[WebAssembly] Add assembly support for final EH proposal (#107917)

This adds the basic assembly generation support for the final EH
proposal, which was newly adopted in Sep 2023 and advanced into Phase 4
in Jul 2024:

https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md

This adds support for the generation of new `try_table` and `throw_ref`
instruction in .s asesmbly format. This does NOT yet include
- Block annotation comment generation for .s format
- .o object file generation
- .s assembly parsing
- Type checking (AsmTypeCheck)
- Disassembler
- Fixing unwind mismatches in CFGStackify

These will be added as follow-up PRs.

---

The format for `TRY_TABLE`, both for `MachineInstr` and `MCInst`, is as
follows:
```
TRY_TABLE type number_of_catches catch_clauses*
```
where `catch_clause` is
```
catch_opcode tag+ destination
```
`catch_opcode` should be one of 0/1/2/3, which denotes
`CATCH`/`CATCH_REF`/`CATCH_ALL`/`CATCH_ALL_REF` respectively. (See
`BinaryFormat/Wasm.h`) `tag` exists when the catch is one of `CATCH` or
`CATCH_REF`.
The MIR format is printed as just the list of raw operands. The
(stack-based) assembly instruction supports pretty-printing, including
printing `catch` clauses by name, in InstPrinter.

In addition to the new instructions `TRY_TABLE` and `THROW_REF`, this
adds four pseudo instructions: `CATCH`, `CATCH_REF`, `CATCH_ALL`, and
`CATCH_ALL_REF`. These are pseudo instructions to simulate block return
values of `catch`, `catch_ref`, `catch_all`, `catch_all_ref` clauses in
`try_table` respectively, given that we don't support block return
values except for one case (`fixEndsAtEndOfFunction` in CFGStackify).
These will be omitted when we lower the instructions to `MCInst` at the
end.

LateEHPrepare now will have one more stage to covert
`CATCH`/`CATCH_ALL`s to `CATCH_REF`/`CATCH_ALL_REF`s when there is a
`RETHROW` to rethrow its exception. The pass also converts `RETHROW`s
into `THROW_REF`. Note that we still use `RETHROW` as an interim pseudo
instruction until we convert them to `THROW_REF` in LateEHPrepare.

CFGStackify has a new `placeTryTableMarker` function, which places
`try_table`/`end_try_table` markers with a necessary `catch` clause and
also `block`/`end_block` markers for the destination of the `catch`
clause.

In MCInstLower, now we need to support one more case for the multivalue
block signature (`catch_ref`'s destination's `(i32, exnref)` return
type).

InstPrinter has a new routine to print the `catch_list` type, which is
used to print `try_table` instructions.

The new test, `exception.ll`'s source is the same as
`exception-legacy.ll`, with the FileCheck expectations changed. One
difference is the commands in this file have `-wasm-enable-exnref` to
test the new format, and don't have `-wasm-disable-explicit-locals
-wasm-keep-registers`, because the new custom InstPrinter routine to
print `catch_list` only works for the stack-based instructions (`_S`),
and we can't use `-wasm-keep-registers` for them.

As in `exception-legacy.ll`, the FileCheck lines for the new tests do
not contain the whole program; they mostly contain only the control flow
instructions for readability.

Author: aheejin

llvm/include/llvm/BinaryFormat/Wasm.h

@@ -144,6 +144,14 @@ enum : unsigned {
   WASM_OPCODE_I32_RMW_CMPXCHG = 0x48,
 };
 
+// Sub-opcodes for catch clauses in a try_table instruction
+enum : unsigned {
+  WASM_OPCODE_CATCH = 0x00,
+  WASM_OPCODE_CATCH_REF = 0x01,
+  WASM_OPCODE_CATCH_ALL = 0x02,
+  WASM_OPCODE_CATCH_ALL_REF = 0x03,
+};
+
 enum : unsigned {
   WASM_LIMITS_FLAG_NONE = 0x0,
   WASM_LIMITS_FLAG_HAS_MAX = 0x1,

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

@@ -45,7 +45,7 @@ namespace {
 /// WebAssemblyOperand - Instances of this class represent the operands in a
 /// parsed Wasm machine instruction.
 struct WebAssemblyOperand : public MCParsedAsmOperand {
-  enum KindTy { Token, Integer, Float, Symbol, BrList } Kind;
+  enum KindTy { Token, Integer, Float, Symbol, BrList, CatchList } Kind;
 
   SMLoc StartLoc, EndLoc;
 
@@ -99,6 +99,7 @@ struct WebAssemblyOperand : public MCParsedAsmOperand {
   bool isMem() const override { return false; }
   bool isReg() const override { return false; }
   bool isBrList() const { return Kind == BrList; }
+  bool isCatchList() const { return Kind == CatchList; }
 
   MCRegister getReg() const override {
     llvm_unreachable("Assembly inspects a register operand");
@@ -151,6 +152,10 @@ struct WebAssemblyOperand : public MCParsedAsmOperand {
       Inst.addOperand(MCOperand::createImm(Br));
   }
 
+  void addCatchListOperands(MCInst &Inst, unsigned N) const {
+    // TODO
+  }
+
   void print(raw_ostream &OS) const override {
     switch (Kind) {
     case Token:
@@ -168,6 +173,9 @@ struct WebAssemblyOperand : public MCParsedAsmOperand {
     case BrList:
       OS << "BrList:" << BrL.List.size();
       break;
+    case CatchList:
+      // TODO
+      break;
     }
   }
 };

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp

@@ -367,3 +367,44 @@ void WebAssemblyInstPrinter::printWebAssemblySignatureOperand(const MCInst *MI,
     }
   }
 }
+
+void WebAssemblyInstPrinter::printCatchList(const MCInst *MI, unsigned OpNo,
+                                            raw_ostream &O) {
+  unsigned OpIdx = OpNo;
+  const MCOperand &Op = MI->getOperand(OpIdx++);
+  unsigned NumCatches = Op.getImm();
+
+  auto PrintTagOp = [&](const MCOperand &Op) {
+    const MCSymbolRefExpr *TagExpr = nullptr;
+    const MCSymbolWasm *TagSym = nullptr;
+    assert(Op.isExpr());
+    TagExpr = dyn_cast<MCSymbolRefExpr>(Op.getExpr());
+    TagSym = cast<MCSymbolWasm>(&TagExpr->getSymbol());
+    O << TagSym->getName() << " ";
+  };
+
+  for (unsigned I = 0; I < NumCatches; I++) {
+    const MCOperand &Op = MI->getOperand(OpIdx++);
+    O << "(";
+    switch (Op.getImm()) {
+    case wasm::WASM_OPCODE_CATCH:
+      O << "catch ";
+      PrintTagOp(MI->getOperand(OpIdx++));
+      break;
+    case wasm::WASM_OPCODE_CATCH_REF:
+      O << "catch_ref ";
+      PrintTagOp(MI->getOperand(OpIdx++));
+      break;
+    case wasm::WASM_OPCODE_CATCH_ALL:
+      O << "catch_all ";
+      break;
+    case wasm::WASM_OPCODE_CATCH_ALL_REF:
+      O << "catch_all_ref ";
+      break;
+    }
+    O << MI->getOperand(OpIdx++).getImm(); // destination
+    O << ")";
+    if (I < NumCatches - 1)
+      O << " ";
+  }
+}

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.h

@@ -47,6 +47,7 @@ class WebAssemblyInstPrinter final : public MCInstPrinter {
                                       raw_ostream &O);
   void printWebAssemblySignatureOperand(const MCInst *MI, unsigned OpNo,
                                         raw_ostream &O);
+  void printCatchList(const MCInst *MI, unsigned OpNo, raw_ostream &O);
 
   // Autogenerated by tblgen.
   std::pair<const char *, uint64_t> getMnemonic(const MCInst *MI) override;

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h

@@ -87,6 +87,8 @@ enum OperandType {
   OPERAND_BRLIST,
   /// 32-bit unsigned table number.
   OPERAND_TABLE,
+  /// A list of catch clauses for try_table.
+  OPERAND_CATCH_LIST,
 };
 } // end namespace WebAssembly
 
@@ -119,6 +121,10 @@ enum TOF {
   // address relative the __table_base wasm global.
   // Only applicable to function symbols.
   MO_TABLE_BASE_REL,
+
+  // On a block signature operand this indicates that this is a destination
+  // block of a (catch_ref) clause in try_table.
+  MO_CATCH_BLOCK_SIG,
 };
 
 } // end namespace WebAssemblyII
@@ -462,6 +468,22 @@ inline bool isMarker(unsigned Opc) {
   case WebAssembly::TRY_S:
   case WebAssembly::END_TRY:
   case WebAssembly::END_TRY_S:
+  case WebAssembly::TRY_TABLE:
+  case WebAssembly::TRY_TABLE_S:
+  case WebAssembly::END_TRY_TABLE:
+  case WebAssembly::END_TRY_TABLE_S:
+    return true;
+  default:
+    return false;
+  }
+}
+
+inline bool isTry(unsigned Opc) {
+  switch (Opc) {
+  case WebAssembly::TRY:
+  case WebAssembly::TRY_S:
+  case WebAssembly::TRY_TABLE:
+  case WebAssembly::TRY_TABLE_S:
     return true;
   default:
     return false;
@@ -474,6 +496,14 @@ inline bool isCatch(unsigned Opc) {
   case WebAssembly::CATCH_LEGACY_S:
   case WebAssembly::CATCH_ALL_LEGACY:
   case WebAssembly::CATCH_ALL_LEGACY_S:
+  case WebAssembly::CATCH:
+  case WebAssembly::CATCH_S:
+  case WebAssembly::CATCH_REF:
+  case WebAssembly::CATCH_REF_S:
+  case WebAssembly::CATCH_ALL:
+  case WebAssembly::CATCH_ALL_S:
+  case WebAssembly::CATCH_ALL_REF:
+  case WebAssembly::CATCH_ALL_REF_S:
     return true;
   default:
     return false;

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTypeUtilities.h

@@ -33,11 +33,15 @@ enum class BlockType : unsigned {
   Externref = unsigned(wasm::ValType::EXTERNREF),
   Funcref = unsigned(wasm::ValType::FUNCREF),
   Exnref = unsigned(wasm::ValType::EXNREF),
-  // Multivalue blocks (and other non-void blocks) are only emitted when the
-  // blocks will never be exited and are at the ends of functions (see
-  // WebAssemblyCFGStackify::fixEndsAtEndOfFunction). They also are never made
-  // to pop values off the stack, so the exact multivalue signature can always
-  // be inferred from the return type of the parent function in MCInstLower.
+  // Multivalue blocks are emitted in two cases:
+  // 1. When the blocks will never be exited and are at the ends of functions
+  //    (see WebAssemblyCFGStackify::fixEndsAtEndOfFunction). In this case the
+  //    exact multivalue signature can always be inferred from the return type
+  //    of the parent function.
+  // 2. (catch_ref ...) clause in try_table instruction. Currently all tags we
+  //    support (cpp_exception and c_longjmp) throws a single i32, so the
+  //    multivalue signature for this case will be (i32, exnref).
+  // The real multivalue siganture will be added in MCInstLower.
   Multivalue = 0xffff,
 };
 

llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

@@ -683,6 +683,17 @@ void WebAssemblyAsmPrinter::emitInstruction(const MachineInstr *MI) {
     // This is a compiler barrier that prevents instruction reordering during
     // backend compilation, and should not be emitted.
     break;
+  case WebAssembly::CATCH:
+  case WebAssembly::CATCH_S:
+  case WebAssembly::CATCH_REF:
+  case WebAssembly::CATCH_REF_S:
+  case WebAssembly::CATCH_ALL:
+  case WebAssembly::CATCH_ALL_S:
+  case WebAssembly::CATCH_ALL_REF:
+  case WebAssembly::CATCH_ALL_REF_S:
+    // These are pseudo instructions to represent catch clauses in try_table
+    // instruction to simulate block return values.
+    break;
   default: {
     WebAssemblyMCInstLower MCInstLowering(OutContext, *this);
     MCInst TmpInst;