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

[Ronan Keryell 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()))
----------------
keryell wrote:

I like this idea too.
I have tried to be as close as the original syntax to avoid disrupting too much 
the tests and the users but there is no reason to do something else.
If there is a consensus from the ClangIR stake-holders, go for it!

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