[CIR] LLVM lowering support for pointers to member functions

This patch adds support for LLVM lowering of pointers to member functions. The
lowering is ABI-specific and this patch only considers Itanium ABI.

Itanium ABI lowers pointers to member functions to a struct with two fields of
type `ptrdiff_t`. To extract fields from such aggregate values, this patch
includes a new operation `cir.extract_member` to accomplish this.

Author: Lancern

clang/include/clang/CIR/Dialect/IR/CIRAttrs.td

@@ -572,6 +572,16 @@ def MethodAttr : CIR_Attr<"Method", "method", [TypedAttrInterface]> {
   let hasCustomAssemblyFormat = 1;
 
   let genVerifyDecl = 1;
+
+  let extraClassDeclaration = [{
+    bool isNull() const {
+      return !getSymbol().has_value() && !getVtableOffset().has_value();
+    }
+
+    bool isVirtual() const {
+      return getVtableOffset().has_value();
+    }
+  }];
 }
 
 //===----------------------------------------------------------------------===//

clang/include/clang/CIR/Dialect/IR/CIROps.td

@@ -2850,6 +2850,64 @@ def GetMemberOp : CIR_Op<"get_member"> {
   let hasVerifier = 1;
 }
 
+//===----------------------------------------------------------------------===//
+// ExtractMemberOp
+//===----------------------------------------------------------------------===//
+
+def ExtractMemberOp : CIR_Op<"extract_member", [Pure]> {
+  let summary = "Extract the value of a member of a struct value";
+  let description = [{
+    The `cir.extract_member` operation extracts the value of a particular member
+    from the input record. Unlike `cir.get_member` which derives pointers, this
+    operation operates on values. It takes a value of record type, and extract
+    the value of the specified record member from the input record value.
+
+    Currently `cir.extract_member` does not work on unions.
+
+    Example:
+
+    ```mlir
+    // Suppose we have a struct with multiple members.
+    !s32i = !cir.int<s, 32>
+    !s8i = !cir.int<s, 32>
+    !struct_ty = !cir.struct<"struct.Bar" {!s32i, !s8i}>
+
+    // And suppose we have a value of the struct type.
+    %0 = cir.const #cir.const_struct<{#cir.int<1> : !s32i, #cir.int<2> : !s8i}> : !struct_ty
+
+    // Extract the value of the second member of the struct.
+    %1 = cir.extract_member %0[1] : !struct_ty -> !s8i
+    ```
+  }];
+
+  let arguments = (ins CIR_StructType:$record, IndexAttr:$index_attr);
+  let results = (outs CIR_AnyType:$result);
+
+  let assemblyFormat = [{
+    $record `[` $index_attr `]` attr-dict
+    `:` qualified(type($record)) `->` qualified(type($result))
+  }];
+
+  let builders = [
+    OpBuilder<(ins "mlir::Type":$type, "mlir::Value":$record, "uint64_t":$index), [{
+      mlir::APInt fieldIdx(64, index);
+      build($_builder, $_state, type, record, fieldIdx);
+    }]>,
+    OpBuilder<(ins "mlir::Value":$record, "uint64_t":$index), [{
+      auto recordTy = mlir::cast<cir::StructType>(record.getType());
+      mlir::Type memberTy = recordTy.getMembers()[index];
+      build($_builder, $_state, memberTy, record, index);
+    }]>
+  ];
+
+  let extraClassDeclaration = [{
+    /// Get the index of the struct member being accessed.
+    uint64_t getIndex() { return getIndexAttr().getZExtValue(); }
+  }];
+
+  let hasVerifier = 1;
+}
+
 //===----------------------------------------------------------------------===//
 // GetRuntimeMemberOp
 //===----------------------------------------------------------------------===//

clang/lib/CIR/Dialect/IR/CIRDialect.cpp

@@ -3575,6 +3575,26 @@ LogicalResult cir::GetMemberOp::verify() {
   return mlir::success();
 }
 
+//===----------------------------------------------------------------------===//
+// ExtractMemberOp Definitions
+//===----------------------------------------------------------------------===//
+
+LogicalResult cir::ExtractMemberOp::verify() {
+  auto recordTy = mlir::cast<cir::StructType>(getRecord().getType());
+  if (recordTy.getKind() == cir::StructType::Union)
+    return emitError()
+           << "cir.extract_member currently does not work on unions";
+  if (recordTy.getMembers().size() <= getIndex())
+    return emitError() << "member index out of bounds";
+
+  // FIXME(cir): member type check is disabled for classes as the codegen for
+  // these still need to be patched.
+  if (!recordTy.isClass() && recordTy.getMembers()[getIndex()] != getType())
+    return emitError() << "member type mismatch";
+
+  return mlir::success();
+}
+
 //===----------------------------------------------------------------------===//
 // GetRuntimeMemberOp Definitions
 //===----------------------------------------------------------------------===//

clang/lib/CIR/Dialect/Transforms/TargetLowering/CIRCXXABI.h

@@ -72,20 +72,40 @@ class CIRCXXABI {
   lowerDataMemberType(cir::DataMemberType type,
                       const mlir::TypeConverter &typeConverter) const = 0;
 
+  /// Lower the given member function pointer type to its ABI type. The returned
+  /// type is also a CIR type.
+  virtual mlir::Type
+  lowerMethodType(cir::MethodType type,
+                  const mlir::TypeConverter &typeConverter) const = 0;
+
   /// Lower the given data member pointer constant to a constant of the ABI
   /// type. The returned constant is represented as an attribute as well.
   virtual mlir::TypedAttr
   lowerDataMemberConstant(cir::DataMemberAttr attr,
                           const mlir::DataLayout &layout,
                           const mlir::TypeConverter &typeConverter) const = 0;
 
+  /// Lower the given member function pointer constant to a constant of the ABI
+  /// type. The returned constant is represented as an attribute as well.
+  virtual mlir::TypedAttr
+  lowerMethodConstant(cir::MethodAttr attr, const mlir::DataLayout &layout,
+                      const mlir::TypeConverter &typeConverter) const = 0;
+
   /// Lower the given cir.get_runtime_member op to a sequence of more
   /// "primitive" CIR operations that act on the ABI types.
   virtual mlir::Operation *
   lowerGetRuntimeMember(cir::GetRuntimeMemberOp op, mlir::Type loweredResultTy,
                         mlir::Value loweredAddr, mlir::Value loweredMember,
                         mlir::OpBuilder &builder) const = 0;
 
+  /// Lower the given cir.get_method op to a sequence of more "primitive" CIR
+  /// operations that act on the ABI types. The lowered result values will be
+  /// stored in the given loweredResults array.
+  virtual void
+  lowerGetMethod(cir::GetMethodOp op, mlir::Value (&loweredResults)[2],
+                 mlir::Value loweredMethod, mlir::Value loweredObjectPtr,
+                 mlir::ConversionPatternRewriter &rewriter) const = 0;
+
   /// Lower the given cir.base_data_member op to a sequence of more "primitive"
   /// CIR operations that act on the ABI types.
   virtual mlir::Value lowerBaseDataMember(cir::BaseDataMemberOp op,