[LLVMdev] New type of smart pointer for LLVM

[David Blaikie]

& 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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