third_party/llvm-7.0/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp - SwiftShader - Git at Google

 //=== WebAssemblyLateEHPrepare.cpp - WebAssembly Exception Preparation -===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// \brief Does various transformations for exception handling.
 ///
 //===----------------------------------------------------------------------===//

 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssembly.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyUtilities.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/WasmEHFuncInfo.h"
 #include "llvm/MC/MCAsmInfo.h"
 using namespace llvm;

 #define DEBUG_TYPE "wasm-exception-prepare"

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

   bool runOnMachineFunction(MachineFunction &MF) override;

   bool replaceFuncletReturns(MachineFunction &MF);
   bool hoistCatches(MachineFunction &MF);
   bool addCatchAlls(MachineFunction &MF);
   bool addRethrows(MachineFunction &MF);
   bool ensureSingleBBTermPads(MachineFunction &MF);
   bool mergeTerminatePads(MachineFunction &MF);
   bool addCatchAllTerminatePads(MachineFunction &MF);

 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 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 *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;
     WL.append(MBB->pred_begin(), MBB->pred_end());
   }
   return EHPad;
 }

 // Erases the given BB and all its children from the function. If other BBs have
 // this BB as a successor, the successor relationships will be deleted as well.
 static void EraseBBAndChildren(MachineBasicBlock *MBB) {
   SmallVector<MachineBasicBlock *, 8> WL;
   WL.push_back(MBB);
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
     for (auto *Pred : MBB->predecessors())
       Pred->removeSuccessor(MBB);
     for (auto *Succ : MBB->successors()) {
       WL.push_back(Succ);
       MBB->removeSuccessor(Succ);
     }
     MBB->eraseFromParent();
   }
 }

 bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
   if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
       ExceptionHandling::Wasm)
     return false;

   bool Changed = false;
   Changed |= addRethrows(MF);
   if (!MF.getFunction().hasPersonalityFn())
     return Changed;
   Changed |= replaceFuncletReturns(MF);
   Changed |= hoistCatches(MF);
   Changed |= addCatchAlls(MF);
   Changed |= ensureSingleBBTermPads(MF);
   Changed |= mergeTerminatePads(MF);
   Changed |= addCatchAllTerminatePads(MF);
   return Changed;
 }

 bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   auto *EHInfo = MF.getWasmEHFuncInfo();

   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
       if (EHInfo->hasThrowUnwindDest(&MBB))
         BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
             .addMBB(EHInfo->getThrowUnwindDest(&MBB));
       else
         BuildMI(MBB, TI, TI->getDebugLoc(),
                 TII.get(WebAssembly::RETHROW_TO_CALLER));

       TI->eraseFromParent();
       Changed = true;
       break;
     }
     }
   }
   return Changed;
 }

 // 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))
         Catches.push_back(&MI);

   for (auto *Catch : Catches) {
     MachineBasicBlock *EHPad = GetMatchingEHPad(Catch);
     assert(EHPad && "No matching EH pad for catch");
     if (EHPad->begin() == Catch)
       continue;
     Changed = true;
     EHPad->insert(EHPad->begin(), 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;
     // This runs after hoistCatches(), so we assume that if there is a catch,
     // that should be the first instruction in an EH pad.
     if (!WebAssembly::isCatch(*MBB.begin())) {
       Changed = true;
       BuildMI(MBB, MBB.begin(), MBB.begin()->getDebugLoc(),
               TII.get(WebAssembly::CATCH_ALL));
     }
   }
   return Changed;
 }

 // Add a 'rethrow' instruction after __cxa_rethrow() call
 bool WebAssemblyLateEHPrepare::addRethrows(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   auto *EHInfo = MF.getWasmEHFuncInfo();

   for (auto &MBB : MF)
     for (auto &MI : MBB) {
       // Check if it is a call to __cxa_rethrow()
       if (!MI.isCall())
         continue;
       MachineOperand &CalleeOp = MI.getOperand(0);
       if (!CalleeOp.isGlobal() ||
           CalleeOp.getGlobal()->getName() != WebAssembly::CxaRethrowFn)
         continue;

       // Now we have __cxa_rethrow() call
       Changed = true;
       auto InsertPt = std::next(MachineBasicBlock::iterator(MI));
       while (InsertPt != MBB.end() && InsertPt->isLabel()) // Skip EH_LABELs
         ++InsertPt;
       MachineInstr *Rethrow = nullptr;
       if (EHInfo->hasThrowUnwindDest(&MBB))
         Rethrow = BuildMI(MBB, InsertPt, MI.getDebugLoc(),
                           TII.get(WebAssembly::RETHROW))
                       .addMBB(EHInfo->getThrowUnwindDest(&MBB));
       else
         Rethrow = BuildMI(MBB, InsertPt, MI.getDebugLoc(),
                           TII.get(WebAssembly::RETHROW_TO_CALLER));

       // Becasue __cxa_rethrow does not return, the instruction after the
       // rethrow should be an unreachable or a branch to another BB that should
       // eventually lead to an unreachable. Delete it because rethrow itself is
       // a terminator, and also delete non-EH pad successors if any.
       MBB.erase(std::next(MachineBasicBlock::iterator(Rethrow)), MBB.end());
       for (auto *Succ : MBB.successors())
         if (!Succ->isEHPad())
           EraseBBAndChildren(Succ);
     }
   return Changed;
 }

 // Terminate pads are an single-BB EH pad in the form of
 // termpad:
 //   %exn = catch 0
 //   call @__clang_call_terminate(%exn)
 //   unreachable
 // (There can be set_local and get_locals 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.
 bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

   // Find calls to __clang_call_terminate()
   SmallVector<MachineInstr *, 8> ClangCallTerminateCalls;
   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)
           ClangCallTerminateCalls.push_back(&MI);
       }

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

     // 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;
     MachineInstr *Catch = &*EHPad->begin();
     // This runs after hoistCatches(), so catch instruction should be at the top
     assert(WebAssembly::isCatch(*Catch));
     // Takes the result register of the catch instruction as argument. There may
     // have been some other set_local/get_locals 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());
     for (auto *Succ : EHPad->successors())
       EraseBBAndChildren(Succ);
   }
   return Changed;
 }

 // In case there are multiple terminate pads, merge them into one for code size.
 // This runs after ensureSingleBBTermPads() and assumes every terminate pad is a
 // single BB.
 // In principle this violates EH scope relationship because it can merge
 // multiple inner EH scopes, each of which is in different outer EH scope. But
 // getEHScopeMembership() function will not be called after this, so it is fine.
 bool WebAssemblyLateEHPrepare::mergeTerminatePads(MachineFunction &MF) {
   SmallVector<MachineBasicBlock *, 8> TermPads;
   for (auto &MBB : MF)
     if (WebAssembly::isCatchTerminatePad(MBB))
       TermPads.push_back(&MBB);
   if (TermPads.empty())
     return false;

   MachineBasicBlock *UniqueTermPad = TermPads.front();
   for (auto *TermPad :
        llvm::make_range(std::next(TermPads.begin()), TermPads.end())) {
     SmallVector<MachineBasicBlock *, 2> Preds(TermPad->pred_begin(),
                                               TermPad->pred_end());
     for (auto *Pred : Preds)
       Pred->replaceSuccessor(TermPad, UniqueTermPad);
     TermPad->eraseFromParent();
   }
   return true;
 }

 // Terminate pads are cleanup pads, so they should start with a 'catch_all'
 // instruction. But in the Itanium model, when we have a C++ exception object,
 // we pass them to __clang_call_terminate function, which calls __cxa_end_catch
 // with the passed exception pointer and then std::terminate. This is the reason
 // that terminate pads are generated with not a catch_all but a catch
 // instruction in clang and earlier llvm passes. Here we append a terminate pad
 // with a catch_all after each existing terminate pad so we can also catch
 // foreign exceptions. For every terminate pad:
 //   %exn = catch 0
 //   call @__clang_call_terminate(%exn)
 //   unreachable
 // We append this BB right after that:
 //   catch_all
 //   call @std::terminate()
 //   unreachable
 bool WebAssemblyLateEHPrepare::addCatchAllTerminatePads(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   SmallVector<MachineBasicBlock *, 8> TermPads;
   for (auto &MBB : MF)
     if (WebAssembly::isCatchTerminatePad(MBB))
       TermPads.push_back(&MBB);
   if (TermPads.empty())
     return false;

   Function *StdTerminateFn =
       MF.getFunction().getParent()->getFunction(WebAssembly::StdTerminateFn);
   assert(StdTerminateFn && "There is no std::terminate() function");
   for (auto *CatchTermPad : TermPads) {
     DebugLoc DL = CatchTermPad->findDebugLoc(CatchTermPad->begin());
     auto *CatchAllTermPad = MF.CreateMachineBasicBlock();
     MF.insert(std::next(MachineFunction::iterator(CatchTermPad)),
               CatchAllTermPad);
     CatchAllTermPad->setIsEHPad();
     BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::CATCH_ALL));
     BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::CALL_VOID))
         .addGlobalAddress(StdTerminateFn);
     BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::UNREACHABLE));

     // Actually this CatchAllTermPad (new terminate pad with a catch_all) is not
     // a successor of an existing terminate pad. CatchAllTermPad should have all
     // predecessors CatchTermPad has instead. This is a hack to force
     // CatchAllTermPad be always sorted right after CatchTermPad; the correct
     // predecessor-successor relationships will be restored in CFGStackify pass.
     CatchTermPad->addSuccessor(CatchAllTermPad);
   }
   return true;
 }
	//=== WebAssemblyLateEHPrepare.cpp - WebAssembly Exception Preparation -===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	///
	/// \file
	/// \brief Does various transformations for exception handling.
	///
	//===----------------------------------------------------------------------===//

	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssembly.h"
	#include "WebAssemblySubtarget.h"
	#include "WebAssemblyUtilities.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/WasmEHFuncInfo.h"
	#include "llvm/MC/MCAsmInfo.h"
	using namespace llvm;

	#define DEBUG_TYPE "wasm-exception-prepare"

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

	bool runOnMachineFunction(MachineFunction &MF) override;

	bool replaceFuncletReturns(MachineFunction &MF);
	bool hoistCatches(MachineFunction &MF);
	bool addCatchAlls(MachineFunction &MF);
	bool addRethrows(MachineFunction &MF);
	bool ensureSingleBBTermPads(MachineFunction &MF);
	bool mergeTerminatePads(MachineFunction &MF);
	bool addCatchAllTerminatePads(MachineFunction &MF);

	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 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 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;
	WL.append(MBB->pred_begin(), MBB->pred_end());
	}
	return EHPad;
	}

	// Erases the given BB and all its children from the function. If other BBs have
	// this BB as a successor, the successor relationships will be deleted as well.
	static void EraseBBAndChildren(MachineBasicBlock *MBB) {
	SmallVector<MachineBasicBlock *, 8> WL;
	WL.push_back(MBB);
	while (!WL.empty()) {
	MachineBasicBlock *MBB = WL.pop_back_val();
	for (auto *Pred : MBB->predecessors())
	Pred->removeSuccessor(MBB);
	for (auto *Succ : MBB->successors()) {
	WL.push_back(Succ);
	MBB->removeSuccessor(Succ);
	}
	MBB->eraseFromParent();
	}
	}

	bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
	if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
	ExceptionHandling::Wasm)
	return false;

	bool Changed = false;
	Changed \|= addRethrows(MF);
	if (!MF.getFunction().hasPersonalityFn())
	return Changed;
	Changed \|= replaceFuncletReturns(MF);
	Changed \|= hoistCatches(MF);
	Changed \|= addCatchAlls(MF);
	Changed \|= ensureSingleBBTermPads(MF);
	Changed \|= mergeTerminatePads(MF);
	Changed \|= addCatchAllTerminatePads(MF);
	return Changed;
	}

	bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
	bool Changed = false;
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	auto *EHInfo = MF.getWasmEHFuncInfo();

	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
	if (EHInfo->hasThrowUnwindDest(&MBB))
	BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
	.addMBB(EHInfo->getThrowUnwindDest(&MBB));
	else
	BuildMI(MBB, TI, TI->getDebugLoc(),
	TII.get(WebAssembly::RETHROW_TO_CALLER));

	TI->eraseFromParent();
	Changed = true;
	break;
	}
	}
	}
	return Changed;
	}

	// 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))
	Catches.push_back(&MI);

	for (auto *Catch : Catches) {
	MachineBasicBlock *EHPad = GetMatchingEHPad(Catch);
	assert(EHPad && "No matching EH pad for catch");
	if (EHPad->begin() == Catch)
	continue;
	Changed = true;
	EHPad->insert(EHPad->begin(), 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;
	// This runs after hoistCatches(), so we assume that if there is a catch,
	// that should be the first instruction in an EH pad.
	if (!WebAssembly::isCatch(*MBB.begin())) {
	Changed = true;
	BuildMI(MBB, MBB.begin(), MBB.begin()->getDebugLoc(),
	TII.get(WebAssembly::CATCH_ALL));
	}
	}
	return Changed;
	}

	// Add a 'rethrow' instruction after __cxa_rethrow() call
	bool WebAssemblyLateEHPrepare::addRethrows(MachineFunction &MF) {
	bool Changed = false;
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	auto *EHInfo = MF.getWasmEHFuncInfo();

	for (auto &MBB : MF)
	for (auto &MI : MBB) {
	// Check if it is a call to __cxa_rethrow()
	if (!MI.isCall())
	continue;
	MachineOperand &CalleeOp = MI.getOperand(0);
	if (!CalleeOp.isGlobal() \|\|
	CalleeOp.getGlobal()->getName() != WebAssembly::CxaRethrowFn)
	continue;

	// Now we have __cxa_rethrow() call
	Changed = true;
	auto InsertPt = std::next(MachineBasicBlock::iterator(MI));
	while (InsertPt != MBB.end() && InsertPt->isLabel()) // Skip EH_LABELs
	++InsertPt;
	MachineInstr *Rethrow = nullptr;
	if (EHInfo->hasThrowUnwindDest(&MBB))
	Rethrow = BuildMI(MBB, InsertPt, MI.getDebugLoc(),
	TII.get(WebAssembly::RETHROW))
	.addMBB(EHInfo->getThrowUnwindDest(&MBB));
	else
	Rethrow = BuildMI(MBB, InsertPt, MI.getDebugLoc(),
	TII.get(WebAssembly::RETHROW_TO_CALLER));

	// Becasue __cxa_rethrow does not return, the instruction after the
	// rethrow should be an unreachable or a branch to another BB that should
	// eventually lead to an unreachable. Delete it because rethrow itself is
	// a terminator, and also delete non-EH pad successors if any.
	MBB.erase(std::next(MachineBasicBlock::iterator(Rethrow)), MBB.end());
	for (auto *Succ : MBB.successors())
	if (!Succ->isEHPad())
	EraseBBAndChildren(Succ);
	}
	return Changed;
	}

	// Terminate pads are an single-BB EH pad in the form of
	// termpad:
	// %exn = catch 0
	// call @__clang_call_terminate(%exn)
	// unreachable
	// (There can be set_local and get_locals 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.
	bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

	// Find calls to __clang_call_terminate()
	SmallVector<MachineInstr *, 8> ClangCallTerminateCalls;
	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)
	ClangCallTerminateCalls.push_back(&MI);
	}

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

	// 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;
	MachineInstr Catch = &EHPad->begin();
	// This runs after hoistCatches(), so catch instruction should be at the top
	assert(WebAssembly::isCatch(*Catch));
	// Takes the result register of the catch instruction as argument. There may
	// have been some other set_local/get_locals 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());
	for (auto *Succ : EHPad->successors())
	EraseBBAndChildren(Succ);
	}
	return Changed;
	}

	// In case there are multiple terminate pads, merge them into one for code size.
	// This runs after ensureSingleBBTermPads() and assumes every terminate pad is a
	// single BB.
	// In principle this violates EH scope relationship because it can merge
	// multiple inner EH scopes, each of which is in different outer EH scope. But
	// getEHScopeMembership() function will not be called after this, so it is fine.
	bool WebAssemblyLateEHPrepare::mergeTerminatePads(MachineFunction &MF) {
	SmallVector<MachineBasicBlock *, 8> TermPads;
	for (auto &MBB : MF)
	if (WebAssembly::isCatchTerminatePad(MBB))
	TermPads.push_back(&MBB);
	if (TermPads.empty())
	return false;

	MachineBasicBlock *UniqueTermPad = TermPads.front();
	for (auto *TermPad :
	llvm::make_range(std::next(TermPads.begin()), TermPads.end())) {
	SmallVector<MachineBasicBlock *, 2> Preds(TermPad->pred_begin(),
	TermPad->pred_end());
	for (auto *Pred : Preds)
	Pred->replaceSuccessor(TermPad, UniqueTermPad);
	TermPad->eraseFromParent();
	}
	return true;
	}

	// Terminate pads are cleanup pads, so they should start with a 'catch_all'
	// instruction. But in the Itanium model, when we have a C++ exception object,
	// we pass them to __clang_call_terminate function, which calls __cxa_end_catch
	// with the passed exception pointer and then std::terminate. This is the reason
	// that terminate pads are generated with not a catch_all but a catch
	// instruction in clang and earlier llvm passes. Here we append a terminate pad
	// with a catch_all after each existing terminate pad so we can also catch
	// foreign exceptions. For every terminate pad:
	// %exn = catch 0
	// call @__clang_call_terminate(%exn)
	// unreachable
	// We append this BB right after that:
	// catch_all
	// call @std::terminate()
	// unreachable
	bool WebAssemblyLateEHPrepare::addCatchAllTerminatePads(MachineFunction &MF) {
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	SmallVector<MachineBasicBlock *, 8> TermPads;
	for (auto &MBB : MF)
	if (WebAssembly::isCatchTerminatePad(MBB))
	TermPads.push_back(&MBB);
	if (TermPads.empty())
	return false;

	Function *StdTerminateFn =
	MF.getFunction().getParent()->getFunction(WebAssembly::StdTerminateFn);
	assert(StdTerminateFn && "There is no std::terminate() function");
	for (auto *CatchTermPad : TermPads) {
	DebugLoc DL = CatchTermPad->findDebugLoc(CatchTermPad->begin());
	auto *CatchAllTermPad = MF.CreateMachineBasicBlock();
	MF.insert(std::next(MachineFunction::iterator(CatchTermPad)),
	CatchAllTermPad);
	CatchAllTermPad->setIsEHPad();
	BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::CATCH_ALL));
	BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::CALL_VOID))
	.addGlobalAddress(StdTerminateFn);
	BuildMI(CatchAllTermPad, DL, TII.get(WebAssembly::UNREACHABLE));

	// Actually this CatchAllTermPad (new terminate pad with a catch_all) is not
	// a successor of an existing terminate pad. CatchAllTermPad should have all
	// predecessors CatchTermPad has instead. This is a hack to force
	// CatchAllTermPad be always sorted right after CatchTermPad; the correct
	// predecessor-successor relationships will be restored in CFGStackify pass.
	CatchTermPad->addSuccessor(CatchAllTermPad);
	}
	return true;
	}