[CIR] Split cir-simplify into two passes (#868)

This PR splits the old `cir-simplify` pass into two new passes, namely
`cir-canonicalize` and `cir-simplify` (the new `cir-simplify`). The
`cir-canonicalize` pass runs transformations that do not affect
CIR-to-source fidelity much, such as operation folding and redundant
operation elimination. On the other hand, the new `cir-simplify` pass
runs transformations that may significantly change the code and break
high-level code analysis passes, such as more aggresive code
optimizations.

This PR also updates the CIR-to-CIR pipeline to fit these two new
passes. The `cir-canonicalize` pass is moved to the very front of the
pipeline, while the new `cir-simplify` pass is moved to the back of the
pipeline (but still before lowering prepare of course). Additionally,
the new `cir-simplify` now only runs when the user specifies a non-zero
optimization level on the frontend.

Also fixed some typos and resolved some `clang-tidy` complaints along
the way.

Resolves #827 .

Author: Lancern

clang/include/clang/CIR/CIRToCIRPasses.h

@@ -34,8 +34,8 @@ mlir::LogicalResult runCIRToCIRPasses(
     llvm::StringRef lifetimeOpts, bool enableIdiomRecognizer,
     llvm::StringRef idiomRecognizerOpts, bool enableLibOpt,
     llvm::StringRef libOptOpts, std::string &passOptParsingFailure,
-    bool flattenCIR, bool emitMLIR, bool enableCallConvLowering,
-    bool enableMem2reg);
+    bool enableCIRSimplify, bool flattenCIR, bool emitMLIR,
+    bool enableCallConvLowering, bool enableMem2reg);
 
 } // namespace cir
 

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

@@ -26,6 +26,7 @@ std::unique_ptr<Pass> createLifetimeCheckPass(ArrayRef<StringRef> remark,
                                               ArrayRef<StringRef> hist,
                                               unsigned hist_limit,
                                               clang::ASTContext *astCtx);
+std::unique_ptr<Pass> createCIRCanonicalizePass();
 std::unique_ptr<Pass> createCIRSimplifyPass();
 std::unique_ptr<Pass> createDropASTPass();
 std::unique_ptr<Pass> createSCFPreparePass();

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

@@ -11,22 +11,33 @@
 
 include "mlir/Pass/PassBase.td"
 
-def CIRSimplify : Pass<"cir-simplify"> {
-  let summary = "Performs CIR simplification";
+def CIRCanonicalize : Pass<"cir-canonicalize"> {
+  let summary = "Performs CIR canonicalization";
   let description = [{
-    The pass rewrites CIR and removes some redundant operations.
+    Perform canonicalizations on CIR and removes some redundant operations.
+
+    This pass performs basic cleanup and canonicalization transformations that
+    hopefully do not affect CIR-to-source fidelity and high-level code analysis
+    passes too much. 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.
+  }];
 
-    For example, due to canonicalize pass is too aggressive for CIR when
-    the pipeline is used for C/C++ analysis, this pass runs some rewrites
-    for scopes, merging some blocks and eliminating unnecessary control-flow.
+  let constructor = "mlir::createCIRCanonicalizePass()";
+  let dependentDialects = ["cir::CIRDialect"];
+}
 
-    Also, the pass removes redundant and/or unneccessary cast and unary not
-    operation e.g.
-    ```mlir
-      %1 = cir.cast(bool_to_int, %0 : !cir.bool), !s32i
-      %2 = cir.cast(int_to_bool, %1 : !s32i), !cir.bool
-    ```
+def CIRSimplify : Pass<"cir-simplify"> {
+  let summary = "Performs CIR simplification and code optimization";
+  let description = [{
+    The pass performs code simplification and optimization on CIR.
 
+    Unlike the `cir-canonicalize` pass, this pass contains more aggresive code
+    transformations that could significantly affect CIR-to-source fidelity.
+    Example transformations performed in this pass include ternary folding,
+    code hoisting, etc.
   }];
   let constructor = "mlir::createCIRSimplifyPass()";
   let dependentDialects = ["cir::CIRDialect"];

clang/lib/CIR/CodeGen/CIRPasses.cpp

@@ -26,11 +26,11 @@ mlir::LogicalResult runCIRToCIRPasses(
     llvm::StringRef lifetimeOpts, bool enableIdiomRecognizer,
     llvm::StringRef idiomRecognizerOpts, bool enableLibOpt,
     llvm::StringRef libOptOpts, std::string &passOptParsingFailure,
-    bool flattenCIR, bool emitMLIR, bool enableCallConvLowering,
-    bool enableMem2Reg) {
+    bool enableCIRSimplify, bool flattenCIR, bool emitMLIR,
+    bool enableCallConvLowering, bool enableMem2Reg) {
 
   mlir::PassManager pm(mlirCtx);
-  pm.addPass(mlir::createCIRSimplifyPass());
+  pm.addPass(mlir::createCIRCanonicalizePass());
 
   // TODO(CIR): Make this actually propagate errors correctly. This is stubbed
   // in to get rebases going.
@@ -66,6 +66,9 @@ mlir::LogicalResult runCIRToCIRPasses(
     pm.addPass(std::move(libOpPass));
   }
 
+  if (enableCIRSimplify)
+    pm.addPass(mlir::createCIRSimplifyPass());
+
   pm.addPass(mlir::createLoweringPreparePass(&astCtx));
 
   // FIXME(cir): This pass should run by default, but it is lacking support for

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

@@ -0,0 +1,187 @@
+//===- CIRSimplify.cpp - performs CIR canonicalization --------------------===//
+//
+// 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 "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"
+
+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();
+
+    if (isa<mlir::cir::LabelOp>(dest->front()))
+      return failure();
+
+    // 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> {
+  using OpRewritePattern<ScopeOp>::OpRewritePattern;
+
+  LogicalResult match(ScopeOp op) const final {
+    return success(op.getRegion().empty() ||
+                   (op.getRegion().getBlocks().size() == 1 &&
+                    op.getRegion().front().empty()));
+  }
+
+  void rewrite(ScopeOp op, PatternRewriter &rewriter) const final {
+    rewriter.eraseOp(op);
+  }
+};
+
+struct RemoveEmptySwitch : public OpRewritePattern<SwitchOp> {
+  using OpRewritePattern<SwitchOp>::OpRewritePattern;
+
+  LogicalResult match(SwitchOp op) const final {
+    return success(op.getRegions().empty());
+  }
+
+  void rewrite(SwitchOp op, PatternRewriter &rewriter) const final {
+    rewriter.eraseOp(op);
+  }
+};
+
+struct RemoveTrivialTry : public OpRewritePattern<TryOp> {
+  using OpRewritePattern<TryOp>::OpRewritePattern;
+
+  LogicalResult match(TryOp op) const final {
+    // FIXME: also check all catch regions are empty
+    // return success(op.getTryRegion().hasOneBlock());
+    return mlir::failure();
+  }
+
+  void rewrite(TryOp op, PatternRewriter &rewriter) const final {
+    // Move try body to the parent.
+    assert(op.getTryRegion().hasOneBlock());
+
+    Block *parentBlock = op.getOperation()->getBlock();
+    mlir::Block *tryBody = &op.getTryRegion().getBlocks().front();
+    YieldOp y = dyn_cast<YieldOp>(tryBody->getTerminator());
+    assert(y && "expected well wrapped up try block");
+    y->erase();
+
+    rewriter.inlineBlockBefore(tryBody, parentBlock, Block::iterator(op));
+    rewriter.eraseOp(op);
+  }
+};
+
+// Remove call exception with empty cleanups
+struct SimplifyCallOp : public OpRewritePattern<CallOp> {
+  using OpRewritePattern<CallOp>::OpRewritePattern;
+
+  LogicalResult match(CallOp op) const final {
+    // Applicable to cir.call exception ... clean { cir.yield }
+    mlir::Region *r = &op.getCleanup();
+    if (r->empty() || !r->hasOneBlock())
+      return failure();
+
+    mlir::Block *b = &r->getBlocks().back();
+    if (&b->back() != &b->front())
+      return failure();
+
+    return success(isa<YieldOp>(&b->getOperations().back()));
+  }
+
+  void rewrite(CallOp op, PatternRewriter &rewriter) const final {
+    mlir::Block *b = &op.getCleanup().back();
+    rewriter.eraseOp(&b->back());
+    rewriter.eraseBlock(b);
+  }
+};
+
+//===----------------------------------------------------------------------===//
+// 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,
+    RemoveEmptySwitch,
+    RemoveTrivialTry,
+    SimplifyCallOp
+  >(patterns.getContext());
+  // clang-format on
+}
+
+void CIRCanonicalizePass::runOnOperation() {
+  // Collect rewrite patterns.
+  RewritePatternSet patterns(&getContext());
+  populateCIRCanonicalizePatterns(patterns);
+
+  // Collect operations to apply patterns.
+  SmallVector<Operation *, 16> ops;
+  getOperation()->walk([&](Operation *op) {
+    // CastOp here is to perform a manual `fold` in
+    // applyOpPatternsAndFold
+    if (isa<BrOp, BrCondOp, ScopeOp, SwitchOp, CastOp, TryOp, UnaryOp, SelectOp,
+            ComplexCreateOp, ComplexRealOp, ComplexImagOp, CallOp>(op))
+      ops.push_back(op);
+  });
+
+  // Apply patterns.
+  if (applyOpPatternsAndFold(ops, std::move(patterns)).failed())
+    signalPassFailure();
+}
+
+} // namespace
+
+std::unique_ptr<Pass> mlir::createCIRCanonicalizePass() {
+  return std::make_unique<CIRCanonicalizePass>();
+}