Add asm.js-style exception handling support for wasm

[aheejin]

Summary

This patch includes asm.js-style exception handling support for WebAssembly. To handle exceptions, this scheme lowers exception-related instructions in order to use JavaScript's try and catch mechanism.

[aheejin]

Delete .clang-format that was uploaded by a mistake

[aheejin]

Re-add .clang-format and revert all changes. I didn't know this file was already in the repo.

[aheejin]

Revert vimrc that was uploaded by mistake.

[aheejin]

Fix typos

[dschuff]

First round; looks pretty good overall.

lib/Target/WebAssembly/WebAssemblyLowerExceptions.cpp

1 ↗

(On Diff #66101)

Maybe we should call this WebAssemblyLowerEmscriptenExceptions so that when we want to experiment with an alternative way to do it, they can both be in the tree at the same time, and it will be unambiguous what they mean. (also applies to the string pass name).

12 ↗

(On Diff #66101)

Let's call this 'Emscripten's Javascript try and catch mechanism', since there could be others.

56 ↗

(On Diff #66101)

labdl -> label

125 ↗

(On Diff #66101)

Let's initialize these to nullptr here, just for general paranoia.

153 ↗

(On Diff #66101)

Remove the comment about emscripten builtins here.

196 ↗

(On Diff #66101)

how about FindMatchingCatches.count(NumClauses)

285 ↗

(On Diff #66101)

We should use the IRBuilder interface (here and below) rather than creating the instructions directly with new or FooInst::create()

314 ↗

(On Diff #66101)

Here you should use auto *II = dyn_cast<InvokeInst>(...) since the type InvokeInst is already stated on that same line and it's obvious. In other cases (e.g. IntegerType *Int1Ty = Type::getInt1Ty(...) or BasicBlock *BB = BasicBlock::Create(...)) you could also use auto * if you wanted but I'd consider it optional.

325 ↗

(On Diff #66101)

auto *F

415 ↗

(On Diff #66101)

The wasm backend has aggregate varargs support so we should update this comment. but we can leave the code the same to be compatible with the JSBackend.

test/CodeGen/WebAssembly/lower-exceptions.ll

5 ↗

(On Diff #66101)

Could we maybe make these regexes a little more specific? So we'd catch the error in case e.g. they get reordered or something.

[aheejin]

Addressed comments

[aheejin]

clang-format

[dschuff]

lib/Target/WebAssembly/WebAssemblyLowerEmscriptenExceptions.cpp

109

let's also update this to "wasm-lower-em-exceptions" or something like that, since it also affects the pass name when run under opt.

149

Also update the description string.

366

Let's also use IRBuilder in this function and everywhere else (in general, it should be used wherever possible). We could construct it outside the loop and just set the insert point here. I know it seems pedantic but IRBuilder automates some useful things (e.g. constant folding) and can result in better code.

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

33

we should also name this flag such that it somehow refers to emscripten.

test/CodeGen/WebAssembly/lower-exceptions.ll

1 ↗

(On Diff #66234)

should be 'RUN:' (i.e. it needs the colon), or it won't actually run the test.

[aheejin]

Address comments

[dschuff]

LGTM

lib/Target/WebAssembly/CMakeLists.txt

add_llvm_target(WebAssemblyCodeGen
   WebAssemblyCFGStackify.cpp
   WebAssemblyFastISel.cpp
   WebAssemblyFixIrreducibleControlFlow.cpp
   WebAssemblyFrameLowering.cpp
   WebAssemblyISelDAGToDAG.cpp
   WebAssemblyISelLowering.cpp
   WebAssemblyInstrInfo.cpp
   WebAssemblyLowerBrUnless.cpp
+  WebAssemblyLowerEmscriptenExceptions.cpp
   WebAssemblyMachineFunctionInfo.cpp
   WebAssemblyMCInstLower.cpp
   WebAssemblyOptimizeLiveIntervals.cpp
   WebAssemblyOptimizeReturned.cpp
   WebAssemblyPeephole.cpp
   WebAssemblyPrepareForLiveIntervals.cpp
   WebAssemblyRegisterInfo.cpp
   WebAssemblyRegColoring.cpp

lib/Target/WebAssembly/WebAssembly.h

 /// \brief This file contains the entry points for global functions defined in
 /// the LLVM WebAssembly back-end.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_WEBASSEMBLY_WEBASSEMBLY_H
 #define LLVM_LIB_TARGET_WEBASSEMBLY_WEBASSEMBLY_H
 
+#include "llvm/PassRegistry.h"
 #include "llvm/Support/CodeGen.h"
 
 namespace llvm {
 
 class WebAssemblyTargetMachine;
+class ModulePass;
 class FunctionPass;
 
 // LLVM IR passes.
+ModulePass *createWebAssemblyLowerEmscriptenExceptions();
+void initializeWebAssemblyLowerEmscriptenExceptionsPass(PassRegistry &);
 FunctionPass *createWebAssemblyOptimizeReturned();
 
 // ISel and immediate followup passes.
 FunctionPass *createWebAssemblyISelDag(WebAssemblyTargetMachine &TM,
                                        CodeGenOpt::Level OptLevel);
 FunctionPass *createWebAssemblyArgumentMove();
 FunctionPass *createWebAssemblySetP2AlignOperands();
 

lib/Target/WebAssembly/WebAssemblyLowerEmscriptenExceptions.cpp

+// WebAssemblyLowerEmscriptenExceptions.cpp - Lower exceptions for Emscripten //
+//
+//                     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 lowers exception-related instructions in order to use
+/// Emscripten's JavaScript try and catch mechanism to handle exceptions.
+///
+/// To handle exceptions, this scheme relies on JavaScript's try and catch
+/// syntax and relevant exception-related libraries implemented in JavaScript
+/// glue code that will be produced by Emscripten. This is similar to the
+/// current Emscripten asm.js exception handling in fastcomp.
+/// For fastcomp's EH scheme, see these files in fastcomp LLVM branch:
+/// (Location: https://github.com/kripken/emscripten-fastcomp)
+/// lib/Target/JSBackend/NaCl/LowerEmExceptionsPass.cpp
+/// lib/Target/JSBackend/JSBackend.cpp
+/// lib/Target/JSBackend/CallHandlers.h
+///
+/// This pass does following things:
+///
+/// 1) Create three global variables: __THREW__, threwValue, and tempRet0.
+///    tempRet0 will be set within ___cxa_find_matching_catch() function in
+///    JS library, and __THREW__ and threwValue will be set in invoke wrappers
+///    in JS glue code. For what invoke wrappers are, refer to 3).
+///
+/// 2) Create setThrew and setTempRet0 functions.
+///    The global variables created in 1) will exist in wasm address space,
+///    but their values should be set in JS code, so we provide these functions
+///    as interfaces to JS glue code. These functions are equivalent to the
+///    following JS functions, which actually exist in asm.js version of JS
+///    library.
+///
+///    function setThrew(threw, value) {
+///      if (__THREW__ == 0) {
+///        __THREW__ = threw;
+///        threwValue = value;
+///      }
+///    }
+///
+///    function setTempRet0(value) {
+///      tempRet0 = value;
+///    }
+///
+/// 3) Lower
+///      invoke @func(arg1, arg2) to label %invoke.cont unwind label %lpad
+///    into
+///      __THREW__ = 0;
+///      call @invoke_SIG(func, arg1, arg2)
+///      %__THREW__.val = __THREW__;
+///      __THREW__ = 0;
+///      br %__THREW__.val, label %lpad, label %invoke.cont
+///    SIG is a mangled string generated based on the LLVM IR-level function
+///    signature. After LLVM IR types are lowered to the target wasm types,
+///    the names for these wrappers will change based on wasm types as well,
+///    as in invoke_vi (function takes an int and returns void). The bodies of
+///    these wrappers will be generated in JS glue code, and inside those
+///    wrappers we use JS try-catch to generate actual exception effects. It
+///    also calls the original callee function. An example wrapper in JS code
+///    would look like this:
+///      function invoke_vi(index,a1) {
+///        try {
+///          Module["dynCall_vi"](index,a1); // This calls original callee
+///        } catch(e) {
+///          if (typeof e !== 'number' && e !== 'longjmp') throw e;
+///          asm["setThrew"](1, 0); // setThrew is called here
+///        }
+///      }
+///    If an exception is thrown, __THREW__ will be set to true in a wrapper,
+///    so we can jump to the right BB based on this value.
+///
+/// 4) Lower
+///      %val = landingpad catch c1 catch c2 catch c3 ...
+///      ... use %val ...
+///    into
+///      %fmc = call @___cxa_find_matching_catch_N(c1, c2, c3, ...)
+///      %val = {%fmc, tempRet0}
+///      ... use %val ...
+///    Here N is a number calculated based on the number of clauses.
+///    Global variable tempRet0 is set within ___cxa_find_matching_catch() in
+///    JS glue code.
+///
+/// 5) Lower
+///      resume {%a, %b}
+///    into
+///      call @___resumeException(%a)
+///    where ___resumeException() is a function in JS glue code.
+///
+/// TODO: Handle i64 types
+///
+///===----------------------------------------------------------------------===//
+
+#include "WebAssembly.h"
+#include "llvm/ADT/IndexedMap.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "wasm-lower-em-exceptions"
+
+namespace {
+class WebAssemblyLowerEmscriptenExceptions final : public ModulePass {
+  const char *getPassName() const override {
+    return "WebAssembly Lower Emscripten Exceptions";
+  }
+
+  bool runOnFunction(Function &F);
+  // Returns ___cxa_find_matching_catch_N function, where N = NumClauses + 2.
+  // This is because a landingpad instruction contains two more arguments,
+  // a personality function and a cleanup bit, and ___cxa_find_matching_catch_N
+  // functions are named after the number of arguments in the original
+  // landingpad instruction.
+  Function *getFindMatchingCatch(Module &M, unsigned NumClauses);
+
+  Function *getInvokeWrapper(Module &M, InvokeInst *II);
+
+  GlobalVariable *ThrewGV;      // __THREW__
+  GlobalVariable *ThrewValueGV; // threwValue
+  GlobalVariable *TempRet0GV;   // tempRet0
+  Function *ResumeF;
+  // ___cxa_find_matching_catch_N functions.
+  // Indexed by the number of clauses in an original landingpad instruction.
+  DenseMap<int, Function *> FindMatchingCatches;
+  // Map of <function signature string, invoke_ wrappers>
+  StringMap<Function *> InvokeWrappers;
+
+public:
+  static char ID;
+
+  WebAssemblyLowerEmscriptenExceptions()
+      : ModulePass(ID), ThrewGV(nullptr), ThrewValueGV(nullptr),
+        TempRet0GV(nullptr) {}
+  bool runOnModule(Module &M) override;
+};
+} // End anonymous namespace
+
+char WebAssemblyLowerEmscriptenExceptions::ID = 0;
+INITIALIZE_PASS(WebAssemblyLowerEmscriptenExceptions, DEBUG_TYPE,
+                "WebAssembly Lower Emscripten Exceptions", false, false)
+
+ModulePass *llvm::createWebAssemblyLowerEmscriptenExceptions() {
+  return new WebAssemblyLowerEmscriptenExceptions();
+}
+
+static bool canThrow(const Value *V) {
+  if (const auto *F = dyn_cast<const Function>(V)) {
+    // Intrinsics cannot throw
+    if (F->isIntrinsic())
+      return false;
+    StringRef Name = F->getName();
+    // leave setjmp and longjmp (mostly) alone, we process them properly later
+    if (Name == "setjmp" || Name == "longjmp")
+      return false;
+    return true;
+  }
+  return true; // not a function, so an indirect call - can throw, we can't tell
+}
+
+// Returns an available name for a global value.
+// If the proposed name already exists in the module, adds '_' at the end of
+// the name until the name is available.
+static inline std::string createGlobalValueName(const Module &M,
+                                                const std::string &Propose) {
+  std::string Name = Propose;
+  while (M.getNamedGlobal(Name))
+    Name += "_";
+  return Name;
+}
+
+// Simple function name mangler.
+// This function simply takes LLVM's string representation of parameter types
+// concatenate them with '_'. There are non-alphanumeric characters but llc is
+// ok with it, and we need to postprocess these names after the lowering phase
+// anyway.
+static std::string getSignature(FunctionType *FTy) {
+  std::string Sig;
+  raw_string_ostream OS(Sig);
+  OS << *FTy->getReturnType();
+  for (Type *ParamTy : FTy->params())
+    OS << "_" << *ParamTy;
+  if (FTy->isVarArg())
+    OS << "_...";
+  Sig = OS.str();
+  Sig.erase(std::remove_if(Sig.begin(), Sig.end(), isspace), Sig.end());
+  return Sig;
+}
+
+Function *WebAssemblyLowerEmscriptenExceptions::getFindMatchingCatch(
+    Module &M, unsigned NumClauses) {
+  if (FindMatchingCatches.count(NumClauses))
+    return FindMatchingCatches[NumClauses];
+  PointerType *Int8PtrTy = Type::getInt8PtrTy(M.getContext());
+  SmallVector<Type *, 16> Args(NumClauses, Int8PtrTy);
+  FunctionType *FTy = FunctionType::get(Int8PtrTy, Args, false);
+  Function *F = Function::Create(
+      FTy, GlobalValue::ExternalLinkage,
+      "___cxa_find_matching_catch_" + Twine(NumClauses + 2), &M);
+  FindMatchingCatches[NumClauses] = F;
+  return F;
+}
+
+Function *
+WebAssemblyLowerEmscriptenExceptions::getInvokeWrapper(Module &M,
+                                                       InvokeInst *II) {
+  SmallVector<Type *, 16> ArgTys;
+  Value *Callee = II->getCalledValue();
+  FunctionType *CalleeFTy;
+  if (auto *F = dyn_cast<Function>(Callee))
+    CalleeFTy = F->getFunctionType();
+  else {
+    auto *CalleeTy = dyn_cast<PointerType>(Callee->getType())->getElementType();
+    CalleeFTy = dyn_cast<FunctionType>(CalleeTy);
+  }
+
+  std::string Sig = getSignature(CalleeFTy);
+  if (InvokeWrappers.find(Sig) != InvokeWrappers.end())
+    return InvokeWrappers[Sig];
+
+  // Put the pointer to the callee as first argument
+  ArgTys.push_back(PointerType::getUnqual(CalleeFTy));
+  // Add argument types
+  ArgTys.append(CalleeFTy->param_begin(), CalleeFTy->param_end());
+
+  FunctionType *FTy = FunctionType::get(CalleeFTy->getReturnType(), ArgTys,
+                                        CalleeFTy->isVarArg());
+  Function *F = Function::Create(FTy, GlobalValue::ExternalLinkage,
+                                 "__invoke_" + Sig, &M);
+  InvokeWrappers[Sig] = F;
+  return F;
+}
+
+bool WebAssemblyLowerEmscriptenExceptions::runOnModule(Module &M) {
+  IRBuilder<> Builder(M.getContext());
+  IntegerType *Int1Ty = Builder.getInt1Ty();
+  PointerType *Int8PtrTy = Builder.getInt8PtrTy();
+  IntegerType *Int32Ty = Builder.getInt32Ty();
+
+  // Create global variables __THREW__, threwValue, and tempRet0
+  ThrewGV = new GlobalVariable(M, Int1Ty, false, GlobalValue::ExternalLinkage,
+                               Builder.getFalse(),
+                               createGlobalValueName(M, "__THREW__"));
+  ThrewValueGV = new GlobalVariable(
+      M, Int32Ty, false, GlobalValue::ExternalLinkage, Builder.getInt32(0),
+      createGlobalValueName(M, "threwValue"));
+  TempRet0GV = new GlobalVariable(
+      M, Int32Ty, false, GlobalValue::ExternalLinkage, Builder.getInt32(0),
+      createGlobalValueName(M, "tempRet0"));
+
+  // Register ___resumeException function
+  Type *VoidTy = Type::getVoidTy(M.getContext());
+  FunctionType *ResumeFTy = FunctionType::get(VoidTy, Int8PtrTy, false);
+  ResumeF = Function::Create(ResumeFTy, GlobalValue::ExternalLinkage,
+                             "___resumeException", &M);
+
+  bool Changed = false;
+  for (Function &F : M) {
+    if (F.isDeclaration())
+      continue;
+    Changed |= runOnFunction(F);
+  }
+
+  if (!Changed)
+    return false;
+
+  assert(!M.getNamedGlobal("setThrew") && "setThrew already exists");
+  assert(!M.getNamedGlobal("setTempRet0") && "setTempRet0 already exists");
+
+  // Create setThrew function
+  SmallVector<Type *, 2> Params = {Int1Ty, Int32Ty};
+  FunctionType *FTy = FunctionType::get(VoidTy, Params, false);
+  Function *F =
+      Function::Create(FTy, GlobalValue::ExternalLinkage, "setThrew", &M);
+  Argument *Arg1 = &*(F->arg_begin());
+  Argument *Arg2 = &*(++F->arg_begin());
+  Arg1->setName("threw");
+  Arg2->setName("value");
+  BasicBlock *EntryBB = BasicBlock::Create(M.getContext(), "entry", F);
+  BasicBlock *ThenBB = BasicBlock::Create(M.getContext(), "if.then", F);
+  BasicBlock *EndBB = BasicBlock::Create(M.getContext(), "if.end", F);
+
+  Builder.SetInsertPoint(EntryBB);
+  Value *Threw = Builder.CreateLoad(ThrewGV, ThrewGV->getName() + ".val");
+  Value *Cmp = Builder.CreateICmpEQ(Threw, Builder.getFalse(), "cmp");
+  Builder.CreateCondBr(Cmp, ThenBB, EndBB);
+
+  Builder.SetInsertPoint(ThenBB);
+  Builder.CreateStore(Arg1, ThrewGV);
+  Builder.CreateStore(Arg2, ThrewValueGV);
+  Builder.CreateBr(EndBB);
+
+  Builder.SetInsertPoint(EndBB);
+  Builder.CreateRetVoid();
+
+  // Create setTempRet0 function
+  Params = {Int32Ty};
+  FTy = FunctionType::get(VoidTy, Params, false);
+  F = Function::Create(FTy, GlobalValue::ExternalLinkage, "setTempRet0", &M);
+  F->arg_begin()->setName("value");
+  EntryBB = BasicBlock::Create(M.getContext(), "entry", F);
+  Builder.SetInsertPoint(EntryBB);
+  Builder.CreateStore(&*F->arg_begin(), TempRet0GV);
+  Builder.CreateRetVoid();
+
+  return true;
+}
+
+bool WebAssemblyLowerEmscriptenExceptions::runOnFunction(Function &F) {
+  Module &M = *F.getParent();
+  IRBuilder<> Builder(M.getContext());
+  bool Changed = false;
+  SmallVector<Instruction *, 64> ToErase;
+  SmallPtrSet<LandingPadInst *, 32> LandingPads;
+
+  for (BasicBlock &BB : F) {
+    auto *II = dyn_cast<InvokeInst>(BB.getTerminator());
+    if (!II)
+      continue;
+    Changed = true;
+    LandingPads.insert(II->getLandingPadInst());
+    Builder.SetInsertPoint(II);
+
+    if (canThrow(II->getCalledValue())) {
+      // If we are calling a function that is noreturn, we must remove that
+      // attribute. The code we insert here does expect it to return, after we
+      // catch the exception.
+      if (II->doesNotReturn()) {
+        if (auto *F = dyn_cast<Function>(II->getCalledValue()))
+          F->removeFnAttr(Attribute::NoReturn);
+        AttributeSet NewAttrs = II->getAttributes();
+        NewAttrs.removeAttribute(M.getContext(), AttributeSet::FunctionIndex,
+                                 Attribute::NoReturn);
+        II->setAttributes(NewAttrs);
+      }
+
+      // Pre-invoke
+      // __THREW__ = 0;
+      Builder.CreateStore(Builder.getFalse(), ThrewGV);
+
+      // Invoke function wrapper in JavaScript
+      SmallVector<Value *, 16> CallArgs;
+      // Put the pointer to the callee as first argument, so it can be called
+      // within the invoke wrapper later
+      CallArgs.push_back(II->getCalledValue());
+      CallArgs.append(II->arg_begin(), II->arg_end());
+      CallInst *NewCall = Builder.CreateCall(getInvokeWrapper(M, II), CallArgs);
+      NewCall->takeName(II);
+      NewCall->setCallingConv(II->getCallingConv());
+      NewCall->setAttributes(II->getAttributes());
+      NewCall->setDebugLoc(II->getDebugLoc());
+      II->replaceAllUsesWith(NewCall);
+      ToErase.push_back(II);
+
+      // Post-invoke
+      // %__THREW__.val = __THREW__; __THREW__ = 0;
+      Value *Threw = Builder.CreateLoad(ThrewGV, ThrewGV->getName() + ".val");
+      Builder.CreateStore(Builder.getFalse(), ThrewGV);
+
+      // Insert a branch based on __THREW__ variable
+      Builder.CreateCondBr(Threw, II->getUnwindDest(), II->getNormalDest());
+
+    } else {
+      // This can't throw, and we don't need this invoke, just replace it with a
+      // call+branch
+      SmallVector<Value *, 16> CallArgs(II->arg_begin(), II->arg_end());
+      CallInst *NewCall = Builder.CreateCall(II->getCalledValue(), CallArgs);
+      NewCall->takeName(II);
+      NewCall->setCallingConv(II->getCallingConv());
+      NewCall->setAttributes(II->getAttributes());
+      NewCall->setDebugLoc(II->getDebugLoc());
+      II->replaceAllUsesWith(NewCall);
+      ToErase.push_back(II);
+
+      Builder.CreateBr(II->getNormalDest());
+
+      // Remove any PHI node entries from the exception destination
+      II->getUnwindDest()->removePredecessor(&BB);
+    }
+  }
+
+  // Process resume instructions
+  for (BasicBlock &BB : F) {
+    // Scan the body of the basic block for resumes
+    for (Instruction &I : BB) {
+      auto *RI = dyn_cast<ResumeInst>(&I);
+      if (!RI)
+        continue;
+
+      // Split the input into legal values
+      Value *Input = RI->getValue();
+      Builder.SetInsertPoint(RI);
+      Value *Low = Builder.CreateExtractValue(Input, 0, "low");
+
+      // Create a call to ___resumeException function
+      Value *Args[] = {Low};
+      Builder.CreateCall(ResumeF, Args);
+
+      // Add a terminator to the block
+      Builder.CreateUnreachable();
+      ToErase.push_back(RI);
+    }
+  }
+
+  // Look for orphan landingpads, can occur in blocks with no predecesors
+  for (BasicBlock &BB : F) {
+    Instruction *I = BB.getFirstNonPHI();
+    if (auto *LPI = dyn_cast<LandingPadInst>(I))
+      LandingPads.insert(LPI);
+  }
+
+  // Handle all the landingpad for this function together, as multiple invokes
+  // may share a single lp
+  for (LandingPadInst *LPI : LandingPads) {
+    Builder.SetInsertPoint(LPI);
+    SmallVector<Value *, 16> FMCArgs;
+    for (unsigned i = 0, e = LPI->getNumClauses(); i < e; ++i) {
+      Constant *Clause = LPI->getClause(i);
+      // As a temporary workaround for the lack of aggregate varargs support
+      // in the interface between JS and wasm, break out filter operands into
+      // their component elements.
+      if (LPI->isFilter(i)) {
+        ArrayType *ATy = cast<ArrayType>(Clause->getType());
+        for (unsigned j = 0, e = ATy->getNumElements(); j < e; ++j) {
+          Value *EV =
+              Builder.CreateExtractValue(Clause, makeArrayRef(j), "filter");
+          FMCArgs.push_back(EV);
+        }
+      } else
+        FMCArgs.push_back(Clause);
+    }
+
+    // Create a call to ___cxa_find_matching_catch_N function
+    Function *FMCF = getFindMatchingCatch(M, FMCArgs.size());
+    CallInst *FMCI = Builder.CreateCall(FMCF, FMCArgs, "fmc");
+    Value *Undef = UndefValue::get(LPI->getType());
+    Value *Pair0 = Builder.CreateInsertValue(Undef, FMCI, 0, "pair0");
+    Value *TempRet0 =
+        Builder.CreateLoad(TempRet0GV, TempRet0GV->getName() + "val");
+    Value *Pair1 = Builder.CreateInsertValue(Pair0, TempRet0, 1, "pair1");
+
+    LPI->replaceAllUsesWith(Pair1);
+    ToErase.push_back(LPI);
+  }
+
+  // Erase everything we no longer need in this function
+  for (Instruction *I : ToErase)
+    I->eraseFromParent();
+
+  return Changed;
+}

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

 #include "llvm/IR/Function.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Transforms/Scalar.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "wasm"
 
+// Emscripten's asm.js-style exception handling
+static cl::opt<bool> EnableEmExceptionHandling(
+    "wasm-em-exception-handling",
+    cl::desc("WebAssembly Emscripten-style exception handling"),
+    cl::init(false));
+
 extern "C" void LLVMInitializeWebAssemblyTarget() {
   // Register the target.
   RegisterTargetMachine<WebAssemblyTargetMachine> X(TheWebAssemblyTarget32);
   RegisterTargetMachine<WebAssemblyTargetMachine> Y(TheWebAssemblyTarget64);
+
+  // Register exception handling pass to opt
+  initializeWebAssemblyLowerEmscriptenExceptionsPass(
+      *PassRegistry::getPassRegistry());
 }
 
 //===----------------------------------------------------------------------===//
 // WebAssembly Lowering public interface.
 //===----------------------------------------------------------------------===//
 
 static Reloc::Model getEffectiveRelocModel(Optional<Reloc::Model> RM) {
   if (!RM.hasValue())
else
     // Expand some atomic operations. WebAssemblyTargetLowering has hooks which
     // control specifically what gets lowered.
     addPass(createAtomicExpandPass(TM));
 
   // Optimize "returned" function attributes.
   if (getOptLevel() != CodeGenOpt::None)
     addPass(createWebAssemblyOptimizeReturned());
 
+  // Handle exceptions.
+  if (EnableEmExceptionHandling)
+    addPass(createWebAssemblyLowerEmscriptenExceptions());
+
   TargetPassConfig::addIRPasses();
 }
 
 bool WebAssemblyPassConfig::addInstSelector() {
   (void)TargetPassConfig::addInstSelector();
   addPass(
       createWebAssemblyISelDag(getWebAssemblyTargetMachine(), getOptLevel()));
   // Run the argument-move pass immediately after the ScheduleDAG scheduler

test/CodeGen/WebAssembly/lower-em-exceptions.ll

+; RUN: opt < %s -wasm-lower-em-exceptions -S | FileCheck %s
+
+@_ZTIi = external constant i8*
+@_ZTIc = external constant i8*
+; CHECK: @[[__THREW__:__THREW__.*]] = global i1 false
+; CHECK: @[[THREWVALUE:threwValue.*]] = global i32 0
+; CHECK: @[[TEMPRET0:tempRet0.*]] = global i32 0
+
+; Test invoke instruction with clauses (try-catch block)
+define void @clause() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+; CHECK-LABEL: @clause(
+entry:
+  invoke void @foo(i32 3)
+          to label %invoke.cont unwind label %lpad
+; CHECK: entry:
+; CHECK-NEXT: store i1 false, i1* @[[__THREW__]]
+; CHECK-NEXT: call void @__invoke_void_i32(void (i32)* @foo, i32 3)
+; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i1, i1* @[[__THREW__]]
+; CHECK-NEXT: store i1 false, i1* @[[__THREW__]]
+; CHECK-NEXT: br i1 %[[__THREW__VAL]], label %lpad, label %invoke.cont
+
+invoke.cont:                                      ; preds = %entry
+  br label %try.cont
+
+lpad:                                             ; preds = %entry
+  %0 = landingpad { i8*, i32 }
+          catch i8* bitcast (i8** @_ZTIi to i8*)
+          catch i8* null
+  %1 = extractvalue { i8*, i32 } %0, 0
+  %2 = extractvalue { i8*, i32 } %0, 1
+  br label %catch.dispatch
+; CHECK: lpad:
+; CHECK-NEXT: %[[FMC:.*]] = call i8* @___cxa_find_matching_catch_4(i8* bitcast (i8** @_ZTIi to i8*), i8* null)
+; CHECK-NEXT: %[[IVI1:.*]] = insertvalue { i8*, i32 } undef, i8* %[[FMC]], 0
+; CHECK-NEXT: %[[TEMPRET0_VAL:.*]] = load i32, i32* @[[TEMPRET0]]
+; CHECK-NEXT: %[[IVI2:.*]] = insertvalue { i8*, i32 } %[[IVI1]], i32 %[[TEMPRET0_VAL]], 1
+; CHECK-NEXT: extractvalue { i8*, i32 } %[[IVI2]], 0
+; CHECK-NEXT: extractvalue { i8*, i32 } %[[IVI2]], 1
+
+catch.dispatch:                                   ; preds = %lpad
+  %3 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*))
+  %matches = icmp eq i32 %2, %3
+  br i1 %matches, label %catch1, label %catch
+
+catch1:                                           ; preds = %catch.dispatch
+  %4 = call i8* @__cxa_begin_catch(i8* %1)
+  %5 = bitcast i8* %4 to i32*
+  %6 = load i32, i32* %5, align 4
+  call void @__cxa_end_catch()
+  br label %try.cont
+
+try.cont:                                         ; preds = %catch, %catch1, %invoke.cont
+  ret void
+
+catch:                                            ; preds = %catch.dispatch
+  %7 = call i8* @__cxa_begin_catch(i8* %1)
+  call void @__cxa_end_catch()
+  br label %try.cont
+}
+
+; Test invoke instruction with filters (functions with throw(...) declaration)
+define void @filter() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+; CHECK-LABEL: @filter(
+entry:
+  invoke void @foo(i32 3)
+          to label %invoke.cont unwind label %lpad
+; CHECK: entry:
+; CHECK-NEXT: store i1 false, i1* @[[__THREW__]]
+; CHECK-NEXT: call void @__invoke_void_i32(void (i32)* @foo, i32 3)
+; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i1, i1* @[[__THREW__]]
+; CHECK-NEXT: store i1 false, i1* @[[__THREW__]]
+; CHECK-NEXT: br i1 %[[__THREW__VAL]], label %lpad, label %invoke.cont
+
+invoke.cont:                                      ; preds = %entry
+  ret void
+
+lpad:                                             ; preds = %entry
+  %0 = landingpad { i8*, i32 }
+          filter [2 x i8*] [i8* bitcast (i8** @_ZTIi to i8*), i8* bitcast (i8** @_ZTIc to i8*)]
+  %1 = extractvalue { i8*, i32 } %0, 0
+  %2 = extractvalue { i8*, i32 } %0, 1
+  br label %filter.dispatch
+; CHECK: lpad:
+; CHECK-NEXT: %[[FMC:.*]] = call i8* @___cxa_find_matching_catch_4(i8* bitcast (i8** @_ZTIi to i8*), i8* bitcast (i8** @_ZTIc to i8*))
+; CHECK-NEXT: %[[IVI1:.*]] = insertvalue { i8*, i32 } undef, i8* %[[FMC]], 0
+; CHECK-NEXT: %[[TEMPRET0_VAL:.*]] = load i32, i32* @[[TEMPRET0]]
+; CHECK-NEXT: %[[IVI2:.*]] = insertvalue { i8*, i32 } %[[IVI1]], i32 %[[TEMPRET0_VAL]], 1
+; CHECK-NEXT: extractvalue { i8*, i32 } %[[IVI2]], 0
+; CHECK-NEXT: extractvalue { i8*, i32 } %[[IVI2]], 1
+
+filter.dispatch:                                  ; preds = %lpad
+  %ehspec.fails = icmp slt i32 %2, 0
+  br i1 %ehspec.fails, label %ehspec.unexpected, label %eh.resume
+
+ehspec.unexpected:                                ; preds = %filter.dispatch
+  call void @__cxa_call_unexpected(i8* %1) #4
+  unreachable
+
+eh.resume:                                        ; preds = %filter.dispatch
+  %lpad.val = insertvalue { i8*, i32 } undef, i8* %1, 0
+  %lpad.val3 = insertvalue { i8*, i32 } %lpad.val, i32 %2, 1
+  resume { i8*, i32 } %lpad.val3
+; CHECK: eh.resume:
+; CHECK-NEXT: insertvalue
+; CHECK-NEXT: %[[LPAD_VAL:.*]] = insertvalue
+; CHECK-NEXT: %[[LOW:.*]] = extractvalue { i8*, i32 } %[[LPAD_VAL]], 0
+; CHECK-NEXT: call void @___resumeException(i8* %[[LOW]])
+; CHECK-NEXT: unreachable
+}
+
+declare void @foo(i32)
+
+declare i32 @__gxx_personality_v0(...)
+declare i32 @llvm.eh.typeid.for(i8*)
+declare i8* @__cxa_begin_catch(i8*)
+declare void @__cxa_end_catch()
+declare void @__cxa_call_unexpected(i8*)
+
+; JS glue functions and invoke wrappers registration
+; CHECK: declare void @___resumeException(i8*)
+; CHECK: declare void @__invoke_void_i32(void (i32)*, i32)
+; CHECK: declare i8* @___cxa_find_matching_catch_4(i8*, i8*)
+
+; setThrew function creation
+; CHECK-LABEL: define void @setThrew(i1 %threw, i32 %value) {
+; CHECK: entry:
+; CHECK-NEXT: %__THREW__.val = load i1, i1* @__THREW__
+; CHECK-NEXT: %cmp = icmp eq i1 %__THREW__.val, false
+; CHECK-NEXT: br i1 %cmp, label %if.then, label %if.end
+; CHECK: if.then:
+; CHECK-NEXT: store i1 %threw, i1* @__THREW__
+; CHECK-NEXT: store i32 %value, i32* @threwValue
+; CHECK-NEXT: br label %if.end
+; CHECK: if.end:
+; CHECK-NEXT: ret void
+; CHECK: }
+
+; setTempRet0 function creation
+; CHECK-LABEL: define void @setTempRet0(i32 %value) {
+; CHECK: entry:
+; CHECK-NEXT: store i32 %value, i32* @tempRet0
+; CHECK-NEXT: ret void
+; CHECK: }