Revert "[Coroutines] Add an O(n) algorithm for computing the cross suspend point"

This reverts commit bb4121e. Sorry for
forgetting adding Differential Revision information. It may worth
reverting this one and commit it again given this is a relative big
patch.

Author: ChuanqiXu9

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -98,57 +98,24 @@ class SuspendCrossingInfo {
     bool Suspend = false;
     bool End = false;
     bool KillLoop = false;
+    bool Changed = false;
   };
   SmallVector<BlockData, SmallVectorThreshold> Block;
 
   iterator_range<pred_iterator> predecessors(BlockData const &BD) const {
     BasicBlock *BB = Mapping.indexToBlock(&BD - &Block[0]);
     return llvm::predecessors(BB);
   }
-  size_t pred_size(BlockData const &BD) const {
-    BasicBlock *BB = Mapping.indexToBlock(&BD - &Block[0]);
-    return llvm::pred_size(BB);
-  }
-  iterator_range<succ_iterator> successors(BlockData const &BD) const {
-    BasicBlock *BB = Mapping.indexToBlock(&BD - &Block[0]);
-    return llvm::successors(BB);
-  }
 
   BlockData &getBlockData(BasicBlock *BB) {
     return Block[Mapping.blockToIndex(BB)];
   }
 
-  /// This algorithm is based on topological sorting. As we know, topological
-  /// sorting is typically used on Directed Acyclic Graph (DAG). However, a
-  /// Control Flow Graph (CFG) may not always be a DAG, as it can contain back
-  /// edges or loops. To handle this, we need to break the back edge when we
-  /// encounter it in order to ensure a valid topological sorting.
-  /// Why do we need an extra traversal when a CFG contains a back edge?
-  /// Firstly, we need to figure out how the Consumes information propagates
-  /// along the back edge. For example,
-  ///
-  ///    A -> B -> C -> D -> H
-  ///         ^         |
-  ///         |         v
-  ///         G <- F <- E
-  ///
-  /// Following the direction of the arrow, we can obtain the traversal
-  /// sequences: A, B, C, D, H, E, F, G or A, B, C, D, E, H, F, G. We know that
-  /// there is a path from C to G after the first traversal. However, we are
-  /// uncertain about the existence of a path from G to C, as the Consumes info
-  /// of G has not yet propagated to C (via B). Therefore, we need a second
-  /// traversal to propagate G's Consumes info to C (via B) and its successors.
-  /// The second traversal allows us to obtain the complete Consumes info. Since
-  /// the computation of the Kills info depends on the Consumes info.
-
-  /// The parameter "EntryNo" represents the index associated with the entry
-  /// block.
-  /// The parameter "BlockPredecessorsNum" represents the number of predecessors
-  /// for each block.
-  /// Returns true if there exists back edges in CFG.
-  template <bool HasBackEdge = false>
-  bool collectConsumeKillInfo(size_t EntryNo,
-                              const SmallVector<size_t> &BlockPredecessorsNum);
+  /// Compute the BlockData for the current function in one iteration.
+  /// Returns whether the BlockData changes in this iteration.
+  /// Initialize - Whether this is the first iteration, we can optimize
+  /// the initial case a little bit by manual loop switch.
+  template <bool Initialize = false> bool computeBlockData();
 
 public:
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
@@ -256,132 +223,84 @@ LLVM_DUMP_METHOD void SuspendCrossingInfo::dump() const {
 }
 #endif
 
-template <bool HasBackEdge>
-bool SuspendCrossingInfo::collectConsumeKillInfo(
-    size_t EntryNo, const SmallVector<size_t> &BlockPredecessorsNum) {
-  bool FoundBackEdge = false;
-  SmallVector<size_t> UnvisitedBlockPredNum = BlockPredecessorsNum;
-  // BlockNo Queue with BlockPredNum[BlockNo] equal to zero.
-  std::queue<size_t> CandidateQueue;
-  // For blocks that maybe has a back edge.
-  DenseSet<size_t> MaybeBackEdgeSet;
-  // Visit BlockNo
-  auto visit = [&](size_t BlockNo) {
-    switch (UnvisitedBlockPredNum[BlockNo]) {
-    // Already visited, not visit again.
-    case 0:
-      break;
-    // If predecessors number of BlockNo is 1, it means all predecessors of
-    // BlockNo have propagated its info to BlockNo. So add BlockNo to
-    // CandidateQueue.
-    case 1: {
-      CandidateQueue.push(BlockNo);
-      MaybeBackEdgeSet.erase(BlockNo);
-      UnvisitedBlockPredNum[BlockNo] = 0;
-      break;
-    }
-    // If predecessors number of BlockNo bigger than 1, it means BlockNo not
-    // collect full Consumes/Kills info yet. So decrease
-    // UnvisitedBlockPredNum[BlockNo] and insert BlockNo into MaybeBackEdgeSet.
-    default: {
-      UnvisitedBlockPredNum[BlockNo]--;
-      MaybeBackEdgeSet.insert(BlockNo);
-      break;
-    }
-    }
-  };
+template <bool Initialize> bool SuspendCrossingInfo::computeBlockData() {
+  const size_t N = Mapping.size();
+  bool Changed = false;
 
-  CandidateQueue.push(EntryNo);
-
-  // Topological sorting.
-  while (!CandidateQueue.empty()) {
-    auto &B = Block[CandidateQueue.front()];
-    CandidateQueue.pop();
-    for (BasicBlock *SI : successors(B)) {
-      auto SuccNo = Mapping.blockToIndex(SI);
-      auto &S = Block[SuccNo];
-
-      // Propagate Kills and Consumes from predecessors into S.
-      S.Consumes |= B.Consumes;
-      S.Kills |= B.Kills;
-
-      if (B.Suspend)
-        S.Kills |= B.Consumes;
-
-      if (S.Suspend) {
-        // If block S is a suspend block, it should kill all of the blocks
-        // it consumes.
-        S.Kills |= S.Consumes;
-      } else if (S.End) {
-        // If block S is an end block, it should not propagate kills as the
-        // blocks following coro.end() are reached during initial invocation
-        // of the coroutine while all the data are still available on the
-        // stack or in the registers.
-        S.Kills.reset();
-      } else {
-        // This is reached when S block it not Suspend nor coro.end and it
-        // need to make sure that it is not in the kill set.
-        S.KillLoop |= S.Kills[SuccNo];
-        S.Kills.reset(SuccNo);
+  for (size_t I = 0; I < N; ++I) {
+    auto &B = Block[I];
+
+    // We don't need to count the predecessors when initialization.
+    if constexpr (!Initialize)
+      // If all the predecessors of the current Block don't change,
+      // the BlockData for the current block must not change too.
+      if (all_of(predecessors(B), [this](BasicBlock *BB) {
+            return !Block[Mapping.blockToIndex(BB)].Changed;
+          })) {
+        B.Changed = false;
+        continue;
       }
-      // visit SuccNo.
-      visit(SuccNo);
-    }
 
-    // If the CandidateQueue is empty but the MaybeBackEdgeSet is not, it
-    // indicates the presence of a back edge that needs to be addressed. In such
-    // cases, it is necessary to break the back edge.
-    if (CandidateQueue.empty() && !MaybeBackEdgeSet.empty()) {
-      FoundBackEdge = true;
-      size_t CandidateNo = -1;
-      if constexpr (HasBackEdge) {
-        auto IsCandidate = [this](size_t I) {
-          for (BasicBlock *PI : llvm::predecessors(Mapping.indexToBlock(I))) {
-            auto PredNo = Mapping.blockToIndex(PI);
-            auto &P = Block[PredNo];
-            // The node I can reach its predecessor. So we found a loop.
-            if (P.Consumes[I])
-              return true;
-          }
+    // Saved Consumes and Kills bitsets so that it is easy to see
+    // if anything changed after propagation.
+    auto SavedConsumes = B.Consumes;
+    auto SavedKills = B.Kills;
 
-          return false;
-        };
+    for (BasicBlock *PI : predecessors(B)) {
+      auto PrevNo = Mapping.blockToIndex(PI);
+      auto &P = Block[PrevNo];
 
-        for (auto I : MaybeBackEdgeSet) {
-          if (IsCandidate(I)) {
-            CandidateNo = I;
-            break;
-          }
-        }
-        assert(CandidateNo != size_t(-1) && "We collected the wrong backegdes");
-      } else
-        // When the value of HasBackEdge is false and we don't have any
-        // information about back edges, we can simply select one block from the
-        // MaybeBackEdgeSet.
-        CandidateNo = *(MaybeBackEdgeSet.begin());
-      CandidateQueue.push(CandidateNo);
-      MaybeBackEdgeSet.erase(CandidateNo);
-      UnvisitedBlockPredNum[CandidateNo] = 0;
+      // Propagate Kills and Consumes from predecessors into B.
+      B.Consumes |= P.Consumes;
+      B.Kills |= P.Kills;
+
+      // If block P is a suspend block, it should propagate kills into block
+      // B for every block P consumes.
+      if (P.Suspend)
+        B.Kills |= P.Consumes;
+    }
+
+    if (B.Suspend) {
+      // If block S is a suspend block, it should kill all of the blocks it
+      // consumes.
+      B.Kills |= B.Consumes;
+    } else if (B.End) {
+      // If block B is an end block, it should not propagate kills as the
+      // blocks following coro.end() are reached during initial invocation
+      // of the coroutine while all the data are still available on the
+      // stack or in the registers.
+      B.Kills.reset();
+    } else {
+      // This is reached when B block it not Suspend nor coro.end and it
+      // need to make sure that it is not in the kill set.
+      B.KillLoop |= B.Kills[I];
+      B.Kills.reset(I);
+    }
+
+    if constexpr (!Initialize) {
+      B.Changed = (B.Kills != SavedKills) || (B.Consumes != SavedConsumes);
+      Changed |= B.Changed;
     }
   }
-  return FoundBackEdge;
+
+  if constexpr (Initialize)
+    return true;
+
+  return Changed;
 }
 
 SuspendCrossingInfo::SuspendCrossingInfo(Function &F, coro::Shape &Shape)
     : Mapping(F) {
   const size_t N = Mapping.size();
   Block.resize(N);
 
-  size_t EntryNo = Mapping.blockToIndex(&(F.getEntryBlock()));
-  SmallVector<size_t> BlockPredecessorsNum(N, 0);
-
   // Initialize every block so that it consumes itself
   for (size_t I = 0; I < N; ++I) {
     auto &B = Block[I];
     B.Consumes.resize(N);
     B.Kills.resize(N);
     B.Consumes.set(I);
-    BlockPredecessorsNum[I] = pred_size(B);
+    B.Changed = true;
   }
 
   // Mark all CoroEnd Blocks. We do not propagate Kills beyond coro.ends as
@@ -406,11 +325,10 @@ SuspendCrossingInfo::SuspendCrossingInfo(Function &F, coro::Shape &Shape)
       markSuspendBlock(Save);
   }
 
-  // We should collect the Consumes and Kills information initially. If there is
-  // a back edge present, it is necessary to perform the collection process
-  // again.
-  if (collectConsumeKillInfo(EntryNo, BlockPredecessorsNum))
-    collectConsumeKillInfo</*HasBackEdge*/ true>(EntryNo, BlockPredecessorsNum);
+  computeBlockData</*Initialize=*/true>();
+
+  while (computeBlockData())
+    ;
 
   LLVM_DEBUG(dump());
 }