llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp - toolchain/llvm-project - Gitiles

 //===-- 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 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.
 ///
 //===----------------------------------------------------------------------===//

 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssembly.h"
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyUtilities.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.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 {
   StringRef 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();
 }

 /// 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,
     DenseMap<const MachineInstr *, MachineInstr *> &BlockTops,
     DenseMap<const MachineInstr *, MachineInstr *> &LoopTops,
     const WebAssemblyInstrInfo &TII,
     const MachineLoopInfo &MLI,
     MachineDominatorTree &MDT,
     WebAssemblyFunctionInfo &MFI) {
   // 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 = &*std::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 = std::next(MachineFunction::iterator(ScopeTop));
       } else {
         // We found a scope level at an appropriate depth.
         Header = ScopeTop;
         break;
       }
     }
   }

   // 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, after any END constructs.
     InsertPos = Header->begin();
     while (InsertPos->getOpcode() == WebAssembly::END_BLOCK ||
            InsertPos->getOpcode() == WebAssembly::END_LOOP)
       ++InsertPos;
   } else {
     // Otherwise, insert the BLOCK as late in Header as we can, but before the
     // beginning of the local expression tree and any nested BLOCKs.
     InsertPos = Header->getFirstTerminator();
     while (InsertPos != Header->begin() &&
            WebAssembly::isChild(*std::prev(InsertPos), MFI) &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::LOOP &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_BLOCK &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
       --InsertPos;
   }

   // Add the BLOCK.
   MachineInstr *Begin = BuildMI(*Header, InsertPos, DebugLoc(),
                                 TII.get(WebAssembly::BLOCK))
       .addImm(int64_t(WebAssembly::ExprType::Void));

   // Mark the end of the block.
   InsertPos = MBB.begin();
   while (InsertPos != MBB.end() &&
          InsertPos->getOpcode() == WebAssembly::END_LOOP &&
          LoopTops[&*InsertPos]->getParent()->getNumber() >= Header->getNumber())
     ++InsertPos;
   MachineInstr *End = BuildMI(MBB, InsertPos, DebugLoc(),
                               TII.get(WebAssembly::END_BLOCK));
   BlockTops[End] = Begin;

   // 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,
     DenseMap<const MachineInstr *, MachineInstr *> &LoopTops,
     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 = std::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 = std::next(MachineFunction::iterator(Bottom));
   }
   MachineBasicBlock *AfterLoop = &*Iter;

   // Mark the beginning of the loop (after the end of any existing loop that
   // ends here).
   auto InsertPos = MBB.begin();
   while (InsertPos != MBB.end() &&
          InsertPos->getOpcode() == WebAssembly::END_LOOP)
     ++InsertPos;
   MachineInstr *Begin = BuildMI(MBB, InsertPos, DebugLoc(),
                                 TII.get(WebAssembly::LOOP))
       .addImm(int64_t(WebAssembly::ExprType::Void));

   // Mark the end of the loop.
   MachineInstr *End = BuildMI(*AfterLoop, AfterLoop->begin(), DebugLoc(),
                               TII.get(WebAssembly::END_LOOP));
   LoopTops[End] = Begin;

   assert((!ScopeTops[AfterLoop->getNumber()] ||
           ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
          "With block sorting the outermost loop for a block should be first.");
   if (!ScopeTops[AfterLoop->getNumber()])
     ScopeTops[AfterLoop->getNumber()] = &MBB;
 }

 static unsigned
 GetDepth(const SmallVectorImpl<const MachineBasicBlock *> &Stack,
          const MachineBasicBlock *MBB) {
   unsigned Depth = 0;
   for (auto X : reverse(Stack)) {
     if (X == MBB)
       break;
     ++Depth;
   }
   assert(Depth < Stack.size() && "Branch destination should be in scope");
   return Depth;
 }

 /// In normal assembly languages, when the end of a function is unreachable,
 /// because the function ends in an infinite loop or a noreturn call or similar,
 /// it isn't necessary to worry about the function return type at the end of
 /// the function, because it's never reached. However, in WebAssembly, blocks
 /// that end at the function end need to have a return type signature that
 /// matches the function signature, even though it's unreachable. This function
 /// checks for such cases and fixes up the signatures.
 static void FixEndsAtEndOfFunction(
     MachineFunction &MF,
     const WebAssemblyFunctionInfo &MFI,
     DenseMap<const MachineInstr *, MachineInstr *> &BlockTops,
     DenseMap<const MachineInstr *, MachineInstr *> &LoopTops) {
   assert(MFI.getResults().size() <= 1);

   if (MFI.getResults().empty())
     return;

   WebAssembly::ExprType retType;
   switch (MFI.getResults().front().SimpleTy) {
   case MVT::i32: retType = WebAssembly::ExprType::I32; break;
   case MVT::i64: retType = WebAssembly::ExprType::I64; break;
   case MVT::f32: retType = WebAssembly::ExprType::F32; break;
   case MVT::f64: retType = WebAssembly::ExprType::F64; break;
   case MVT::v16i8: retType = WebAssembly::ExprType::I8x16; break;
   case MVT::v8i16: retType = WebAssembly::ExprType::I16x8; break;
   case MVT::v4i32: retType = WebAssembly::ExprType::I32x4; break;
   case MVT::v4f32: retType = WebAssembly::ExprType::F32x4; break;
   default: llvm_unreachable("unexpected return type");
   }

   for (MachineBasicBlock &MBB : reverse(MF)) {
     for (MachineInstr &MI : reverse(MBB)) {
       if (MI.isPosition() || MI.isDebugValue())
         continue;
       if (MI.getOpcode() == WebAssembly::END_BLOCK) {
         BlockTops[&MI]->getOperand(0).setImm(int32_t(retType));
         continue;
       }
       if (MI.getOpcode() == WebAssembly::END_LOOP) {
         LoopTops[&MI]->getOperand(0).setImm(int32_t(retType));
         continue;
       }
       // Something other than an `end`. We're done.
       return;
     }
   }
 }

 // WebAssembly functions end with an end instruction, as if the function body
 // were a block.
 static void AppendEndToFunction(
     MachineFunction &MF,
     const WebAssemblyInstrInfo &TII) {
   BuildMI(MF.back(), MF.back().end(), DebugLoc(),
           TII.get(WebAssembly::END_FUNCTION));
 }

 /// Insert LOOP and BLOCK markers at appropriate places.
 static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
                          const WebAssemblyInstrInfo &TII,
                          MachineDominatorTree &MDT,
                          WebAssemblyFunctionInfo &MFI) {
   // 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 each LOOP_END, the corresponding LOOP.
   DenseMap<const MachineInstr *, MachineInstr *> LoopTops;

   // For each END_BLOCK, the corresponding BLOCK.
   DenseMap<const MachineInstr *, MachineInstr *> BlockTops;

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

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

   // Now rewrite references to basic blocks to be depth immediates.
   SmallVector<const MachineBasicBlock *, 8> Stack;
   for (auto &MBB : reverse(MF)) {
     for (auto &MI : reverse(MBB)) {
       switch (MI.getOpcode()) {
       case WebAssembly::BLOCK:
         assert(ScopeTops[Stack.back()->getNumber()]->getNumber() <= MBB.getNumber() &&
                "Block should be balanced");
         Stack.pop_back();
         break;
       case WebAssembly::LOOP:
         assert(Stack.back() == &MBB && "Loop top should be balanced");
         Stack.pop_back();
         break;
       case WebAssembly::END_BLOCK:
         Stack.push_back(&MBB);
         break;
       case WebAssembly::END_LOOP:
         Stack.push_back(LoopTops[&MI]->getParent());
         break;
       default:
         if (MI.isTerminator()) {
           // Rewrite MBB operands to be depth immediates.
           SmallVector<MachineOperand, 4> Ops(MI.operands());
           while (MI.getNumOperands() > 0)
             MI.RemoveOperand(MI.getNumOperands() - 1);
           for (auto MO : Ops) {
             if (MO.isMBB())
               MO = MachineOperand::CreateImm(GetDepth(Stack, MO.getMBB()));
             MI.addOperand(MF, MO);
           }
         }
         break;
       }
     }
   }
   assert(Stack.empty() && "Control flow should be balanced");

   // Fix up block/loop signatures at the end of the function to conform to
   // WebAssembly's rules.
   FixEndsAtEndOfFunction(MF, MFI, BlockTops, LoopTops);

   // Add an end instruction at the end of the function body.
   if (!MF.getSubtarget<WebAssemblySubtarget>()
         .getTargetTriple().isOSBinFormatELF())
     AppendEndToFunction(MF, TII);
 }

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

   const auto &MLI = getAnalysis<MachineLoopInfo>();
   auto &MDT = getAnalysis<MachineDominatorTree>();
   // Liveness is not tracked for VALUE_STACK physreg.
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
   MF.getRegInfo().invalidateLiveness();

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

   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 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.
	///
	//===----------------------------------------------------------------------===//

	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssembly.h"
	#include "WebAssemblyMachineFunctionInfo.h"
	#include "WebAssemblySubtarget.h"
	#include "WebAssemblyUtilities.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/CodeGen/MachineRegisterInfo.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 {
	StringRef 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();
	}

	/// 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,
	DenseMap<const MachineInstr , MachineInstr > &BlockTops,
	DenseMap<const MachineInstr , MachineInstr > &LoopTops,
	const WebAssemblyInstrInfo &TII,
	const MachineLoopInfo &MLI,
	MachineDominatorTree &MDT,
	WebAssemblyFunctionInfo &MFI) {
	// 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 = &std::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 = std::next(MachineFunction::iterator(ScopeTop));
	} else {
	// We found a scope level at an appropriate depth.
	Header = ScopeTop;
	break;
	}
	}
	}

	// 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, after any END constructs.
	InsertPos = Header->begin();
	while (InsertPos->getOpcode() == WebAssembly::END_BLOCK \|\|
	InsertPos->getOpcode() == WebAssembly::END_LOOP)
	++InsertPos;
	} else {
	// Otherwise, insert the BLOCK as late in Header as we can, but before the
	// beginning of the local expression tree and any nested BLOCKs.
	InsertPos = Header->getFirstTerminator();
	while (InsertPos != Header->begin() &&
	WebAssembly::isChild(*std::prev(InsertPos), MFI) &&
	std::prev(InsertPos)->getOpcode() != WebAssembly::LOOP &&
	std::prev(InsertPos)->getOpcode() != WebAssembly::END_BLOCK &&
	std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
	--InsertPos;
	}

	// Add the BLOCK.
	MachineInstr Begin = BuildMI(Header, InsertPos, DebugLoc(),
	TII.get(WebAssembly::BLOCK))
	.addImm(int64_t(WebAssembly::ExprType::Void));

	// Mark the end of the block.
	InsertPos = MBB.begin();
	while (InsertPos != MBB.end() &&
	InsertPos->getOpcode() == WebAssembly::END_LOOP &&
	LoopTops[&*InsertPos]->getParent()->getNumber() >= Header->getNumber())
	++InsertPos;
	MachineInstr *End = BuildMI(MBB, InsertPos, DebugLoc(),
	TII.get(WebAssembly::END_BLOCK));
	BlockTops[End] = Begin;

	// 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,
	DenseMap<const MachineInstr , MachineInstr > &LoopTops,
	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 = std::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 = std::next(MachineFunction::iterator(Bottom));
	}
	MachineBasicBlock AfterLoop = &Iter;

	// Mark the beginning of the loop (after the end of any existing loop that
	// ends here).
	auto InsertPos = MBB.begin();
	while (InsertPos != MBB.end() &&
	InsertPos->getOpcode() == WebAssembly::END_LOOP)
	++InsertPos;
	MachineInstr *Begin = BuildMI(MBB, InsertPos, DebugLoc(),
	TII.get(WebAssembly::LOOP))
	.addImm(int64_t(WebAssembly::ExprType::Void));

	// Mark the end of the loop.
	MachineInstr End = BuildMI(AfterLoop, AfterLoop->begin(), DebugLoc(),
	TII.get(WebAssembly::END_LOOP));
	LoopTops[End] = Begin;

	assert((!ScopeTops[AfterLoop->getNumber()] \|\|
	ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
	"With block sorting the outermost loop for a block should be first.");
	if (!ScopeTops[AfterLoop->getNumber()])
	ScopeTops[AfterLoop->getNumber()] = &MBB;
	}

	static unsigned
	GetDepth(const SmallVectorImpl<const MachineBasicBlock *> &Stack,
	const MachineBasicBlock *MBB) {
	unsigned Depth = 0;
	for (auto X : reverse(Stack)) {
	if (X == MBB)
	break;
	++Depth;
	}
	assert(Depth < Stack.size() && "Branch destination should be in scope");
	return Depth;
	}

	/// In normal assembly languages, when the end of a function is unreachable,
	/// because the function ends in an infinite loop or a noreturn call or similar,
	/// it isn't necessary to worry about the function return type at the end of
	/// the function, because it's never reached. However, in WebAssembly, blocks
	/// that end at the function end need to have a return type signature that
	/// matches the function signature, even though it's unreachable. This function
	/// checks for such cases and fixes up the signatures.
	static void FixEndsAtEndOfFunction(
	MachineFunction &MF,
	const WebAssemblyFunctionInfo &MFI,
	DenseMap<const MachineInstr , MachineInstr > &BlockTops,
	DenseMap<const MachineInstr , MachineInstr > &LoopTops) {
	assert(MFI.getResults().size() <= 1);

	if (MFI.getResults().empty())
	return;

	WebAssembly::ExprType retType;
	switch (MFI.getResults().front().SimpleTy) {
	case MVT::i32: retType = WebAssembly::ExprType::I32; break;
	case MVT::i64: retType = WebAssembly::ExprType::I64; break;
	case MVT::f32: retType = WebAssembly::ExprType::F32; break;
	case MVT::f64: retType = WebAssembly::ExprType::F64; break;
	case MVT::v16i8: retType = WebAssembly::ExprType::I8x16; break;
	case MVT::v8i16: retType = WebAssembly::ExprType::I16x8; break;
	case MVT::v4i32: retType = WebAssembly::ExprType::I32x4; break;
	case MVT::v4f32: retType = WebAssembly::ExprType::F32x4; break;
	default: llvm_unreachable("unexpected return type");
	}

	for (MachineBasicBlock &MBB : reverse(MF)) {
	for (MachineInstr &MI : reverse(MBB)) {
	if (MI.isPosition() \|\| MI.isDebugValue())
	continue;
	if (MI.getOpcode() == WebAssembly::END_BLOCK) {
	BlockTops[&MI]->getOperand(0).setImm(int32_t(retType));
	continue;
	}
	if (MI.getOpcode() == WebAssembly::END_LOOP) {
	LoopTops[&MI]->getOperand(0).setImm(int32_t(retType));
	continue;
	}
	// Something other than an `end`. We're done.
	return;
	}
	}
	}

	// WebAssembly functions end with an end instruction, as if the function body
	// were a block.
	static void AppendEndToFunction(
	MachineFunction &MF,
	const WebAssemblyInstrInfo &TII) {
	BuildMI(MF.back(), MF.back().end(), DebugLoc(),
	TII.get(WebAssembly::END_FUNCTION));
	}

	/// Insert LOOP and BLOCK markers at appropriate places.
	static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
	const WebAssemblyInstrInfo &TII,
	MachineDominatorTree &MDT,
	WebAssemblyFunctionInfo &MFI) {
	// 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 each LOOP_END, the corresponding LOOP.
	DenseMap<const MachineInstr , MachineInstr > LoopTops;

	// For each END_BLOCK, the corresponding BLOCK.
	DenseMap<const MachineInstr , MachineInstr > BlockTops;

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

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

	// Now rewrite references to basic blocks to be depth immediates.
	SmallVector<const MachineBasicBlock *, 8> Stack;
	for (auto &MBB : reverse(MF)) {
	for (auto &MI : reverse(MBB)) {
	switch (MI.getOpcode()) {
	case WebAssembly::BLOCK:
	assert(ScopeTops[Stack.back()->getNumber()]->getNumber() <= MBB.getNumber() &&
	"Block should be balanced");
	Stack.pop_back();
	break;
	case WebAssembly::LOOP:
	assert(Stack.back() == &MBB && "Loop top should be balanced");
	Stack.pop_back();
	break;
	case WebAssembly::END_BLOCK:
	Stack.push_back(&MBB);
	break;
	case WebAssembly::END_LOOP:
	Stack.push_back(LoopTops[&MI]->getParent());
	break;
	default:
	if (MI.isTerminator()) {
	// Rewrite MBB operands to be depth immediates.
	SmallVector<MachineOperand, 4> Ops(MI.operands());
	while (MI.getNumOperands() > 0)
	MI.RemoveOperand(MI.getNumOperands() - 1);
	for (auto MO : Ops) {
	if (MO.isMBB())
	MO = MachineOperand::CreateImm(GetDepth(Stack, MO.getMBB()));
	MI.addOperand(MF, MO);
	}
	}
	break;
	}
	}
	}
	assert(Stack.empty() && "Control flow should be balanced");

	// Fix up block/loop signatures at the end of the function to conform to
	// WebAssembly's rules.
	FixEndsAtEndOfFunction(MF, MFI, BlockTops, LoopTops);

	// Add an end instruction at the end of the function body.
	if (!MF.getSubtarget<WebAssemblySubtarget>()
	.getTargetTriple().isOSBinFormatELF())
	AppendEndToFunction(MF, TII);
	}

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

	const auto &MLI = getAnalysis<MachineLoopInfo>();
	auto &MDT = getAnalysis<MachineDominatorTree>();
	// Liveness is not tracked for VALUE_STACK physreg.
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
	MF.getRegInfo().invalidateLiveness();

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

	return true;
	}