[llvm] [WebAssembly] Replace Reachability with SCCs in Irreducible CFG Fixer (PR #179722)

[Heejin Ahn via llvm-commits]

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@@ -120,65 +113,138 @@ class ReachabilityGraph {
     assert(I != LoopEnterers.end());
     return I->second;
   }
+  unsigned getSCCId(MachineBasicBlock *MBB) const {
+    return SccId[getIndex(MBB)];
+  }
 
 private:
   MachineBasicBlock *Entry;
   const BlockSet &Blocks;
+  DenseMap<MachineBasicBlock *, unsigned> BlockIndex;
 
   BlockSet Loopers, LoopEntries;
   DenseMap<MachineBasicBlock *, BlockSet> LoopEnterers;
 
   bool inRegion(MachineBasicBlock *MBB) const { return Blocks.count(MBB); }
 
-  // Maps a block to all the other blocks it can reach.
-  DenseMap<MachineBasicBlock *, BlockSet> Reachable;
+  // Per-node adjacency in the region (excluding edges to Entry).
+  SmallVector<SmallVector<unsigned, 4>, 0> Succs;
+  SmallVector<bool, 0> SelfLoop;
+
+  // SCC = Strongly-connected component
+  // Map of an MBB's index (provided by `BlockIndex`) to it's assigned SCC
+  SmallVector<unsigned, 0> SccId; 
+  // The number of elements in each SCC
+  SmallVector<unsigned, 0> SccSize; 
+
+  unsigned getIndex(MachineBasicBlock *MBB) const {
+    auto It = BlockIndex.find(MBB);
+    assert(It != BlockIndex.end());
+    return It->second;
+  }
 
   void calculate() {
-    // Reachability computation work list. Contains pairs of recent additions
-    // (A, B) where we just added a link A => B.
-    using BlockPair = std::pair<MachineBasicBlock *, MachineBasicBlock *>;
-    SmallVector<BlockPair, 4> WorkList;
+    auto NumBlocks = Blocks.size();
+    Succs.assign(NumBlocks, {});
+    SelfLoop.assign(NumBlocks, false);
+
+    unsigned MMBIdx = 0;
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aheejin wrote:

Was this meant to be `MBBIdx`?

https://github.com/llvm/llvm-project/pull/179722