[CIR] Upstream initial support for CIR flattening

The ClangIR CFG has to be flat before it can be lowered to LLVM IR. That is,
there can be no nested regions and all blocks in a region must belong to the
parent region. Currently only cir.scope operations violate these rules, so
the initial implementation of the cir-flatten-cfg pass only has to transform
scope operations.

Author: andykaylor

clang/include/clang/CIR/Dialect/CMakeLists.txt

@@ -1 +1,7 @@
 add_subdirectory(IR)
+
+set(LLVM_TARGET_DEFINITIONS Passes.td)
+mlir_tablegen(Passes.h.inc -gen-pass-decls -name CIR)
+mlir_tablegen(Passes.capi.h.inc -gen-pass-capi-header --prefix CIR)
+mlir_tablegen(Passes.capi.cpp.inc -gen-pass-capi-impl --prefix CIR)
+add_public_tablegen_target(MLIRCIRPassIncGen)

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

@@ -428,6 +428,46 @@ def ScopeOp : CIR_Op<"scope", [
   ];
 }
 
+//===----------------------------------------------------------------------===//
+// BrOp
+//===----------------------------------------------------------------------===//
+
+def BrOp : CIR_Op<"br",
+      [DeclareOpInterfaceMethods<BranchOpInterface, ["getSuccessorForOperands"]>,
+     Pure, Terminator]> {
+  let summary = "Unconditional branch";
+  let description = [{
+    The `cir.br` branches unconditionally to a block. Used to represent C/C++
+    goto's and general block branching.
+
+    Note that for source level `goto`'s crossing scope boundaries, those are
+    usually represented with the "symbolic" `cir.goto` operation.
+
+    Example:
+
+    ```mlir
+      ...
+        cir.br ^bb3
+      ^bb3:
+        cir.return
+    ```
+  }];
+
+  let builders = [
+    OpBuilder<(ins "mlir::Block *":$dest,
+              CArg<"mlir::ValueRange", "{}">:$destOperands), [{
+      $_state.addSuccessors(dest);
+      $_state.addOperands(destOperands);
+    }]>
+  ];
+
+  let arguments = (ins Variadic<CIR_AnyType>:$destOperands);
+  let successors = (successor AnySuccessor:$dest);
+  let assemblyFormat = [{
+    $dest (`(` $destOperands^ `:` type($destOperands) `)`)? attr-dict
+  }];
+}
+
 //===----------------------------------------------------------------------===//
 // GlobalOp
 //===----------------------------------------------------------------------===//

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

@@ -0,0 +1,39 @@
+//===- Passes.h - CIR pass entry points -------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This header file defines prototypes that expose pass constructors.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef CLANG_CIR_DIALECT_PASSES_H
+#define CLANG_CIR_DIALECT_PASSES_H
+
+#include "mlir/Pass/Pass.h"
+
+namespace clang {
+class ASTContext;
+}
+namespace mlir {
+
+std::unique_ptr<Pass> createCIRFlattenCFGPass();
+
+void populateCIRPreLoweringPasses(mlir::OpPassManager &pm);
+
+//===----------------------------------------------------------------------===//
+// Registration
+//===----------------------------------------------------------------------===//
+
+void registerCIRDialectTranslation(mlir::MLIRContext &context);
+
+/// Generate the code for registering passes.
+#define GEN_PASS_REGISTRATION
+#include "clang/CIR/Dialect/Passes.h.inc"
+
+} // namespace mlir
+
+#endif // CLANG_CIR_DIALECT_PASSES_H

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

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef CLANG_CIR_DIALECT_PASSES_TD
+#define CLANG_CIR_DIALECT_PASSES_TD
+
+include "mlir/Pass/PassBase.td"
+
+def CIRFlattenCFG : Pass<"cir-flatten-cfg"> {
+  let summary = "Produces flatten CFG";
+  let description = [{
+    This pass transforms CIR and inline all the nested regions. Thus,
+    the next post condtions are met after the pass applied:
+    - there is not any nested region in a function body
+    - all the blocks in a function belong to the parent region
+    In other words, this pass removes such CIR operations like IfOp, LoopOp,
+    ScopeOp and etc. and produces a flat CIR.
+  }];
+  let constructor = "mlir::createCIRFlattenCFGPass()";
+  let dependentDialects = ["cir::CIRDialect"];
+}
+
+#endif // CLANG_CIR_DIALECT_PASSES_TD

clang/include/clang/CIR/MissingFeatures.h

@@ -82,6 +82,7 @@ struct MissingFeatures {
   static bool objCLifetime() { return false; }
   static bool emitNullabilityCheck() { return false; }
   static bool astVarDeclInterface() { return false; }
+  static bool stackSaveOp() { return false; }
 };
 
 } // namespace cir

clang/lib/CIR/Dialect/CMakeLists.txt

@@ -1 +1,2 @@
 add_subdirectory(IR)
+add_subdirectory(Transforms)

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

@@ -268,6 +268,19 @@ LogicalResult cir::ScopeOp::verify() {
   return success();
 }
 
+//===----------------------------------------------------------------------===//
+// BrOp
+//===----------------------------------------------------------------------===//
+
+mlir::SuccessorOperands cir::BrOp::getSuccessorOperands(unsigned index) {
+  assert(index == 0 && "invalid successor index");
+  return mlir::SuccessorOperands(getDestOperandsMutable());
+}
+
+Block *cir::BrOp::getSuccessorForOperands(ArrayRef<Attribute>) {
+  return getDest();
+}
+
 //===----------------------------------------------------------------------===//
 // GlobalOp
 //===----------------------------------------------------------------------===//

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

@@ -11,6 +11,7 @@ add_clang_library(MLIRCIR
 
   LINK_LIBS PUBLIC
   MLIRIR
+  MLIRCIRInterfaces
   MLIRDLTIDialect
   MLIRDataLayoutInterfaces
   MLIRFuncDialect

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

@@ -0,0 +1,18 @@
+add_clang_library(MLIRCIRTransforms
+  FlattenCFG.cpp
+
+  DEPENDS
+  MLIRCIRPassIncGen
+
+  LINK_LIBS PUBLIC
+  clangAST
+  clangBasic
+
+  MLIRAnalysis
+  MLIRIR
+  MLIRPass
+  MLIRTransformUtils
+
+  MLIRCIR
+  MLIRCIRInterfaces
+)

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

@@ -0,0 +1,114 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements pass that inlines CIR operations regions into the parent
+// function region.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/Support/LogicalResult.h"
+#include "mlir/Transforms/DialectConversion.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "clang/CIR/Dialect/Passes.h"
+#include "clang/CIR/MissingFeatures.h"
+
+using namespace mlir;
+using namespace cir;
+
+namespace {
+
+struct CIRFlattenCFGPass : public CIRFlattenCFGBase<CIRFlattenCFGPass> {
+
+  CIRFlattenCFGPass() = default;
+  void runOnOperation() override;
+};
+
+class CIRScopeOpFlattening : public mlir::OpRewritePattern<cir::ScopeOp> {
+public:
+  using OpRewritePattern<cir::ScopeOp>::OpRewritePattern;
+
+  mlir::LogicalResult
+  matchAndRewrite(cir::ScopeOp scopeOp,
+                  mlir::PatternRewriter &rewriter) const override {
+    mlir::OpBuilder::InsertionGuard guard(rewriter);
+    mlir::Location loc = scopeOp.getLoc();
+
+    // Empty scope: just remove it.
+    // TODO: Remove this logic once CIR uses MLIR infrastructure to remove
+    // trivially dead operations
+    if (scopeOp.isEmpty()) {
+      rewriter.eraseOp(scopeOp);
+      return mlir::success();
+    }
+
+    // Split the current block before the ScopeOp to create the inlining
+    // point.
+    mlir::Block *currentBlock = rewriter.getInsertionBlock();
+    mlir::Block *continueBlock =
+        rewriter.splitBlock(currentBlock, rewriter.getInsertionPoint());
+    if (scopeOp.getNumResults() > 0)
+      continueBlock->addArguments(scopeOp.getResultTypes(), loc);
+
+    // Inline body region.
+    mlir::Block *beforeBody = &scopeOp.getScopeRegion().front();
+    mlir::Block *afterBody = &scopeOp.getScopeRegion().back();
+    rewriter.inlineRegionBefore(scopeOp.getScopeRegion(), continueBlock);
+
+    // Save stack and then branch into the body of the region.
+    rewriter.setInsertionPointToEnd(currentBlock);
+    assert(!cir::MissingFeatures::stackSaveOp());
+    rewriter.create<cir::BrOp>(loc, mlir::ValueRange(), beforeBody);
+
+    // Replace the scopeop return with a branch that jumps out of the body.
+    // Stack restore before leaving the body region.
+    rewriter.setInsertionPointToEnd(afterBody);
+    if (auto yieldOp = dyn_cast<cir::YieldOp>(afterBody->getTerminator())) {
+      rewriter.replaceOpWithNewOp<cir::BrOp>(yieldOp, yieldOp.getArgs(),
+                                             continueBlock);
+    }
+
+    // Replace the op with values return from the body region.
+    rewriter.replaceOp(scopeOp, continueBlock->getArguments());
+
+    return mlir::success();
+  }
+};
+
+void populateFlattenCFGPatterns(RewritePatternSet &patterns) {
+  patterns.add<CIRScopeOpFlattening>(patterns.getContext());
+}
+
+void CIRFlattenCFGPass::runOnOperation() {
+  RewritePatternSet patterns(&getContext());
+  populateFlattenCFGPatterns(patterns);
+
+  // Collect operations to apply patterns.
+  llvm::SmallVector<Operation *, 16> ops;
+  getOperation()->walk<mlir::WalkOrder::PostOrder>([&](Operation *op) {
+    if (isa<ScopeOp>(op))
+      ops.push_back(op);
+  });
+
+  // Apply patterns.
+  if (applyOpPatternsGreedily(ops, std::move(patterns)).failed())
+    signalPassFailure();
+}
+
+} // namespace
+
+namespace mlir {
+
+std::unique_ptr<Pass> createCIRFlattenCFGPass() {
+  return std::make_unique<CIRFlattenCFGPass>();
+}
+
+} // namespace mlir