wrappers lifetime if ownership of a
>>>>>>> raw pointer was not taken away during the lifetime of the wrapper.
>>>>>>> The main difference from unique_ptr is an ability to access the
>>>>>>> wrapped pointer after the ownership is transferred.
>>>>>>> To make the ownership clearer the wrapper is designed for
>>>>>>> local-scope usage only.
>>>>>>>
>>>>>>
>>>>>>  I'm still concerned this isn't the right direction, and that we
>>>>>> instead want a more formal "maybe owning pointer". The specific use case
>>>>>> you've designed for here, in my experience, doesn't seem to come up often
>>>>>> enough to justify a custom ADT - but the more general tool of
>>>>>> sometimes-owning smart pointer seems common enough (& problematic enough)
>>>>>> to warrant a generic data structure (and such a structure would also be
>>>>>> appliable to the TGParser use case where this came up).
>>>>>>
>>>>>   David, could you, please, clarify the concept of the "maybe owning
>>>>> pointer"?
>>>>>
>>>>
>>>> See my reply to Chandler with a list of classes that hold {T*, bool}
>>>> members where the bool indicates whether the T* needs to be deleted or not.
>>>> My original goal here was to provide an API to make those more robust (more
>>>> precisely: to remove the need to call "release()" and allow us to stay in a
>>>> clear(er) owning domain).
>>>>
>>>>
>>>>>
>>>>>
>>>>>
>>>>>> I'd love to hear some more opinions, but maybe we're not going to get
>>>>>> them...
>>>>>>
>>>>>
>>>>> I strongly agree that the example here isn't compelling.
>>>>>
>>>>>  I think it is a very fundamental design problem when there is a need
>>>>> for a pointer value after it has been deallocated...
>>>>>
>>>>> Not deallocated but stored to the long-living storage. I agree, the
>>>>> problem is in design, the suggested wrapper is an intermediate solution, it
>>>>> was just designed to replace the existing ugly fixes.
>>>>>
>>>>>  I even question whether we need a "maybe owning" smart pointer, or
>>>>> whether this is just an indication that the underlying data structure is
>>>>> *wrong*. The idea of "maybe" and "owning" going to gether, in any sense,
>>>>> seems flat out broken to me from a design perspective, and so I'm not
>>>>> enthused about providing abstractions that make it easier to build systems
>>>>> with unclear ownership semantics.
>>>>>
>>>>>
>>>>> --
>>>>> Anton
>>>>>
>>>>>
>>>>
>>>
>>
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