[cfe-dev] Inlining temporary destructor calls...

[Manuel Klimek]

On Sat, Jun 21, 2014 at 3:05 AM, Jordan Rose <jordan_rose at apple.com> wrote:

>
> On Jun 20, 2014, at 13:25 , Manuel Klimek <klimek at google.com> wrote:
>
> On Fri, Jun 20, 2014 at 7:17 PM, Jordan Rose <jordan_rose at apple.com>
> wrote:
>
>>
>> On Jun 18, 2014, at 9:08 , Manuel Klimek <klimek at google.com> wrote:
>>
>> > Hi Jordan,
>> >
>> > I've started to look into inlining temporary destructors, but I'm
>> hitting a wall.
>> > Specifically, I think I fail at finding the right MemRegion in
>> ProcessTemporaryDtor (there is a comment in there, that says we need the
>> region for the VisitCXXDestructor call).
>> >
>> > Any help / pointers / brain-dumps on how to do it would be highly
>> appreciated.
>>
>> Hm. I haven't thought about it enough to give you a fool-proof plan, but
>> one possibility could be to store the MemRegion in the state at the point
>> of the CXXBindTemporaryExpr, rather than just a boolean flag. I don't quite
>> know what the ramifications of that would be, though.
>>
>> (By the time we get to the destructor, the value of the
>> CXXBindTemporaryExpr is almost certainly no longer in the Environment.)
>>
>
> Are there other cases in which we store the MemRegion so we make sure it
> doesn't get expunged from the Environment, so I can take a look at similar
> code?
>
>
> It's not a "so it doesn't get purged from the Environment"—it's most
> likely leaving the Environment no matter what. Instead, you just keep a
> reference to the MemRegion. CStringChecker does something a bit like this.
>
> ...but wait, we need the contents of the region to not be cleaned out of
> the state either. So maybe there's a better way to go: change the
> LiveVariables analysis to not consider the CXXBindTemporaryExpr dead until
> the end of the whole full-expression. (And then do, uh, something about
> lifetime extension, which isn't quite handled correctly right now anyway.)
>

How does this currently work for normal variables?
I expect in:
{
  A a;
  ...
}
the MemRegion for 'a' is still available at the end of the block? At least,
if I look into the handing of all other types of destructors in
ExprEngine.cpp, it looks like they just unconditionally conjure up the
MemRegion for the destructed object.

Also, do you have any tips on how to effectively debug those issues? I'm
basically trying to dump the state etc, but for temporaries it's quite hard
to see which part of the state belongs to them (I'll probably need to write
a patch to make that better? :)
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