[llvm] [Xtensa] Implement base CallConvention. (PR #83280)

[Sergei Barannikov via llvm-commits]

================
@@ -0,0 +1,28 @@
+//===- XtensaCallingConv.td - Xtensa Calling Conventions -*- tablegen ---*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// This describes the calling conventions for the Xtensa ABI.
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+// Xtensa return value calling convention
+//===----------------------------------------------------------------------===//
+def RetCC_Xtensa : CallingConv<[
+  CCIfType<[i1, i8, i16], CCPromoteToType<i32>>,
----------------
s-barannikov wrote:

This is a no-op because all these types are illegal. Same for f32.

https://github.com/llvm/llvm-project/pull/83280