third_party/llvm-10.0/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp - SwiftShader - 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/SmallSet.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/WasmEHFuncInfo.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/Support/Debug.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 addCatches(MachineFunction &MF);
   bool replaceFuncletReturns(MachineFunction &MF);
   bool removeUnnecessaryUnreachables(MachineFunction &MF);
   bool addExceptionExtraction(MachineFunction &MF);
   bool restoreStackPointer(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 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.
 static 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;
 }

 // 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());
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
     if (!MBB->pred_empty())
       continue;
     SmallVector<MachineBasicBlock *, 4> Succs(MBB->succ_begin(),
                                               MBB->succ_end());
     WL.append(MBB->succ_begin(), MBB->succ_end());
     for (auto *Succ : Succs)
       MBB->removeSuccessor(Succ);
     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 |= addCatches(MF);
     Changed |= replaceFuncletReturns(MF);
   }
   Changed |= removeUnnecessaryUnreachables(MF);
   if (MF.getFunction().hasPersonalityFn()) {
     Changed |= addExceptionExtraction(MF);
     Changed |= restoreStackPointer(MF);
   }
   return Changed;
 }

 // Add catch instruction to beginning of catchpads and cleanuppads.
 bool WebAssemblyLateEHPrepare::addCatches(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   MachineRegisterInfo &MRI = MF.getRegInfo();
   for (auto &MBB : MF) {
     if (MBB.isEHPad()) {
       Changed = true;
       auto InsertPos = MBB.begin();
       if (InsertPos->isEHLabel()) // EH pad starts with an EH label
         ++InsertPos;
       Register DstReg = MRI.createVirtualRegister(&WebAssembly::EXNREFRegClass);
       BuildMI(MBB, InsertPos, MBB.begin()->getDebugLoc(),
               TII.get(WebAssembly::CATCH), DstReg);
     }
   }
   return Changed;
 }

 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:
     case WebAssembly::RETHROW_IN_CATCH: {
       // Replace a cleanupret/rethrow_in_catch with a rethrow
       auto *EHPad = getMatchingEHPad(TI);
       auto CatchPos = EHPad->begin();
       if (CatchPos->isEHLabel()) // EH pad starts with an EH label
         ++CatchPos;
       MachineInstr *Catch = &*CatchPos;
       Register ExnReg = Catch->getOperand(0).getReg();
       BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
           .addReg(ExnReg);
       TI->eraseFromParent();
       Changed = true;
       break;
     }
     }
   }
   return Changed;
 }

 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.succ_begin(),
                                                 MBB.succ_end());
       for (auto *Succ : Succs)
         if (!Succ->isEHPad())
           MBB.removeSuccessor(Succ);
       eraseDeadBBsAndChildren(Succs);
     }
   }

   return Changed;
 }

 // Wasm uses 'br_on_exn' instruction to check the tag of an exception. It takes
 // exnref type object returned by 'catch', and branches to the destination if it
 // matches a given tag. We currently use __cpp_exception symbol to represent the
 // tag for all C++ exceptions.
 //
 // block $l (result i32)
 //   ...
 //   ;; exnref $e is on the stack at this point
 //   br_on_exn $l $e ;; branch to $l with $e's arguments
 //   ...
 // end
 // ;; Here we expect the extracted values are on top of the wasm value stack
 // ... Handle exception using values ...
 //
 // br_on_exn takes an exnref object and branches if it matches the given tag.
 // There can be multiple br_on_exn instructions if we want to match for another
 // tag, but for now we only test for __cpp_exception tag, and if it does not
 // match, i.e., it is a foreign exception, we rethrow it.
 //
 // In the destination BB that's the target of br_on_exn, extracted exception
 // values (in C++'s case a single i32, which represents an exception pointer)
 // are placed on top of the wasm stack. Because we can't model wasm stack in
 // LLVM instruction, we use 'extract_exception' pseudo instruction to retrieve
 // it. The pseudo instruction will be deleted later.
 bool WebAssemblyLateEHPrepare::addExceptionExtraction(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   MachineRegisterInfo &MRI = MF.getRegInfo();
   auto *EHInfo = MF.getWasmEHFuncInfo();
   SmallVector<MachineInstr *, 16> ExtractInstrs;
   SmallVector<MachineInstr *, 8> ToDelete;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
         if (MI.getOperand(0).isDead())
           ToDelete.push_back(&MI);
         else
           ExtractInstrs.push_back(&MI);
       }
     }
   }
   bool Changed = !ToDelete.empty() || !ExtractInstrs.empty();
   for (auto *MI : ToDelete)
     MI->eraseFromParent();
   if (ExtractInstrs.empty())
     return Changed;

   // Find terminate pads.
   SmallSet<MachineBasicBlock *, 8> TerminatePads;
   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)
           TerminatePads.insert(getMatchingEHPad(&MI));
       }
     }
   }

   for (auto *Extract : ExtractInstrs) {
     MachineBasicBlock *EHPad = getMatchingEHPad(Extract);
     assert(EHPad && "No matching EH pad for extract_exception");
     auto CatchPos = EHPad->begin();
     if (CatchPos->isEHLabel()) // EH pad starts with an EH label
       ++CatchPos;
     MachineInstr *Catch = &*CatchPos;

     if (Catch->getNextNode() != Extract)
       EHPad->insert(Catch->getNextNode(), Extract->removeFromParent());

     // - Before:
     // ehpad:
     //   %exnref:exnref = catch
     //   %exn:i32 = extract_exception
     //   ... use exn ...
     //
     // - After:
     // ehpad:
     //   %exnref:exnref = catch
     //   br_on_exn %thenbb, $__cpp_exception, %exnref
     //   br %elsebb
     // elsebb:
     //   rethrow
     // thenbb:
     //   %exn:i32 = extract_exception
     //   ... use exn ...
     Register ExnReg = Catch->getOperand(0).getReg();
     auto *ThenMBB = MF.CreateMachineBasicBlock();
     auto *ElseMBB = MF.CreateMachineBasicBlock();
     MF.insert(std::next(MachineFunction::iterator(EHPad)), ElseMBB);
     MF.insert(std::next(MachineFunction::iterator(ElseMBB)), ThenMBB);
     ThenMBB->splice(ThenMBB->end(), EHPad, Extract, EHPad->end());
     ThenMBB->transferSuccessors(EHPad);
     EHPad->addSuccessor(ThenMBB);
     EHPad->addSuccessor(ElseMBB);

     DebugLoc DL = Extract->getDebugLoc();
     const char *CPPExnSymbol = MF.createExternalSymbolName("__cpp_exception");
     BuildMI(EHPad, DL, TII.get(WebAssembly::BR_ON_EXN))
         .addMBB(ThenMBB)
         .addExternalSymbol(CPPExnSymbol)
         .addReg(ExnReg);
     BuildMI(EHPad, DL, TII.get(WebAssembly::BR)).addMBB(ElseMBB);

     // When this is a terminate pad with __clang_call_terminate() call, we don't
     // rethrow it anymore and call __clang_call_terminate() with a nullptr
     // argument, which will call std::terminate().
     //
     // - Before:
     // ehpad:
     //   %exnref:exnref = catch
     //   %exn:i32 = extract_exception
     //   call @__clang_call_terminate(%exn)
     //   unreachable
     //
     // - After:
     // ehpad:
     //   %exnref:exnref = catch
     //   br_on_exn %thenbb, $__cpp_exception, %exnref
     //   br %elsebb
     // elsebb:
     //   call @__clang_call_terminate(0)
     //   unreachable
     // thenbb:
     //   %exn:i32 = extract_exception
     //   call @__clang_call_terminate(%exn)
     //   unreachable
     if (TerminatePads.count(EHPad)) {
       Function *ClangCallTerminateFn =
           MF.getFunction().getParent()->getFunction(
               WebAssembly::ClangCallTerminateFn);
       assert(ClangCallTerminateFn &&
              "There is no __clang_call_terminate() function");
       Register Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::CONST_I32), Reg).addImm(0);
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::CALL_VOID))
           .addGlobalAddress(ClangCallTerminateFn)
           .addReg(Reg);
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::UNREACHABLE));

     } else {
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::RETHROW)).addReg(ExnReg);
       if (EHInfo->hasEHPadUnwindDest(EHPad))
         ElseMBB->addSuccessor(EHInfo->getEHPadUnwindDest(EHPad));
     }
   }

   return true;
 }

 // 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();
     if (InsertPos->isEHLabel()) // EH pad starts with an EH label
       ++InsertPos;
     if (InsertPos->getOpcode() == WebAssembly::CATCH)
       ++InsertPos;
     FrameLowering->writeSPToGlobal(WebAssembly::SP32, 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/SmallSet.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/WasmEHFuncInfo.h"
	#include "llvm/MC/MCAsmInfo.h"
	#include "llvm/Support/Debug.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 addCatches(MachineFunction &MF);
	bool replaceFuncletReturns(MachineFunction &MF);
	bool removeUnnecessaryUnreachables(MachineFunction &MF);
	bool addExceptionExtraction(MachineFunction &MF);
	bool restoreStackPointer(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 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.
	static 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;
	}

	// 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());
	while (!WL.empty()) {
	MachineBasicBlock *MBB = WL.pop_back_val();
	if (!MBB->pred_empty())
	continue;
	SmallVector<MachineBasicBlock *, 4> Succs(MBB->succ_begin(),
	MBB->succ_end());
	WL.append(MBB->succ_begin(), MBB->succ_end());
	for (auto *Succ : Succs)
	MBB->removeSuccessor(Succ);
	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 \|= addCatches(MF);
	Changed \|= replaceFuncletReturns(MF);
	}
	Changed \|= removeUnnecessaryUnreachables(MF);
	if (MF.getFunction().hasPersonalityFn()) {
	Changed \|= addExceptionExtraction(MF);
	Changed \|= restoreStackPointer(MF);
	}
	return Changed;
	}

	// Add catch instruction to beginning of catchpads and cleanuppads.
	bool WebAssemblyLateEHPrepare::addCatches(MachineFunction &MF) {
	bool Changed = false;
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	MachineRegisterInfo &MRI = MF.getRegInfo();
	for (auto &MBB : MF) {
	if (MBB.isEHPad()) {
	Changed = true;
	auto InsertPos = MBB.begin();
	if (InsertPos->isEHLabel()) // EH pad starts with an EH label
	++InsertPos;
	Register DstReg = MRI.createVirtualRegister(&WebAssembly::EXNREFRegClass);
	BuildMI(MBB, InsertPos, MBB.begin()->getDebugLoc(),
	TII.get(WebAssembly::CATCH), DstReg);
	}
	}
	return Changed;
	}

	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:
	case WebAssembly::RETHROW_IN_CATCH: {
	// Replace a cleanupret/rethrow_in_catch with a rethrow
	auto *EHPad = getMatchingEHPad(TI);
	auto CatchPos = EHPad->begin();
	if (CatchPos->isEHLabel()) // EH pad starts with an EH label
	++CatchPos;
	MachineInstr Catch = &CatchPos;
	Register ExnReg = Catch->getOperand(0).getReg();
	BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
	.addReg(ExnReg);
	TI->eraseFromParent();
	Changed = true;
	break;
	}
	}
	}
	return Changed;
	}

	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.succ_begin(),
	MBB.succ_end());
	for (auto *Succ : Succs)
	if (!Succ->isEHPad())
	MBB.removeSuccessor(Succ);
	eraseDeadBBsAndChildren(Succs);
	}
	}

	return Changed;
	}

	// Wasm uses 'br_on_exn' instruction to check the tag of an exception. It takes
	// exnref type object returned by 'catch', and branches to the destination if it
	// matches a given tag. We currently use __cpp_exception symbol to represent the
	// tag for all C++ exceptions.
	//
	// block $l (result i32)
	// ...
	// ;; exnref $e is on the stack at this point
	// br_on_exn $l $e ;; branch to $l with $e's arguments
	// ...
	// end
	// ;; Here we expect the extracted values are on top of the wasm value stack
	// ... Handle exception using values ...
	//
	// br_on_exn takes an exnref object and branches if it matches the given tag.
	// There can be multiple br_on_exn instructions if we want to match for another
	// tag, but for now we only test for __cpp_exception tag, and if it does not
	// match, i.e., it is a foreign exception, we rethrow it.
	//
	// In the destination BB that's the target of br_on_exn, extracted exception
	// values (in C++'s case a single i32, which represents an exception pointer)
	// are placed on top of the wasm stack. Because we can't model wasm stack in
	// LLVM instruction, we use 'extract_exception' pseudo instruction to retrieve
	// it. The pseudo instruction will be deleted later.
	bool WebAssemblyLateEHPrepare::addExceptionExtraction(MachineFunction &MF) {
	const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
	MachineRegisterInfo &MRI = MF.getRegInfo();
	auto *EHInfo = MF.getWasmEHFuncInfo();
	SmallVector<MachineInstr *, 16> ExtractInstrs;
	SmallVector<MachineInstr *, 8> ToDelete;
	for (auto &MBB : MF) {
	for (auto &MI : MBB) {
	if (MI.getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
	if (MI.getOperand(0).isDead())
	ToDelete.push_back(&MI);
	else
	ExtractInstrs.push_back(&MI);
	}
	}
	}
	bool Changed = !ToDelete.empty() \|\| !ExtractInstrs.empty();
	for (auto *MI : ToDelete)
	MI->eraseFromParent();
	if (ExtractInstrs.empty())
	return Changed;

	// Find terminate pads.
	SmallSet<MachineBasicBlock *, 8> TerminatePads;
	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)
	TerminatePads.insert(getMatchingEHPad(&MI));
	}
	}
	}

	for (auto *Extract : ExtractInstrs) {
	MachineBasicBlock *EHPad = getMatchingEHPad(Extract);
	assert(EHPad && "No matching EH pad for extract_exception");
	auto CatchPos = EHPad->begin();
	if (CatchPos->isEHLabel()) // EH pad starts with an EH label
	++CatchPos;
	MachineInstr Catch = &CatchPos;

	if (Catch->getNextNode() != Extract)
	EHPad->insert(Catch->getNextNode(), Extract->removeFromParent());

	// - Before:
	// ehpad:
	// %exnref:exnref = catch
	// %exn:i32 = extract_exception
	// ... use exn ...
	//
	// - After:
	// ehpad:
	// %exnref:exnref = catch
	// br_on_exn %thenbb, $__cpp_exception, %exnref
	// br %elsebb
	// elsebb:
	// rethrow
	// thenbb:
	// %exn:i32 = extract_exception
	// ... use exn ...
	Register ExnReg = Catch->getOperand(0).getReg();
	auto *ThenMBB = MF.CreateMachineBasicBlock();
	auto *ElseMBB = MF.CreateMachineBasicBlock();
	MF.insert(std::next(MachineFunction::iterator(EHPad)), ElseMBB);
	MF.insert(std::next(MachineFunction::iterator(ElseMBB)), ThenMBB);
	ThenMBB->splice(ThenMBB->end(), EHPad, Extract, EHPad->end());
	ThenMBB->transferSuccessors(EHPad);
	EHPad->addSuccessor(ThenMBB);
	EHPad->addSuccessor(ElseMBB);

	DebugLoc DL = Extract->getDebugLoc();
	const char *CPPExnSymbol = MF.createExternalSymbolName("__cpp_exception");
	BuildMI(EHPad, DL, TII.get(WebAssembly::BR_ON_EXN))
	.addMBB(ThenMBB)
	.addExternalSymbol(CPPExnSymbol)
	.addReg(ExnReg);
	BuildMI(EHPad, DL, TII.get(WebAssembly::BR)).addMBB(ElseMBB);

	// When this is a terminate pad with __clang_call_terminate() call, we don't
	// rethrow it anymore and call __clang_call_terminate() with a nullptr
	// argument, which will call std::terminate().
	//
	// - Before:
	// ehpad:
	// %exnref:exnref = catch
	// %exn:i32 = extract_exception
	// call @__clang_call_terminate(%exn)
	// unreachable
	//
	// - After:
	// ehpad:
	// %exnref:exnref = catch
	// br_on_exn %thenbb, $__cpp_exception, %exnref
	// br %elsebb
	// elsebb:
	// call @__clang_call_terminate(0)
	// unreachable
	// thenbb:
	// %exn:i32 = extract_exception
	// call @__clang_call_terminate(%exn)
	// unreachable
	if (TerminatePads.count(EHPad)) {
	Function *ClangCallTerminateFn =
	MF.getFunction().getParent()->getFunction(
	WebAssembly::ClangCallTerminateFn);
	assert(ClangCallTerminateFn &&
	"There is no __clang_call_terminate() function");
	Register Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
	BuildMI(ElseMBB, DL, TII.get(WebAssembly::CONST_I32), Reg).addImm(0);
	BuildMI(ElseMBB, DL, TII.get(WebAssembly::CALL_VOID))
	.addGlobalAddress(ClangCallTerminateFn)
	.addReg(Reg);
	BuildMI(ElseMBB, DL, TII.get(WebAssembly::UNREACHABLE));

	} else {
	BuildMI(ElseMBB, DL, TII.get(WebAssembly::RETHROW)).addReg(ExnReg);
	if (EHInfo->hasEHPadUnwindDest(EHPad))
	ElseMBB->addSuccessor(EHInfo->getEHPadUnwindDest(EHPad));
	}
	}

	return true;
	}

	// 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();
	if (InsertPos->isEHLabel()) // EH pad starts with an EH label
	++InsertPos;
	if (InsertPos->getOpcode() == WebAssembly::CATCH)
	++InsertPos;
	FrameLowering->writeSPToGlobal(WebAssembly::SP32, MF, MBB, InsertPos,
	MBB.begin()->getDebugLoc());
	}
	return Changed;
	}