[cfe-commits] r118991 - in /cfe/trunk: lib/Analysis/CFG.cpp test/Analysis/temp-obj-dtors-cfg-output.cpp

[Marcin Świderski]

W dniu 14 listopada 2010 03:15 użytkownik Zhongxing Xu <
xuzhongxing at gmail.com> napisał:

>
>
> 2010/11/14 Marcin Świderski <marcin.sfider at gmail.com>
>
>> W dniu 14 listopada 2010 00:48 użytkownik Zhongxing Xu <
>> xuzhongxing at gmail.com> napisał:
>>
>>
>>>
>>> 2010/11/14 Marcin Świderski <marcin.sfider at gmail.com>
>>>
>>> W dniu 13 listopada 2010 12:37 użytkownik Zhongxing Xu <
>>>> xuzhongxing at gmail.com> napisał:
>>>>
>>>> I see some unbalancing before this patch. (See the test case that was
>>>>> modified.) Could you provide an example that is unbalanced by this patch?
>>>>>
>>>>> 2010/11/13 Marcin Świderski <marcin.sfider at gmail.com>
>>>>>
>>>>> Hi Zhongxing
>>>>>>
>>>>>> Won't it lead to unbalancing ctors/dtors like I've written in
>>>>>> discusion after commit r118159?
>>>>>>
>>>>>>
>>>>>> When I wrote about unbalancing I meant that removing elidable
>>>> CXXConstructExpr and then removing destructors for temporaries that where
>>>> created with those expressions will unbalance ctors/dtors calls. Because not
>>>> every call to the ctor is a block level statement this is not visualised in
>>>> the CFG.
>>>>
>>>> Example of this is simple:
>>>>
>>>> class A {
>>>> public:
>>>>   A() {}
>>>>   ~A() {}
>>>> };
>>>>
>>>> A foo() { return A(); }
>>>>
>>>>  AST for function foo() looks like this:
>>>>
>>>> A foo() (CompoundStmt 0x105048b98 <main.cpp:7:9, col:23>
>>>>   (ReturnStmt 0x105048b70 <col:11, col:20>
>>>>     (CXXExprWithTemporaries 0x105048b38 <col:18, col:20> 'class A'
>>>>       (CXXTemporary 0x105048a80)
>>>>       (CXXConstructExpr 0x105048af0 <col:18, col:20> 'class A''void
>>>> (const class A &) throw()' elidable
>>>>         (ImplicitCastExpr 0x105048ad0 <col:18, col:20> 'const class A'
>>>> <NoOp>
>>>>           (CXXBindTemporaryExpr 0x105048a88 <col:18, col:20> 'class A'
>>>> (CXXTemporary 0x105048a80)
>>>>             (CXXTemporaryObjectExpr 0x105048a38 <col:18, col:20> 'class
>>>> A''void (void)')))))))
>>>>
>>>> If we want to ensure that the CFG is constructed from AST with all
>>>> elidable constructors elided, for above example we would have to ignore
>>>> both CXXConstructExpr and CXXBindTemporaryExpr. In your implementation only
>>>> the first will be elided.
>>>>
>>>
>>> Hi Marcin,
>>>
>>> I still do not understand your point. The current implementation works
>>> well for the above example. One block-level expr for CXXTemporaryObjectExpr.
>>> One temporary dtor for CXXBindTemporaryExpr.
>>>
>>
>> If CXXConstructExpr that constructs the return value will be elided,
>> object created with CXXTemporaryObjectExpr
>> will be the return value and should not be destroyed. So eliding copy
>> constructor should also prevent destroying
>> temporary from which it would copy, not the other way around.
>>
>
> I see your point. This could be prevented by other means, for example, in
> CodeGen there is a LifetimeFlag in AggValueSlot, which controls whether the
> temporary would be cleaned up.
>
> But this reminds me that probably we should not include dtors for
> temporaries in the CFG, since not all temporaries require destruction.
>
> Another option is to create a dtor for every CXXBindTemporaryExpr and let
> the client decide whether to ignore it. What do you think?
>
> Which temporaries, other than those elided, don't require destruction?

I think that we should leave destructors for temporaries as an option. Also
I think we shouldn't optimize elidable constructors and destructors they
impact in the CFG, because this will force clients to also account for those
optimizations. This will complicate things without any gain for analysis
outcome.
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