lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp - platform/external/llvm - Git at Google

 //===-- WebAssemblyCFGStackify.cpp - CFG Stackification -------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// \brief This file implements a CFG stacking pass.
 ///
 /// This pass reorders the blocks in a function to put them into a reverse
 /// post-order [0], with special care to keep the order as similar as possible
 /// to the original order, and to keep loops contiguous even in the case of
 /// split backedges.
 ///
 /// Then, it inserts BLOCK and LOOP markers to mark the start of scopes, since
 /// scope boundaries serve as the labels for WebAssembly's control transfers.
 ///
 /// This is sufficient to convert arbitrary CFGs into a form that works on
 /// WebAssembly, provided that all loops are single-entry.
 ///
 /// [0] https://en.wikipedia.org/wiki/Depth-first_search#Vertex_orderings
 ///
 //===----------------------------------------------------------------------===//

 #include "WebAssembly.h"
 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssemblySubtarget.h"
 #include "llvm/ADT/SCCIterator.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

 #define DEBUG_TYPE "wasm-cfg-stackify"

 namespace {
 class WebAssemblyCFGStackify final : public MachineFunctionPass {
   const char *getPassName() const override {
     return "WebAssembly CFG Stackify";
   }

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<MachineDominatorTree>();
     AU.addPreserved<MachineDominatorTree>();
     AU.addRequired<MachineLoopInfo>();
     AU.addPreserved<MachineLoopInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }

   bool runOnMachineFunction(MachineFunction &MF) override;

 public:
   static char ID; // Pass identification, replacement for typeid
   WebAssemblyCFGStackify() : MachineFunctionPass(ID) {}
 };
 } // end anonymous namespace

 char WebAssemblyCFGStackify::ID = 0;
 FunctionPass *llvm::createWebAssemblyCFGStackify() {
   return new WebAssemblyCFGStackify();
 }

 static void EliminateMultipleEntryLoops(MachineFunction &MF,
                                         const MachineLoopInfo &MLI) {
   SmallPtrSet<MachineBasicBlock *, 8> InSet;
   for (scc_iterator<MachineFunction *> I = scc_begin(&MF), E = scc_end(&MF);
        I != E; ++I) {
     const std::vector<MachineBasicBlock *> &CurrentSCC = *I;

     // Skip trivial SCCs.
     if (CurrentSCC.size() == 1)
       continue;

     InSet.insert(CurrentSCC.begin(), CurrentSCC.end());
     MachineBasicBlock *Header = nullptr;
     for (MachineBasicBlock *MBB : CurrentSCC) {
       for (MachineBasicBlock *Pred : MBB->predecessors()) {
         if (InSet.count(Pred))
           continue;
         if (!Header) {
           Header = MBB;
           break;
         }
         // TODO: Implement multiple-entry loops.
         report_fatal_error("multiple-entry loops are not supported yet");
       }
     }
     assert(MLI.isLoopHeader(Header));

     InSet.clear();
   }
 }

 namespace {
 /// Post-order traversal stack entry.
 struct POStackEntry {
   MachineBasicBlock *MBB;
   SmallVector<MachineBasicBlock *, 0> Succs;

   POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
                const MachineLoopInfo &MLI);
 };
 } // end anonymous namespace

 static bool LoopContains(const MachineLoop *Loop,
                          const MachineBasicBlock *MBB) {
   return Loop ? Loop->contains(MBB) : true;
 }

 POStackEntry::POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
                            const MachineLoopInfo &MLI)
     : MBB(MBB), Succs(MBB->successors()) {
   // RPO is not a unique form, since at every basic block with multiple
   // successors, the DFS has to pick which order to visit the successors in.
   // Sort them strategically (see below).
   MachineLoop *Loop = MLI.getLoopFor(MBB);
   MachineFunction::iterator Next = next(MachineFunction::iterator(MBB));
   MachineBasicBlock *LayoutSucc = Next == MF.end() ? nullptr : &*Next;
   std::stable_sort(
       Succs.begin(), Succs.end(),
       [=, &MLI](const MachineBasicBlock *A, const MachineBasicBlock *B) {
         if (A == B)
           return false;

         // Keep loops contiguous by preferring the block that's in the same
         // loop.
         bool LoopContainsA = LoopContains(Loop, A);
         bool LoopContainsB = LoopContains(Loop, B);
         if (LoopContainsA && !LoopContainsB)
           return true;
         if (!LoopContainsA && LoopContainsB)
           return false;

         // Minimize perturbation by preferring the block which is the immediate
         // layout successor.
         if (A == LayoutSucc)
           return true;
         if (B == LayoutSucc)
           return false;

         // TODO: More sophisticated orderings may be profitable here.

         return false;
       });
 }

 /// Return the "bottom" block of a loop. This differs from
 /// MachineLoop::getBottomBlock in that it works even if the loop is
 /// discontiguous.
 static MachineBasicBlock *LoopBottom(const MachineLoop *Loop) {
   MachineBasicBlock *Bottom = Loop->getHeader();
   for (MachineBasicBlock *MBB : Loop->blocks())
     if (MBB->getNumber() > Bottom->getNumber())
       Bottom = MBB;
   return Bottom;
 }

 /// Sort the blocks in RPO, taking special care to make sure that loops are
 /// contiguous even in the case of split backedges.
 ///
 /// TODO: Determine whether RPO is actually worthwhile, or whether we should
 /// move to just a stable-topological-sort-based approach that would preserve
 /// more of the original order.
 static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
   // Note that we do our own RPO rather than using
   // "llvm/ADT/PostOrderIterator.h" because we want control over the order that
   // successors are visited in (see above). Also, we can sort the blocks in the
   // MachineFunction as we go.
   SmallPtrSet<MachineBasicBlock *, 16> Visited;
   SmallVector<POStackEntry, 16> Stack;

   MachineBasicBlock *EntryBlock = &*MF.begin();
   Visited.insert(EntryBlock);
   Stack.push_back(POStackEntry(EntryBlock, MF, MLI));

   for (;;) {
     POStackEntry &Entry = Stack.back();
     SmallVectorImpl<MachineBasicBlock *> &Succs = Entry.Succs;
     if (!Succs.empty()) {
       MachineBasicBlock *Succ = Succs.pop_back_val();
       if (Visited.insert(Succ).second)
         Stack.push_back(POStackEntry(Succ, MF, MLI));
       continue;
     }

     // Put the block in its position in the MachineFunction.
     MachineBasicBlock &MBB = *Entry.MBB;
     MBB.moveBefore(&*MF.begin());

     // Branch instructions may utilize a fallthrough, so update them if a
     // fallthrough has been added or removed.
     if (!MBB.empty() && MBB.back().isTerminator() && !MBB.back().isBranch() &&
         !MBB.back().isBarrier())
       report_fatal_error(
           "Non-branch terminator with fallthrough cannot yet be rewritten");
     if (MBB.empty() || !MBB.back().isTerminator() || MBB.back().isBranch())
       MBB.updateTerminator();

     Stack.pop_back();
     if (Stack.empty())
       break;
   }

   // Now that we've sorted the blocks in RPO, renumber them.
   MF.RenumberBlocks();

 #ifndef NDEBUG
   SmallSetVector<MachineLoop *, 8> OnStack;

   // Insert a sentinel representing the degenerate loop that starts at the
   // function entry block and includes the entire function as a "loop" that
   // executes once.
   OnStack.insert(nullptr);

   for (auto &MBB : MF) {
     assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");

     MachineLoop *Loop = MLI.getLoopFor(&MBB);
     if (Loop && &MBB == Loop->getHeader()) {
       // Loop header. The loop predecessor should be sorted above, and the other
       // predecessors should be backedges below.
       for (auto Pred : MBB.predecessors())
         assert(
             (Pred->getNumber() < MBB.getNumber() || Loop->contains(Pred)) &&
             "Loop header predecessors must be loop predecessors or backedges");
       assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
     } else {
       // Not a loop header. All predecessors should be sorted above.
       for (auto Pred : MBB.predecessors())
         assert(Pred->getNumber() < MBB.getNumber() &&
                "Non-loop-header predecessors should be topologically sorted");
       assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
              "Blocks must be nested in their loops");
     }
     while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
       OnStack.pop_back();
   }
   assert(OnStack.pop_back_val() == nullptr &&
          "The function entry block shouldn't actually be a loop header");
   assert(OnStack.empty() &&
          "Control flow stack pushes and pops should be balanced.");
 #endif
 }

 /// Test whether Pred has any terminators explicitly branching to MBB, as
 /// opposed to falling through. Note that it's possible (eg. in unoptimized
 /// code) for a branch instruction to both branch to a block and fallthrough
 /// to it, so we check the actual branch operands to see if there are any
 /// explicit mentions.
 static bool ExplicitlyBranchesTo(MachineBasicBlock *Pred, MachineBasicBlock *MBB) {
   for (MachineInstr &MI : Pred->terminators())
     for (MachineOperand &MO : MI.explicit_operands())
       if (MO.isMBB() && MO.getMBB() == MBB)
         return true;
   return false;
 }

 /// Insert a BLOCK marker for branches to MBB (if needed).
 static void PlaceBlockMarker(MachineBasicBlock &MBB, MachineFunction &MF,
                              SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
                              const WebAssemblyInstrInfo &TII,
                              const MachineLoopInfo &MLI,
                              MachineDominatorTree &MDT) {
   // First compute the nearest common dominator of all forward non-fallthrough
   // predecessors so that we minimize the time that the BLOCK is on the stack,
   // which reduces overall stack height.
   MachineBasicBlock *Header = nullptr;
   bool IsBranchedTo = false;
   int MBBNumber = MBB.getNumber();
   for (MachineBasicBlock *Pred : MBB.predecessors())
     if (Pred->getNumber() < MBBNumber) {
       Header = Header ? MDT.findNearestCommonDominator(Header, Pred) : Pred;
       if (ExplicitlyBranchesTo(Pred, &MBB))
         IsBranchedTo = true;
     }
   if (!Header)
     return;
   if (!IsBranchedTo)
     return;

   assert(&MBB != &MF.front() && "Header blocks shouldn't have predecessors");
   MachineBasicBlock *LayoutPred = &*prev(MachineFunction::iterator(&MBB));

   // If the nearest common dominator is inside a more deeply nested context,
   // walk out to the nearest scope which isn't more deeply nested.
   for (MachineFunction::iterator I(LayoutPred), E(Header); I != E; --I) {
     if (MachineBasicBlock *ScopeTop = ScopeTops[I->getNumber()]) {
       if (ScopeTop->getNumber() > Header->getNumber()) {
         // Skip over an intervening scope.
         I = next(MachineFunction::iterator(ScopeTop));
       } else {
         // We found a scope level at an appropriate depth.
         Header = ScopeTop;
         break;
       }
     }
   }

   // If there's a loop which ends just before MBB which contains Header, we can
   // reuse its label instead of inserting a new BLOCK.
   for (MachineLoop *Loop = MLI.getLoopFor(LayoutPred);
        Loop && Loop->contains(LayoutPred); Loop = Loop->getParentLoop())
     if (Loop && LoopBottom(Loop) == LayoutPred && Loop->contains(Header))
       return;

   // Decide where in Header to put the BLOCK.
   MachineBasicBlock::iterator InsertPos;
   MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
   if (HeaderLoop && MBB.getNumber() > LoopBottom(HeaderLoop)->getNumber()) {
     // Header is the header of a loop that does not lexically contain MBB, so
     // the BLOCK needs to be above the LOOP.
     InsertPos = Header->begin();
   } else {
     // Otherwise, insert the BLOCK as late in Header as we can, but before any
     // existing BLOCKs.
     InsertPos = Header->getFirstTerminator();
     while (InsertPos != Header->begin() &&
            prev(InsertPos)->getOpcode() == WebAssembly::BLOCK)
       --InsertPos;
   }

   // Add the BLOCK.
   BuildMI(*Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
       .addMBB(&MBB);

   // Track the farthest-spanning scope that ends at this point.
   int Number = MBB.getNumber();
   if (!ScopeTops[Number] ||
       ScopeTops[Number]->getNumber() > Header->getNumber())
     ScopeTops[Number] = Header;
 }

 /// Insert a LOOP marker for a loop starting at MBB (if it's a loop header).
 static void PlaceLoopMarker(MachineBasicBlock &MBB, MachineFunction &MF,
                             SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
                             const WebAssemblyInstrInfo &TII,
                             const MachineLoopInfo &MLI) {
   MachineLoop *Loop = MLI.getLoopFor(&MBB);
   if (!Loop || Loop->getHeader() != &MBB)
     return;

   // The operand of a LOOP is the first block after the loop. If the loop is the
   // bottom of the function, insert a dummy block at the end.
   MachineBasicBlock *Bottom = LoopBottom(Loop);
   auto Iter = next(MachineFunction::iterator(Bottom));
   if (Iter == MF.end()) {
     MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
     // Give it a fake predecessor so that AsmPrinter prints its label.
     Label->addSuccessor(Label);
     MF.push_back(Label);
     Iter = next(MachineFunction::iterator(Bottom));
   }
   MachineBasicBlock *AfterLoop = &*Iter;
   BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
       .addMBB(AfterLoop);

   // Emit a special no-op telling the asm printer that we need a label to close
   // the loop scope, even though the destination is only reachable by
   // fallthrough.
   if (!Bottom->back().isBarrier())
     BuildMI(*Bottom, Bottom->end(), DebugLoc(), TII.get(WebAssembly::LOOP_END));

   assert((!ScopeTops[AfterLoop->getNumber()] ||
           ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
          "With RPO we should visit the outer-most loop for a block first.");
   if (!ScopeTops[AfterLoop->getNumber()])
     ScopeTops[AfterLoop->getNumber()] = &MBB;
 }

 /// Insert LOOP and BLOCK markers at appropriate places.
 static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
                          const WebAssemblyInstrInfo &TII,
                          MachineDominatorTree &MDT) {
   // For each block whose label represents the end of a scope, record the block
   // which holds the beginning of the scope. This will allow us to quickly skip
   // over scoped regions when walking blocks. We allocate one more than the
   // number of blocks in the function to accommodate for the possible fake block
   // we may insert at the end.
   SmallVector<MachineBasicBlock *, 8> ScopeTops(MF.getNumBlockIDs() + 1);

   for (auto &MBB : MF) {
     // Place the LOOP for MBB if MBB is the header of a loop.
     PlaceLoopMarker(MBB, MF, ScopeTops, TII, MLI);

     // Place the BLOCK for MBB if MBB is branched to from above.
     PlaceBlockMarker(MBB, MF, ScopeTops, TII, MLI, MDT);
   }
 }

 #ifndef NDEBUG
 static bool
 IsOnStack(const SmallVectorImpl<std::pair<MachineBasicBlock *, bool>> &Stack,
           const MachineBasicBlock *MBB) {
   for (const auto &Pair : Stack)
     if (Pair.first == MBB)
       return true;
   return false;
 }
 #endif

 bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
   DEBUG(dbgs() << "********** CFG Stackifying **********\n"
                   "********** Function: "
                << MF.getName() << '\n');

   const auto &MLI = getAnalysis<MachineLoopInfo>();
   auto &MDT = getAnalysis<MachineDominatorTree>();
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

   // RPO sorting needs all loops to be single-entry.
   EliminateMultipleEntryLoops(MF, MLI);

   // Sort the blocks in RPO, with contiguous loops.
   SortBlocks(MF, MLI);

   // Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
   PlaceMarkers(MF, MLI, TII, MDT);

 #ifndef NDEBUG
   // Verify that block and loop beginnings and endings are in LIFO order, and
   // that all references to blocks are to blocks on the stack at the point of
   // the reference.
   SmallVector<std::pair<MachineBasicBlock *, bool>, 0> Stack;
   for (auto &MBB : MF) {
     while (!Stack.empty() && Stack.back().first == &MBB)
       if (Stack.back().second) {
         assert(Stack.size() >= 2);
         Stack.pop_back();
         Stack.pop_back();
       } else {
         assert(Stack.size() >= 1);
         Stack.pop_back();
       }
     for (auto &MI : MBB)
       switch (MI.getOpcode()) {
       case WebAssembly::LOOP:
         Stack.push_back(std::make_pair(&MBB, false));
         Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), true));
         break;
       case WebAssembly::BLOCK:
         Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), false));
         break;
       default:
         // Verify that all referenced blocks are in scope. A reference to a
         // block with a negative number is invalid, but can happen with inline
         // asm, so we shouldn't assert on it, but instead let CodeGen properly
         // fail on it.
         for (const MachineOperand &MO : MI.explicit_operands())
           if (MO.isMBB() && MO.getMBB()->getNumber() >= 0)
             assert(IsOnStack(Stack, MO.getMBB()));
         break;
       }
   }
   assert(Stack.empty());
 #endif

   return true;
 }
	//===-- WebAssemblyCFGStackify.cpp - CFG Stackification -------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	///
	/// \file
	/// \brief This file implements a CFG stacking pass.
	///
	/// This pass reorders the blocks in a function to put them into a reverse
	/// post-order [0], with special care to keep the order as similar as possible
	/// to the original order, and to keep loops contiguous even in the case of
	/// split backedges.
	///
	/// Then, it inserts BLOCK and LOOP markers to mark the start of scopes, since
	/// scope boundaries serve as the labels for WebAssembly's control transfers.
	///
	/// This is sufficient to convert arbitrary CFGs into a form that works on
	/// WebAssembly, provided that all loops are single-entry.
	///
	/// [0] https://en.wikipedia.org/wiki/Depth-first_search#Vertex_orderings
	///
	//===----------------------------------------------------------------------===//

	#include "WebAssembly.h"
	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssemblySubtarget.h"
	#include "llvm/ADT/SCCIterator.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"
	using namespace llvm;

	#define DEBUG_TYPE "wasm-cfg-stackify"

	namespace {
	class WebAssemblyCFGStackify final : public MachineFunctionPass {
	const char *getPassName() const override {
	return "WebAssembly CFG Stackify";
	}

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.setPreservesCFG();
	AU.addRequired<MachineDominatorTree>();
	AU.addPreserved<MachineDominatorTree>();
	AU.addRequired<MachineLoopInfo>();
	AU.addPreserved<MachineLoopInfo>();
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	bool runOnMachineFunction(MachineFunction &MF) override;

	public:
	static char ID; // Pass identification, replacement for typeid
	WebAssemblyCFGStackify() : MachineFunctionPass(ID) {}
	};
	} // end anonymous namespace

	char WebAssemblyCFGStackify::ID = 0;
	FunctionPass *llvm::createWebAssemblyCFGStackify() {
	return new WebAssemblyCFGStackify();
	}

	static void EliminateMultipleEntryLoops(MachineFunction &MF,
	const MachineLoopInfo &MLI) {
	SmallPtrSet<MachineBasicBlock *, 8> InSet;
	for (scc_iterator<MachineFunction *> I = scc_begin(&MF), E = scc_end(&MF);
	I != E; ++I) {
	const std::vector<MachineBasicBlock > &CurrentSCC = I;

	// Skip trivial SCCs.
	if (CurrentSCC.size() == 1)
	continue;

	InSet.insert(CurrentSCC.begin(), CurrentSCC.end());
	MachineBasicBlock *Header = nullptr;
	for (MachineBasicBlock *MBB : CurrentSCC) {
	for (MachineBasicBlock *Pred : MBB->predecessors()) {
	if (InSet.count(Pred))
	continue;
	if (!Header) {
	Header = MBB;
	break;
	}
	// TODO: Implement multiple-entry loops.
	report_fatal_error("multiple-entry loops are not supported yet");
	}
	}
	assert(MLI.isLoopHeader(Header));

	InSet.clear();
	}
	}

	namespace {
	/// Post-order traversal stack entry.
	struct POStackEntry {
	MachineBasicBlock *MBB;
	SmallVector<MachineBasicBlock *, 0> Succs;

	POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
	const MachineLoopInfo &MLI);
	};
	} // end anonymous namespace

	static bool LoopContains(const MachineLoop *Loop,
	const MachineBasicBlock *MBB) {
	return Loop ? Loop->contains(MBB) : true;
	}

	POStackEntry::POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
	const MachineLoopInfo &MLI)
	: MBB(MBB), Succs(MBB->successors()) {
	// RPO is not a unique form, since at every basic block with multiple
	// successors, the DFS has to pick which order to visit the successors in.
	// Sort them strategically (see below).
	MachineLoop *Loop = MLI.getLoopFor(MBB);
	MachineFunction::iterator Next = next(MachineFunction::iterator(MBB));
	MachineBasicBlock LayoutSucc = Next == MF.end() ? nullptr : &Next;
	std::stable_sort(
	Succs.begin(), Succs.end(),
	[=, &MLI](const MachineBasicBlock A, const MachineBasicBlock B) {
	if (A == B)
	return false;

	// Keep loops contiguous by preferring the block that's in the same
	// loop.
	bool LoopContainsA = LoopContains(Loop, A);
	bool LoopContainsB = LoopContains(Loop, B);
	if (LoopContainsA && !LoopContainsB)
	return true;
	if (!LoopContainsA && LoopContainsB)
	return false;

	// Minimize perturbation by preferring the block which is the immediate
	// layout successor.
	if (A == LayoutSucc)
	return true;
	if (B == LayoutSucc)
	return false;

	// TODO: More sophisticated orderings may be profitable here.

	return false;
	});
	}

	/// Return the "bottom" block of a loop. This differs from
	/// MachineLoop::getBottomBlock in that it works even if the loop is
	/// discontiguous.
	static MachineBasicBlock LoopBottom(const MachineLoop Loop) {
	MachineBasicBlock *Bottom = Loop->getHeader();
	for (MachineBasicBlock *MBB : Loop->blocks())
	if (MBB->getNumber() > Bottom->getNumber())
	Bottom = MBB;
	return Bottom;
	}

	/// Sort the blocks in RPO, taking special care to make sure that loops are
	/// contiguous even in the case of split backedges.
	///
	/// TODO: Determine whether RPO is actually worthwhile, or whether we should
	/// move to just a stable-topological-sort-based approach that would preserve
	/// more of the original order.
	static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
	// Note that we do our own RPO rather than using
	// "llvm/ADT/PostOrderIterator.h" because we want control over the order that
	// successors are visited in (see above). Also, we can sort the blocks in the
	// MachineFunction as we go.
	SmallPtrSet<MachineBasicBlock *, 16> Visited;
	SmallVector<POStackEntry, 16> Stack;

	MachineBasicBlock EntryBlock = &MF.begin();
	Visited.insert(EntryBlock);
	Stack.push_back(POStackEntry(EntryBlock, MF, MLI));

	for (;;) {
	POStackEntry &Entry = Stack.back();
	SmallVectorImpl<MachineBasicBlock *> &Succs = Entry.Succs;
	if (!Succs.empty()) {
	MachineBasicBlock *Succ = Succs.pop_back_val();
	if (Visited.insert(Succ).second)
	Stack.push_back(POStackEntry(Succ, MF, MLI));
	continue;
	}

	// Put the block in its position in the MachineFunction.
	MachineBasicBlock &MBB = *Entry.MBB;
	MBB.moveBefore(&*MF.begin());

	// Branch instructions may utilize a fallthrough, so update them if a
	// fallthrough has been added or removed.
	if (!MBB.empty() && MBB.back().isTerminator() && !MBB.back().isBranch() &&
	!MBB.back().isBarrier())
	report_fatal_error(
	"Non-branch terminator with fallthrough cannot yet be rewritten");
	if (MBB.empty() \|\| !MBB.back().isTerminator() \|\| MBB.back().isBranch())
	MBB.updateTerminator();

	Stack.pop_back();
	if (Stack.empty())
	break;
	}

	// Now that we've sorted the blocks in RPO, renumber them.
	MF.RenumberBlocks();

	#ifndef NDEBUG
	SmallSetVector<MachineLoop *, 8> OnStack;

	// Insert a sentinel representing the degenerate loop that starts at the
	// function entry block and includes the entire function as a "loop" that
	// executes once.
	OnStack.insert(nullptr);

	for (auto &MBB : MF) {
	assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");

	MachineLoop *Loop = MLI.getLoopFor(&MBB);
	if (Loop && &MBB == Loop->getHeader()) {
	// Loop header. The loop predecessor should be sorted above, and the other
	// predecessors should be backedges below.
	for (auto Pred : MBB.predecessors())
	assert(
	(Pred->getNumber() < MBB.getNumber() \|\| Loop->contains(Pred)) &&
	"Loop header predecessors must be loop predecessors or backedges");
	assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
	} else {
	// Not a loop header. All predecessors should be sorted above.
	for (auto Pred : MBB.predecessors())
	assert(Pred->getNumber() < MBB.getNumber() &&
	"Non-loop-header predecessors should be topologically sorted");
	assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
	"Blocks must be nested in their loops");
	}
	while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
	OnStack.pop_back();
	}
	assert(OnStack.pop_back_val() == nullptr &&
	"The function entry block shouldn't actually be a loop header");
	assert(OnStack.empty() &&
	"Control flow stack pushes and pops should be balanced.");
	#endif
	}

	/// Test whether Pred has any terminators explicitly branching to MBB, as
	/// opposed to falling through. Note that it's possible (eg. in unoptimized
	/// code) for a branch instruction to both branch to a block and fallthrough
	/// to it, so we check the actual branch operands to see if there are any
	/// explicit mentions.
	static bool ExplicitlyBranchesTo(MachineBasicBlock Pred, MachineBasicBlock MBB) {
	for (MachineInstr &MI : Pred->terminators())
	for (MachineOperand &MO : MI.explicit_operands())
	if (MO.isMBB() && MO.getMBB() == MBB)
	return true;
	return false;
	}

	/// Insert a BLOCK marker for branches to MBB (if needed).
	static void PlaceBlockMarker(MachineBasicBlock &MBB, MachineFunction &MF,
	SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
	const WebAssemblyInstrInfo &TII,
	const MachineLoopInfo &MLI,
	MachineDominatorTree &MDT) {
	// First compute the nearest common dominator of all forward non-fallthrough
	// predecessors so that we minimize the time that the BLOCK is on the stack,
	// which reduces overall stack height.
	MachineBasicBlock *Header = nullptr;
	bool IsBranchedTo = false;
	int MBBNumber = MBB.getNumber();
	for (MachineBasicBlock *Pred : MBB.predecessors())
	if (Pred->getNumber() < MBBNumber) {
	Header = Header ? MDT.findNearestCommonDominator(Header, Pred) : Pred;
	if (ExplicitlyBranchesTo(Pred, &MBB))
	IsBranchedTo = true;
	}
	if (!Header)
	return;
	if (!IsBranchedTo)
	return;

	assert(&MBB != &MF.front() && "Header blocks shouldn't have predecessors");
	MachineBasicBlock LayoutPred = &prev(MachineFunction::iterator(&MBB));

	// If the nearest common dominator is inside a more deeply nested context,
	// walk out to the nearest scope which isn't more deeply nested.
	for (MachineFunction::iterator I(LayoutPred), E(Header); I != E; --I) {
	if (MachineBasicBlock *ScopeTop = ScopeTops[I->getNumber()]) {
	if (ScopeTop->getNumber() > Header->getNumber()) {
	// Skip over an intervening scope.
	I = next(MachineFunction::iterator(ScopeTop));
	} else {
	// We found a scope level at an appropriate depth.
	Header = ScopeTop;
	break;
	}
	}
	}

	// If there's a loop which ends just before MBB which contains Header, we can
	// reuse its label instead of inserting a new BLOCK.
	for (MachineLoop *Loop = MLI.getLoopFor(LayoutPred);
	Loop && Loop->contains(LayoutPred); Loop = Loop->getParentLoop())
	if (Loop && LoopBottom(Loop) == LayoutPred && Loop->contains(Header))
	return;

	// Decide where in Header to put the BLOCK.
	MachineBasicBlock::iterator InsertPos;
	MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
	if (HeaderLoop && MBB.getNumber() > LoopBottom(HeaderLoop)->getNumber()) {
	// Header is the header of a loop that does not lexically contain MBB, so
	// the BLOCK needs to be above the LOOP.
	InsertPos = Header->begin();
	} else {
	// Otherwise, insert the BLOCK as late in Header as we can, but before any
	// existing BLOCKs.
	InsertPos = Header->getFirstTerminator();
	while (InsertPos != Header->begin() &&
	prev(InsertPos)->getOpcode() == WebAssembly::BLOCK)
	--InsertPos;
	}

	// Add the BLOCK.
	BuildMI(*Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
	.addMBB(&MBB);

	// Track the farthest-spanning scope that ends at this point.
	int Number = MBB.getNumber();
	if (!ScopeTops[Number] \|\|
	ScopeTops[Number]->getNumber() > Header->getNumber())
	ScopeTops[Number] = Header;
	}

	/// Insert a LOOP marker for a loop starting at MBB (if it's a loop header).
	static void PlaceLoopMarker(MachineBasicBlock &MBB, MachineFunction &MF,
	SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
	const WebAssemblyInstrInfo &TII,
	const MachineLoopInfo &MLI) {
	MachineLoop *Loop = MLI.getLoopFor(&MBB);
	if (!Loop \|\| Loop->getHeader() != &MBB)
	return;

	// The operand of a LOOP is the first block after the loop. If the loop is the
	// bottom of the function, insert a dummy block at the end.
	MachineBasicBlock *Bottom = LoopBottom(Loop);
	auto Iter = next(MachineFunction::iterator(Bottom));
	if (Iter == MF.end()) {
	MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
	// Give it a fake predecessor so that AsmPrinter prints its label.
	Label->addSuccessor(Label);
	MF.push_back(Label);
	Iter = next(MachineFunction::iterator(Bottom));
	}
	MachineBasicBlock AfterLoop = &Iter;
	BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
	.addMBB(AfterLoop);

	// Emit a special no-op telling the asm printer that we need a label to close
	// the loop scope, even though the destination is only reachable by
	// fallthrough.
	if (!Bottom->back().isBarrier())
	BuildMI(*Bottom, Bottom->end(), DebugLoc(), TII.get(WebAssembly::LOOP_END));

	assert((!ScopeTops[AfterLoop->getNumber()] \|\|
	ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
	"With RPO we should visit the outer-most loop for a block first.");
	if (!ScopeTops[AfterLoop->getNumber()])
	ScopeTops[AfterLoop->getNumber()] = &MBB;
	}

	/// Insert LOOP and BLOCK markers at appropriate places.
	static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
	const WebAssemblyInstrInfo &TII,
	MachineDominatorTree &MDT) {
	// For each block whose label represents the end of a scope, record the block
	// which holds the beginning of the scope. This will allow us to quickly skip
	// over scoped regions when walking blocks. We allocate one more than the
	// number of blocks in the function to accommodate for the possible fake block
	// we may insert at the end.
	SmallVector<MachineBasicBlock *, 8> ScopeTops(MF.getNumBlockIDs() + 1);

	for (auto &MBB : MF) {
	// Place the LOOP for MBB if MBB is the header of a loop.
	PlaceLoopMarker(MBB, MF, ScopeTops, TII, MLI);

	// Place the BLOCK for MBB if MBB is branched to from above.
	PlaceBlockMarker(MBB, MF, ScopeTops, TII, MLI, MDT);
	}
	}

	#ifndef NDEBUG
	static bool
	IsOnStack(const SmallVectorImpl<std::pair<MachineBasicBlock *, bool>> &Stack,
	const MachineBasicBlock *MBB) {
	for (const auto &Pair : Stack)
	if (Pair.first == MBB)
	return true;
	return false;
	}
	#endif

	bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
	DEBUG(dbgs() << "******** CFG Stackifying ********\n"
	"********** Function: "
	<< MF.getName() << '\n');

	const auto &MLI = getAnalysis<MachineLoopInfo>();
	auto &MDT = getAnalysis<MachineDominatorTree>();
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

	// RPO sorting needs all loops to be single-entry.
	EliminateMultipleEntryLoops(MF, MLI);

	// Sort the blocks in RPO, with contiguous loops.
	SortBlocks(MF, MLI);

	// Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
	PlaceMarkers(MF, MLI, TII, MDT);

	#ifndef NDEBUG
	// Verify that block and loop beginnings and endings are in LIFO order, and
	// that all references to blocks are to blocks on the stack at the point of
	// the reference.
	SmallVector<std::pair<MachineBasicBlock *, bool>, 0> Stack;
	for (auto &MBB : MF) {
	while (!Stack.empty() && Stack.back().first == &MBB)
	if (Stack.back().second) {
	assert(Stack.size() >= 2);
	Stack.pop_back();
	Stack.pop_back();
	} else {
	assert(Stack.size() >= 1);
	Stack.pop_back();
	}
	for (auto &MI : MBB)
	switch (MI.getOpcode()) {
	case WebAssembly::LOOP:
	Stack.push_back(std::make_pair(&MBB, false));
	Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), true));
	break;
	case WebAssembly::BLOCK:
	Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), false));
	break;
	default:
	// Verify that all referenced blocks are in scope. A reference to a
	// block with a negative number is invalid, but can happen with inline
	// asm, so we shouldn't assert on it, but instead let CodeGen properly
	// fail on it.
	for (const MachineOperand &MO : MI.explicit_operands())
	if (MO.isMBB() && MO.getMBB()->getNumber() >= 0)
	assert(IsOnStack(Stack, MO.getMBB()));
	break;
	}
	}
	assert(Stack.empty());
	#endif

	return true;
	}