[CIR][Lowering] Add an empty LoweringPrepare pass (#236)

This change is a prerequisite of
#235

Author: htyu

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

@@ -28,6 +28,8 @@ std::unique_ptr<Pass> createLifetimeCheckPass(ArrayRef<StringRef> remark,
                                               clang::ASTContext *astCtx);
 std::unique_ptr<Pass> createMergeCleanupsPass();
 std::unique_ptr<Pass> createDropASTPass();
+std::unique_ptr<Pass> createLoweringPreparePass();
+std::unique_ptr<Pass> createLoweringPreparePass(clang::ASTContext *astCtx);
 
 //===----------------------------------------------------------------------===//
 // Registration

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

@@ -65,4 +65,14 @@ def DropAST : Pass<"cir-drop-ast"> {
   let dependentDialects = ["cir::CIRDialect"];
 }
 
+def LoweringPrepare : Pass<"cir-lowering-prepare"> {
+  let summary = "Preparation work before lowering to LLVM dialect";
+  let description = [{
+    This pass does preparation work for LLVM lowering. For example, it may
+    expand the global variable initialziation in a more ABI-friendly form.
+  }];
+  let constructor = "mlir::createLoweringPreparePass()";
+  let dependentDialects = ["cir::CIRDialect"];
+}
+
 #endif // MLIR_DIALECT_CIR_PASSES

clang/lib/CIR/CodeGen/CIRPasses.cpp

@@ -38,6 +38,8 @@ mlir::LogicalResult runCIRToCIRPasses(mlir::ModuleOp theModule,
     pm.addPass(std::move(lifetimePass));
   }
 
+  pm.addPass(mlir::createLoweringPreparePass(&astCtx));
+
   // FIXME: once CIRCodenAction fixes emission other than CIR we
   // need to run this right before dialect emission.
   pm.addPass(mlir::createDropASTPass());

clang/lib/CIR/Dialect/Transforms/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_clang_library(MLIRCIRTransforms
   LifetimeCheck.cpp
+  LoweringPrepare.cpp
   MergeCleanups.cpp
   DropAST.cpp
 

clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp

@@ -0,0 +1,51 @@
+//===- LoweringPrepare.cpp - pareparation work for LLVM lowering ----------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "clang/CIR/Dialect/Passes.h"
+
+using namespace mlir;
+using namespace cir;
+
+namespace {
+struct LoweringPreparePass : public LoweringPrepareBase<LoweringPreparePass> {
+  LoweringPreparePass() = default;
+  void runOnOperation() override;
+
+  ///
+  /// AST related
+  /// -----------
+
+  clang::ASTContext *astCtx;
+  void setASTContext(clang::ASTContext *c) { astCtx = c; }
+
+  /// Tracks current module.
+  ModuleOp theModule;
+};
+} // namespace
+
+
+void LoweringPreparePass::runOnOperation() {
+  assert(astCtx && "Missing ASTContext, please construct with the right ctor");
+  auto* op = getOperation();
+  if (isa<::mlir::ModuleOp>(op)) {
+    theModule = cast<::mlir::ModuleOp>(op);
+  }
+}
+
+std::unique_ptr<Pass> mlir::createLoweringPreparePass() {
+  return std::make_unique<LoweringPreparePass>();
+}
+
+std::unique_ptr<Pass> mlir::createLoweringPreparePass(clang::ASTContext *astCtx) {
+  auto pass = std::make_unique<LoweringPreparePass>();
+  pass->setASTContext(astCtx);
+  return std::move(pass);
+}

clang/test/CIR/mlirprint.c

@@ -10,6 +10,8 @@ int foo(void) {
 
 // CIR:  IR Dump After MergeCleanups (cir-merge-cleanups)
 // CIR:  cir.func @foo() -> !s32i
+// CIR:  IR Dump After LoweringPrepare (cir-lowering-prepare)
+// CIR:  cir.func @foo() -> !s32i
 // CIR:  IR Dump After DropAST (cir-drop-ast)
 // CIR:  cir.func @foo() -> !s32i
 // LLVM: IR Dump After cir::direct::ConvertCIRToLLVMPass (cir-to-llvm)