[cfe-dev] Adding local scope and automatic object destructors

[Manuel Klimek]

On Wed, Jul 30, 2014 at 4:50 AM, Jordan Rose <jordan_rose at apple.com> wrote:

>
> On Jul 29, 2014, at 8:13 , Manuel Klimek <klimek at google.com> wrote:
>
> -cfe-commits, +cfe-dev (sorry, /me unable to handle mailing lists)
>
> On Tue, Jul 29, 2014 at 3:51 PM, Manuel Klimek <klimek at google.com> wrote:
>
>> Hi Jordan,
>>
>> I am working on a fix for lifetime extended temporaries, and I have a
>> hard time understanding some of the code.
>> 1. addLocalScopeForVarDecl already has a FIXME; I have no idea how to
>> apply that though - it seems like we always add the VarDecl to the scope,
>> even if the type we look at is from the lifetime extended temporary; as
>> noted, we need to be able to handle the scope of multiple lifetime extended
>> temporaries; do we need a completely different approach here then?
>> 2. addAutomaticObjDtors seems to work on what addLocalScopeForVarDecl
>> does, so it seems like we'll need to adapt this after solving (1)
>>
>> My main problem is that I don't have enough insight into what else the
>> local scope is used for, so I'm not sure how we'd want to change it to
>> account for lifetime extended temporaries.
>>
>
> After some investigation, it looks to me like we'll want to add some way
> to CFGAutomaticObjDtor to allow not only specifying VarDecls, but somehow
> directly referencing the lifetime extended expressions (?)
>
>
> Hm. That's interesting, because we'd really like to clean up those
> expressions, but if we had to keep them alive I guess we could. For the
> AggregateRef test case in http://reviews.llvm.org/D4696, we'd be able to
> recover all information from the VarDecl: this member came from this
> expression, which was a temporary, so destroy that.
>

But to do that we'd need to dig arbitrarily deep into the objects of the
variable. It seems to me like it'd be much clearer code-wise that the
temporary dtor is still a temporary dtor when it's lifetime extended - it's
just executed at a later point. At least that matches my mental model
better.


> Likewise for the subobject test I just suggested: go up to the base
> region, which was a temporary, so destroy it. *Combining* those seems a
> bit iffy, but not impossible.
>
> I'm not sure we'd need to stick strictly to the existing destructor CFG
> elements either. Let's come up with a model that works, and then figure out
> which CFG elements we need for it. (For example, what I just said above is
> easy enough in the analyzer if you just did all that work at the
> CFGAutomaticObjDtor, but a bit harder to model in the CFG.)
>

I think the model that temporaries are destroyed by CFGTemporaryDtor,
either at the end of the full statment, or, when lifetime extended, at then
end of the lifetime extension, makes a lot of sense. Any concerns why you
think this is the wrong model?

Cheers,
/Manuel
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