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

[Jordan Rose]

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. 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.)

Jordan
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