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

 //=== WebAssemblyLateEHPrepare.cpp - WebAssembly Exception Preparation -===//
 //
 // 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
 /// \brief Does various transformations for exception handling.
 ///
 //===----------------------------------------------------------------------===//

 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssembly.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyUtilities.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/WasmEHFuncInfo.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Target/TargetMachine.h"
 using namespace llvm;

 #define DEBUG_TYPE "wasm-late-eh-prepare"

 namespace {
 class WebAssemblyLateEHPrepare final : public MachineFunctionPass {
   StringRef getPassName() const override {
     return "WebAssembly Late Prepare Exception";
   }

   bool runOnMachineFunction(MachineFunction &MF) override;
   bool removeUnreachableEHPads(MachineFunction &MF);
   void recordCatchRetBBs(MachineFunction &MF);
   bool hoistCatches(MachineFunction &MF);
   bool addCatchAlls(MachineFunction &MF);
   bool replaceFuncletReturns(MachineFunction &MF);
   bool removeUnnecessaryUnreachables(MachineFunction &MF);
   bool ensureSingleBBTermPads(MachineFunction &MF);
   bool restoreStackPointer(MachineFunction &MF);

   MachineBasicBlock *getMatchingEHPad(MachineInstr *MI);
   SmallPtrSet<MachineBasicBlock *, 8> CatchRetBBs;

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

 char WebAssemblyLateEHPrepare::ID = 0;
 INITIALIZE_PASS(WebAssemblyLateEHPrepare, DEBUG_TYPE,
                 "WebAssembly Late Exception Preparation", false, false)

 FunctionPass *llvm::createWebAssemblyLateEHPrepare() {
   return new WebAssemblyLateEHPrepare();
 }

 // Returns the nearest EH pad that dominates this instruction. This does not use
 // dominator analysis; it just does BFS on its predecessors until arriving at an
 // EH pad. This assumes valid EH scopes so the first EH pad it arrives in all
 // possible search paths should be the same.
 // Returns nullptr in case it does not find any EH pad in the search, or finds
 // multiple different EH pads.
 MachineBasicBlock *
 WebAssemblyLateEHPrepare::getMatchingEHPad(MachineInstr *MI) {
   MachineFunction *MF = MI->getParent()->getParent();
   SmallVector<MachineBasicBlock *, 2> WL;
   SmallPtrSet<MachineBasicBlock *, 2> Visited;
   WL.push_back(MI->getParent());
   MachineBasicBlock *EHPad = nullptr;
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
     if (Visited.count(MBB))
       continue;
     Visited.insert(MBB);
     if (MBB->isEHPad()) {
       if (EHPad && EHPad != MBB)
         return nullptr;
       EHPad = MBB;
       continue;
     }
     if (MBB == &MF->front())
       return nullptr;
     for (auto *Pred : MBB->predecessors())
       if (!CatchRetBBs.count(Pred)) // We don't go into child scopes
         WL.push_back(Pred);
   }
   return EHPad;
 }

 // Erase the specified BBs if the BB does not have any remaining predecessors,
 // and also all its dead children.
 template <typename Container>
 static void eraseDeadBBsAndChildren(const Container &MBBs) {
   SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
   SmallPtrSet<MachineBasicBlock *, 8> Deleted;
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
     if (Deleted.count(MBB) || !MBB->pred_empty())
       continue;
     SmallVector<MachineBasicBlock *, 4> Succs(MBB->successors());
     WL.append(MBB->succ_begin(), MBB->succ_end());
     for (auto *Succ : Succs)
       MBB->removeSuccessor(Succ);
     // To prevent deleting the same BB multiple times, which can happen when
     // 'MBBs' contain both a parent and a child
     Deleted.insert(MBB);
     MBB->eraseFromParent();
   }
 }

 bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** Late EH Prepare **********\n"
                        "********** Function: "
                     << MF.getName() << '\n');

   if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
       ExceptionHandling::Wasm)
     return false;

   bool Changed = false;
   if (MF.getFunction().hasPersonalityFn()) {
     Changed |= removeUnreachableEHPads(MF);
     recordCatchRetBBs(MF);
     Changed |= hoistCatches(MF);
     Changed |= addCatchAlls(MF);
     Changed |= replaceFuncletReturns(MF);
     Changed |= ensureSingleBBTermPads(MF);
   }
   Changed |= removeUnnecessaryUnreachables(MF);
   if (MF.getFunction().hasPersonalityFn())
     Changed |= restoreStackPointer(MF);
   return Changed;
 }

 // Remove unreachable EH pads and its children. If they remain, CFG
 // stackification can be tricky.
 bool WebAssemblyLateEHPrepare::removeUnreachableEHPads(MachineFunction &MF) {
   SmallVector<MachineBasicBlock *, 4> ToDelete;
   for (auto &MBB : MF)
     if (MBB.isEHPad() && MBB.pred_empty())
       ToDelete.push_back(&MBB);
   eraseDeadBBsAndChildren(ToDelete);
   return !ToDelete.empty();
 }

 // Record which BB ends with catchret instruction, because this will be replaced
 // with 'br's later. This set of catchret BBs is necessary in 'getMatchingEHPad'
 // function.
 void WebAssemblyLateEHPrepare::recordCatchRetBBs(MachineFunction &MF) {
   CatchRetBBs.clear();
   for (auto &MBB : MF) {
     auto Pos = MBB.getFirstTerminator();
     if (Pos == MBB.end())
       continue;
     MachineInstr *TI = &*Pos;
     if (TI->getOpcode() == WebAssembly::CATCHRET)
       CatchRetBBs.insert(&MBB);
   }
 }

 // Hoist catch instructions to the beginning of their matching EH pad BBs in
 // case,
 // (1) catch instruction is not the first instruction in EH pad.
 // ehpad:
 //   some_other_instruction
 //   ...
 //   %exn = catch 0
 // (2) catch instruction is in a non-EH pad BB. For example,
 // ehpad:
 //   br bb0
 // bb0:
 //   %exn = catch 0
 bool WebAssemblyLateEHPrepare::hoistCatches(MachineFunction &MF) {
   bool Changed = false;
   SmallVector<MachineInstr *, 16> Catches;
   for (auto &MBB : MF)
     for (auto &MI : MBB)
       if (WebAssembly::isCatch(MI.getOpcode()))
         Catches.push_back(&MI);

   for (auto *Catch : Catches) {
     MachineBasicBlock *EHPad = getMatchingEHPad(Catch);
     assert(EHPad && "No matching EH pad for catch");
     auto InsertPos = EHPad->begin();
     // Skip EH_LABELs in the beginning of an EH pad if present. We don't use
     // these labels at the moment, but other targets also seem to have an
     // EH_LABEL instruction in the beginning of an EH pad.
     while (InsertPos != EHPad->end() && InsertPos->isEHLabel())
       InsertPos++;
     if (InsertPos == Catch)
       continue;
     Changed = true;
     EHPad->insert(InsertPos, Catch->removeFromParent());
   }
   return Changed;
 }

 // Add catch_all to beginning of cleanup pads.
 bool WebAssemblyLateEHPrepare::addCatchAlls(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

   for (auto &MBB : MF) {
     if (!MBB.isEHPad())
       continue;
     auto InsertPos = MBB.begin();
     // Skip EH_LABELs in the beginning of an EH pad if present.
     while (InsertPos != MBB.end() && InsertPos->isEHLabel())
       InsertPos++;
     // This runs after hoistCatches(), so we assume that if there is a catch,
     // that should be the first non-EH-label instruction in an EH pad.
     if (InsertPos == MBB.end() ||
         !WebAssembly::isCatch(InsertPos->getOpcode())) {
       Changed = true;
       BuildMI(MBB, InsertPos, InsertPos->getDebugLoc(),
               TII.get(WebAssembly::CATCH_ALL));
     }
   }
   return Changed;
 }

 // Replace pseudo-instructions catchret and cleanupret with br and rethrow
 // respectively.
 bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

   for (auto &MBB : MF) {
     auto Pos = MBB.getFirstTerminator();
     if (Pos == MBB.end())
       continue;
     MachineInstr *TI = &*Pos;

     switch (TI->getOpcode()) {
     case WebAssembly::CATCHRET: {
       // Replace a catchret with a branch
       MachineBasicBlock *TBB = TI->getOperand(0).getMBB();
       if (!MBB.isLayoutSuccessor(TBB))
         BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::BR))
             .addMBB(TBB);
       TI->eraseFromParent();
       Changed = true;
       break;
     }
     case WebAssembly::CLEANUPRET: {
       // Replace a cleanupret with a rethrow. For C++ support, currently
       // rethrow's immediate argument is always 0 (= the latest exception).
       BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
           .addImm(0);
       TI->eraseFromParent();
       Changed = true;
       break;
     }
     }
   }
   return Changed;
 }

 // Remove unnecessary unreachables after a throw or rethrow.
 bool WebAssemblyLateEHPrepare::removeUnnecessaryUnreachables(
     MachineFunction &MF) {
   bool Changed = false;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.getOpcode() != WebAssembly::THROW &&
           MI.getOpcode() != WebAssembly::RETHROW)
         continue;
       Changed = true;

       // The instruction after the throw should be an unreachable or a branch to
       // another BB that should eventually lead to an unreachable. Delete it
       // because throw itself is a terminator, and also delete successors if
       // any.
       MBB.erase(std::next(MI.getIterator()), MBB.end());
       SmallVector<MachineBasicBlock *, 8> Succs(MBB.successors());
       for (auto *Succ : Succs)
         if (!Succ->isEHPad())
           MBB.removeSuccessor(Succ);
       eraseDeadBBsAndChildren(Succs);
     }
   }

   return Changed;
 }

 // Clang-generated terminate pads are an single-BB EH pad in the form of
 // termpad:
 //   %exn = catch $__cpp_exception
 //   call @__clang_call_terminate(%exn)
 //   unreachable
 // (There can be local.set and local.gets before the call if we didn't run
 // RegStackify)
 // But code transformations can change or add more control flow, so the call to
 // __clang_call_terminate() function may not be in the original EH pad anymore.
 // This ensures every terminate pad is a single BB in the form illustrated
 // above.
 //
 // This is preparation work for the HandleEHTerminatePads pass later, which
 // duplicates terminate pads both for 'catch' and 'catch_all'. Refer to
 // WebAssemblyHandleEHTerminatePads.cpp for details.
 bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

   // Find calls to __clang_call_terminate()
   SmallVector<MachineInstr *, 8> ClangCallTerminateCalls;
   SmallPtrSet<MachineBasicBlock *, 8> TermPads;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.isCall()) {
         const MachineOperand &CalleeOp = MI.getOperand(0);
         if (CalleeOp.isGlobal() && CalleeOp.getGlobal()->getName() ==
                                        WebAssembly::ClangCallTerminateFn) {
           MachineBasicBlock *EHPad = getMatchingEHPad(&MI);
           assert(EHPad && "No matching EH pad for __clang_call_terminate");
           // In case a __clang_call_terminate call is duplicated during code
           // transformation so one terminate pad contains multiple
           // __clang_call_terminate calls, we only count one of them
           if (TermPads.insert(EHPad).second)
             ClangCallTerminateCalls.push_back(&MI);
         }
       }
     }
   }

   bool Changed = false;
   for (auto *Call : ClangCallTerminateCalls) {
     MachineBasicBlock *EHPad = getMatchingEHPad(Call);
     assert(EHPad && "No matching EH pad for __clang_call_terminate");

     // If it is already the form we want, skip it
     if (Call->getParent() == EHPad &&
         Call->getNextNode()->getOpcode() == WebAssembly::UNREACHABLE)
       continue;

     // In case the __clang_call_terminate() call is not in its matching EH pad,
     // move the call to the end of EH pad and add an unreachable instruction
     // after that. Delete all successors and their children if any, because here
     // the program terminates.
     Changed = true;
     // This runs after hoistCatches(), so catch instruction should be at the top
     MachineInstr *Catch = WebAssembly::findCatch(EHPad);
     assert(Catch && "EH pad does not have a catch instruction");
     // Takes the result register of the catch instruction as argument. There may
     // have been some other local.set/local.gets in between, but at this point
     // we don't care.
     Call->getOperand(1).setReg(Catch->getOperand(0).getReg());
     auto InsertPos = std::next(MachineBasicBlock::iterator(Catch));
     EHPad->insert(InsertPos, Call->removeFromParent());
     BuildMI(*EHPad, InsertPos, Call->getDebugLoc(),
             TII.get(WebAssembly::UNREACHABLE));
     EHPad->erase(InsertPos, EHPad->end());
     SmallVector<MachineBasicBlock *, 8> Succs(EHPad->successors());
     for (auto *Succ : Succs)
       EHPad->removeSuccessor(Succ);
     eraseDeadBBsAndChildren(Succs);
   }
   return Changed;
 }

 // After the stack is unwound due to a thrown exception, the __stack_pointer
 // global can point to an invalid address. This inserts instructions that
 // restore __stack_pointer global.
 bool WebAssemblyLateEHPrepare::restoreStackPointer(MachineFunction &MF) {
   const auto *FrameLowering = static_cast<const WebAssemblyFrameLowering *>(
       MF.getSubtarget().getFrameLowering());
   if (!FrameLowering->needsPrologForEH(MF))
     return false;
   bool Changed = false;

   for (auto &MBB : MF) {
     if (!MBB.isEHPad())
       continue;
     Changed = true;

     // Insert __stack_pointer restoring instructions at the beginning of each EH
     // pad, after the catch instruction. Here it is safe to assume that SP32
     // holds the latest value of __stack_pointer, because the only exception for
     // this case is when a function uses the red zone, but that only happens
     // with leaf functions, and we don't restore __stack_pointer in leaf
     // functions anyway.
     auto InsertPos = MBB.begin();
     // Skip EH_LABELs in the beginning of an EH pad if present.
     while (InsertPos != MBB.end() && InsertPos->isEHLabel())
       InsertPos++;
     assert(InsertPos != MBB.end() &&
            WebAssembly::isCatch(InsertPos->getOpcode()) &&
            "catch/catch_all should be present in every EH pad at this point");
     ++InsertPos; // Skip the catch instruction
     FrameLowering->writeSPToGlobal(FrameLowering->getSPReg(MF), MF, MBB,
                                    InsertPos, MBB.begin()->getDebugLoc());
   }
   return Changed;
 }
	//=== WebAssemblyLateEHPrepare.cpp - WebAssembly Exception Preparation -===//
	//
	// 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
	/// \brief Does various transformations for exception handling.
	///
	//===----------------------------------------------------------------------===//

	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssembly.h"
	#include "WebAssemblySubtarget.h"
	#include "WebAssemblyUtilities.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/WasmEHFuncInfo.h"
	#include "llvm/MC/MCAsmInfo.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Target/TargetMachine.h"
	using namespace llvm;

	#define DEBUG_TYPE "wasm-late-eh-prepare"

	namespace {
	class WebAssemblyLateEHPrepare final : public MachineFunctionPass {
	StringRef getPassName() const override {
	return "WebAssembly Late Prepare Exception";
	}

	bool runOnMachineFunction(MachineFunction &MF) override;
	bool removeUnreachableEHPads(MachineFunction &MF);
	void recordCatchRetBBs(MachineFunction &MF);
	bool hoistCatches(MachineFunction &MF);
	bool addCatchAlls(MachineFunction &MF);
	bool replaceFuncletReturns(MachineFunction &MF);
	bool removeUnnecessaryUnreachables(MachineFunction &MF);
	bool ensureSingleBBTermPads(MachineFunction &MF);
	bool restoreStackPointer(MachineFunction &MF);

	MachineBasicBlock getMatchingEHPad(MachineInstr MI);
	SmallPtrSet<MachineBasicBlock *, 8> CatchRetBBs;

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

	char WebAssemblyLateEHPrepare::ID = 0;
	INITIALIZE_PASS(WebAssemblyLateEHPrepare, DEBUG_TYPE,
	"WebAssembly Late Exception Preparation", false, false)

	FunctionPass *llvm::createWebAssemblyLateEHPrepare() {
	return new WebAssemblyLateEHPrepare();
	}

	// Returns the nearest EH pad that dominates this instruction. This does not use
	// dominator analysis; it just does BFS on its predecessors until arriving at an
	// EH pad. This assumes valid EH scopes so the first EH pad it arrives in all
	// possible search paths should be the same.
	// Returns nullptr in case it does not find any EH pad in the search, or finds
	// multiple different EH pads.
	MachineBasicBlock *
	WebAssemblyLateEHPrepare::getMatchingEHPad(MachineInstr *MI) {
	MachineFunction *MF = MI->getParent()->getParent();
	SmallVector<MachineBasicBlock *, 2> WL;
	SmallPtrSet<MachineBasicBlock *, 2> Visited;
	WL.push_back(MI->getParent());
	MachineBasicBlock *EHPad = nullptr;
	while (!WL.empty()) {
	MachineBasicBlock *MBB = WL.pop_back_val();
	if (Visited.count(MBB))
	continue;
	Visited.insert(MBB);
	if (MBB->isEHPad()) {
	if (EHPad && EHPad != MBB)
	return nullptr;
	EHPad = MBB;
	continue;
	}
	if (MBB == &MF->front())
	return nullptr;
	for (auto *Pred : MBB->predecessors())
	if (!CatchRetBBs.count(Pred)) // We don't go into child scopes
	WL.push_back(Pred);
	}
	return EHPad;
	}

	// Erase the specified BBs if the BB does not have any remaining predecessors,
	// and also all its dead children.
	template <typename Container>
	static void eraseDeadBBsAndChildren(const Container &MBBs) {
	SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
	SmallPtrSet<MachineBasicBlock *, 8> Deleted;
	while (!WL.empty()) {
	MachineBasicBlock *MBB = WL.pop_back_val();
	if (Deleted.count(MBB) \|\| !MBB->pred_empty())
	continue;
	SmallVector<MachineBasicBlock *, 4> Succs(MBB->successors());
	WL.append(MBB->succ_begin(), MBB->succ_end());
	for (auto *Succ : Succs)
	MBB->removeSuccessor(Succ);
	// To prevent deleting the same BB multiple times, which can happen when
	// 'MBBs' contain both a parent and a child
	Deleted.insert(MBB);
	MBB->eraseFromParent();
	}
	}

	bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
	LLVM_DEBUG(dbgs() << "******** Late EH Prepare ********\n"
	"********** Function: "
	<< MF.getName() << '\n');

	if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
	ExceptionHandling::Wasm)
	return false;

	bool Changed = false;
	if (MF.getFunction().hasPersonalityFn()) {
	Changed \|= removeUnreachableEHPads(MF);
	recordCatchRetBBs(MF);
	Changed \|= hoistCatches(MF);
	Changed \|= addCatchAlls(MF);
	Changed \|= replaceFuncletReturns(MF);
	Changed \|= ensureSingleBBTermPads(MF);
	}
	Changed \|= removeUnnecessaryUnreachables(MF);
	if (MF.getFunction().hasPersonalityFn())
	Changed \|= restoreStackPointer(MF);
	return Changed;
	}

	// Remove unreachable EH pads and its children. If they remain, CFG
	// stackification can be tricky.
	bool WebAssemblyLateEHPrepare::removeUnreachableEHPads(MachineFunction &MF) {
	SmallVector<MachineBasicBlock *, 4> ToDelete;
	for (auto &MBB : MF)
	if (MBB.isEHPad() && MBB.pred_empty())
	ToDelete.push_back(&MBB);
	eraseDeadBBsAndChildren(ToDelete);
	return !ToDelete.empty();
	}

	// Record which BB ends with catchret instruction, because this will be replaced
	// with 'br's later. This set of catchret BBs is necessary in 'getMatchingEHPad'
	// function.
	void WebAssemblyLateEHPrepare::recordCatchRetBBs(MachineFunction &MF) {
	CatchRetBBs.clear();
	for (auto &MBB : MF) {
	auto Pos = MBB.getFirstTerminator();
	if (Pos == MBB.end())
	continue;
	MachineInstr TI = &Pos;
	if (TI->getOpcode() == WebAssembly::CATCHRET)
	CatchRetBBs.insert(&MBB);
	}
	}

	// Hoist catch instructions to the beginning of their matching EH pad BBs in
	// case,
	// (1) catch instruction is not the first instruction in EH pad.
	// ehpad:
	// some_other_instruction
	// ...
	// %exn = catch 0
	// (2) catch instruction is in a non-EH pad BB. For example,
	// ehpad:
	// br bb0
	// bb0:
	// %exn = catch 0
	bool WebAssemblyLateEHPrepare::hoistCatches(MachineFunction &MF) {
	bool Changed = false;
	SmallVector<MachineInstr *, 16> Catches;
	for (auto &MBB : MF)
	for (auto &MI : MBB)
	if (WebAssembly::isCatch(MI.getOpcode()))
	Catches.push_back(&MI);

	for (auto *Catch : Catches) {
	MachineBasicBlock *EHPad = getMatchingEHPad(Catch);
	assert(EHPad && "No matching EH pad for catch");
	auto InsertPos = EHPad->begin();
	// Skip EH_LABELs in the beginning of an EH pad if present. We don't use
	// these labels at the moment, but other targets also seem to have an
	// EH_LABEL instruction in the beginning of an EH pad.
	while (InsertPos != EHPad->end() && InsertPos->isEHLabel())
	InsertPos++;
	if (InsertPos == Catch)
	continue;
	Changed = true;
	EHPad->insert(InsertPos, Catch->removeFromParent());
	}
	return Changed;
	}

	// Add catch_all to beginning of cleanup pads.
	bool WebAssemblyLateEHPrepare::addCatchAlls(MachineFunction &MF) {
	bool Changed = false;
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

	for (auto &MBB : MF) {
	if (!MBB.isEHPad())
	continue;
	auto InsertPos = MBB.begin();
	// Skip EH_LABELs in the beginning of an EH pad if present.
	while (InsertPos != MBB.end() && InsertPos->isEHLabel())
	InsertPos++;
	// This runs after hoistCatches(), so we assume that if there is a catch,
	// that should be the first non-EH-label instruction in an EH pad.
	if (InsertPos == MBB.end() \|\|
	!WebAssembly::isCatch(InsertPos->getOpcode())) {
	Changed = true;
	BuildMI(MBB, InsertPos, InsertPos->getDebugLoc(),
	TII.get(WebAssembly::CATCH_ALL));
	}
	}
	return Changed;
	}

	// Replace pseudo-instructions catchret and cleanupret with br and rethrow
	// respectively.
	bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
	bool Changed = false;
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

	for (auto &MBB : MF) {
	auto Pos = MBB.getFirstTerminator();
	if (Pos == MBB.end())
	continue;
	MachineInstr TI = &Pos;

	switch (TI->getOpcode()) {
	case WebAssembly::CATCHRET: {
	// Replace a catchret with a branch
	MachineBasicBlock *TBB = TI->getOperand(0).getMBB();
	if (!MBB.isLayoutSuccessor(TBB))
	BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::BR))
	.addMBB(TBB);
	TI->eraseFromParent();
	Changed = true;
	break;
	}
	case WebAssembly::CLEANUPRET: {
	// Replace a cleanupret with a rethrow. For C++ support, currently
	// rethrow's immediate argument is always 0 (= the latest exception).
	BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
	.addImm(0);
	TI->eraseFromParent();
	Changed = true;
	break;
	}
	}
	}
	return Changed;
	}

	// Remove unnecessary unreachables after a throw or rethrow.
	bool WebAssemblyLateEHPrepare::removeUnnecessaryUnreachables(
	MachineFunction &MF) {
	bool Changed = false;
	for (auto &MBB : MF) {
	for (auto &MI : MBB) {
	if (MI.getOpcode() != WebAssembly::THROW &&
	MI.getOpcode() != WebAssembly::RETHROW)
	continue;
	Changed = true;

	// The instruction after the throw should be an unreachable or a branch to
	// another BB that should eventually lead to an unreachable. Delete it
	// because throw itself is a terminator, and also delete successors if
	// any.
	MBB.erase(std::next(MI.getIterator()), MBB.end());
	SmallVector<MachineBasicBlock *, 8> Succs(MBB.successors());
	for (auto *Succ : Succs)
	if (!Succ->isEHPad())
	MBB.removeSuccessor(Succ);
	eraseDeadBBsAndChildren(Succs);
	}
	}

	return Changed;
	}

	// Clang-generated terminate pads are an single-BB EH pad in the form of
	// termpad:
	// %exn = catch $__cpp_exception
	// call @__clang_call_terminate(%exn)
	// unreachable
	// (There can be local.set and local.gets before the call if we didn't run
	// RegStackify)
	// But code transformations can change or add more control flow, so the call to
	// __clang_call_terminate() function may not be in the original EH pad anymore.
	// This ensures every terminate pad is a single BB in the form illustrated
	// above.
	//
	// This is preparation work for the HandleEHTerminatePads pass later, which
	// duplicates terminate pads both for 'catch' and 'catch_all'. Refer to
	// WebAssemblyHandleEHTerminatePads.cpp for details.
	bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

	// Find calls to __clang_call_terminate()
	SmallVector<MachineInstr *, 8> ClangCallTerminateCalls;
	SmallPtrSet<MachineBasicBlock *, 8> TermPads;
	for (auto &MBB : MF) {
	for (auto &MI : MBB) {
	if (MI.isCall()) {
	const MachineOperand &CalleeOp = MI.getOperand(0);
	if (CalleeOp.isGlobal() && CalleeOp.getGlobal()->getName() ==
	WebAssembly::ClangCallTerminateFn) {
	MachineBasicBlock *EHPad = getMatchingEHPad(&MI);
	assert(EHPad && "No matching EH pad for __clang_call_terminate");
	// In case a __clang_call_terminate call is duplicated during code
	// transformation so one terminate pad contains multiple
	// __clang_call_terminate calls, we only count one of them
	if (TermPads.insert(EHPad).second)
	ClangCallTerminateCalls.push_back(&MI);
	}
	}
	}
	}

	bool Changed = false;
	for (auto *Call : ClangCallTerminateCalls) {
	MachineBasicBlock *EHPad = getMatchingEHPad(Call);
	assert(EHPad && "No matching EH pad for __clang_call_terminate");

	// If it is already the form we want, skip it
	if (Call->getParent() == EHPad &&
	Call->getNextNode()->getOpcode() == WebAssembly::UNREACHABLE)
	continue;

	// In case the __clang_call_terminate() call is not in its matching EH pad,
	// move the call to the end of EH pad and add an unreachable instruction
	// after that. Delete all successors and their children if any, because here
	// the program terminates.
	Changed = true;
	// This runs after hoistCatches(), so catch instruction should be at the top
	MachineInstr *Catch = WebAssembly::findCatch(EHPad);
	assert(Catch && "EH pad does not have a catch instruction");
	// Takes the result register of the catch instruction as argument. There may
	// have been some other local.set/local.gets in between, but at this point
	// we don't care.
	Call->getOperand(1).setReg(Catch->getOperand(0).getReg());
	auto InsertPos = std::next(MachineBasicBlock::iterator(Catch));
	EHPad->insert(InsertPos, Call->removeFromParent());
	BuildMI(*EHPad, InsertPos, Call->getDebugLoc(),
	TII.get(WebAssembly::UNREACHABLE));
	EHPad->erase(InsertPos, EHPad->end());
	SmallVector<MachineBasicBlock *, 8> Succs(EHPad->successors());
	for (auto *Succ : Succs)
	EHPad->removeSuccessor(Succ);
	eraseDeadBBsAndChildren(Succs);
	}
	return Changed;
	}

	// After the stack is unwound due to a thrown exception, the __stack_pointer
	// global can point to an invalid address. This inserts instructions that
	// restore __stack_pointer global.
	bool WebAssemblyLateEHPrepare::restoreStackPointer(MachineFunction &MF) {
	const auto FrameLowering = static_cast<const WebAssemblyFrameLowering >(
	MF.getSubtarget().getFrameLowering());
	if (!FrameLowering->needsPrologForEH(MF))
	return false;
	bool Changed = false;

	for (auto &MBB : MF) {
	if (!MBB.isEHPad())
	continue;
	Changed = true;

	// Insert __stack_pointer restoring instructions at the beginning of each EH
	// pad, after the catch instruction. Here it is safe to assume that SP32
	// holds the latest value of __stack_pointer, because the only exception for
	// this case is when a function uses the red zone, but that only happens
	// with leaf functions, and we don't restore __stack_pointer in leaf
	// functions anyway.
	auto InsertPos = MBB.begin();
	// Skip EH_LABELs in the beginning of an EH pad if present.
	while (InsertPos != MBB.end() && InsertPos->isEHLabel())
	InsertPos++;
	assert(InsertPos != MBB.end() &&
	WebAssembly::isCatch(InsertPos->getOpcode()) &&
	"catch/catch_all should be present in every EH pad at this point");
	++InsertPos; // Skip the catch instruction
	FrameLowering->writeSPToGlobal(FrameLowering->getSPReg(MF), MF, MBB,
	InsertPos, MBB.begin()->getDebugLoc());
	}
	return Changed;
	}