[llvm-dev] Heap Exhaustion during 'DAGCombiner::Run'

[Nirav Davé via llvm-dev]

Martin:

It sounds like you are doing is more akin to shuffle selection than fusion
and therefore it's a better fit for instruction selection than
DAGCombining. Try movign it to <Target>ISelDAGToDAG's Select (or
potentially PreprocessISelDAG).

Th

-Nirav




On Tue, Mar 6, 2018 at 4:05 PM Martin J. O'Riordan <MartinO at theheart.ie>
wrote:

> We discovered what is happening.
>
>
>
> SDAGCombiner essentially looks at various combinations of nodes to do with
> vectors, and when it can, it creates a vector shuffle.  The problem is,
> that our vector shuffle lowering builds new trees with vector element, or
> vector sub-vector insert sequences.  The generic DAGCombiner, reconstructs
> these into a new shuffle, and so the loop continues - we reduce it, and
> DAGCombiner re-abstracts it.
>
>
>
> Our shuffle lowering produces (produced) very optimal code sequences for
> our target, and has not been changed significantly since LLVM v3.4; but
> changes between v5.0 and v6.0 have introduced this DAG reduction dependency
> loop.
>
>
>
> Is there any advice to Out-of-Tree implementations about how to re-write
> their lowering code for shuffle so as to avoid this kind of infinite
> dependency coupling?
>
>
>
> Thanks,
>
>
>
>     MartinO
>
>
>
> *From:* Nirav Davé [mailto:niravd at google.com]
> *Sent:* 01 March 2018 20:45
> *To:* MartinO at theheart.ie
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Heap Exhaustion during 'DAGCombiner::Run'
>
>
>
> Martin:
>
>
>
> I suspect this is an issue with post-DAG legalization store merging in the
> DAGCombiner. If you have a custom lowered type the DAGCombiner may end up
> merging a set of stores and immediately splitting them up in legalization.
> You should be able to disable this pass universally by overriding
> mergeStoresAfterLegalization() or conditionally for cases that shouldn't
> match with canMergeStoresTo.
>
>
>
> You should able able to verify by finding the loop of nodes considered
> with "-debug" on.
>
>
>
> -Nirav
>
>
>
>
>
>
>
> On Sun, Feb 25, 2018 at 9:36 AM Martin J. O'Riordan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi LLVM-Devs,
>
> I am in the process of updating our out-of-tree implementation from v5.0 to
> v6.0 RC3, and while it builds and mostly runs, I am having trouble with a
> small number of tests where the 'WorklistMap' in 'DAGCombiner::Run' never
> becomes empty.  This is resulting in a runaway state of continuous heap
> allocation until the process exhausts all system memory.
>
> But I can't get a handle on why it is doing this, and it is not obvious to
> me that the changes between v5.0 and v6.0 RC3 invalidate our implementation
> in a way that might cause this.  The only time I see our code entered is
> when lowering is called for vector element insert by 'LegalizeOp'.  Does
> anybody have an advice on how I should approach debugging this?
>
> Thanks,
>
>         MartinO
>
>
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