[clang] [CIR] Add support for accessing members of base classes (PR #143195)

[Bruno Cardoso Lopes via cfe-commits]

================
@@ -2231,4 +2231,48 @@ def VecTernaryOp : CIR_Op<"vec.ternary",
   let hasVerifier = 1;
 }
 
+//===----------------------------------------------------------------------===//
+// BaseClassAddrOp
+//===----------------------------------------------------------------------===//
+
+def BaseClassAddrOp : CIR_Op<"base_class_addr"> {
+  let summary = "Get the base class address for a class/struct";
+  let description = [{
+    The `cir.base_class_addr` operaration gets the address of a particular
+    non-virtual base class given a derived class pointer. The offset in bytes
+    of the base class must be passed in, since it is easier for the front end
+    to calculate that than the MLIR passes. The operation contains a flag for
+    whether or not the operand may be nullptr. That depends on the context and
+    cannot be known by the operation, and that information affects how the
+    operation is lowered.
+
+    Example:
+    ```c++
+    struct Base { };
+    struct Derived : Base { };
+    Derived d;
+    Base& b = d;
+    ```
+    will generate
+    ```mlir
+    %3 = cir.base_class_addr (%1 : !cir.ptr<!rec_Derived> nonnull) [0] -> !cir.ptr<!rec_Base>
+    ```
+  }];
+
+  // The validity of the relationship of derived and base cannot yet be
+  // verified, currently not worth adding a verifier.
+  let arguments = (ins
+    Arg<CIR_PointerType, "derived class pointer", [MemRead]>:$derived_addr,
+    IndexAttr:$offset, UnitAttr:$assume_not_null);
+
+  let results = (outs Res<CIR_PointerType, "">:$base_addr);
+
+  let assemblyFormat = [{
+    `(`
----------------
bcardosolopes wrote:

This could be a good opportunity to get rid of the `(` and `)` (https://llvm.github.io/clangir/Dialect/cir-style-guide.html).

https://github.com/llvm/llvm-project/pull/143195