[WebAssembly] Use Windows EH instructions for Wasm EH

Summary:
Because wasm control flow needs to be structured, using WinEH
instructions to support wasm EH brings several benefits. This patch
makes wasm EH uses Windows EH instructions, with some changes:
1. Because wasm uses a single catch block to catch all C++ exceptions,
   this merges all catch clauses into a single catchpad, within which we
   test the EH selector as in Itanium EH.
2. Generates a call to `__clang_call_terminate` in case a cleanup
   throws. Wasm does not have a runtime to handle this.
3. In case there is no catch-all clause, inserts a call to
   `__cxa_rethrow` at the end of a catchpad in order to unwind to an
   enclosing EH scope.

Reviewers: majnemer, dschuff

Subscribers: jfb, sbc100, jgravelle-google, sunfish, cfe-commits

Differential Revision: https://reviews.llvm.org/D44931

llvm-svn: 333703

clang/lib/CodeGen/CGCXXABI.h

@@ -607,6 +607,17 @@ CGCXXABI *CreateItaniumCXXABI(CodeGenModule &CGM);
 /// Creates a Microsoft-family ABI.
 CGCXXABI *CreateMicrosoftCXXABI(CodeGenModule &CGM);
 
+struct CatchRetScope final : EHScopeStack::Cleanup {
+  llvm::CatchPadInst *CPI;
+
+  CatchRetScope(llvm::CatchPadInst *CPI) : CPI(CPI) {}
+
+  void Emit(CodeGenFunction &CGF, Flags flags) override {
+    llvm::BasicBlock *BB = CGF.createBasicBlock("catchret.dest");
+    CGF.Builder.CreateCatchRet(CPI, BB);
+    CGF.EmitBlock(BB);
+  }
+};
 }
 }
 

clang/lib/CodeGen/CGCleanup.cpp

@@ -971,16 +971,21 @@ void CodeGenFunction::PopCleanupBlock(bool FallthroughIsBranchThrough) {
     SaveAndRestore<llvm::Instruction *> RestoreCurrentFuncletPad(
         CurrentFuncletPad);
     llvm::CleanupPadInst *CPI = nullptr;
-    if (!EHPersonality::get(*this).usesFuncletPads()) {
-      EHStack.pushTerminate();
-      PushedTerminate = true;
-    } else {
+
+    const EHPersonality &Personality = EHPersonality::get(*this);
+    if (Personality.usesFuncletPads()) {
       llvm::Value *ParentPad = CurrentFuncletPad;
       if (!ParentPad)
         ParentPad = llvm::ConstantTokenNone::get(CGM.getLLVMContext());
       CurrentFuncletPad = CPI = Builder.CreateCleanupPad(ParentPad);
     }
 
+    // Non-MSVC personalities need to terminate when an EH cleanup throws.
+    if (!Personality.isMSVCPersonality()) {
+      EHStack.pushTerminate();
+      PushedTerminate = true;
+    }
+
     // We only actually emit the cleanup code if the cleanup is either
     // active or was used before it was deactivated.
     if (EHActiveFlag.isValid() || IsActive) {

clang/lib/CodeGen/CGCleanup.h

@@ -627,16 +627,21 @@ struct EHPersonality {
   static const EHPersonality MSVC_except_handler;
   static const EHPersonality MSVC_C_specific_handler;
   static const EHPersonality MSVC_CxxFrameHandler3;
+  static const EHPersonality GNU_Wasm_CPlusPlus;
 
   /// Does this personality use landingpads or the family of pad instructions
   /// designed to form funclets?
-  bool usesFuncletPads() const { return isMSVCPersonality(); }
+  bool usesFuncletPads() const {
+    return isMSVCPersonality() || isWasmPersonality();
+  }
 
   bool isMSVCPersonality() const {
     return this == &MSVC_except_handler || this == &MSVC_C_specific_handler ||
            this == &MSVC_CxxFrameHandler3;
   }
 
+  bool isWasmPersonality() const { return this == &GNU_Wasm_CPlusPlus; }
+
   bool isMSVCXXPersonality() const { return this == &MSVC_CxxFrameHandler3; }
 };
 }

clang/lib/CodeGen/CGException.cpp

@@ -111,6 +111,8 @@ const EHPersonality
 EHPersonality::MSVC_C_specific_handler = { "__C_specific_handler", nullptr };
 const EHPersonality
 EHPersonality::MSVC_CxxFrameHandler3 = { "__CxxFrameHandler3", nullptr };
+const EHPersonality
+EHPersonality::GNU_Wasm_CPlusPlus = { "__gxx_wasm_personality_v0", nullptr };
 
 static const EHPersonality &getCPersonality(const llvm::Triple &T,
                                             const LangOptions &L) {
@@ -161,6 +163,9 @@ static const EHPersonality &getCXXPersonality(const llvm::Triple &T,
     return EHPersonality::MSVC_CxxFrameHandler3;
   if (L.SEHExceptions)
     return EHPersonality::GNU_CPlusPlus_SEH;
+  if (T.getArch() == llvm::Triple::wasm32 ||
+      T.getArch() == llvm::Triple::wasm64)
+    return EHPersonality::GNU_Wasm_CPlusPlus;
   return EHPersonality::GNU_CPlusPlus;
 }
 
@@ -574,7 +579,7 @@ void CodeGenFunction::EnterCXXTryStmt(const CXXTryStmt &S, bool IsFnTryBlock) {
 llvm::BasicBlock *
 CodeGenFunction::getEHDispatchBlock(EHScopeStack::stable_iterator si) {
   if (EHPersonality::get(*this).usesFuncletPads())
-    return getMSVCDispatchBlock(si);
+    return getFuncletEHDispatchBlock(si);
 
   // The dispatch block for the end of the scope chain is a block that
   // just resumes unwinding.
@@ -622,7 +627,7 @@ CodeGenFunction::getEHDispatchBlock(EHScopeStack::stable_iterator si) {
 }
 
 llvm::BasicBlock *
-CodeGenFunction::getMSVCDispatchBlock(EHScopeStack::stable_iterator SI) {
+CodeGenFunction::getFuncletEHDispatchBlock(EHScopeStack::stable_iterator SI) {
   // Returning nullptr indicates that the previous dispatch block should unwind
   // to caller.
   if (SI == EHStack.stable_end())
@@ -916,10 +921,121 @@ static void emitCatchPadBlock(CodeGenFunction &CGF, EHCatchScope &CatchScope) {
   CGF.Builder.restoreIP(SavedIP);
 }
 
+// Wasm uses Windows-style EH instructions, but it merges all catch clauses into
+// one big catchpad, within which we use Itanium's landingpad-style selector
+// comparison instructions.
+static void emitWasmCatchPadBlock(CodeGenFunction &CGF,
+                                  EHCatchScope &CatchScope) {
+  llvm::BasicBlock *DispatchBlock = CatchScope.getCachedEHDispatchBlock();
+  assert(DispatchBlock);
+
+  CGBuilderTy::InsertPoint SavedIP = CGF.Builder.saveIP();
+  CGF.EmitBlockAfterUses(DispatchBlock);
+
+  llvm::Value *ParentPad = CGF.CurrentFuncletPad;
+  if (!ParentPad)
+    ParentPad = llvm::ConstantTokenNone::get(CGF.getLLVMContext());
+  llvm::BasicBlock *UnwindBB =
+      CGF.getEHDispatchBlock(CatchScope.getEnclosingEHScope());
+
+  unsigned NumHandlers = CatchScope.getNumHandlers();
+  llvm::CatchSwitchInst *CatchSwitch =
+      CGF.Builder.CreateCatchSwitch(ParentPad, UnwindBB, NumHandlers);
+
+  // We don't use a landingpad instruction, so generate intrinsic calls to
+  // provide exception and selector values.
+  llvm::BasicBlock *WasmCatchStartBlock = CGF.createBasicBlock("catch.start");
+  CatchSwitch->addHandler(WasmCatchStartBlock);
+  CGF.EmitBlockAfterUses(WasmCatchStartBlock);
+
+  // Create a catchpad instruction.
+  SmallVector<llvm::Value *, 4> CatchTypes;
+  for (unsigned I = 0, E = NumHandlers; I < E; ++I) {
+    const EHCatchScope::Handler &Handler = CatchScope.getHandler(I);
+    CatchTypeInfo TypeInfo = Handler.Type;
+    if (!TypeInfo.RTTI)
+      TypeInfo.RTTI = llvm::Constant::getNullValue(CGF.VoidPtrTy);
+    CatchTypes.push_back(TypeInfo.RTTI);
+  }
+  auto *CPI = CGF.Builder.CreateCatchPad(CatchSwitch, CatchTypes);
+
+  // Create calls to wasm.get.exception and wasm.get.ehselector intrinsics.
+  // Before they are lowered appropriately later, they provide values for the
+  // exception and selector.
+  llvm::Value *GetExnFn =
+      CGF.CGM.getIntrinsic(llvm::Intrinsic::wasm_get_exception);
+  llvm::Value *GetSelectorFn =
+      CGF.CGM.getIntrinsic(llvm::Intrinsic::wasm_get_ehselector);
+  llvm::CallInst *Exn = CGF.Builder.CreateCall(GetExnFn, CPI);
+  CGF.Builder.CreateStore(Exn, CGF.getExceptionSlot());
+  llvm::CallInst *Selector = CGF.Builder.CreateCall(GetSelectorFn, CPI);
+
+  llvm::Value *TypeIDFn = CGF.CGM.getIntrinsic(llvm::Intrinsic::eh_typeid_for);
+
+  // If there's only a single catch-all, branch directly to its handler.
+  if (CatchScope.getNumHandlers() == 1 &&
+      CatchScope.getHandler(0).isCatchAll()) {
+    CGF.Builder.CreateBr(CatchScope.getHandler(0).Block);
+    CGF.Builder.restoreIP(SavedIP);
+    return;
+  }
+
+  // Test against each of the exception types we claim to catch.
+  for (unsigned I = 0, E = NumHandlers;; ++I) {
+    assert(I < E && "ran off end of handlers!");
+    const EHCatchScope::Handler &Handler = CatchScope.getHandler(I);
+    CatchTypeInfo TypeInfo = Handler.Type;
+    if (!TypeInfo.RTTI)
+      TypeInfo.RTTI = llvm::Constant::getNullValue(CGF.VoidPtrTy);
+
+    // Figure out the next block.
+    llvm::BasicBlock *NextBlock;
+
+    bool EmitNextBlock = false, NextIsEnd = false;
+
+    // If this is the last handler, we're at the end, and the next block is a
+    // block that contains a call to the rethrow function, so we can unwind to
+    // the enclosing EH scope. The call itself will be generated later.
+    if (I + 1 == E) {
+      NextBlock = CGF.createBasicBlock("rethrow");
+      EmitNextBlock = true;
+      NextIsEnd = true;
+
+      // If the next handler is a catch-all, we're at the end, and the
+      // next block is that handler.
+    } else if (CatchScope.getHandler(I + 1).isCatchAll()) {
+      NextBlock = CatchScope.getHandler(I + 1).Block;
+      NextIsEnd = true;
+
+      // Otherwise, we're not at the end and we need a new block.
+    } else {
+      NextBlock = CGF.createBasicBlock("catch.fallthrough");
+      EmitNextBlock = true;
+    }
+
+    // Figure out the catch type's index in the LSDA's type table.
+    llvm::CallInst *TypeIndex = CGF.Builder.CreateCall(TypeIDFn, TypeInfo.RTTI);
+    TypeIndex->setDoesNotThrow();
+
+    llvm::Value *MatchesTypeIndex =
+        CGF.Builder.CreateICmpEQ(Selector, TypeIndex, "matches");
+    CGF.Builder.CreateCondBr(MatchesTypeIndex, Handler.Block, NextBlock);
+
+    if (EmitNextBlock)
+      CGF.EmitBlock(NextBlock);
+    if (NextIsEnd)
+      break;
+  }
+
+  CGF.Builder.restoreIP(SavedIP);
+}
+
 /// Emit the structure of the dispatch block for the given catch scope.
 /// It is an invariant that the dispatch block already exists.
 static void emitCatchDispatchBlock(CodeGenFunction &CGF,
                                    EHCatchScope &catchScope) {
+  if (EHPersonality::get(CGF).isWasmPersonality())
+    return emitWasmCatchPadBlock(CGF, catchScope);
   if (EHPersonality::get(CGF).usesFuncletPads())
     return emitCatchPadBlock(CGF, catchScope);
 
@@ -1007,6 +1123,7 @@ void CodeGenFunction::ExitCXXTryStmt(const CXXTryStmt &S, bool IsFnTryBlock) {
   unsigned NumHandlers = S.getNumHandlers();
   EHCatchScope &CatchScope = cast<EHCatchScope>(*EHStack.begin());
   assert(CatchScope.getNumHandlers() == NumHandlers);
+  llvm::BasicBlock *DispatchBlock = CatchScope.getCachedEHDispatchBlock();
 
   // If the catch was not required, bail out now.
   if (!CatchScope.hasEHBranches()) {
@@ -1039,14 +1156,32 @@ void CodeGenFunction::ExitCXXTryStmt(const CXXTryStmt &S, bool IsFnTryBlock) {
     doImplicitRethrow = isa<CXXDestructorDecl>(CurCodeDecl) ||
                         isa<CXXConstructorDecl>(CurCodeDecl);
 
+  // Wasm uses Windows-style EH instructions, but merges all catch clauses into
+  // one big catchpad. So we save the old funclet pad here before we traverse
+  // each catch handler.
+  SaveAndRestore<llvm::Instruction *> RestoreCurrentFuncletPad(
+      CurrentFuncletPad);
+  llvm::BasicBlock *WasmCatchStartBlock = nullptr;
+  if (EHPersonality::get(*this).isWasmPersonality()) {
+    auto *CatchSwitch =
+        cast<llvm::CatchSwitchInst>(DispatchBlock->getFirstNonPHI());
+    WasmCatchStartBlock = CatchSwitch->hasUnwindDest()
+                              ? CatchSwitch->getSuccessor(1)
+                              : CatchSwitch->getSuccessor(0);
+    auto *CPI = cast<llvm::CatchPadInst>(WasmCatchStartBlock->getFirstNonPHI());
+    CurrentFuncletPad = CPI;
+  }
+
   // Perversely, we emit the handlers backwards precisely because we
   // want them to appear in source order.  In all of these cases, the
   // catch block will have exactly one predecessor, which will be a
   // particular block in the catch dispatch.  However, in the case of
   // a catch-all, one of the dispatch blocks will branch to two
   // different handlers, and EmitBlockAfterUses will cause the second
   // handler to be moved before the first.
+  bool HasCatchAll = false;
   for (unsigned I = NumHandlers; I != 0; --I) {
+    HasCatchAll |= Handlers[I - 1].isCatchAll();
     llvm::BasicBlock *CatchBlock = Handlers[I-1].Block;
     EmitBlockAfterUses(CatchBlock);
 
@@ -1091,6 +1226,27 @@ void CodeGenFunction::ExitCXXTryStmt(const CXXTryStmt &S, bool IsFnTryBlock) {
       Builder.CreateBr(ContBB);
   }
 
+  // Because in wasm we merge all catch clauses into one big catchpad, in case
+  // none of the types in catch handlers matches after we test against each of
+  // them, we should unwind to the next EH enclosing scope. We generate a call
+  // to rethrow function here to do that.
+  if (EHPersonality::get(*this).isWasmPersonality() && !HasCatchAll) {
+    assert(WasmCatchStartBlock);
+    // Navigate for the "rethrow" block we created in emitWasmCatchPadBlock().
+    // Wasm uses landingpad-style conditional branches to compare selectors, so
+    // we follow the false destination for each of the cond branches to reach
+    // the rethrow block.
+    llvm::BasicBlock *RethrowBlock = WasmCatchStartBlock;
+    while (llvm::TerminatorInst *TI = RethrowBlock->getTerminator()) {
+      auto *BI = cast<llvm::BranchInst>(TI);
+      assert(BI->isConditional());
+      RethrowBlock = BI->getSuccessor(1);
+    }
+    assert(RethrowBlock != WasmCatchStartBlock && RethrowBlock->empty());
+    Builder.SetInsertPoint(RethrowBlock);
+    CGM.getCXXABI().emitRethrow(*this, /*isNoReturn=*/true);
+  }
+
   EmitBlock(ContBB);
   incrementProfileCounter(&S);
 }
@@ -1369,8 +1525,17 @@ llvm::BasicBlock *CodeGenFunction::getTerminateFunclet() {
   CurrentFuncletPad = Builder.CreateCleanupPad(ParentPad);
 
   // Emit the __std_terminate call.
+  llvm::Value *Exn = nullptr;
+  // In case of wasm personality, we need to pass the exception value to
+  // __clang_call_terminate function.
+  if (getLangOpts().CPlusPlus &&
+      EHPersonality::get(*this).isWasmPersonality()) {
+    llvm::Value *GetExnFn =
+        CGM.getIntrinsic(llvm::Intrinsic::wasm_get_exception);
+    Exn = Builder.CreateCall(GetExnFn, CurrentFuncletPad);
+  }
   llvm::CallInst *terminateCall =
-      CGM.getCXXABI().emitTerminateForUnexpectedException(*this, nullptr);
+      CGM.getCXXABI().emitTerminateForUnexpectedException(*this, Exn);
   terminateCall->setDoesNotReturn();
   Builder.CreateUnreachable();
 

clang/lib/CodeGen/CodeGenFunction.h

@@ -877,7 +877,8 @@ class CodeGenFunction : public CodeGenTypeCache {
 
   llvm::BasicBlock *getEHResumeBlock(bool isCleanup);
   llvm::BasicBlock *getEHDispatchBlock(EHScopeStack::stable_iterator scope);
-  llvm::BasicBlock *getMSVCDispatchBlock(EHScopeStack::stable_iterator scope);
+  llvm::BasicBlock *
+  getFuncletEHDispatchBlock(EHScopeStack::stable_iterator scope);
 
   /// An object to manage conditionally-evaluated expressions.
   class ConditionalEvaluation {

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -469,6 +469,7 @@ class WebAssemblyCXXABI final : public ItaniumCXXABI {
   explicit WebAssemblyCXXABI(CodeGen::CodeGenModule &CGM)
       : ItaniumCXXABI(CGM, /*UseARMMethodPtrABI=*/true,
                       /*UseARMGuardVarABI=*/true) {}
+  void emitBeginCatch(CodeGenFunction &CGF, const CXXCatchStmt *C) override;
 
 private:
   bool HasThisReturn(GlobalDecl GD) const override {
@@ -4098,3 +4099,10 @@ ItaniumCXXABI::LoadVTablePtr(CodeGenFunction &CGF, Address This,
                              const CXXRecordDecl *RD) {
   return {CGF.GetVTablePtr(This, CGM.Int8PtrTy, RD), RD};
 }
+
+void WebAssemblyCXXABI::emitBeginCatch(CodeGenFunction &CGF,
+                                       const CXXCatchStmt *C) {
+  CGF.EHStack.pushCleanup<CatchRetScope>(
+      NormalCleanup, cast<llvm::CatchPadInst>(CGF.CurrentFuncletPad));
+  ItaniumCXXABI::emitBeginCatch(CGF, C);
+}

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -859,20 +859,6 @@ void MicrosoftCXXABI::emitRethrow(CodeGenFunction &CGF, bool isNoReturn) {
     CGF.EmitRuntimeCallOrInvoke(Fn, Args);
 }
 
-namespace {
-struct CatchRetScope final : EHScopeStack::Cleanup {
-  llvm::CatchPadInst *CPI;
-
-  CatchRetScope(llvm::CatchPadInst *CPI) : CPI(CPI) {}
-
-  void Emit(CodeGenFunction &CGF, Flags flags) override {
-    llvm::BasicBlock *BB = CGF.createBasicBlock("catchret.dest");
-    CGF.Builder.CreateCatchRet(CPI, BB);
-    CGF.EmitBlock(BB);
-  }
-};
-}
-
 void MicrosoftCXXABI::emitBeginCatch(CodeGenFunction &CGF,
                                      const CXXCatchStmt *S) {
   // In the MS ABI, the runtime handles the copy, and the catch handler is