New Solution for memory leak in tblgen
David Blaikie
dblaikie at gmail.com
Mon Nov 17 14:20:45 PST 2014
Is it possible for you to give a small complete example of a handful of
tree nodes showing the full ownership graph?
Some of the things I'm unclear on:
This thing is described as a tree - is it? Trees should be able to be
represented with single ownership (unique_ptr), not even shared or cyclic.
That's not to say a node can't have parent pointers/backedges, but so long
as they just point up into things owned by someone else, that should be
fine.
Who owns the roots of the tree - it looks like it's
CodeGenDAGPatterns::PatternFragments, yes? (and indeed,
/those/ TreePatterns are currently deleted correctly, by the sounds of
things - which goes against the previous idea that these were all leaked
except for some on the stack)
Is everything accessible from the root? Can we detect when they become
disconnected easily and destroy them?
The backedges (which form cycles) are the "PatFragRec" pointers - do these
need to be owning?
You mention/show that two TreePatternNodes can have PredicateFns with
PatFragRecs that refer to some common TreePattern. Is it possible that
these PredicateFns/PatFragRecs should be separately duplicated so they
refer to their own TreePattern? Alternatively, perhaps it's guaranteed that
these two TreePatternNodes will be within the same TreePattern - in which
case the PatFragRec is just a parent (or grandparent) pointer and doesn't
need to be owning.
I guess what I'm eventually saying is if all the
TreePatterns/TreePatternNodes are kept alive via chains of
CodeGenDAGPatterns::PatternFragments -> TreePattern -> TreePattern::Trees
-> (TreePatternNode -> TreePatternNode::Children)*
(ie: we don't ever clone a TreePatternNode, then lose the original
TreePattern that was its PatFragRec and still go looking for it via the
cloned TreePatternNode)
If that is true, then we don't need PatFragRec to own - it's just a raw
pointer. We would use unique_ptrs in CodeGenDAGPatterns::PatternFragments,
TreePattern::Trees, and TreePatternNode::Children - and we'd be done...
So, with that in mind, I've unique_ptr'd the first two
(CodeGenDAGPatterns::PatternFragments and TreePattern::Trees), and the 3rd
I haven't touched yet. There are other places that the elements from Trees
get passed to, so you'll find that my patches (r222183, r221926) did leave
a few 'release()' calls around that could be cleaner/cleaned up, too.
Perhaps, if you want to pursue this, you could try doing some more
incremental patches, like the ones I've committed - I think you could tidy
up the release() calls I introduced in r222183 by pushing unique_ptr
through a few more interfaces.
The last step, of using unique_ptr in TreePatternNode::Children might be a
bit more monolithic - but hopefully it can be broken into fairly
obvious/beneficial steps.
On Tue, Nov 11, 2014 at 6:54 PM, wuhui1973 <wuhui1973 at 163.com> wrote:
>
> At 2014-11-12 10:22:22, "David Blaikie" <dblaikie at gmail.com> wrote:
>
> OK, so you're saying all the heap allocated TreePatterns and
> TreePatternNodes are currently leaked? And we create some on the stack
> (that obviously don't leak)?
>
> >> [huiwu] Yes, almost all. And quite a few TreePatterns are created on
> stack, which will not leak.
>
> And from previous discussions we've seen that TreePatterns and
> TreePatternNodes form unbreakable(?) cycles. (are they unbreakable - could
> we use weak_ptr or raw pointer in there somewhere to break them safely?
> I'll have to go back & look at the prior threads or you could copy/paste
> the summary of the cycles you saw? - though the only reason this would be
> worth understanding is if we actually destroy things at some point other
> than the end - I don't know anything about tablegen or when nodes are
> created and destroyed, I'm not sure if they're destroyed at any time other
> than "we've finished doing all the work, cleanup now")
>
> >> [huiwu]
> >> TreePatternNode has field PredicateFns , which contains PatFragRec,
> the pointer of TreePattern
>
> >> TreePattern has Trees which is std::vector<TreePatternNodePtr>
>
>
> >> The cycles exist among PatFragRec & Trees. May like this:
>
> >> TreePatternNode
> <---------------------------------------------------------------+
> >> PredicateFns (std::vector<TreePredicateFn>)
> |
> >> PatFragRec ------> TreePattern
> |
> >> ^ Trees
> (std::vector<TreePatternNodePtr>) +------------> Other TreePatternNode
> >> |
> >> Other TreePatternNode |
> >> PredicateFns |
> >> PatFragRec -----+
>
> >> In previous solution, every TreePatternNode & TreePattern hold the
> reference count of objects pointed by PatFragRec & Trees. If they find
> the reference count reaches 0,
> >> the associated object will be freed.
> >> At the end of the processing, I found around 700 TreePatternNodes & 100
> TreePatterns (I forget the exact number) remains. Obviously, they form
> cycles which can't be
> >> broken by reference count.
> >> That is why we need FreeCycleRef, in which we can't explicit delete
> those objects, but just assign NULL to PatFragRec & Trees, their owner
> will do the rest for us.
>
> >> But in current design, all heap allocated TreePatternNode &
> TreePattern will be recorded, and there is the mechanism (the operator
> delete) to remove the record if the object is
> >> freed manually somewhere in the source. Then at the end of the process
> (the dtor of CodeGenDAGPatterns), these records are all leaked objects.
> We just go through both
> >> containers to free them.
> >> So as a requirement, TreePatternNode's dtor is not allowed to delete
> pointers within PatFragRec & Children (std::vector<TreePatternNode*>),
> and
> >> TreePattern's dtor is not allowed to delete pointers within Trees. All
> these objects are assumed to be deleted by MemReclaimer::reclaim.
>
>
> If that's all the case, then it seems obvious enough that we should just
> have a big list of all the heap allocated ones, that goes away when we're
> done.
>
> On Tue, Nov 11, 2014 at 6:07 PM, wuhui1973 <wuhui1973 at 163.com> wrote:
>
>> Sorry David:
>>
>> I make a mistake in yesterday explanation.
>>
>> Here is some data I collect during building Tablegen, the data is from
>> x86 target
>>
>> llvm[3]: Building X86.td instruction information with tblgen
>> NumTPNAllocCur: 120754, NumTPNAllocAccu: 121469 <-- it indicates only
>> 121469-120754=715 heap alloc TreePatternNodes get freed during processing
>> (~0.5%)
>> NumTPAllocCur: 9488, NumTPAllocAccu: 9488 <-- in fact *NO*
>> heap alloc TreePatterns get freed during processing
>> NumTPNAllocCur: 0, NumTPNDel: 121469
>> NumTPAllocCur: 0, NumTPDel: 9488
>> TPN dtorNum: 121469, TP dtorNum: 16821 <-- the diff "
>> NumTPAllocAccu"-"TPN dtorNum"=0 indicates that all TreePatternNodes are
>> heap allocated
>> TPN ctorNum: 121469, TP ctorNum: 16821 <-- the diff "TP
>> ctorNum"-"NumTPDel"=7333 indicates that 7333 TreePatterns are placed
>> into stack!
>>
>> llvm[3]: Building X86.td DAG instruction selector implementation with
>> tblgen
>> NumTPNAllocCur: 180382, NumTPNAllocAccu: 197494 <-- 17112
>> TreePatternNodes (~8%) get freed
>> NumTPAllocCur: 9488, NumTPAllocAccu: 9488 <-- *NO* heap
>> alloc TreePatterns get freed during processing
>> NumTPNAllocCur: 0, NumTPNDel: 197494 <-- "NumTPNDel"
>> == "TPN ctorNum": all TreePatternNodes all heap allocated
>> NumTPAllocCur: 0, NumTPDel: 9488
>> TPN dtorNum: 197494, TP dtorNum: 16821 <-- the diff 16821-
>> 9488=7333 indicates that 7333 TreePatterns are placed into stack!
>> TPN ctorNum: 197494, TP ctorNum: 16821
>>
>> So more than 43% TreePatterns are stack residue for both processing.
>>
>> And other target also gives similar data, for instance AMDGPU
>>
>> llvm[3]: Building AMDGPU.td instruction information with tblgen
>> NumTPNAllocCur: 28248, NumTPNAllocAccu: 28348 <-- 100 TreePatternNodes
>> (~0.3%) get freed
>> NumTPAllocCur: 2422, NumTPAllocAccu: 2422 <-- *NO* heap
>> alloc TreePatterns get freed during processing
>> NumTPNAllocCur: 0, NumTPNDel: 28348
>> NumTPAllocCur: 0, NumTPDel: 2422
>> TPN dtorNum: 28348, TP dtorNum: 3792
>> TPN ctorNum: 28348, TP ctorNum: 3792 <-- 1370
>> TreePatterns are inside stack (>36%)
>> llvm[3]: Building AMDGPU.td DAG instruction selector implementation with
>> tblgen
>> NumTPNAllocCur: 44076, NumTPNAllocAccu: 47030 <-- 2954
>> TreePatternNodes (~6%) get freed
>> NumTPAllocCur: 2422, NumTPAllocAccu: 2422 <-- *NO* heap
>> alloc TreePatterns get freed during processing
>> NumTPNAllocCur: 0, NumTPNDel: 47030
>> NumTPAllocCur: 0, NumTPDel: 2422
>> TPN dtorNum: 47030, TP dtorNum: 3792
>> TPN ctorNum: 47030, TP ctorNum: 3792 <-- 1370 TreePatterns
>> are inside stack (>36%)
>>
>> There are several functions in CodeGenDAGPatterns.cpp will allocate
>> TreePatternNodes & TreePatterns from heap.
>> ParsePatternFragments (5 spots allocate TreePatternNodes)
>> ParsePatterns (2 spots allocate TreePatternNodes, 2 spots allocate
>> TreePatterns)
>> CombineChildVariants (1 spot allocate TreePatternNodes)
>> clone (2 spots allocate TreePatternNodes)
>>
>> So I don't think there is any special construction path.
>>
>> Regards
>> Hui Wu
>>
>> 在 2014-11-12 06:33:37，"David Blaikie" <dblaikie at gmail.com> 写道：
>>
>>
>>
>> On Mon, Nov 10, 2014 at 11:36 PM, wuhui1973 <wuhui1973 at 163.com> wrote:
>>
>>> Hope I can explain it clear enough :-)
>>>
>>> > MemReclaimer has two containers to hold pointers of TreePatternNode &
>>> TreePattern which are allocated from heap.
>>> > Only one global instance of MemReclaimer is declared -- memReclaimer.
>>> > operator new & operator delete are defined for TreePatternNode &
>>> TreePattern
>>> new: push the new allocated object (pointer) into
>>> containers in memReclaimer.
>>> delete: remove the pointer from the containers in
>>> memReclaimer.
>>>
>>> So we can guarantee that objects allocated from heap can
>>> always be recorded by memReclaimer.
>>> And no double free may incur, as some places in the source
>>> delete some pointers manually (maybe we can remove these lines).
>>> > So memReclaimer only keeps the objects that are leaked in current
>>> design.
>>> > the author of CodeGenDAGPattern create nodes, but destroy part of
>>> them (for TreePattern, the nodes destroyed manually account for more
>>> than 30%,
>>>
>>
>> Could you provide stack traces/explanation for these case where the
>> TreePatterns are destroyed?
>>
>>
>>> for some target even over 50%; while for TreePatternNode only
>>> around 1%
>>>
>>
>> And for the TreePatternNodes?
>>
>> If they're mostly just on special case construction paths, perhaps we
>> could special case their removal from the list we could add to
>> CodeGenDAGPattern - or, if they're close enough to construction time, we
>> might be able to avoid adding them to such a list in the first place?
>>
>>
>>> ) and make the rest leaked deliberately.
>>> > dtor of CodeGenDAGPattern now is the place to destroy all not freed
>>> objects
>>> > move the functionality of MemReclaimer into CodeGenDAGPattern is
>>> definitely feasible I think.
>>>
>>
>>> Regards
>>> Hui Wu
>>>
>>>
>>> At 2014-11-11 14:20:25, "David Blaikie" <dblaikie at gmail.com> wrote:
>>>
>>> Could you describe the high level design here?
>>>
>>> It looks like there's a static pool
>>> (CodeGenDAGPatterns.cpp::memReclaimer) of instances that's used as some
>>> kind of last-chance cleanup? Instances of TreePatternNode mostly manage
>>> their own lifetime but then if any haven't been destroyed by the time
>>> the CodeGenDAGPatterns dtor runs, the remaining elements are destroyed.
>>> This is to handle cycles, I take it?
>>>
>>> It still seems like a bit of a GC-esque workaround to handle this case
>>> when there might be something better... but I don't know much about
>>> tablegen, perhaps there isn't.
>>>
>>> How often are nodes destroyed by themselves? Should we just give up
>>> owning these anywhere else & move all ownership into CodeGenDAGPatterns and
>>> clean them up in CodeGenDAGPatterns' dtor? (just have a vector of
>>> unique_ptrs (maybe even a list or deque of nodes directly owned, rather
>>> than via unique_ptr), only create nodes, never destroy them, then destroy
>>> them all at the end)
>>>
>>> On Mon, Nov 10, 2014 at 9:58 PM, wuhui1973 <wuhui1973 at 163.com> wrote:
>>>
>>>> Hi Andrew, David and Hal:
>>>>
>>>> I have made a new solution for this memory leak issue, which is much
>>>> simpler than previous one.
>>>>
>>>> I have tested it, it works well!
>>>>
>>>> Please have a look, and appreciate any comment.
>>>>
>>>> Thanks & Regards
>>>>
>>>> Hui Wu
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>>
>
>
>
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