xref: /llvm-project-15.0.7/mlir/lib/Transforms/ControlFlowSink.cpp (revision 36d3efea)

//===- ControlFlowSink.cpp - Code to perform control-flow sinking ---------===//
//
// 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 a basic control-flow sink pass. Control-flow sinking
// moves operations whose only uses are in conditionally-executed blocks in to
// those blocks so that they aren't executed on paths where their results are
// not needed.
//
//===----------------------------------------------------------------------===//

#include "PassDetail.h"
#include "mlir/IR/Dominance.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"
#include "mlir/Transforms/ControlFlowSinkUtils.h"
#include "mlir/Transforms/Passes.h"

using namespace mlir;

namespace {
/// A control-flow sink pass.
struct ControlFlowSink : public ControlFlowSinkBase<ControlFlowSink> {
  void runOnOperation() override;
};
} // end anonymous namespace

/// Returns true if the given operation is side-effect free as are all of its
/// nested operations.
static bool isSideEffectFree(Operation *op) {
  if (auto memInterface = dyn_cast<MemoryEffectOpInterface>(op)) {
    // If the op has side-effects, it cannot be moved.
    if (!memInterface.hasNoEffect())
      return false;
    // If the op does not have recursive side effects, then it can be moved.
    if (!op->hasTrait<OpTrait::HasRecursiveSideEffects>())
      return true;
  } else if (!op->hasTrait<OpTrait::HasRecursiveSideEffects>()) {
    // Otherwise, if the op does not implement the memory effect interface and
    // it does not have recursive side effects, then it cannot be known that the
    // op is moveable.
    return false;
  }

  // Recurse into the regions and ensure that all nested ops can also be moved.
  for (Region &region : op->getRegions())
    for (Operation &op : region.getOps())
      if (!isSideEffectFree(&op))
        return false;
  return true;
}

void ControlFlowSink::runOnOperation() {
  auto &domInfo = getAnalysis<DominanceInfo>();
  getOperation()->walk([&](RegionBranchOpInterface branch) {
    SmallVector<Region *> regionsToSink;
    // Get the regions are that known to be executed at most once.
    getSinglyExecutedRegionsToSink(branch, regionsToSink);
    // Sink side-effect free operations.
    numSunk = controlFlowSink(
        regionsToSink, domInfo,
        [](Operation *op, Region *) { return isSideEffectFree(op); },
        [](Operation *op, Region *region) {
          // Move the operation to the beginning of the region's entry block.
          // This guarantees the preservation of SSA dominance of all of the
          // operation's uses are in the region.
          op->moveBefore(&region->front(), region->front().begin());
        });
  });
}

std::unique_ptr<Pass> mlir::createControlFlowSinkPass() {
  return std::make_unique<ControlFlowSink>();
}