revng-revng/lib/DataLayoutAnalysis/Middleend/DLADeduplicateUnionFields.cpp

//
// Copyright (c) rev.ng Srls. See LICENSE.md for details.
//

#include <algorithm>
#include <compare>
#include <cstdint>
#include <iterator>

#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Support/Debug.h"

#include "revng/Support/Assert.h"
#include "revng/Support/Debug.h"

#include "revng-c/DataLayoutAnalysis/DLATypeSystem.h"

#include "DLAStep.h"

using LTSN = dla::LayoutTypeSystemNode;
using order = std::strong_ordering;
using Link = dla::LayoutTypeSystemNode::Link;
using EdgeList = std::vector<Link>;
using Tag = dla::TypeLinkTag;
using NonPointerFilterT = EdgeFilteredGraph<LTSN *, dla::isNotPointerEdge>;

using namespace llvm;

static Logger<> Log("dla-deduplicate-union-fields");
static Logger<> CmpLog("dla-duf-comparisons");

namespace dla {

/// Strong ordering for nodes: order by size, then by number of successors
static order cmpNodes(const LTSN *A, const LTSN *B) {
  if (A == B)
    return order::equal;

  if (not A)
    return order::less;

  if (not B)
    return order::greater;

  const auto SizeCmp = A->Size <=> B->Size;
  if (SizeCmp != order::equal) {
    revng_log(CmpLog, "Different sizes");
    return SizeCmp;
  }

  size_t NChild1 = A->Successors.size();
  size_t NChild2 = B->Successors.size();
  const auto NChildCmp = NChild1 <=> NChild2;
  if (NChildCmp != order::equal) {
    revng_log(CmpLog,
              "Different number of successors:  node "
                << A->ID << " has " << NChild1 << " successors, node " << B->ID
                << " has" << NChild2 << " successors");
    return NChildCmp;
  }

  return order::equal;
}

/// Strong ordering for edges: order by kind, then by offset expression
static order cmpEdgeTags(const Tag *A, const Tag *B) {
  if (A == B)
    return order::equal;
  revng_assert(A != nullptr and B != nullptr);

  auto KindA = A->getKind();
  auto KindB = B->getKind();

  // If at least one is a pointer, only look at the kind
  if (KindA == TypeLinkTag::LK_Pointer or KindB == TypeLinkTag::LK_Pointer)
    return KindA <=> KindB;

  OffsetExpression OffA = A->getOffsetExpr();
  OffsetExpression OffB = B->getOffsetExpr();

  const auto KindCmp = KindA <=> KindB;
  if (KindCmp != order::equal)
    return KindCmp;

  const auto OffsetCmp = OffA.Offset <=> OffB.Offset;
  if (OffsetCmp != order::equal)
    return OffsetCmp;

  const auto StrideSizeCmp = OffA.Strides.size() <=> OffB.Strides.size();
  if (StrideSizeCmp != order::equal)
    return StrideSizeCmp;
  for (const auto &[StrA, StrB] : llvm::zip(OffA.Strides, OffB.Strides)) {
    const auto StrideCmp = StrA <=> StrB;
    if (StrideCmp != order::equal)
      return StrideCmp;
  }

  const auto TCSizeCmp = OffA.TripCounts.size() <=> OffB.TripCounts.size();
  if (TCSizeCmp != order::equal)
    return TCSizeCmp;
  for (const auto &[TCA, TCB] : llvm::zip(OffA.TripCounts, OffB.TripCounts)) {
    if (not TCA and not TCB)
      continue;
    if (TCA and not TCB)
      return order::less;
    if (not TCA and TCB)
      return order::greater;

    if (TCA and TCB) {
      const auto TCCmp = *TCA <=> *TCB;
      if (TCCmp != order::equal)
        return TCCmp;
    }
  }

  return order::equal;
}

/// Strong ordering for links: compare edge tags and destination node
static order cmpLinks(const Link &A, const Link &B) {
  const order EdgeOrder = cmpEdgeTags(A.second, B.second);
  if (EdgeOrder != order::equal)
    return EdgeOrder;

  // Pointer edges are equivalent only if they correspond to the same node
  if (isPointerEdge(A)) {
    revng_assert(isPointerEdge(B));
    return A.first->ID <=> B.first->ID;
  }

  const order NodeOrder = cmpNodes(A.first, B.first);
  if (NodeOrder != order::equal)
    return NodeOrder;

  return order::equal;
}

/// Compare two subtrees, saving the visited nodes onto two stacks
static std::tuple<order, EdgeList, EdgeList>
exploreAndCompare(const Link &Child1, const Link &Child2);

/// Recursively define an ordering between children of a node
static bool linkOrderLess(const Link &A, const Link &B) {
  if (A == B)
    return false;

  const order LinkOrder = cmpLinks(A, B);
  if (LinkOrder != order::equal)
    return LinkOrder < 0;

  revng_log(CmpLog,
            "No order between " << A.first->ID << " and " << B.first->ID
                                << ", must recur");
  // In case the two nodes are equivalent, explore the whole subtree
  // TODO: cache the result of this comparison?
  const auto [SubtreeOrder, _, __] = exploreAndCompare(A, B);
  revng_assert(SubtreeOrder != order::equal);
  return SubtreeOrder == order::less;
}

static std::tuple<order, EdgeList, EdgeList>
exploreAndCompare(const Link &Child1, const Link &Child2) {
  if (Child1.first->ID == Child2.first->ID)
    return { order::equal, { Child1 }, { Child2 } };

  EdgeList VisitStack1{ Child1 }, VisitStack2{ Child2 };
  EdgeList NextToVisit1, NextToVisit2;
  size_t CurIdx = 0;
  do {
    // Append the newly found nodes to the visit stack of each subtree
    size_t NextSize = NextToVisit1.size();
    revng_assert(NextSize == NextToVisit2.size());

    if (NextSize > 0) {
      size_t PrevSize = VisitStack1.size();
      VisitStack1.reserve(PrevSize + NextSize);
      VisitStack2.reserve(PrevSize + NextSize);
      VisitStack1.insert(VisitStack1.end(),
                         std::make_move_iterator(NextToVisit1.begin()),
                         std::make_move_iterator(NextToVisit1.end()));
      VisitStack2.insert(VisitStack2.end(),
                         std::make_move_iterator(NextToVisit2.begin()),
                         std::make_move_iterator(NextToVisit2.end()));
      NextToVisit1.clear();
      NextToVisit2.clear();
    }

    // Perform bfs on new nodes
    for (; CurIdx < VisitStack1.size(); CurIdx++) {
      const Link &L1 = VisitStack1[CurIdx];
      const Link &L2 = VisitStack2[CurIdx];
      const auto &[Node1, Edge1] = L1;
      const auto &[Node2, Edge2] = L2;
      revng_log(CmpLog, "Comparing " << Node1->ID << " with " << Node2->ID);

      if (Node1->ID == Node2->ID)
        continue;

      // Return if the links are different
      const order LinkOrder = cmpLinks(L1, L2);
      if (LinkOrder != order::equal)
        return { LinkOrder, VisitStack1, VisitStack2 };

      revng_log(CmpLog, "Could not tell the difference");

      if (not isPointerEdge(L1)) {
        // Enqueue the successors of the current nodes
        revng_assert(not isPointerEdge(L2));
        NextToVisit1.reserve(NextToVisit1.size() + Node1->Successors.size());
        NextToVisit2.reserve(NextToVisit2.size() + Node2->Successors.size());
        llvm::copy(Node1->Successors, std::back_inserter(NextToVisit1));
        llvm::copy(Node2->Successors, std::back_inserter(NextToVisit2));

        // Sort the newly enqueued nodes
        size_t NChildren = Node1->Successors.size();
        revng_assert(NChildren == Node2->Successors.size());

        std::sort(NextToVisit1.end() - NChildren,
                  NextToVisit1.end(),
                  linkOrderLess);
        std::sort(NextToVisit2.end() - NChildren,
                  NextToVisit2.end(),
                  linkOrderLess);
      }
    }
  } while (NextToVisit1.size() > 0);

  return { order::equal, VisitStack1, VisitStack2 };
}

/// Check if two subtrees are equivalent, saving the visited nodes in the
/// order in which they were compared.
static std::tuple<bool, EdgeList, EdgeList>
areEquivSubtrees(const Link &Child1, const Link &Child2) {
  auto [Result, Visited1, Visited2] = exploreAndCompare(Child1, Child2);
  bool AreSubtreesEqual = Result == order::equal;

  return { AreSubtreesEqual, Visited1, Visited2 };
}

/// Visit the two subtrees of \a Child1 and \a Child2. If they are
/// equivalent, merge each node with the one it has been compared to.
///
///\return true if the two nodes were merged, and merged subtree
///\param TS the graph in which the comparison should be performed
///\param Child1 the root of the first subtree
///\param Child2 the root of the second subtree, will be collapsed if
///           equivalent to the subtree of \a Child1
static std::tuple<bool, EdgeList, std::set<LTSN *>>
mergeIfTopologicallyEq(LayoutTypeSystem &TS,
                       const Link &Child1,
                       const Link &Child2) {
  if (Child1.first == Child2.first) {
    revng_log(CmpLog, "Same Node!");
    return { false, {}, {} };
  }

  auto [AreEquiv, Subtree1, Subtree2] = areEquivSubtrees(Child1, Child2);
  if (not AreEquiv) {
    revng_log(CmpLog, "Different!");
    return { false, {}, {} };
  }

  revng_log(CmpLog, "Equivalent!");

  // Create a map between nodes to merge and the corresponding merge
  // destination, in order to:
  // 1. avoid duplicates in merging list
  // 2. check that a node is never merged into two separate nodes
  // 3. handle the case in which the merge destination has to be merged itself
  std::map</*to merge*/ LTSN *, /*to keep*/ LTSN *> MergeMap;
  for (const auto &[Link1, Link2] : llvm::zip(Subtree1, Subtree2)) {
    auto *NodeToKeep = Link1.first;
    auto *NodeToMerge = Link2.first;

    const auto &[_, Inserted] = MergeMap.insert({ NodeToMerge, NodeToKeep });
    revng_assert(Inserted or MergeMap.at(NodeToMerge) == NodeToKeep);
  }

  // Redirect chains of nodes that have to be merged together
  llvm::SmallPtrSet<LTSN *, 8> Subtree1MergedNodes;
  for (auto &[NodeToMerge, NodeToKeep] : MergeMap) {
    if (NodeToKeep == NodeToMerge or Subtree1MergedNodes.contains(NodeToMerge))
      continue;

    auto MapEntry = MergeMap.find(NodeToKeep);
    llvm::SmallPtrSet<LTSN *, 8> MergeChain;

    // Find chains of nodes to merge
    while (MapEntry != MergeMap.end()) {
      Subtree1MergedNodes.insert(MapEntry->first);
      const auto &[_, Inserted] = MergeChain.insert(NodeToKeep);
      // Avoid loops
      if (not Inserted)
        break;

      NodeToKeep = MapEntry->second;

      // Go to next node of the chain
      MapEntry = MergeMap.find(NodeToKeep);
    }

    // Update the merge destination of all the nodes of the chain
    for (auto *N : MergeChain)
      MergeMap.at(N) = NodeToKeep;
  }

  // Execute merge
  std::set<LTSN *> ErasedNodes;
  for (auto &[NodeToMerge, NodeToKeep] : MergeMap) {
    if (NodeToKeep == NodeToMerge)
      continue;

    // TODO: light merge
    TS.mergeNodes({ NodeToKeep, NodeToMerge });
    ErasedNodes.insert(NodeToMerge);
  }

  // Remove merged nodes from subtree1
  if (Subtree1MergedNodes.size() > 0) {
    for (auto It = Subtree1.begin(); It != Subtree1.end();) {
      if (Subtree1MergedNodes.contains(It->first))
        It = Subtree1.erase(It);
      else
        ++It;
    }
  }

  return { true, Subtree1, ErasedNodes };
}

/// Remove conflicting edges and collapse single children after merging.
static bool
postProcessMerge(LayoutTypeSystem &TS, const EdgeList &MergedSubtree) {
  bool Modified = false;

  // Merging nodes together might have created conflicting edges, i.e.
  // instance-offset-0 edges that connect two nodes with an already
  // existing inheritance edges: remove them.
  for (auto &E : MergedSubtree) {
    // Materialize predecessors to avoid iterator invalidation
    llvm::SmallVector<LTSN *, 8> PredNodes;
    for (auto &PredLink : E.first->Predecessors)
      PredNodes.push_back(PredLink.first);
  }

  // Merging nodes and removing conflicts might have created situations in
  // which a node has a single collapsible child: collapse it into its parent.
  LTSN *SubtreeRoot = MergedSubtree.begin()->first;
  for (auto &N : post_order(NonPointerFilterT(SubtreeRoot)))
    Modified |= CollapseSingleChild::collapseSingle(TS, N);

  return Modified;
}

static auto getSuccEdgesToChild(LTSN *Parent, LTSN *Child) {
  auto &Succ = Parent->Successors;
  return llvm::iterator_range(Succ.lower_bound({ Child, nullptr }),
                              Succ.upper_bound({ std::next(Child), nullptr }));
}

bool DeduplicateUnionFields::runOnTypeSystem(LayoutTypeSystem &TS) {
  bool TypeSystemChanged = false;
  if (VerifyLog.isEnabled())
    revng_assert(TS.verifyDAG());

  if (Log.isEnabled())
    TS.dumpDotOnFile("before-deduplicate-union-fields.dot");

  llvm::SmallPtrSet<LTSN *, 16> VisitedUnions;

  for (LTSN *Root : llvm::nodes(&TS)) {
    revng_assert(Root != nullptr);
    if (not isRoot(Root))
      continue;

    for (LTSN *Node : post_order(NonPointerFilterT(Root)))
      TypeSystemChanged |= CollapseSingleChild::collapseSingle(TS, Node);

    llvm::SmallVector<LTSN *, 8> PostOrderFromRoot;
    for (LTSN *UnionNode : post_order(NonPointerFilterT(Root))) {
      if (UnionNode->InterferingInfo != AllChildrenAreInterfering
          or VisitedUnions.contains(UnionNode))
        continue;

      revng_log(Log, "****** Union Node found: " << UnionNode->ID);
      VisitedUnions.insert(UnionNode);
      PostOrderFromRoot.push_back(UnionNode);
    }

    // Visit all Union nodes in post-order. The post-order needs to be cached
    // because children can be merged during traversal, which would invalidate
    // iterators in llvm::post_order if we use it vanilla.
    for (LTSN *UnionNode : PostOrderFromRoot) {
      revng_log(Log,
                "****** Try to dedup children of UnionNode with ID: "
                  << UnionNode->ID);

      // Since a node can be connected to the parent union by more than one
      // edge, we keep track of the **nodes** that we have to visit and the
      // **edges** we visited. In this way, when comparing subtrees, we consider
      // all the edges incoming from the parent node, so that, if we merge
      // two nodes, we don't have to update other links in the worklist.
      llvm::SmallSetVector<LTSN *, 8> UnionChildrenToCompare;
      llvm::SmallSet<LTSN *, 8> OriginalUnionChildren;
      llvm::SmallSet<LTSN *, 8> AnalyzedNodesNotMerged;

      // We keep a separate list of successors since we might need to re-enqueue
      // some of them.
      for (const Link &L : UnionNode->Successors) {
        UnionChildrenToCompare.insert(L.first);
        OriginalUnionChildren.insert(L.first);
      }

      bool UnionNodeChanged = false;
      while (UnionChildrenToCompare.size() > 0) {
        LTSN *CurChild = UnionChildrenToCompare.pop_back_val();

        // The CurChild can be connected to UnionNode with more than one edge
        // (inheritance and instance at offset 0), so consider them all when
        // comparing CurChild with the AnalyzedNotMerged.
        // TODO: turn this into an iterator range
        bool UnionChildrenMerged = false;
        auto CurChildEdges = getSuccEdgesToChild(UnionNode, CurChild);
        for (auto &CurLink : CurChildEdges) {
          revng_assert(isInstanceEdge(CurLink));

          // We want to compare CurChild with all the other nodes that we have
          // looked at in previous iterations, and try to merge it with one of
          // them.
          for (LTSN *NotMergedNode : AnalyzedNodesNotMerged) {
            auto MergedEdges = getSuccEdgesToChild(UnionNode, NotMergedNode);

            bool AnalyzedNotMergedInvalidated = false;
            for (const Link &NotMergedLink : MergedEdges) {
              revng_assert(isInstanceEdge(NotMergedLink));

              auto [IsMerged,
                    Preserved,
                    Erased] = mergeIfTopologicallyEq(TS,
                                                     NotMergedLink,
                                                     CurLink);
              if (not IsMerged)
                continue;

              // If we merged something, there should be at least one preserved
              // node and one erase one
              revng_assert(not Preserved.empty());
              revng_assert(not Erased.empty());

              TypeSystemChanged = true;
              UnionNodeChanged = true;
              revng_log(Log, "Merged!");

              // The following call coul remove stuff from Preserved and add it
              // to Erased.
              // BUT:
              //  - postProcessMerge only calls
              //    - CollapseSingleChild::collapseSingle only removes nodes
              //    with if these two are safe we're good
              //  - CollapseSingleChild::collapseSingle only removes nodes with
              //    exactly one parent, so it cannot remove nodes that were not
              //    originally children of the union, because if they were they
              //    would have had more than one incoming edge so they wouldn't
              //    be removed.
              postProcessMerge(TS, Preserved);

              for (auto &ErasedNode : Erased) {
                // The ErasedNode has been deleted while merging, so we never
                // want it to be processed again.
                bool Erased = UnionChildrenToCompare.remove(ErasedNode);
                UnionChildrenMerged |= Erased;
                Erased = AnalyzedNodesNotMerged.erase(ErasedNode);
                AnalyzedNotMergedInvalidated |= Erased;
              }

              UnionChildrenMerged |= Erased.contains(CurChild);

              // This should always be true, since whenever we merge we are at
              // least erasing CurChild, merging it with NotMergedNode.
              revng_assert(UnionChildrenMerged);

              for (auto &[PreservedNode, _] : Preserved) {
                // The PreservedNode is preserved (not erased) by merge, but
                // the merge process might have changed it.
                // So, if it'a an original children of the UnionNode, we need
                // to re-process it, hence we add it to UnionChildreToCompare,
                // and remove it from AnalyzedNodesNotMerged.
                if (OriginalUnionChildren.count(PreservedNode)) {
                  UnionChildrenToCompare.insert(PreservedNode);
                  bool Erased = AnalyzedNodesNotMerged.erase(PreservedNode);
                  AnalyzedNotMergedInvalidated |= Erased;
                }
              }

              // This should always be true, since whenever we merge we are at
              // least preserving the NotMergedNode, which might have been
              // changed by the merge.
              revng_assert(AnalyzedNotMergedInvalidated);

              // We have merged the NotMergedNode into CurChild, we have to
              // brake out of all the loops looking at CurChild and at
              // AnalyzedNodesNotMerged, since both of these might have
              // changed.
              // looking at the next node in UnionChildrenToCompare.
              break;
            }

            if (AnalyzedNotMergedInvalidated) {
              // If we just merged CurChild into NotMergedNode we have
              // invalidated the AnalyzedNodesNotMerged iterators.
              // So we have to exit this loop and re-start iterating on
              // UnionChildrenToCompare.
              break;
            }
          }

          // If the children of the UnionNode have been changed by the merge,
          // the CurChildEdges iterator ranges have been invalidated. So we
          // have to break out of this loop as well.
          if (UnionChildrenMerged)
            break;
        }

        // If we haven't merged CurChild with anything we can mark it as
        // analyzed and not merged.
        if (not UnionChildrenMerged) {
          AnalyzedNodesNotMerged.insert(CurChild);
          revng_log(Log, "Child " << CurChild->ID << " not merged");
        }
      }

      // Collapse the union node if we are left with only one member
      if (UnionNodeChanged)
        TypeSystemChanged |= CollapseSingleChild::collapseSingle(TS, UnionNode);
    }
  }

  if (Log.isEnabled())
    TS.dumpDotOnFile("after-deduplicate-union-fields.dot");

  if (VerifyLog.isEnabled())
    revng_assert(TS.verifyDAG());

  return TypeSystemChanged;
}

} // end namespace dla