[llvm] r312022 - [DAG] Bound loop dependence check in merge optimization.

[Hans Wennborg via llvm-commits]

Merged to 5.0 in r312041.

On Tue, Aug 29, 2017 at 11:41 AM, Hans Wennborg via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: hans
> Date: Tue Aug 29 11:41:00 2017
> New Revision: 312022
>
> URL: http://llvm.org/viewvc/llvm-project?rev=312022&view=rev
> Log:
> [DAG] Bound loop dependence check in merge optimization.
>
> The loop dependence check looks for dependencies between store merge
> candidates not captured by the chain sub-DAG doing a check of
> predecessors which may be very large. Conservatively bound number of
> nodes checked for compilation time. (Resolves PR34326).
>
> Landing on behalf of Nirav Dave to unblock the 5.0.0 release.
>
> Differential Revision: https://reviews.llvm.org/D37220
>
> Modified:
>     llvm/trunk/include/llvm/CodeGen/SelectionDAGNodes.h
>     llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
> Modified: llvm/trunk/include/llvm/CodeGen/SelectionDAGNodes.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/SelectionDAGNodes.h?rev=312022&r1=312021&r2=312022&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/CodeGen/SelectionDAGNodes.h (original)
> +++ llvm/trunk/include/llvm/CodeGen/SelectionDAGNodes.h Tue Aug 29 11:41:00 2017
> @@ -799,7 +799,8 @@ public:
>    /// if DAG changes.
>    static bool hasPredecessorHelper(const SDNode *N,
>                                     SmallPtrSetImpl<const SDNode *> &Visited,
> -                                   SmallVectorImpl<const SDNode *> &Worklist) {
> +                                   SmallVectorImpl<const SDNode *> &Worklist,
> +                                   unsigned int MaxSteps = 0) {
>      if (Visited.count(N))
>        return true;
>      while (!Worklist.empty()) {
> @@ -814,6 +815,8 @@ public:
>        }
>        if (Found)
>          return true;
> +      if (MaxSteps != 0 && Visited.size() >= MaxSteps)
> +        return false;
>      }
>      return false;
>    }
>
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=312022&r1=312021&r2=312022&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Aug 29 11:41:00 2017
> @@ -12780,25 +12780,37 @@ void DAGCombiner::getStoreMergeCandidate
>          }
>  }
>
> -// We need to check that merging these stores does not cause a loop
> -// in the DAG. Any store candidate may depend on another candidate
> +// We need to check that merging these stores does not cause a loop in
> +// the DAG. Any store candidate may depend on another candidate
>  // indirectly through its operand (we already consider dependencies
>  // through the chain). Check in parallel by searching up from
>  // non-chain operands of candidates.
> +
>  bool DAGCombiner::checkMergeStoreCandidatesForDependencies(
>      SmallVectorImpl<MemOpLink> &StoreNodes, unsigned NumStores) {
> +
> +  // FIXME: We should be able to truncate a full search of
> +  // predecessors by doing a BFS and keeping tabs the originating
> +  // stores from which worklist nodes come from in a similar way to
> +  // TokenFactor simplfication.
> +
>    SmallPtrSet<const SDNode *, 16> Visited;
>    SmallVector<const SDNode *, 8> Worklist;
> -  // search ops of store candidates
> +  unsigned int Max = 8192;
> +  // Search Ops of store candidates.
>    for (unsigned i = 0; i < NumStores; ++i) {
>      SDNode *n = StoreNodes[i].MemNode;
>      // Potential loops may happen only through non-chain operands
>      for (unsigned j = 1; j < n->getNumOperands(); ++j)
>        Worklist.push_back(n->getOperand(j).getNode());
>    }
> -  // search through DAG. We can stop early if we find a storenode
> +  // Search through DAG. We can stop early if we find a store node.
>    for (unsigned i = 0; i < NumStores; ++i) {
> -    if (SDNode::hasPredecessorHelper(StoreNodes[i].MemNode, Visited, Worklist))
> +    if (SDNode::hasPredecessorHelper(StoreNodes[i].MemNode, Visited, Worklist,
> +                                     Max))
> +      return false;
> +    // Check if we ended early, failing conservatively if so.
> +    if (Visited.size() >= Max)
>        return false;
>    }
>    return true;
>
>
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