[LLVMdev] dominator, post-dominator and memory leak

Bin Tzeng
Fri, 15 Nov 2013 04:29:26 -0000

Hi Henrique,

I have tried using -mergereturn and inserting a free into the predecessors
of dominance frontier of malloc block and it caused double free. It is
possible for multiple free's to be inserted on the path from malloc to an
exit. For example, in the following CFG:

                 BB10 (malloc)
                 /       \
        BB11      BB12
...     /        \       /        \        ...
  \    /          \     /           \      /
   BB13     BB14       BB15
                      |      ...
                      |     /
                   BB16

The block BB10 dominates BB11, BB12 and BB14. The dominance frontier of
BB10 contains BB13, BB15 and BB16. So if the predecessors of the dominance
frontier contains BB11, BB12 and BB14. If a call to free is inserted into
BB11, BB12, and BB14, double free would occur. The problem boils down to
this: how can we ensure exactly ONE call to free is inserted at each path
from malloc to all the blocks in dominance frontier?

This problem has some other usage. Frame pointer is needed because there
are variable length objects allocated through alloca. Otherwise, frame
pointer can be omitted as an optimization. If we can ensure that they are
deallocated correctly, frame pointers can be omitted even if variable
length objects exist.

On Wed, Nov 13, 2013 at 5:01 AM, Henrique Santos <
henrique.nazare.santos at gmail.com> wrote:

> It seems that placing the calls to free at the predecessors of dominance
>> frontier is inadequate. It is possible that there are exit blocks that
are
>> dominated by BB12 (calls to malloc). I guess we can also insert calls
to
>> free at these exit blocks too.
>
> That crossed my mind a few minutes later. : )
>
> If you're interested, PRE.cpp existed last at r25315. It calculates the
> "availability frontier" which is probably what you're looking
for.
> I suggest, however, that you try coming up with another solution instead.
> You might consider using -mergereturn.
>
> H.
>
>
> On Wed, Nov 13, 2013 at 2:13 AM, Bin Tzeng <bintzeng at gmail.com>
wrote:
>
>> Hi Henrique,
>> Thanks for the quick reply!
>>
>> On Tue, Nov 12, 2013 at 9:13 PM, Henrique Santos <
>> henrique.nazare.santos at gmail.com> wrote:
>>
>>> PRE normally uses a latest placement algorithm to do something of
the
>>> sort.
>>> I don't know about GVN/PRE, but older version of PRE might have
it.
>>> Just placing the calls to free at the predecessors (dominated by
BB12)
>>> of the dominance frontier of BB12 seems to work, however. Is there
anything
>>> wrong with this?
>>>
>> It seems that placing the calls to free at the predecessors of
dominance
>> frontier is inadequate. It is possible that there are exit blocks that
are
>> dominated by BB12 (calls to malloc). I guess we can also insert calls
to
>> free at these exit blocks too.
>>
>>> H.
>>>
>>>
>>> On Tue, Nov 12, 2013 at 11:30 PM, Bin Tzeng <bintzeng at
gmail.com> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I have been writing a pass to heapify some alloca's (it is
>>>> pessimistization, not optimization). For example, in the
following control
>>>> flow graph, there is a call to malloc inserted in block BB12.
In order to
>>>> avoid memory leak, free's are needed. The free cannot be
inserted in BB23
>>>> because BB23 is not dominated by BB12. There are two ways to go
I can think
>>>> of here. One way is to insert a new basic block, say BB24, to
connect both
>>>> BB21 and BB22 and a free can be inserted into the new block
BB24. The new
>>>> block BB24 has to post-dominate BB12 and all the users of
malloc have to
>>>> happen before BB24. Another way to go is to insert a free in
both BB21 and
>>>> BB22. That is, a free is inserted in all the paths from BB12 to
all exits
>>>> after all users of malloc to avoid memory leak. I wonder
whether there is
>>>> any pass that does similar analysis in order to avoid
duplication of
>>>> efforts.
>>>>
>>>>                BB10 (entry)
>>>>               /       \
>>>>         BB11       BB12 (malloc)
>>>>        /               /      \
>>>>   BB13           /     BB15
>>>>       \             /         /       \
>>>>        \           /     BB18  BB19
>>>>         \         /           \       /
>>>>       BB20  BB21      BB22
>>>>              \      |         /
>>>>               \     |       /
>>>>                \    |     /
>>>>                 \   |   /
>>>>                  BB23 (exit)
>>>>
>>>> Any advice is appreciated. Thanks in advance!
>>>> Bill
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>>
>>>
>>
>
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