[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.

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp b/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
index 24a9ad6..5299e6e 100644
--- a/llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
+++ b/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;
     }
   }
 };