[PATCH] Proposal on how to fix temporary dtors.

[Manuel Klimek]

On Mon, Jul 7, 2014 at 10:15 PM, Jordan Rose <jordan_rose at apple.com> wrote:

> Ah...then I'm glad we added the assertion. :-) Lifetime-extended
> temporaries aren't quite implemented correctly yet, but we should probably
> be removing or not even adding the state when the temporary is
> lifetime-extended.
>
> Unfortunately I don't have this paged in, but there's logic in processing
> auto destructors to handle this, and a bug (
> http://llvm.org/bugs/show_bug.cgi?id=19539) about how it's wrong.
>

Ok, I dug into this a bit, and if I'm not missing something I think it's
not possible to switch this off without implementing lifetime-extended
temporaries correctly (or at least a similarly sized implementation effort):
The only link between the MaterializeTempooraryExpr (which has the
information whether the lifetime was extended) and the CXXBindTemporaryExpr
is the underlying object storage. I looked into CodeGen, and if I
understand it correctly, it looks like it basically stores for the
destination whether the destructor was already handled (for example from
the MaterializeTemporary flow). To get that information, I'd guess we need
to do something similar in the static analyzer.

I hope that my analysis is wrong and you tell me a better way to fix this :)

Thanks!
/Manuel


>
> Jordan
>
>
> On Jul 7, 2014, at 13:13 , Manuel Klimek <klimek at google.com> wrote:
>
> Sigh. That triggers in Analysis/dtor.cpp for this CXXBindTemporaryExpr:
> CXXBindTemporaryExpr 0x3c195d8 'class
> LifetimeExtension::SaveOnVirtualDestruct' (CXXTemporary 0x3c195d0)
> `-CXXTemporaryObjectExpr 0x3c19590 'class
> LifetimeExtension::SaveOnVirtualDestruct' 'void (void)'
>
> I assume something fishy is going on for lifetime extended temporaries...
>
>
> On Mon, Jul 7, 2014 at 8:38 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>
>>
>> On Jul 7, 2014, at 11:37 , Manuel Klimek <klimek at google.com> wrote:
>>
>> On Mon, Jul 7, 2014 at 8:05 PM, Jordan Rose <jordan_rose at apple.com>
>> wrote:
>>
>>>
>>> On Jul 7, 2014, at 10:50 , Manuel Klimek <klimek at google.com> wrote:
>>>
>>> On Mon, Jul 7, 2014 at 7:42 PM, Jordan Rose <jordan_rose at apple.com>
>>> wrote:
>>>
>>>>
>>>> On Jul 7, 2014, at 10:41 , Manuel Klimek <klimek at google.com> wrote:
>>>>
>>>> On Mon, Jul 7, 2014 at 7:38 PM, Jordan Rose <jordan_rose at apple.com>
>>>> wrote:
>>>>
>>>>>
>>>>> On Jul 7, 2014, at 10:37 , Manuel Klimek <klimek at google.com> wrote:
>>>>>
>>>>> On Mon, Jul 7, 2014 at 7:29 PM, Jordan Rose <jordan_rose at apple.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>> On Jul 7, 2014, at 10:28 , Manuel Klimek <klimek at google.com> wrote:
>>>>>>
>>>>>> On Mon, Jul 7, 2014 at 6:48 PM, Jordan Rose <jordan_rose at apple.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Can you add an assertion at the end of a block that there are no
>>>>>>> outstanding temporary destructors in the current stack frame? That seems
>>>>>>> useful.
>>>>>>>
>>>>>>
>>>>>> Do you mean at the end of a VisitBlockDecl?
>>>>>>
>>>>>>
>>>>>> No, during the path-sensitive run, so handleBlockExit.
>>>>>>
>>>>>
>>>>> So you mean at the end of a CFG block? But here we might have
>>>>> outstanding temporary dtors open (?)
>>>>>
>>>>>
>>>>> Oops, right. Was thinking too much in terms of AST structure. How
>>>>> about at the end of a function (inlined or not)?
>>>>>
>>>>
>>>> Could we say every time we transition from a block with a temp dtor
>>>> terminator to a block that does not have a temp dtor terminator (or an
>>>> unconditional terminator) we check?
>>>>
>>>>
>>>> That sounds correct, but misses the case where we built the CFG wrong
>>>> (forgetting to add the branch in the correct place and thus never getting
>>>> to the temp dtor block at all).
>>>>
>>>
>>> Makes sense. Do you have a hint where the right place on function exit
>>> to check it would be? :)
>>>
>>>
>>> *checks* ExprEngine::processEndOfFunction.
>>>
>>
>> Hm, so we'll need to adjust the data structure to be indexed by stack
>> frame somehow (use a map) instead of the pair<expr, stack-frame>?
>>
>>
>> Eh, since it's an assertion I'd be fine with just iterating over it in a
>> helper function. Or better, using std::find_if (if it has proper begin/end
>> members).
>>
>
>
>
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