[CIR] Add runAtStartOfConvertCIRToLLVMPass() to ConvertCIRToLLVMPass

This allows injecting some dialects and patterns to the
cir::direct::ConvertCIRToLLVMPass lowering pass.

Author: keryell

clang/include/clang/CIR/LowerToLLVM.h

@@ -13,7 +13,9 @@
 #define CLANG_CIR_LOWERTOLLVM_H
 
 #include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/DialectConversion.h"
 
+#include <functional>
 #include <memory>
 
 namespace llvm {
@@ -34,6 +36,9 @@ mlir::ModuleOp lowerDirectlyFromCIRToLLVMDialect(mlir::ModuleOp theModule,
                                                  bool disableCCLowering = false,
                                                  bool disableDebugInfo = false);
 
+void runAtStartOfConvertCIRToLLVMPass(
+    std::function<void(mlir::ConversionTarget)>);
+
 // Lower directly from pristine CIR to LLVMIR.
 std::unique_ptr<llvm::Module> lowerDirectlyFromCIRToLLVMIR(
     mlir::ModuleOp theModule, llvm::LLVMContext &llvmCtx,

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -1381,6 +1381,8 @@ struct ConvertCIRToLLVMPass
 
   void processCIRAttrs(mlir::ModuleOp moduleOp);
 
+  inline static std::function<void(mlir::ConversionTarget)> runAtStartHook;
+
   StringRef getDescription() const override {
     return "Convert the prepared CIR dialect module to LLVM dialect";
   }
@@ -4741,6 +4743,9 @@ void ConvertCIRToLLVMPass::runOnOperation() {
                     // ,YieldOp
                     >();
   // clang-format on
+  if (runAtStartHook)
+    runAtStartHook(target);
+
   target.addLegalDialect<mlir::LLVM::LLVMDialect>();
   target.addIllegalDialect<mlir::BuiltinDialect, cir::CIRDialect,
                            mlir::func::FuncDialect>();
@@ -4778,6 +4783,13 @@ void ConvertCIRToLLVMPass::runOnOperation() {
   processCIRAttrs(module);
 }
 
+/// Set a hook to be called just before applying the dialect conversion so other
+/// dialects or patterns can be added
+void runAtStartOfConvertCIRToLLVMPass(
+    std::function<void(mlir::ConversionTarget)> hook) {
+  ConvertCIRToLLVMPass::runAtStartHook = std::move(hook);
+}
+
 std::unique_ptr<mlir::Pass> createConvertCIRToLLVMPass() {
   return std::make_unique<ConvertCIRToLLVMPass>();
 }