[mlir][NFC] Expose the `CFGToSCFInterface` for the `ControlFlowToSCF` pass

[zero9178]

Summary

This is useful for any downstream users who may just want to use slightly different ops than the pass or need to take into account other ops in the input while still dealing with ControlFlow ops for the most part.
This also helps writing test implementations for transformCFGToSCF.

Since the implementation is now public, comments noting the post conditions of the methods have been added to ensure downstream users can stay compatible with the existing implementation.

[definelicht]

LGTM with grammar fixes 🤠

mlir/include/mlir/Conversion/ControlFlowToSCF/ControlFlowToSCF.h

27–29

34

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53

mlir/include/mlir/Conversion/ControlFlowToSCF/ControlFlowToSCF.h

 //
 //===----------------------------------------------------------------------===//
 
 #ifndef MLIR_CONVERSION_CONTROLFLOWTOSCF_CONTROLFLOWTOSCF_H
 #define MLIR_CONVERSION_CONTROLFLOWTOSCF_CONTROLFLOWTOSCF_H
 
 #include <memory>
 
+#include "mlir/Transforms/CFGToSCF.h"
+
 namespace mlir {
 class Pass;
 
+/// Implementation of `CFGToSCFInterface` used to lift Control Flow Dialect
+/// operations to SCF Dialect operations.
+class ControlFlowToSCFTransformation : public CFGToSCFInterface {
+public:
+  /// Creates an `scf.if` op if `controlFlowCondOp` is a `cf.cond_br` op or
+  /// an `scf.index_switch` if `controlFlowCondOp` is a `cf.switch`.
+  /// Returns failure otherwise.
+  FailureOr<Operation *> createStructuredBranchRegionOp(
+      OpBuilder &builder, Operation *controlFlowCondOp, TypeRange resultTypes,
+      MutableArrayRef<Region> regions) override;
+
+  /// Creates an `scf.yield` op returning the given results.
+  LogicalResult
+  createStructuredBranchRegionTerminatorOp(Location loc, OpBuilder &builder,
+                                           Operation *branchRegionOp,
+                                           ValueRange results) override;
+
+  /// Creates an `scf.while` op. The loop body is made the before-region of the
+  /// while op and terminated with an `scf.condition` op. The after-region does
+  /// nothing but forward the iteration variables.
+  FailureOr<Operation *>
+  createStructuredDoWhileLoopOp(OpBuilder &builder, Operation *replacedOp,
+                                ValueRange loopVariablesInit, Value condition,
+                                ValueRange loopVariablesNextIter,
+                                Region &&loopBody) override;
+
+  /// Creates an `arith.constant` with an i32 attribute of the given value.
+  Value getCFGSwitchValue(Location loc, OpBuilder &builder,
+                          unsigned value) override;
+
+  /// Creates a `cf.switch` op with the given cases and flag.
+  void createCFGSwitchOp(Location loc, OpBuilder &builder, Value flag,
+                         ArrayRef<unsigned> caseValues,
+                         BlockRange caseDestinations,
+                         ArrayRef<ValueRange> caseArguments, Block *defaultDest,
+                         ValueRange defaultArgs) override;
+
+  /// Creates a `ub.poison` op of the given type.
+  Value getUndefValue(Location loc, OpBuilder &builder, Type type) override;
+
+  /// Creates a `func.return` op with poison for each of the return values of
+  /// the function. It is guaranteed to be directly within the function body.
+  /// TODO: This can be made independent of the `func` dialect once the UB
+  ///       dialect has a `ub.unreachable` op.
+  FailureOr<Operation *> createUnreachableTerminator(Location loc,
+                                                     OpBuilder &builder,
+                                                     Region &region) override;
+};
+
 #define GEN_PASS_DECL_LIFTCONTROLFLOWTOSCFPASS
 #include "mlir/Conversion/Passes.h.inc"
 
 } // namespace mlir
 
 #endif // MLIR_CONVERSION_CONTROLFLOWTOSCF_CONTROLFLOWTOSCF_H

mlir/lib/Conversion/ControlFlowToSCF/ControlFlowToSCF.cpp

 
 namespace mlir {
 #define GEN_PASS_DEF_LIFTCONTROLFLOWTOSCFPASS
 #include "mlir/Conversion/Passes.h.inc"
 } // namespace mlir
 
 using namespace mlir;
 
-namespace {
-
-class ControlFlowToSCFTransformation : public CFGToSCFInterface {
-public:
-  FailureOr<Operation *> createStructuredBranchRegionOp(
-      OpBuilder &builder, Operation *controlFlowCondOp, TypeRange resultTypes,
-      MutableArrayRef<Region> regions) override {
-    if (auto condBrOp = dyn_cast<cf::CondBranchOp>(controlFlowCondOp)) {
-      assert(regions.size() == 2);
-      auto ifOp = builder.create<scf::IfOp>(
-          controlFlowCondOp->getLoc(), resultTypes, condBrOp.getCondition());
-      ifOp.getThenRegion().takeBody(regions[0]);
-      ifOp.getElseRegion().takeBody(regions[1]);
-      return ifOp.getOperation();
-    }
+FailureOr<Operation *>
+ControlFlowToSCFTransformation::createStructuredBranchRegionOp(
+    MutableArrayRef<Region> regions) {
+    auto ifOp = builder.create<scf::IfOp>(controlFlowCondOp->getLoc(),
+                                          resultTypes, condBrOp.getCondition());
 
-    if (auto switchOp = dyn_cast<cf::SwitchOp>(controlFlowCondOp)) {
-      // `getCFGSwitchValue` returns an i32 that we need to convert to index
-      // fist.
-      auto cast = builder.create<arith::IndexCastUIOp>(
-          controlFlowCondOp->getLoc(), builder.getIndexType(),
-          switchOp.getFlag());
-      SmallVector<int64_t> cases;
-      if (auto caseValues = switchOp.getCaseValues())
-        llvm::append_range(
-            cases, llvm::map_range(*caseValues, [](const llvm::APInt &apInt) {
-              return apInt.getZExtValue();
-            }));
 
-      assert(regions.size() == cases.size() + 1);
 
-      auto indexSwitchOp = builder.create<scf::IndexSwitchOp>(
-          controlFlowCondOp->getLoc(), resultTypes, cast, cases, cases.size());
 
-      indexSwitchOp.getDefaultRegion().takeBody(regions[0]);
-      for (auto &&[targetRegion, sourceRegion] :
-           llvm::zip(indexSwitchOp.getCaseRegions(), llvm::drop_begin(regions)))
-        targetRegion.takeBody(sourceRegion);
 
-      return indexSwitchOp.getOperation();
-    }
 
-    controlFlowCondOp->emitOpError(
-        "Cannot convert unknown control flow op to structured control flow");
-    return failure();
-  }
 
-  LogicalResult
-  createStructuredBranchRegionTerminatorOp(Location loc, OpBuilder &builder,
-                                           Operation *branchRegionOp,
-                                           ValueRange results) override {
-    builder.create<scf::YieldOp>(loc, results);
-    return success();
-  }
+ControlFlowToSCFTransformation::createStructuredBranchRegionTerminatorOp(
+    Location loc, OpBuilder &builder, Operation *branchRegionOp,
+    ValueRange results) {
 
-  FailureOr<Operation *>
-  createStructuredDoWhileLoopOp(OpBuilder &builder, Operation *replacedOp,
-                                ValueRange loopVariablesInit, Value condition,
-                                ValueRange loopVariablesNextIter,
-                                Region &&loopBody) override {
-    Location loc = replacedOp->getLoc();
-    auto whileOp = builder.create<scf::WhileOp>(
-        loc, loopVariablesInit.getTypes(), loopVariablesInit);
+ControlFlowToSCFTransformation::createStructuredDoWhileLoopOp(
+    OpBuilder &builder, Operation *replacedOp, ValueRange loopVariablesInit,
+    Value condition, ValueRange loopVariablesNextIter, Region &&loopBody) {
+  auto whileOp = builder.create<scf::WhileOp>(loc, loopVariablesInit.getTypes(),
+                                              loopVariablesInit);
 
-    whileOp.getBefore().takeBody(loopBody);
 
-    builder.setInsertionPointToEnd(&whileOp.getBefore().back());
-    // `getCFGSwitchValue` returns a i32. We therefore need to truncate the
-    // condition to i1 first. It is guaranteed to be either 0 or 1 already.
-    builder.create<scf::ConditionOp>(
-        loc,
-        builder.create<arith::TruncIOp>(loc, builder.getI1Type(), condition),
-        loopVariablesNextIter);
+      loc, builder.create<arith::TruncIOp>(loc, builder.getI1Type(), condition),
 
-    auto *afterBlock = new Block;
-    whileOp.getAfter().push_back(afterBlock);
-    afterBlock->addArguments(
-        loopVariablesInit.getTypes(),
-        SmallVector<Location>(loopVariablesInit.size(), loc));
-    builder.setInsertionPointToEnd(afterBlock);
-    builder.create<scf::YieldOp>(loc, afterBlock->getArguments());
 
-    return whileOp.getOperation();
-  }
 
-  Value getCFGSwitchValue(Location loc, OpBuilder &builder,
-                          unsigned int value) override {
-    return builder.create<arith::ConstantOp>(loc,
-                                             builder.getI32IntegerAttr(value));
-  }
+Value ControlFlowToSCFTransformation::getCFGSwitchValue(Location loc,
+                                                        OpBuilder &builder,
+                                                        unsigned int value) {
 
-  void createCFGSwitchOp(Location loc, OpBuilder &builder, Value flag,
-                         ArrayRef<unsigned int> caseValues,
-                         BlockRange caseDestinations,
-                         ArrayRef<ValueRange> caseArguments, Block *defaultDest,
-                         ValueRange defaultArgs) override {
-    builder.create<cf::SwitchOp>(loc, flag, defaultDest, defaultArgs,
-                                 llvm::to_vector_of<int32_t>(caseValues),
-                                 caseDestinations, caseArguments);
-  }
+void ControlFlowToSCFTransformation::createCFGSwitchOp(
+    Location loc, OpBuilder &builder, Value flag,
+    ArrayRef<unsigned int> caseValues, BlockRange caseDestinations,
+    ValueRange defaultArgs) {
 
-  Value getUndefValue(Location loc, OpBuilder &builder, Type type) override {
-    return builder.create<ub::PoisonOp>(loc, type, nullptr);
-  }
+Value ControlFlowToSCFTransformation::getUndefValue(Location loc,
+                                                    OpBuilder &builder,
+                                                    Type type) {
 
-  FailureOr<Operation *> createUnreachableTerminator(Location loc,
+FailureOr<Operation *>
+ControlFlowToSCFTransformation::createUnreachableTerminator(Location loc,
                                                             OpBuilder &builder,
-                                                     Region &region) override {
+                                                            Region &region) {
 
-    // TODO: This should create a `ub.unreachable` op. Once such an operation
-    //       exists to make the pass can be made independent of the func
-    //       dialect. For now just return poison values.
-    auto funcOp = dyn_cast<func::FuncOp>(region.getParentOp());
-    if (!funcOp)
-      return emitError(loc, "Expected '")
-             << func::FuncOp::getOperationName() << "' as top level operation";
+  //       exists to make the pass independent of the func dialect. For now just
+  //       return poison values.
 
-    return builder
-        .create<func::ReturnOp>(
-            loc, llvm::map_to_vector(funcOp.getResultTypes(),
-                                     [&](Type type) {
-                                       return getUndefValue(loc, builder, type);
-                                     }))
-        .getOperation();
-  }
-};
+
+namespace {
 
 struct LiftControlFlowToSCF
     : public impl::LiftControlFlowToSCFPassBase<LiftControlFlowToSCF> {
 
   using Base::Base;
 
   void runOnOperation() override {
     ControlFlowToSCFTransformation transformation;