[CIR][WIP] Add ABI lowering pass

This patch adds a new pass cir-abi-lowering to the CIR dialect. This pass runs
before the CallConvLowering pass, and it expands all ABI-dependent types and
operations inside a function to their ABI-independent equivalences according to
the ABI specification.

This patch also moves the lowering code of the following types and operations
from the LLVM lowering conversion to the new pass:
  - The pointer-to-data-member type `cir.data_member`;
  - The pointer-to-member-function type `cir.method`;
  - All operations working on operands of the above types.

Author: Lancern

clang/include/clang/CIR/Dialect/Passes.h

@@ -40,6 +40,9 @@ std::unique_ptr<Pass> createFlattenCFGPass();
 std::unique_ptr<Pass> createHoistAllocasPass();
 std::unique_ptr<Pass> createGotoSolverPass();
 
+/// Create a pass to expand ABI-dependent types and operations.
+std::unique_ptr<Pass> createABILoweringPass();
+
 /// Create a pass to lower ABI-independent function definitions/calls.
 std::unique_ptr<Pass> createCallConvLoweringPass();
 

clang/include/clang/CIR/Dialect/Passes.td

@@ -180,6 +180,27 @@ def LibOpt : Pass<"cir-lib-opt"> {
   ];
 }
 
+def ABILowering : Pass<"cir-abi-lowering"> {
+  let summary = "Expands ABI-dependent types and operations";
+  let description = [{
+    This pass expands ABI-dependent CIR types and operations to more "primitive"
+    ABI-independent CIR types and operations according to the target ABI
+    specification.
+
+    Some CIR types, such as pointers to members, may have different layouts and
+    representations under different target ABIs. This pass expands these types
+    to their underlying representations as specified by the target ABI. For
+    example, when targeting Itanium ABI, this pass will replace pointers to
+    member functions with a struct with two ptrdiff_t fields.
+
+    Similarly, some CIR operations may also behave differently under different
+    target ABIs. This pass also expands these operations to more "primitive"
+    CIR operations as specified by the target ABI.
+  }];
+  let constructor = "mlir::createABILoweringPass()";
+  let dependentDialects = ["cir::CIRDialect"];
+}
+
 def CallConvLowering : Pass<"cir-call-conv-lowering"> {
   let summary = "Handle calling conventions for CIR functions";
   let description = [{

clang/lib/CIR/CodeGen/CIRPasses.cpp

@@ -97,6 +97,7 @@ mlir::LogicalResult runCIRToCIRPasses(
 namespace mlir {
 
 void populateCIRPreLoweringPasses(OpPassManager &pm, bool useCCLowering) {
+  pm.addPass(createABILoweringPass());
   if (useCCLowering)
     pm.addPass(createCallConvLoweringPass());
   pm.addPass(createHoistAllocasPass());