[WebAssembly] Add Wasm SjLj support

This add support for SjLj using Wasm exception handling instructions:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/exception-handling/Exceptions.md

This does not yet support the mixed use of EH and SjLj within a
function. It will be added in a follow-up CL.

This currently passes all SjLj Emscripten tests for wasm0/1/2/3/s,
except for the below:
- `test_longjmp_standalone`: Uses Node
- `test_dlfcn_longjmp`: Uses NodeRAWFS
- `test_longjmp_throw`: Mixes EH and SjLj
- `test_exceptions_longjmp1`: Mixes EH and SjLj
- `test_exceptions_longjmp2`: Mixes EH and SjLj
- `test_exceptions_longjmp3`: Mixes EH and SjLj

Reviewed By: dschuff, tlively

Differential Revision: https://reviews.llvm.org/D108960

Author: aheejin

llvm/lib/CodeGen/AsmPrinter/WasmException.cpp

@@ -18,16 +18,18 @@
 using namespace llvm;
 
 void WasmException::endModule() {
-  // This is the symbol used in 'throw' and 'catch' instruction to denote this
-  // is a C++ exception. This symbol has to be emitted somewhere once in the
-  // module.  Check if the symbol has already been created, i.e., we have at
-  // least one 'throw' or 'catch' instruction in the module, and emit the symbol
-  // only if so.
-  SmallString<60> NameStr;
-  Mangler::getNameWithPrefix(NameStr, "__cpp_exception", Asm->getDataLayout());
-  if (Asm->OutContext.lookupSymbol(NameStr)) {
-    MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol("__cpp_exception");
-    Asm->OutStreamer->emitLabel(ExceptionSym);
+  // These are symbols used to throw/catch C++ exceptions and C longjmps. These
+  // symbols have to be emitted somewhere once in the module. Check if each of
+  // the symbols has already been created, i.e., we have at least one 'throw' or
+  // 'catch' instruction with the symbol in the module, and emit the symbol only
+  // if so.
+  for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) {
+    SmallString<60> NameStr;
+    Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout());
+    if (Asm->OutContext.lookupSymbol(NameStr)) {
+      MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName);
+      Asm->OutStreamer->emitLabel(ExceptionSym);
+    }
   }
 }
 

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -1226,7 +1226,10 @@ static void mapWasmLandingPadIndex(MachineBasicBlock *MBB,
   bool IsSingleCatchAllClause =
       CPI->getNumArgOperands() == 1 &&
       cast<Constant>(CPI->getArgOperand(0))->isNullValue();
-  if (!IsSingleCatchAllClause) {
+  // cathchpads for longjmp use an empty type list, e.g. catchpad within %0 []
+  // and they don't need LSDA info
+  bool IsCatchLongjmp = CPI->getNumArgOperands() == 0;
+  if (!IsSingleCatchAllClause && !IsCatchLongjmp) {
     // Create a mapping from landing pad label to landing pad index.
     bool IntrFound = false;
     for (const User *U : CPI->users()) {

llvm/lib/MC/WasmObjectWriter.cpp

@@ -1644,7 +1644,8 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
         LLVM_DEBUG(dbgs() << " -> table index: "
                           << WasmIndices.find(&WS)->second << "\n");
       } else if (WS.isTag()) {
-        // C++ exception symbol (__cpp_exception)
+        // C++ exception symbol (__cpp_exception) or longjmp symbol
+        // (__c_longjmp)
         unsigned Index;
         if (WS.isDefined()) {
           Index = NumTagImports + Tags.size();

llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

@@ -236,7 +236,7 @@ MCSymbol *WebAssemblyAsmPrinter::getOrCreateWasmSymbol(StringRef Name) {
 
   SmallVector<wasm::ValType, 4> Returns;
   SmallVector<wasm::ValType, 4> Params;
-  if (Name == "__cpp_exception") {
+  if (Name == "__cpp_exception" || Name == "__c_longjmp") {
     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_TAG);
     // We can't confirm its signature index for now because there can be
     // imported exceptions. Set it to be 0 for now.
@@ -248,12 +248,14 @@ MCSymbol *WebAssemblyAsmPrinter::getOrCreateWasmSymbol(StringRef Name) {
     WasmSym->setWeak(true);
     WasmSym->setExternal(true);
 
-    // All C++ exceptions are assumed to have a single i32 (for wasm32) or i64
-    // (for wasm64) param type and void return type. The reaon is, all C++
-    // exception values are pointers, and to share the type section with
-    // functions, exceptions are assumed to have void return type.
-    Params.push_back(Subtarget.hasAddr64() ? wasm::ValType::I64
-                                           : wasm::ValType::I32);
+    // Currently both C++ exceptions and C longjmps have a single pointer type
+    // param. For C++ exceptions it is a pointer to an exception object, and for
+    // C longjmps it is pointer to a struct that contains a setjmp buffer and a
+    // longjmp return value. We may consider using multiple value parameters for
+    // longjmps later when multivalue support is ready.
+    wasm::ValType AddrType =
+        Subtarget.hasAddr64() ? wasm::ValType::I64 : wasm::ValType::I32;
+    Params.push_back(AddrType);
   } else { // Function symbols
     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
     getLibcallSignature(Subtarget, Name, Returns, Params);

llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp

@@ -89,11 +89,13 @@ void WebAssemblyDAGToDAGISel::PreprocessISelDAG() {
 }
 
 static SDValue getTagSymNode(int Tag, SelectionDAG *DAG) {
-  assert(Tag == WebAssembly::CPP_EXCEPTION);
+  assert(Tag == WebAssembly::CPP_EXCEPTION || WebAssembly::C_LONGJMP);
   auto &MF = DAG->getMachineFunction();
   const auto &TLI = DAG->getTargetLoweringInfo();
   MVT PtrVT = TLI.getPointerTy(DAG->getDataLayout());
-  const char *SymName = MF.createExternalSymbolName("__cpp_exception");
+  const char *SymName = Tag == WebAssembly::CPP_EXCEPTION
+                            ? MF.createExternalSymbolName("__cpp_exception")
+                            : MF.createExternalSymbolName("__c_longjmp");
   return DAG->getTargetExternalSymbol(SymName, PtrVT);
 }