[clang] [CIR] Better handling of `void` function return (PR #128089)

[Erich Keane via cfe-commits]

================
@@ -331,9 +335,38 @@ FuncType FuncType::clone(TypeRange inputs, TypeRange results) const {
   return get(llvm::to_vector(inputs), results[0], isVarArg());
 }
 
-mlir::ParseResult parseFuncTypeArgs(mlir::AsmParser &p,
-                                    llvm::SmallVector<mlir::Type> &params,
-                                    bool &isVarArg) {
+// A special parser is needed for function returning void to handle the missing
+// type.
+static mlir::ParseResult parseFuncTypeReturn(mlir::AsmParser &p,
+                                             mlir::Type &optionalReturnType) {
+  if (succeeded(p.parseOptionalLParen())) {
+    // If we have already a '(', the function has no return type
+    optionalReturnType = {};
+    return mlir::success();
+  }
+  mlir::Type type;
+  if (p.parseType(type))
+    return mlir::failure();
+  if (isa<cir::VoidType>(type))
+    // An explicit !cir.void means also no return type.
+    optionalReturnType = {};
+  else
+    // Otherwise use the actual type.
+    optionalReturnType = type;
+  return p.parseLParen();
+}
+
+// A special pretty-printer for function returning or not a result.
+static void printFuncTypeReturn(mlir::AsmPrinter &p,
+                                mlir::Type optionalReturnType) {
+  if (optionalReturnType)
+    p << optionalReturnType << ' ';
+  p << '(';
+}
+
+static mlir::ParseResult
+parseFuncTypeArgs(mlir::AsmParser &p, llvm::SmallVector<mlir::Type> &params,
+                  bool &isVarArg) {
   isVarArg = false;
   // `(` `)`
   if (succeeded(p.parseOptionalRParen()))
----------------
erichkeane wrote:

The only question is what the 'void' type looks like?  Could/should we still use the `void` as the return type in the textual representation, or alternatively just omit entirely? So:

`!cir.func<(args) -> void>`
vs: 
`!cir.func<(args)>`

That way we do `parse arguments`, then `if arrow, parse return type, else is void`.

The 'trailing return type' syntax is nice as it is consistent with the C++ language.

https://github.com/llvm/llvm-project/pull/128089