[CIR] Upstream cir-canonicalize pass (#131891)

This change introduces the cir-canonicalize pass. This is a simple
cir-to-cir transformation that eliminates empty scopes and redundant
branches. It will be expanded in future changes to simplify other
redundant instruction sequences.

MLIR verification and mlir-specific command-line option handling is also
introduced here.

Author: andykaylor

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -386,4 +386,12 @@ def warn_hlsl_langstd_minimal :
   Warning<"support for HLSL language version %0 is incomplete, "
           "recommend using %1 instead">,
   InGroup<HLSLDXCCompat>;
+
+// ClangIR frontend errors
+def err_cir_to_cir_transform_failed : Error<
+    "CIR-to-CIR transformation failed">, DefaultFatal;
+
+def err_cir_verification_failed_pre_passes : Error<
+    "CIR module verification error before running CIR-to-CIR passes">,
+    DefaultFatal;
 }

clang/include/clang/CIR/CIRGenerator.h

@@ -55,6 +55,10 @@ class CIRGenerator : public clang::ASTConsumer {
   void Initialize(clang::ASTContext &astContext) override;
   bool HandleTopLevelDecl(clang::DeclGroupRef group) override;
   mlir::ModuleOp getModule() const;
+  mlir::MLIRContext &getMLIRContext() { return *mlirContext; };
+  const mlir::MLIRContext &getMLIRContext() const { return *mlirContext; };
+
+  bool verifyModule() const;
 };
 
 } // namespace cir

clang/include/clang/CIR/CIRToCIRPasses.h

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 declares an interface for running CIR-to-CIR passes.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef CLANG_CIR_CIRTOCIRPASSES_H
+#define CLANG_CIR_CIRTOCIRPASSES_H
+
+#include "mlir/Pass/Pass.h"
+
+#include <memory>
+
+namespace clang {
+class ASTContext;
+}
+
+namespace mlir {
+class MLIRContext;
+class ModuleOp;
+} // namespace mlir
+
+namespace cir {
+
+// Run set of cleanup/prepare/etc passes CIR <-> CIR.
+mlir::LogicalResult runCIRToCIRPasses(mlir::ModuleOp theModule,
+                                      mlir::MLIRContext &mlirCtx,
+                                      clang::ASTContext &astCtx,
+                                      bool enableVerifier);
+
+} // namespace cir
+
+#endif // CLANG_CIR_CIRTOCIRPASSES_H_

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

@@ -20,6 +20,7 @@ class ASTContext;
 }
 namespace mlir {
 
+std::unique_ptr<Pass> createCIRCanonicalizePass();
 std::unique_ptr<Pass> createCIRFlattenCFGPass();
 
 void populateCIRPreLoweringPasses(mlir::OpPassManager &pm);

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

@@ -11,6 +11,24 @@
 
 include "mlir/Pass/PassBase.td"
 
+def CIRCanonicalize : Pass<"cir-canonicalize"> {
+  let summary = "Performs CIR canonicalization";
+  let description = [{
+    Perform canonicalizations on CIR and removes some redundant operations.
+
+    This pass performs basic cleanup and canonicalization transformations that
+    are not intended to affect CIR-to-source fidelity and high-level code
+    analysis passes. Example transformations performed in this pass include
+    empty scope cleanup, trivial `try` cleanup, redundant branch cleanup, etc.
+    Those more "heavyweight" transformations and those transformations that
+    could significantly affect CIR-to-source fidelity are performed in the
+    `cir-simplify` pass.
+  }];
+
+  let constructor = "mlir::createCIRCanonicalizePass()";
+  let dependentDialects = ["cir::CIRDialect"];
+}
+
 def CIRFlattenCFG : Pass<"cir-flatten-cfg"> {
   let summary = "Produces flatten CFG";
   let description = [{

clang/include/clang/CIR/MissingFeatures.h

@@ -103,6 +103,18 @@ struct MissingFeatures {
   static bool scalableVectors() { return false; }
   static bool unsizedTypes() { return false; }
   static bool vectorType() { return false; }
+
+  // Future CIR operations
+  static bool labelOp() { return false; }
+  static bool brCondOp() { return false; }
+  static bool switchOp() { return false; }
+  static bool tryOp() { return false; }
+  static bool unaryOp() { return false; }
+  static bool selectOp() { return false; }
+  static bool complexCreateOp() { return false; }
+  static bool complexRealOp() { return false; }
+  static bool complexImagOp() { return false; }
+  static bool callOp() { return false; }
 };
 
 } // namespace cir

clang/include/clang/Driver/Options.td

@@ -2978,6 +2978,15 @@ def fapple_link_rtlib : Flag<["-"], "fapple-link-rtlib">, Group<f_Group>,
   HelpText<"Force linking the clang builtins runtime library">;
 
 /// ClangIR-specific options - BEGIN
+def clangir_disable_passes : Flag<["-"], "clangir-disable-passes">,
+  Visibility<[ClangOption, CC1Option]>,
+  HelpText<"Disable CIR transformations pipeline">,
+  MarshallingInfoFlag<FrontendOpts<"ClangIRDisablePasses">>;
+def clangir_disable_verifier : Flag<["-"], "clangir-disable-verifier">,
+  Visibility<[ClangOption, CC1Option]>,
+  HelpText<"ClangIR: Disable MLIR module verifier">,
+  MarshallingInfoFlag<FrontendOpts<"ClangIRDisableCIRVerifier">>;
+
 defm clangir : BoolFOption<"clangir",
   FrontendOpts<"UseClangIRPipeline">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option], "Use the ClangIR pipeline to compile">,
@@ -4822,8 +4831,9 @@ def : Joined<["-"], "mllvm=">,
   Visibility<[ClangOption, CLOption, DXCOption, FlangOption]>, Alias<mllvm>,
   HelpText<"Alias for -mllvm">, MetaVarName<"<arg>">;
 def mmlir : Separate<["-"], "mmlir">,
-  Visibility<[ClangOption, CLOption, FC1Option, FlangOption]>,
-  HelpText<"Additional arguments to forward to MLIR's option processing">;
+  Visibility<[ClangOption, CC1Option, FC1Option, FlangOption]>,
+  HelpText<"Additional arguments to forward to MLIR's option processing">,
+  MarshallingInfoStringVector<FrontendOpts<"MLIRArgs">>;
 def ffuchsia_api_level_EQ : Joined<["-"], "ffuchsia-api-level=">,
   Group<m_Group>, Visibility<[ClangOption, CC1Option]>,
   HelpText<"Set Fuchsia API level">,

clang/include/clang/Frontend/FrontendOptions.h

@@ -412,6 +412,14 @@ class FrontendOptions {
   LLVM_PREFERRED_TYPE(bool)
   unsigned UseClangIRPipeline : 1;
 
+  /// Disable Clang IR specific (CIR) passes
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned ClangIRDisablePasses : 1;
+
+  /// Disable Clang IR (CIR) verifier
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned ClangIRDisableCIRVerifier : 1;
+
   CodeCompleteOptions CodeCompleteOpts;
 
   /// Specifies the output format of the AST.
@@ -488,6 +496,10 @@ class FrontendOptions {
   /// should only be used for debugging and experimental features.
   std::vector<std::string> LLVMArgs;
 
+  /// A list of arguments to forward to MLIR's option processing; this
+  /// should only be used for debugging and experimental features.
+  std::vector<std::string> MLIRArgs;
+
   /// File name of the file that will provide record layouts
   /// (in the format produced by -fdump-record-layouts).
   std::string OverrideRecordLayoutsFile;
@@ -533,7 +545,8 @@ class FrontendOptions {
         EmitExtensionSymbolGraphs(false),
         EmitSymbolGraphSymbolLabelsForTesting(false),
         EmitPrettySymbolGraphs(false), GenReducedBMI(false),
-        UseClangIRPipeline(false), TimeTraceGranularity(500),
+        UseClangIRPipeline(false), ClangIRDisablePasses(false),
+        ClangIRDisableCIRVerifier(false), TimeTraceGranularity(500),
         TimeTraceVerbose(false) {}
 
   /// getInputKindForExtension - Return the appropriate input kind for a file

clang/lib/CIR/CodeGen/CIRGenModule.cpp

@@ -23,6 +23,7 @@
 #include "mlir/IR/BuiltinOps.h"
 #include "mlir/IR/Location.h"
 #include "mlir/IR/MLIRContext.h"
+#include "mlir/IR/Verifier.h"
 
 using namespace clang;
 using namespace clang::CIRGen;
@@ -488,6 +489,13 @@ mlir::Type CIRGenModule::convertType(QualType type) {
   return genTypes.convertType(type);
 }
 
+bool CIRGenModule::verifyModule() const {
+  // Verify the module after we have finished constructing it, this will
+  // check the structural properties of the IR and invoke any specific
+  // verifiers we have on the CIR operations.
+  return mlir::verify(theModule).succeeded();
+}
+
 DiagnosticBuilder CIRGenModule::errorNYI(SourceLocation loc,
                                          llvm::StringRef feature) {
   unsigned diagID = diags.getCustomDiagID(

clang/lib/CIR/CodeGen/CIRGenModule.h

@@ -91,6 +91,8 @@ class CIRGenModule : public CIRGenTypeCache {
 
   void emitTopLevelDecl(clang::Decl *decl);
 
+  bool verifyModule() const;
+
   /// Return the address of the given function. If funcType is non-null, then
   /// this function will use the specified type if it has to create it.
   // TODO: this is a bit weird as `GetAddr` given we give back a FuncOp?

clang/lib/CIR/CodeGen/CIRGenerator.cpp

@@ -40,6 +40,8 @@ void CIRGenerator::Initialize(ASTContext &astContext) {
       *mlirContext.get(), astContext, codeGenOpts, diags);
 }
 
+bool CIRGenerator::verifyModule() const { return cgm->verifyModule(); }
+
 mlir::ModuleOp CIRGenerator::getModule() const { return cgm->getModule(); }
 
 bool CIRGenerator::HandleTopLevelDecl(DeclGroupRef group) {

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

@@ -0,0 +1,145 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 canonicalizes CIR operations, eliminating
+// redundant branches, empty scopes, and other unnecessary operations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/IR/Block.h"
+#include "mlir/IR/Operation.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/IR/Region.h"
+#include "mlir/Support/LogicalResult.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 {
+
+/// Removes branches between two blocks if it is the only branch.
+///
+/// From:
+///   ^bb0:
+///     cir.br ^bb1
+///   ^bb1:  // pred: ^bb0
+///     cir.return
+///
+/// To:
+///   ^bb0:
+///     cir.return
+struct RemoveRedundantBranches : public OpRewritePattern<BrOp> {
+  using OpRewritePattern<BrOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(BrOp op,
+                                PatternRewriter &rewriter) const final {
+    Block *block = op.getOperation()->getBlock();
+    Block *dest = op.getDest();
+
+    assert(!cir::MissingFeatures::labelOp());
+
+    // Single edge between blocks: merge it.
+    if (block->getNumSuccessors() == 1 &&
+        dest->getSinglePredecessor() == block) {
+      rewriter.eraseOp(op);
+      rewriter.mergeBlocks(dest, block);
+      return success();
+    }
+
+    return failure();
+  }
+};
+
+struct RemoveEmptyScope
+    : public OpRewritePattern<ScopeOp>::SplitMatchAndRewrite {
+  using SplitMatchAndRewrite::SplitMatchAndRewrite;
+
+  LogicalResult match(ScopeOp op) const final {
+    // TODO: Remove this logic once CIR uses MLIR infrastructure to remove
+    // trivially dead operations
+    if (op.isEmpty())
+      return success();
+
+    Region &region = op.getScopeRegion();
+    if (region.getBlocks().front().getOperations().size() == 1)
+      return success(isa<YieldOp>(region.getBlocks().front().front()));
+
+    return failure();
+  }
+
+  void rewrite(ScopeOp op, PatternRewriter &rewriter) const final {
+    rewriter.eraseOp(op);
+  }
+};
+
+//===----------------------------------------------------------------------===//
+// CIRCanonicalizePass
+//===----------------------------------------------------------------------===//
+
+struct CIRCanonicalizePass : public CIRCanonicalizeBase<CIRCanonicalizePass> {
+  using CIRCanonicalizeBase::CIRCanonicalizeBase;
+
+  // The same operation rewriting done here could have been performed
+  // by CanonicalizerPass (adding hasCanonicalizer for target Ops and
+  // implementing the same from above in CIRDialects.cpp). However, it's
+  // currently too aggressive for static analysis purposes, since it might
+  // remove things where a diagnostic can be generated.
+  //
+  // FIXME: perhaps we can add one more mode to GreedyRewriteConfig to
+  // disable this behavior.
+  void runOnOperation() override;
+};
+
+void populateCIRCanonicalizePatterns(RewritePatternSet &patterns) {
+  // clang-format off
+  patterns.add<
+    RemoveRedundantBranches,
+    RemoveEmptyScope
+  >(patterns.getContext());
+  // clang-format on
+}
+
+void CIRCanonicalizePass::runOnOperation() {
+  // Collect rewrite patterns.
+  RewritePatternSet patterns(&getContext());
+  populateCIRCanonicalizePatterns(patterns);
+
+  // Collect operations to apply patterns.
+  llvm::SmallVector<Operation *, 16> ops;
+  getOperation()->walk([&](Operation *op) {
+    assert(!cir::MissingFeatures::brCondOp());
+    assert(!cir::MissingFeatures::switchOp());
+    assert(!cir::MissingFeatures::tryOp());
+    assert(!cir::MissingFeatures::unaryOp());
+    assert(!cir::MissingFeatures::selectOp());
+    assert(!cir::MissingFeatures::complexCreateOp());
+    assert(!cir::MissingFeatures::complexRealOp());
+    assert(!cir::MissingFeatures::complexImagOp());
+    assert(!cir::MissingFeatures::callOp());
+    // CastOp here is to perform a manual `fold` in
+    // applyOpPatternsGreedily
+    if (isa<BrOp, ScopeOp, CastOp>(op))
+      ops.push_back(op);
+  });
+
+  // Apply patterns.
+  if (applyOpPatternsGreedily(ops, std::move(patterns)).failed())
+    signalPassFailure();
+}
+
+} // namespace
+
+std::unique_ptr<Pass> mlir::createCIRCanonicalizePass() {
+  return std::make_unique<CIRCanonicalizePass>();
+}

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

@@ -1,4 +1,5 @@
 add_clang_library(MLIRCIRTransforms
+  CIRCanonicalize.cpp
   FlattenCFG.cpp
 
   DEPENDS