lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp - llvm-project/llvm - Git at Google

 //=- WebAssemblyFixIrreducibleControlFlow.cpp - Fix irreducible control flow -//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file implements a pass that removes irreducible control flow.
 /// Irreducible control flow means multiple-entry loops, which this pass
 /// transforms to have a single entry.
 ///
 /// Note that LLVM has a generic pass that lowers irreducible control flow, but
 /// it linearizes control flow, turning diamonds into two triangles, which is
 /// both unnecessary and undesirable for WebAssembly.
 ///
 /// The big picture: We recursively process each "region", defined as a group
 /// of blocks with a single entry and no branches back to that entry. A region
 /// may be the entire function body, or the inner part of a loop, i.e., the
 /// loop's body without branches back to the loop entry. In each region we fix
 /// up multi-entry loops by adding a new block that can dispatch to each of the
 /// loop entries, based on the value of a label "helper" variable, and we
 /// replace direct branches to the entries with assignments to the label
 /// variable and a branch to the dispatch block. Then the dispatch block is the
 /// single entry in the loop containing the previous multiple entries. After
 /// ensuring all the loops in a region are reducible, we recurse into them. The
 /// total time complexity of this pass is:
 ///
 ///   O(NumBlocks * NumNestedLoops * NumIrreducibleLoops +
 ///     NumLoops * NumLoops)
 ///
 /// This pass is similar to what the Relooper [1] does. Both identify looping
 /// code that requires multiple entries, and resolve it in a similar way (in
 /// Relooper terminology, we implement a Multiple shape in a Loop shape). Note
 /// also that like the Relooper, we implement a "minimal" intervention: we only
 /// use the "label" helper for the blocks we absolutely must and no others. We
 /// also prioritize code size and do not duplicate code in order to resolve
 /// irreducibility. The graph algorithms for finding loops and entries and so
 /// forth are also similar to the Relooper. The main differences between this
 /// pass and the Relooper are:
 ///
 ///  * We just care about irreducibility, so we just look at loops.
 ///  * The Relooper emits structured control flow (with ifs etc.), while we
 ///    emit a CFG.
 ///
 /// [1] Alon Zakai. 2011. Emscripten: an LLVM-to-JavaScript compiler. In
 /// Proceedings of the ACM international conference companion on Object oriented
 /// programming systems languages and applications companion (SPLASH '11). ACM,
 /// New York, NY, USA, 301-312. DOI=10.1145/2048147.2048224
 /// http://doi.acm.org/10.1145/2048147.2048224
 ///
 //===----------------------------------------------------------------------===//

 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssembly.h"
 #include "WebAssemblySubtarget.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/Support/Debug.h"
 using namespace llvm;

 #define DEBUG_TYPE "wasm-fix-irreducible-control-flow"

 namespace {

 using BlockVector = SmallVector<MachineBasicBlock *, 4>;
 using BlockSet = SmallPtrSet<MachineBasicBlock *, 4>;

 static BlockVector getSortedEntries(const BlockSet &Entries) {
   BlockVector SortedEntries(Entries.begin(), Entries.end());
   llvm::sort(SortedEntries,
              [](const MachineBasicBlock *A, const MachineBasicBlock *B) {
                auto ANum = A->getNumber();
                auto BNum = B->getNumber();
                return ANum < BNum;
              });
   return SortedEntries;
 }

 // Calculates reachability in a region. Ignores branches to blocks outside of
 // the region, and ignores branches to the region entry (for the case where
 // the region is the inner part of a loop).
 class ReachabilityGraph {
 public:
   ReachabilityGraph(MachineBasicBlock *Entry, const BlockSet &Blocks)
       : Entry(Entry), Blocks(Blocks) {
 #ifndef NDEBUG
     // The region must have a single entry.
     for (auto *MBB : Blocks) {
       if (MBB != Entry) {
         for (auto *Pred : MBB->predecessors()) {
           assert(inRegion(Pred));
         }
       }
     }
 #endif
     calculate();
   }

   bool canReach(MachineBasicBlock *From, MachineBasicBlock *To) const {
     assert(inRegion(From) && inRegion(To));
     auto I = Reachable.find(From);
     if (I == Reachable.end())
       return false;
     return I->second.count(To);
   }

   // "Loopers" are blocks that are in a loop. We detect these by finding blocks
   // that can reach themselves.
   const BlockSet &getLoopers() const { return Loopers; }

   // Get all blocks that are loop entries.
   const BlockSet &getLoopEntries() const { return LoopEntries; }

   // Get all blocks that enter a particular loop from outside.
   const BlockSet &getLoopEnterers(MachineBasicBlock *LoopEntry) const {
     assert(inRegion(LoopEntry));
     auto I = LoopEnterers.find(LoopEntry);
     assert(I != LoopEnterers.end());
     return I->second;
   }

 private:
   MachineBasicBlock *Entry;
   const BlockSet &Blocks;

   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;

   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;

     // Add all relevant direct branches.
     for (auto *MBB : Blocks) {
       for (auto *Succ : MBB->successors()) {
         if (Succ != Entry && inRegion(Succ)) {
           Reachable[MBB].insert(Succ);
           WorkList.emplace_back(MBB, Succ);
         }
       }
     }

     while (!WorkList.empty()) {
       MachineBasicBlock *MBB, *Succ;
       std::tie(MBB, Succ) = WorkList.pop_back_val();
       assert(inRegion(MBB) && Succ != Entry && inRegion(Succ));
       if (MBB != Entry) {
         // We recently added MBB => Succ, and that means we may have enabled
         // Pred => MBB => Succ.
         for (auto *Pred : MBB->predecessors()) {
           if (Reachable[Pred].insert(Succ).second) {
             WorkList.emplace_back(Pred, Succ);
           }
         }
       }
     }

     // Blocks that can return to themselves are in a loop.
     for (auto *MBB : Blocks) {
       if (canReach(MBB, MBB)) {
         Loopers.insert(MBB);
       }
     }
     assert(!Loopers.count(Entry));

     // Find the loop entries - loopers reachable from blocks not in that loop -
     // and those outside blocks that reach them, the "loop enterers".
     for (auto *Looper : Loopers) {
       for (auto *Pred : Looper->predecessors()) {
         // Pred can reach Looper. If Looper can reach Pred, it is in the loop;
         // otherwise, it is a block that enters into the loop.
         if (!canReach(Looper, Pred)) {
           LoopEntries.insert(Looper);
           LoopEnterers[Looper].insert(Pred);
         }
       }
     }
   }
 };

 // Finds the blocks in a single-entry loop, given the loop entry and the
 // list of blocks that enter the loop.
 class LoopBlocks {
 public:
   LoopBlocks(MachineBasicBlock *Entry, const BlockSet &Enterers)
       : Entry(Entry), Enterers(Enterers) {
     calculate();
   }

   BlockSet &getBlocks() { return Blocks; }

 private:
   MachineBasicBlock *Entry;
   const BlockSet &Enterers;

   BlockSet Blocks;

   void calculate() {
     // Going backwards from the loop entry, if we ignore the blocks entering
     // from outside, we will traverse all the blocks in the loop.
     BlockVector WorkList;
     BlockSet AddedToWorkList;
     Blocks.insert(Entry);
     for (auto *Pred : Entry->predecessors()) {
       if (!Enterers.count(Pred)) {
         WorkList.push_back(Pred);
         AddedToWorkList.insert(Pred);
       }
     }

     while (!WorkList.empty()) {
       auto *MBB = WorkList.pop_back_val();
       assert(!Enterers.count(MBB));
       if (Blocks.insert(MBB).second) {
         for (auto *Pred : MBB->predecessors()) {
           if (!AddedToWorkList.count(Pred)) {
             WorkList.push_back(Pred);
             AddedToWorkList.insert(Pred);
           }
         }
       }
     }
   }
 };

 class WebAssemblyFixIrreducibleControlFlow final : public MachineFunctionPass {
   StringRef getPassName() const override {
     return "WebAssembly Fix Irreducible Control Flow";
   }

   bool runOnMachineFunction(MachineFunction &MF) override;

   bool processRegion(MachineBasicBlock *Entry, BlockSet &Blocks,
                      MachineFunction &MF);

   void makeSingleEntryLoop(BlockSet &Entries, BlockSet &Blocks,
                            MachineFunction &MF, const ReachabilityGraph &Graph);

 public:
   static char ID; // Pass identification, replacement for typeid
   WebAssemblyFixIrreducibleControlFlow() : MachineFunctionPass(ID) {}
 };

 bool WebAssemblyFixIrreducibleControlFlow::processRegion(
     MachineBasicBlock *Entry, BlockSet &Blocks, MachineFunction &MF) {
   bool Changed = false;
   // Remove irreducibility before processing child loops, which may take
   // multiple iterations.
   while (true) {
     ReachabilityGraph Graph(Entry, Blocks);

     bool FoundIrreducibility = false;

     for (auto *LoopEntry : getSortedEntries(Graph.getLoopEntries())) {
       // Find mutual entries - all entries which can reach this one, and
       // are reached by it (that always includes LoopEntry itself). All mutual
       // entries must be in the same loop, so if we have more than one, then we
       // have irreducible control flow.
       //
       // (Note that we need to sort the entries here, as otherwise the order can
       // matter: being mutual is a symmetric relationship, and each set of
       // mutuals will be handled properly no matter which we see first. However,
       // there can be multiple disjoint sets of mutuals, and which we process
       // first changes the output.)
       //
       // Note that irreducibility may involve inner loops, e.g. imagine A
       // starts one loop, and it has B inside it which starts an inner loop.
       // If we add a branch from all the way on the outside to B, then in a
       // sense B is no longer an "inner" loop, semantically speaking. We will
       // fix that irreducibility by adding a block that dispatches to either
       // either A or B, so B will no longer be an inner loop in our output.
       // (A fancier approach might try to keep it as such.)
       //
       // Note that we still need to recurse into inner loops later, to handle
       // the case where the irreducibility is entirely nested - we would not
       // be able to identify that at this point, since the enclosing loop is
       // a group of blocks all of whom can reach each other. (We'll see the
       // irreducibility after removing branches to the top of that enclosing
       // loop.)
       BlockSet MutualLoopEntries;
       MutualLoopEntries.insert(LoopEntry);
       for (auto *OtherLoopEntry : Graph.getLoopEntries()) {
         if (OtherLoopEntry != LoopEntry &&
             Graph.canReach(LoopEntry, OtherLoopEntry) &&
             Graph.canReach(OtherLoopEntry, LoopEntry)) {
           MutualLoopEntries.insert(OtherLoopEntry);
         }
       }

       if (MutualLoopEntries.size() > 1) {
         makeSingleEntryLoop(MutualLoopEntries, Blocks, MF, Graph);
         FoundIrreducibility = true;
         Changed = true;
         break;
       }
     }
     // Only go on to actually process the inner loops when we are done
     // removing irreducible control flow and changing the graph. Modifying
     // the graph as we go is possible, and that might let us avoid looking at
     // the already-fixed loops again if we are careful, but all that is
     // complex and bug-prone. Since irreducible loops are rare, just starting
     // another iteration is best.
     if (FoundIrreducibility) {
       continue;
     }

     for (auto *LoopEntry : Graph.getLoopEntries()) {
       LoopBlocks InnerBlocks(LoopEntry, Graph.getLoopEnterers(LoopEntry));
       // Each of these calls to processRegion may change the graph, but are
       // guaranteed not to interfere with each other. The only changes we make
       // to the graph are to add blocks on the way to a loop entry. As the
       // loops are disjoint, that means we may only alter branches that exit
       // another loop, which are ignored when recursing into that other loop
       // anyhow.
       if (processRegion(LoopEntry, InnerBlocks.getBlocks(), MF)) {
         Changed = true;
       }
     }

     return Changed;
   }
 }

 // Given a set of entries to a single loop, create a single entry for that
 // loop by creating a dispatch block for them, routing control flow using
 // a helper variable. Also updates Blocks with any new blocks created, so
 // that we properly track all the blocks in the region. But this does not update
 // ReachabilityGraph; this will be updated in the caller of this function as
 // needed.
 void WebAssemblyFixIrreducibleControlFlow::makeSingleEntryLoop(
     BlockSet &Entries, BlockSet &Blocks, MachineFunction &MF,
     const ReachabilityGraph &Graph) {
   assert(Entries.size() >= 2);

   // Sort the entries to ensure a deterministic build.
   BlockVector SortedEntries = getSortedEntries(Entries);

 #ifndef NDEBUG
   for (auto Block : SortedEntries)
     assert(Block->getNumber() != -1);
   if (SortedEntries.size() > 1) {
     for (auto I = SortedEntries.begin(), E = SortedEntries.end() - 1; I != E;
          ++I) {
       auto ANum = (*I)->getNumber();
       auto BNum = (*(std::next(I)))->getNumber();
       assert(ANum != BNum);
     }
   }
 #endif

   // Create a dispatch block which will contain a jump table to the entries.
   MachineBasicBlock *Dispatch = MF.CreateMachineBasicBlock();
   MF.insert(MF.end(), Dispatch);
   Blocks.insert(Dispatch);

   // Add the jump table.
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   MachineInstrBuilder MIB =
       BuildMI(Dispatch, DebugLoc(), TII.get(WebAssembly::BR_TABLE_I32));

   // Add the register which will be used to tell the jump table which block to
   // jump to.
   MachineRegisterInfo &MRI = MF.getRegInfo();
   Register Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
   MIB.addReg(Reg);

   // Compute the indices in the superheader, one for each bad block, and
   // add them as successors.
   DenseMap<MachineBasicBlock *, unsigned> Indices;
   for (auto *Entry : SortedEntries) {
     auto Pair = Indices.insert(std::make_pair(Entry, 0));
     assert(Pair.second);

     unsigned Index = MIB.getInstr()->getNumExplicitOperands() - 1;
     Pair.first->second = Index;

     MIB.addMBB(Entry);
     Dispatch->addSuccessor(Entry);
   }

   // Rewrite the problematic successors for every block that wants to reach
   // the bad blocks. For simplicity, we just introduce a new block for every
   // edge we need to rewrite. (Fancier things are possible.)

   BlockVector AllPreds;
   for (auto *Entry : SortedEntries) {
     for (auto *Pred : Entry->predecessors()) {
       if (Pred != Dispatch) {
         AllPreds.push_back(Pred);
       }
     }
   }

   // This set stores predecessors within this loop.
   DenseSet<MachineBasicBlock *> InLoop;
   for (auto *Pred : AllPreds) {
     for (auto *Entry : Pred->successors()) {
       if (!Entries.count(Entry))
         continue;
       if (Graph.canReach(Entry, Pred)) {
         InLoop.insert(Pred);
         break;
       }
     }
   }

   // Record if each entry has a layout predecessor. This map stores
   // <<loop entry, Predecessor is within the loop?>, layout predecessor>
   DenseMap<PointerIntPair<MachineBasicBlock *, 1, bool>, MachineBasicBlock *>
       EntryToLayoutPred;
   for (auto *Pred : AllPreds) {
     bool PredInLoop = InLoop.count(Pred);
     for (auto *Entry : Pred->successors())
       if (Entries.count(Entry) && Pred->isLayoutSuccessor(Entry))
         EntryToLayoutPred[{Entry, PredInLoop}] = Pred;
   }

   // We need to create at most two routing blocks per entry: one for
   // predecessors outside the loop and one for predecessors inside the loop.
   // This map stores
   // <<loop entry, Predecessor is within the loop?>, routing block>
   DenseMap<PointerIntPair<MachineBasicBlock *, 1, bool>, MachineBasicBlock *>
       Map;
   for (auto *Pred : AllPreds) {
     bool PredInLoop = InLoop.count(Pred);
     for (auto *Entry : Pred->successors()) {
       if (!Entries.count(Entry) || Map.count({Entry, PredInLoop}))
         continue;
       // If there exists a layout predecessor of this entry and this predecessor
       // is not that, we rather create a routing block after that layout
       // predecessor to save a branch.
       if (auto *OtherPred = EntryToLayoutPred.lookup({Entry, PredInLoop}))
         if (OtherPred != Pred)
           continue;

       // This is a successor we need to rewrite.
       MachineBasicBlock *Routing = MF.CreateMachineBasicBlock();
       MF.insert(Pred->isLayoutSuccessor(Entry)
                     ? MachineFunction::iterator(Entry)
                     : MF.end(),
                 Routing);
       Blocks.insert(Routing);

       // Set the jump table's register of the index of the block we wish to
       // jump to, and jump to the jump table.
       BuildMI(Routing, DebugLoc(), TII.get(WebAssembly::CONST_I32), Reg)
           .addImm(Indices[Entry]);
       BuildMI(Routing, DebugLoc(), TII.get(WebAssembly::BR)).addMBB(Dispatch);
       Routing->addSuccessor(Dispatch);
       Map[{Entry, PredInLoop}] = Routing;
     }
   }

   for (auto *Pred : AllPreds) {
     bool PredInLoop = InLoop.count(Pred);
     // Remap the terminator operands and the successor list.
     for (MachineInstr &Term : Pred->terminators())
       for (auto &Op : Term.explicit_uses())
         if (Op.isMBB() && Indices.count(Op.getMBB()))
           Op.setMBB(Map[{Op.getMBB(), PredInLoop}]);

     for (auto *Succ : Pred->successors()) {
       if (!Entries.count(Succ))
         continue;
       auto *Routing = Map[{Succ, PredInLoop}];
       Pred->replaceSuccessor(Succ, Routing);
     }
   }

   // Create a fake default label, because br_table requires one.
   MIB.addMBB(MIB.getInstr()
                  ->getOperand(MIB.getInstr()->getNumExplicitOperands() - 1)
                  .getMBB());
 }

 } // end anonymous namespace

 char WebAssemblyFixIrreducibleControlFlow::ID = 0;
 INITIALIZE_PASS(WebAssemblyFixIrreducibleControlFlow, DEBUG_TYPE,
                 "Removes irreducible control flow", false, false)

 FunctionPass *llvm::createWebAssemblyFixIrreducibleControlFlow() {
   return new WebAssemblyFixIrreducibleControlFlow();
 }

 bool WebAssemblyFixIrreducibleControlFlow::runOnMachineFunction(
     MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** Fixing Irreducible Control Flow **********\n"
                        "********** Function: "
                     << MF.getName() << '\n');

   // Start the recursive process on the entire function body.
   BlockSet AllBlocks;
   for (auto &MBB : MF) {
     AllBlocks.insert(&MBB);
   }

   if (LLVM_UNLIKELY(processRegion(&*MF.begin(), AllBlocks, MF))) {
     // We rewrote part of the function; recompute relevant things.
     MF.getRegInfo().invalidateLiveness();
     MF.RenumberBlocks();
     return true;
   }

   return false;
 }
	//=- WebAssemblyFixIrreducibleControlFlow.cpp - Fix irreducible control flow -//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//
	///
	/// \file
	/// This file implements a pass that removes irreducible control flow.
	/// Irreducible control flow means multiple-entry loops, which this pass
	/// transforms to have a single entry.
	///
	/// Note that LLVM has a generic pass that lowers irreducible control flow, but
	/// it linearizes control flow, turning diamonds into two triangles, which is
	/// both unnecessary and undesirable for WebAssembly.
	///
	/// The big picture: We recursively process each "region", defined as a group
	/// of blocks with a single entry and no branches back to that entry. A region
	/// may be the entire function body, or the inner part of a loop, i.e., the
	/// loop's body without branches back to the loop entry. In each region we fix
	/// up multi-entry loops by adding a new block that can dispatch to each of the
	/// loop entries, based on the value of a label "helper" variable, and we
	/// replace direct branches to the entries with assignments to the label
	/// variable and a branch to the dispatch block. Then the dispatch block is the
	/// single entry in the loop containing the previous multiple entries. After
	/// ensuring all the loops in a region are reducible, we recurse into them. The
	/// total time complexity of this pass is:
	///
	/// O(NumBlocks * NumNestedLoops * NumIrreducibleLoops +
	/// NumLoops * NumLoops)
	///
	/// This pass is similar to what the Relooper [1] does. Both identify looping
	/// code that requires multiple entries, and resolve it in a similar way (in
	/// Relooper terminology, we implement a Multiple shape in a Loop shape). Note
	/// also that like the Relooper, we implement a "minimal" intervention: we only
	/// use the "label" helper for the blocks we absolutely must and no others. We
	/// also prioritize code size and do not duplicate code in order to resolve
	/// irreducibility. The graph algorithms for finding loops and entries and so
	/// forth are also similar to the Relooper. The main differences between this
	/// pass and the Relooper are:
	///
	/// * We just care about irreducibility, so we just look at loops.
	/// * The Relooper emits structured control flow (with ifs etc.), while we
	/// emit a CFG.
	///
	/// [1] Alon Zakai. 2011. Emscripten: an LLVM-to-JavaScript compiler. In
	/// Proceedings of the ACM international conference companion on Object oriented
	/// programming systems languages and applications companion (SPLASH '11). ACM,
	/// New York, NY, USA, 301-312. DOI=10.1145/2048147.2048224
	/// http://doi.acm.org/10.1145/2048147.2048224
	///
	//===----------------------------------------------------------------------===//

	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssembly.h"
	#include "WebAssemblySubtarget.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/Support/Debug.h"
	using namespace llvm;

	#define DEBUG_TYPE "wasm-fix-irreducible-control-flow"

	namespace {

	using BlockVector = SmallVector<MachineBasicBlock *, 4>;
	using BlockSet = SmallPtrSet<MachineBasicBlock *, 4>;

	static BlockVector getSortedEntries(const BlockSet &Entries) {
	BlockVector SortedEntries(Entries.begin(), Entries.end());
	llvm::sort(SortedEntries,
	[](const MachineBasicBlock A, const MachineBasicBlock B) {
	auto ANum = A->getNumber();
	auto BNum = B->getNumber();
	return ANum < BNum;
	});
	return SortedEntries;
	}

	// Calculates reachability in a region. Ignores branches to blocks outside of
	// the region, and ignores branches to the region entry (for the case where
	// the region is the inner part of a loop).
	class ReachabilityGraph {
	public:
	ReachabilityGraph(MachineBasicBlock *Entry, const BlockSet &Blocks)
	: Entry(Entry), Blocks(Blocks) {
	#ifndef NDEBUG
	// The region must have a single entry.
	for (auto *MBB : Blocks) {
	if (MBB != Entry) {
	for (auto *Pred : MBB->predecessors()) {
	assert(inRegion(Pred));
	}
	}
	}
	#endif
	calculate();
	}

	bool canReach(MachineBasicBlock From, MachineBasicBlock To) const {
	assert(inRegion(From) && inRegion(To));
	auto I = Reachable.find(From);
	if (I == Reachable.end())
	return false;
	return I->second.count(To);
	}

	// "Loopers" are blocks that are in a loop. We detect these by finding blocks
	// that can reach themselves.
	const BlockSet &getLoopers() const { return Loopers; }

	// Get all blocks that are loop entries.
	const BlockSet &getLoopEntries() const { return LoopEntries; }

	// Get all blocks that enter a particular loop from outside.
	const BlockSet &getLoopEnterers(MachineBasicBlock *LoopEntry) const {
	assert(inRegion(LoopEntry));
	auto I = LoopEnterers.find(LoopEntry);
	assert(I != LoopEnterers.end());
	return I->second;
	}

	private:
	MachineBasicBlock *Entry;
	const BlockSet &Blocks;

	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;

	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;

	// Add all relevant direct branches.
	for (auto *MBB : Blocks) {
	for (auto *Succ : MBB->successors()) {
	if (Succ != Entry && inRegion(Succ)) {
	Reachable[MBB].insert(Succ);
	WorkList.emplace_back(MBB, Succ);
	}
	}
	}

	while (!WorkList.empty()) {
	MachineBasicBlock MBB, Succ;
	std::tie(MBB, Succ) = WorkList.pop_back_val();
	assert(inRegion(MBB) && Succ != Entry && inRegion(Succ));
	if (MBB != Entry) {
	// We recently added MBB => Succ, and that means we may have enabled
	// Pred => MBB => Succ.
	for (auto *Pred : MBB->predecessors()) {
	if (Reachable[Pred].insert(Succ).second) {
	WorkList.emplace_back(Pred, Succ);
	}
	}
	}
	}

	// Blocks that can return to themselves are in a loop.
	for (auto *MBB : Blocks) {
	if (canReach(MBB, MBB)) {
	Loopers.insert(MBB);
	}
	}
	assert(!Loopers.count(Entry));

	// Find the loop entries - loopers reachable from blocks not in that loop -
	// and those outside blocks that reach them, the "loop enterers".
	for (auto *Looper : Loopers) {
	for (auto *Pred : Looper->predecessors()) {
	// Pred can reach Looper. If Looper can reach Pred, it is in the loop;
	// otherwise, it is a block that enters into the loop.
	if (!canReach(Looper, Pred)) {
	LoopEntries.insert(Looper);
	LoopEnterers[Looper].insert(Pred);
	}
	}
	}
	}
	};

	// Finds the blocks in a single-entry loop, given the loop entry and the
	// list of blocks that enter the loop.
	class LoopBlocks {
	public:
	LoopBlocks(MachineBasicBlock *Entry, const BlockSet &Enterers)
	: Entry(Entry), Enterers(Enterers) {
	calculate();
	}

	BlockSet &getBlocks() { return Blocks; }

	private:
	MachineBasicBlock *Entry;
	const BlockSet &Enterers;

	BlockSet Blocks;

	void calculate() {
	// Going backwards from the loop entry, if we ignore the blocks entering
	// from outside, we will traverse all the blocks in the loop.
	BlockVector WorkList;
	BlockSet AddedToWorkList;
	Blocks.insert(Entry);
	for (auto *Pred : Entry->predecessors()) {
	if (!Enterers.count(Pred)) {
	WorkList.push_back(Pred);
	AddedToWorkList.insert(Pred);
	}
	}

	while (!WorkList.empty()) {
	auto *MBB = WorkList.pop_back_val();
	assert(!Enterers.count(MBB));
	if (Blocks.insert(MBB).second) {
	for (auto *Pred : MBB->predecessors()) {
	if (!AddedToWorkList.count(Pred)) {
	WorkList.push_back(Pred);
	AddedToWorkList.insert(Pred);
	}
	}
	}
	}
	}
	};

	class WebAssemblyFixIrreducibleControlFlow final : public MachineFunctionPass {
	StringRef getPassName() const override {
	return "WebAssembly Fix Irreducible Control Flow";
	}

	bool runOnMachineFunction(MachineFunction &MF) override;

	bool processRegion(MachineBasicBlock *Entry, BlockSet &Blocks,
	MachineFunction &MF);

	void makeSingleEntryLoop(BlockSet &Entries, BlockSet &Blocks,
	MachineFunction &MF, const ReachabilityGraph &Graph);

	public:
	static char ID; // Pass identification, replacement for typeid
	WebAssemblyFixIrreducibleControlFlow() : MachineFunctionPass(ID) {}
	};

	bool WebAssemblyFixIrreducibleControlFlow::processRegion(
	MachineBasicBlock *Entry, BlockSet &Blocks, MachineFunction &MF) {
	bool Changed = false;
	// Remove irreducibility before processing child loops, which may take
	// multiple iterations.
	while (true) {
	ReachabilityGraph Graph(Entry, Blocks);

	bool FoundIrreducibility = false;

	for (auto *LoopEntry : getSortedEntries(Graph.getLoopEntries())) {
	// Find mutual entries - all entries which can reach this one, and
	// are reached by it (that always includes LoopEntry itself). All mutual
	// entries must be in the same loop, so if we have more than one, then we
	// have irreducible control flow.
	//
	// (Note that we need to sort the entries here, as otherwise the order can
	// matter: being mutual is a symmetric relationship, and each set of
	// mutuals will be handled properly no matter which we see first. However,
	// there can be multiple disjoint sets of mutuals, and which we process
	// first changes the output.)
	//
	// Note that irreducibility may involve inner loops, e.g. imagine A
	// starts one loop, and it has B inside it which starts an inner loop.
	// If we add a branch from all the way on the outside to B, then in a
	// sense B is no longer an "inner" loop, semantically speaking. We will
	// fix that irreducibility by adding a block that dispatches to either
	// either A or B, so B will no longer be an inner loop in our output.
	// (A fancier approach might try to keep it as such.)
	//
	// Note that we still need to recurse into inner loops later, to handle
	// the case where the irreducibility is entirely nested - we would not
	// be able to identify that at this point, since the enclosing loop is
	// a group of blocks all of whom can reach each other. (We'll see the
	// irreducibility after removing branches to the top of that enclosing
	// loop.)
	BlockSet MutualLoopEntries;
	MutualLoopEntries.insert(LoopEntry);
	for (auto *OtherLoopEntry : Graph.getLoopEntries()) {
	if (OtherLoopEntry != LoopEntry &&
	Graph.canReach(LoopEntry, OtherLoopEntry) &&
	Graph.canReach(OtherLoopEntry, LoopEntry)) {
	MutualLoopEntries.insert(OtherLoopEntry);
	}
	}

	if (MutualLoopEntries.size() > 1) {
	makeSingleEntryLoop(MutualLoopEntries, Blocks, MF, Graph);
	FoundIrreducibility = true;
	Changed = true;
	break;
	}
	}
	// Only go on to actually process the inner loops when we are done
	// removing irreducible control flow and changing the graph. Modifying
	// the graph as we go is possible, and that might let us avoid looking at
	// the already-fixed loops again if we are careful, but all that is
	// complex and bug-prone. Since irreducible loops are rare, just starting
	// another iteration is best.
	if (FoundIrreducibility) {
	continue;
	}

	for (auto *LoopEntry : Graph.getLoopEntries()) {
	LoopBlocks InnerBlocks(LoopEntry, Graph.getLoopEnterers(LoopEntry));
	// Each of these calls to processRegion may change the graph, but are
	// guaranteed not to interfere with each other. The only changes we make
	// to the graph are to add blocks on the way to a loop entry. As the
	// loops are disjoint, that means we may only alter branches that exit
	// another loop, which are ignored when recursing into that other loop
	// anyhow.
	if (processRegion(LoopEntry, InnerBlocks.getBlocks(), MF)) {
	Changed = true;
	}
	}

	return Changed;
	}
	}

	// Given a set of entries to a single loop, create a single entry for that
	// loop by creating a dispatch block for them, routing control flow using
	// a helper variable. Also updates Blocks with any new blocks created, so
	// that we properly track all the blocks in the region. But this does not update
	// ReachabilityGraph; this will be updated in the caller of this function as
	// needed.
	void WebAssemblyFixIrreducibleControlFlow::makeSingleEntryLoop(
	BlockSet &Entries, BlockSet &Blocks, MachineFunction &MF,
	const ReachabilityGraph &Graph) {
	assert(Entries.size() >= 2);

	// Sort the entries to ensure a deterministic build.
	BlockVector SortedEntries = getSortedEntries(Entries);

	#ifndef NDEBUG
	for (auto Block : SortedEntries)
	assert(Block->getNumber() != -1);
	if (SortedEntries.size() > 1) {
	for (auto I = SortedEntries.begin(), E = SortedEntries.end() - 1; I != E;
	++I) {
	auto ANum = (*I)->getNumber();
	auto BNum = (*(std::next(I)))->getNumber();
	assert(ANum != BNum);
	}
	}
	#endif

	// Create a dispatch block which will contain a jump table to the entries.
	MachineBasicBlock *Dispatch = MF.CreateMachineBasicBlock();
	MF.insert(MF.end(), Dispatch);
	Blocks.insert(Dispatch);

	// Add the jump table.
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	MachineInstrBuilder MIB =
	BuildMI(Dispatch, DebugLoc(), TII.get(WebAssembly::BR_TABLE_I32));

	// Add the register which will be used to tell the jump table which block to
	// jump to.
	MachineRegisterInfo &MRI = MF.getRegInfo();
	Register Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
	MIB.addReg(Reg);

	// Compute the indices in the superheader, one for each bad block, and
	// add them as successors.
	DenseMap<MachineBasicBlock *, unsigned> Indices;
	for (auto *Entry : SortedEntries) {
	auto Pair = Indices.insert(std::make_pair(Entry, 0));
	assert(Pair.second);

	unsigned Index = MIB.getInstr()->getNumExplicitOperands() - 1;
	Pair.first->second = Index;

	MIB.addMBB(Entry);
	Dispatch->addSuccessor(Entry);
	}

	// Rewrite the problematic successors for every block that wants to reach
	// the bad blocks. For simplicity, we just introduce a new block for every
	// edge we need to rewrite. (Fancier things are possible.)

	BlockVector AllPreds;
	for (auto *Entry : SortedEntries) {
	for (auto *Pred : Entry->predecessors()) {
	if (Pred != Dispatch) {
	AllPreds.push_back(Pred);
	}
	}
	}

	// This set stores predecessors within this loop.
	DenseSet<MachineBasicBlock *> InLoop;
	for (auto *Pred : AllPreds) {
	for (auto *Entry : Pred->successors()) {
	if (!Entries.count(Entry))
	continue;
	if (Graph.canReach(Entry, Pred)) {
	InLoop.insert(Pred);
	break;
	}
	}
	}

	// Record if each entry has a layout predecessor. This map stores
	// <<loop entry, Predecessor is within the loop?>, layout predecessor>
	DenseMap<PointerIntPair<MachineBasicBlock , 1, bool>, MachineBasicBlock >
	EntryToLayoutPred;
	for (auto *Pred : AllPreds) {
	bool PredInLoop = InLoop.count(Pred);
	for (auto *Entry : Pred->successors())
	if (Entries.count(Entry) && Pred->isLayoutSuccessor(Entry))
	EntryToLayoutPred[{Entry, PredInLoop}] = Pred;
	}

	// We need to create at most two routing blocks per entry: one for
	// predecessors outside the loop and one for predecessors inside the loop.
	// This map stores
	// <<loop entry, Predecessor is within the loop?>, routing block>
	DenseMap<PointerIntPair<MachineBasicBlock , 1, bool>, MachineBasicBlock >
	Map;
	for (auto *Pred : AllPreds) {
	bool PredInLoop = InLoop.count(Pred);
	for (auto *Entry : Pred->successors()) {
	if (!Entries.count(Entry) \|\| Map.count({Entry, PredInLoop}))
	continue;
	// If there exists a layout predecessor of this entry and this predecessor
	// is not that, we rather create a routing block after that layout
	// predecessor to save a branch.
	if (auto *OtherPred = EntryToLayoutPred.lookup({Entry, PredInLoop}))
	if (OtherPred != Pred)
	continue;

	// This is a successor we need to rewrite.
	MachineBasicBlock *Routing = MF.CreateMachineBasicBlock();
	MF.insert(Pred->isLayoutSuccessor(Entry)
	? MachineFunction::iterator(Entry)
	: MF.end(),
	Routing);
	Blocks.insert(Routing);

	// Set the jump table's register of the index of the block we wish to
	// jump to, and jump to the jump table.
	BuildMI(Routing, DebugLoc(), TII.get(WebAssembly::CONST_I32), Reg)
	.addImm(Indices[Entry]);
	BuildMI(Routing, DebugLoc(), TII.get(WebAssembly::BR)).addMBB(Dispatch);
	Routing->addSuccessor(Dispatch);
	Map[{Entry, PredInLoop}] = Routing;
	}
	}

	for (auto *Pred : AllPreds) {
	bool PredInLoop = InLoop.count(Pred);
	// Remap the terminator operands and the successor list.
	for (MachineInstr &Term : Pred->terminators())
	for (auto &Op : Term.explicit_uses())
	if (Op.isMBB() && Indices.count(Op.getMBB()))
	Op.setMBB(Map[{Op.getMBB(), PredInLoop}]);

	for (auto *Succ : Pred->successors()) {
	if (!Entries.count(Succ))
	continue;
	auto *Routing = Map[{Succ, PredInLoop}];
	Pred->replaceSuccessor(Succ, Routing);
	}
	}

	// Create a fake default label, because br_table requires one.
	MIB.addMBB(MIB.getInstr()
	->getOperand(MIB.getInstr()->getNumExplicitOperands() - 1)
	.getMBB());
	}

	} // end anonymous namespace

	char WebAssemblyFixIrreducibleControlFlow::ID = 0;
	INITIALIZE_PASS(WebAssemblyFixIrreducibleControlFlow, DEBUG_TYPE,
	"Removes irreducible control flow", false, false)

	FunctionPass *llvm::createWebAssemblyFixIrreducibleControlFlow() {
	return new WebAssemblyFixIrreducibleControlFlow();
	}

	bool WebAssemblyFixIrreducibleControlFlow::runOnMachineFunction(
	MachineFunction &MF) {
	LLVM_DEBUG(dbgs() << "******** Fixing Irreducible Control Flow ********\n"
	"********** Function: "
	<< MF.getName() << '\n');

	// Start the recursive process on the entire function body.
	BlockSet AllBlocks;
	for (auto &MBB : MF) {
	AllBlocks.insert(&MBB);
	}

	if (LLVM_UNLIKELY(processRegion(&*MF.begin(), AllBlocks, MF))) {
	// We rewrote part of the function; recompute relevant things.
	MF.getRegInfo().invalidateLiveness();
	MF.RenumberBlocks();
	return true;
	}

	return false;
	}