llvm/mlir/lib/Transforms/Utils/LoopUtils.cpp

//===- LoopUtils.cpp ---- Misc utilities for loop transformation ----------===//
//
// Copyright 2019 The MLIR Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// =============================================================================
//
// This file implements miscellaneous loop transformation routines.
//
//===----------------------------------------------------------------------===//

#include "mlir/Transforms/LoopUtils.h"

#include "mlir/AffineOps/AffineOps.h"
#include "mlir/Analysis/AffineAnalysis.h"
#include "mlir/Analysis/AffineStructures.h"
#include "mlir/Analysis/LoopAnalysis.h"
#include "mlir/IR/AffineExpr.h"
#include "mlir/IR/AffineMap.h"
#include "mlir/IR/BlockAndValueMapping.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/Operation.h"
#include "mlir/StandardOps/Ops.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Support/Debug.h"

#define DEBUG_TYPE "LoopUtils"

using namespace mlir;

/// Computes the cleanup loop lower bound of the loop being unrolled with
/// the specified unroll factor; this bound will also be upper bound of the main
/// part of the unrolled loop. Computes the bound as an AffineMap with its
/// operands or a null map when the trip count can't be expressed as an affine
/// expression.
void mlir::getCleanupLoopLowerBound(AffineForOp forOp, unsigned unrollFactor,
                                    AffineMap *map,
                                    SmallVectorImpl<Value *> *operands,
                                    FuncBuilder *b) {
  auto lbMap = forOp.getLowerBoundMap();

  // Single result lower bound map only.
  if (lbMap.getNumResults() != 1) {
    *map = AffineMap();
    return;
  }

  AffineMap tripCountMap;
  SmallVector<Value *, 4> tripCountOperands;
  buildTripCountMapAndOperands(forOp, &tripCountMap, &tripCountOperands);

  // Sometimes the trip count cannot be expressed as an affine expression.
  if (!tripCountMap) {
    *map = AffineMap();
    return;
  }

  unsigned step = forOp.getStep();

  SmallVector<Value *, 4> lbOperands(forOp.getLowerBoundOperands());
  auto lb = b->create<AffineApplyOp>(forOp.getLoc(), lbMap, lbOperands);

  // For each upper bound expr, get the range.
  // Eg: affine.for %i = lb to min (ub1, ub2),
  // where tripCountExprs yield (tr1, tr2), we create affine.apply's:
  // lb + tr1 - tr1 % ufactor, lb + tr2 - tr2 % ufactor; the results of all
  // these affine.apply's make up the cleanup loop lower bound.
  SmallVector<AffineExpr, 4> bumpExprs(tripCountMap.getNumResults());
  SmallVector<Value *, 4> bumpValues(tripCountMap.getNumResults());
  for (unsigned i = 0, e = tripCountMap.getNumResults(); i < e; i++) {
    auto tripCountExpr = tripCountMap.getResult(i);
    bumpExprs[i] = (tripCountExpr - tripCountExpr % unrollFactor) * step;
    auto bumpMap =
        b->getAffineMap(tripCountMap.getNumDims(), tripCountMap.getNumSymbols(),
                        bumpExprs[i], {});
    bumpValues[i] =
        b->create<AffineApplyOp>(forOp.getLoc(), bumpMap, tripCountOperands);
  }

  SmallVector<AffineExpr, 4> newUbExprs(tripCountMap.getNumResults());
  for (unsigned i = 0, e = bumpExprs.size(); i < e; i++)
    newUbExprs[i] = b->getAffineDimExpr(0) + b->getAffineDimExpr(i + 1);

  operands->clear();
  operands->push_back(lb);
  operands->append(bumpValues.begin(), bumpValues.end());
  *map = b->getAffineMap(1 + tripCountMap.getNumResults(), 0, newUbExprs, {});
  // Simplify the map + operands.
  fullyComposeAffineMapAndOperands(map, operands);
  *map = simplifyAffineMap(*map);
  canonicalizeMapAndOperands(map, operands);
  // Remove any affine.apply's that became dead from the simplification above.
  for (auto *v : bumpValues) {
    if (v->use_empty()) {
      v->getDefiningOp()->erase();
    }
  }
  if (lb.use_empty())
    lb.erase();
}

/// Promotes the loop body of a forOp to its containing block if the forOp
/// was known to have a single iteration.
// TODO(bondhugula): extend this for arbitrary affine bounds.
LogicalResult mlir::promoteIfSingleIteration(AffineForOp forOp) {
  Optional<uint64_t> tripCount = getConstantTripCount(forOp);
  if (!tripCount.hasValue() || tripCount.getValue() != 1)
    return failure();

  // TODO(mlir-team): there is no builder for a max.
  if (forOp.getLowerBoundMap().getNumResults() != 1)
    return failure();

  // Replaces all IV uses to its single iteration value.
  auto *iv = forOp.getInductionVar();
  Operation *op = forOp.getOperation();
  if (!iv->use_empty()) {
    if (forOp.hasConstantLowerBound()) {
      auto *mlFunc = op->getFunction();
      FuncBuilder topBuilder(mlFunc);
      auto constOp = topBuilder.create<ConstantIndexOp>(
          forOp.getLoc(), forOp.getConstantLowerBound());
      iv->replaceAllUsesWith(constOp);
    } else {
      AffineBound lb = forOp.getLowerBound();
      SmallVector<Value *, 4> lbOperands(lb.operand_begin(), lb.operand_end());
      FuncBuilder builder(op->getBlock(), Block::iterator(op));
      if (lb.getMap() == builder.getDimIdentityMap()) {
        // No need of generating an affine.apply.
        iv->replaceAllUsesWith(lbOperands[0]);
      } else {
        auto affineApplyOp = builder.create<AffineApplyOp>(
            op->getLoc(), lb.getMap(), lbOperands);
        iv->replaceAllUsesWith(affineApplyOp);
      }
    }
  }
  // Move the loop body operations, except for terminator, to the loop's
  // containing block.
  auto *block = op->getBlock();
  forOp.getBody()->getOperations().back().erase();
  block->getOperations().splice(Block::iterator(op),
                                forOp.getBody()->getOperations());
  forOp.erase();
  return success();
}

/// Promotes all single iteration for op's in the Function, i.e., moves
/// their body into the containing Block.
void mlir::promoteSingleIterationLoops(Function *f) {
  // Gathers all innermost loops through a post order pruned walk.
  f->walk<AffineForOp>(
      [](AffineForOp forOp) { promoteIfSingleIteration(forOp); });
}

/// Generates a 'affine.for' op with the specified lower and upper bounds
/// while generating the right IV remappings for the shifted operations. The
/// operation blocks that go into the loop are specified in instGroupQueue
/// starting from the specified offset, and in that order; the first element of
/// the pair specifies the shift applied to that group of operations; note
/// that the shift is multiplied by the loop step before being applied. Returns
/// nullptr if the generated loop simplifies to a single iteration one.
static AffineForOp
generateLoop(AffineMap lbMap, AffineMap ubMap,
             const std::vector<std::pair<uint64_t, ArrayRef<Operation *>>>
                 &instGroupQueue,
             unsigned offset, AffineForOp srcForInst, FuncBuilder *b) {
  SmallVector<Value *, 4> lbOperands(srcForInst.getLowerBoundOperands());
  SmallVector<Value *, 4> ubOperands(srcForInst.getUpperBoundOperands());

  assert(lbMap.getNumInputs() == lbOperands.size());
  assert(ubMap.getNumInputs() == ubOperands.size());

  auto loopChunk =
      b->create<AffineForOp>(srcForInst.getLoc(), lbOperands, lbMap, ubOperands,
                             ubMap, srcForInst.getStep());
  auto *loopChunkIV = loopChunk.getInductionVar();
  auto *srcIV = srcForInst.getInductionVar();

  BlockAndValueMapping operandMap;

  FuncBuilder bodyBuilder = loopChunk.getBodyBuilder();
  for (auto it = instGroupQueue.begin() + offset, e = instGroupQueue.end();
       it != e; ++it) {
    uint64_t shift = it->first;
    auto insts = it->second;
    // All 'same shift' operations get added with their operands being
    // remapped to results of cloned operations, and their IV used remapped.
    // Generate the remapping if the shift is not zero: remappedIV = newIV -
    // shift.
    if (!srcIV->use_empty() && shift != 0) {
      auto ivRemap = bodyBuilder.create<AffineApplyOp>(
          srcForInst.getLoc(),
          bodyBuilder.getSingleDimShiftAffineMap(
              -static_cast<int64_t>(srcForInst.getStep() * shift)),
          loopChunkIV);
      operandMap.map(srcIV, ivRemap);
    } else {
      operandMap.map(srcIV, loopChunkIV);
    }
    for (auto *op : insts) {
      if (!op->isa<AffineTerminatorOp>())
        bodyBuilder.clone(*op, operandMap);
    }
  };
  if (succeeded(promoteIfSingleIteration(loopChunk)))
    return AffineForOp();
  return loopChunk;
}

/// Skew the operations in the body of a 'affine.for' operation with the
/// specified operation-wise shifts. The shifts are with respect to the
/// original execution order, and are multiplied by the loop 'step' before being
/// applied. A shift of zero for each operation will lead to no change.
// The skewing of operations with respect to one another can be used for
// example to allow overlap of asynchronous operations (such as DMA
// communication) with computation, or just relative shifting of operations
// for better register reuse, locality or parallelism. As such, the shifts are
// typically expected to be at most of the order of the number of operations.
// This method should not be used as a substitute for loop distribution/fission.
// This method uses an algorithm// in time linear in the number of operations
// in the body of the for loop - (using the 'sweep line' paradigm). This method
// asserts preservation of SSA dominance. A check for that as well as that for
// memory-based depedence preservation check rests with the users of this
// method.
LogicalResult mlir::instBodySkew(AffineForOp forOp, ArrayRef<uint64_t> shifts,
                                 bool unrollPrologueEpilogue) {
  if (forOp.getBody()->begin() == std::prev(forOp.getBody()->end()))
    return success();

  // If the trip counts aren't constant, we would need versioning and
  // conditional guards (or context information to prevent such versioning). The
  // better way to pipeline for such loops is to first tile them and extract
  // constant trip count "full tiles" before applying this.
  auto mayBeConstTripCount = getConstantTripCount(forOp);
  if (!mayBeConstTripCount.hasValue()) {
    LLVM_DEBUG(forOp.emitNote("non-constant trip count loop not handled"));
    return success();
  }
  uint64_t tripCount = mayBeConstTripCount.getValue();

  assert(isInstwiseShiftValid(forOp, shifts) &&
         "shifts will lead to an invalid transformation\n");

  int64_t step = forOp.getStep();

  unsigned numChildInsts = forOp.getBody()->getOperations().size();

  // Do a linear time (counting) sort for the shifts.
  uint64_t maxShift = 0;
  for (unsigned i = 0; i < numChildInsts; i++) {
    maxShift = std::max(maxShift, shifts[i]);
  }
  // Such large shifts are not the typical use case.
  if (maxShift >= numChildInsts) {
    forOp.emitWarning("not shifting because shifts are unrealistically large");
    return success();
  }

  // An array of operation groups sorted by shift amount; each group has all
  // operations with the same shift in the order in which they appear in the
  // body of the 'affine.for' op.
  std::vector<std::vector<Operation *>> sortedInstGroups(maxShift + 1);
  unsigned pos = 0;
  for (auto &op : *forOp.getBody()) {
    auto shift = shifts[pos++];
    sortedInstGroups[shift].push_back(&op);
  }

  // Unless the shifts have a specific pattern (which actually would be the
  // common use case), prologue and epilogue are not meaningfully defined.
  // Nevertheless, if 'unrollPrologueEpilogue' is set, we will treat the first
  // loop generated as the prologue and the last as epilogue and unroll these
  // fully.
  AffineForOp prologue;
  AffineForOp epilogue;

  // Do a sweep over the sorted shifts while storing open groups in a
  // vector, and generating loop portions as necessary during the sweep. A block
  // of operations is paired with its shift.
  std::vector<std::pair<uint64_t, ArrayRef<Operation *>>> instGroupQueue;

  auto origLbMap = forOp.getLowerBoundMap();
  uint64_t lbShift = 0;
  FuncBuilder b(forOp.getOperation());
  for (uint64_t d = 0, e = sortedInstGroups.size(); d < e; ++d) {
    // If nothing is shifted by d, continue.
    if (sortedInstGroups[d].empty())
      continue;
    if (!instGroupQueue.empty()) {
      assert(d >= 1 &&
             "Queue expected to be empty when the first block is found");
      // The interval for which the loop needs to be generated here is:
      // [lbShift, min(lbShift + tripCount, d)) and the body of the
      // loop needs to have all operations in instQueue in that order.
      AffineForOp res;
      if (lbShift + tripCount * step < d * step) {
        res = generateLoop(
            b.getShiftedAffineMap(origLbMap, lbShift),
            b.getShiftedAffineMap(origLbMap, lbShift + tripCount * step),
            instGroupQueue, 0, forOp, &b);
        // Entire loop for the queued op groups generated, empty it.
        instGroupQueue.clear();
        lbShift += tripCount * step;
      } else {
        res = generateLoop(b.getShiftedAffineMap(origLbMap, lbShift),
                           b.getShiftedAffineMap(origLbMap, d), instGroupQueue,
                           0, forOp, &b);
        lbShift = d * step;
      }
      if (!prologue && res)
        prologue = res;
      epilogue = res;
    } else {
      // Start of first interval.
      lbShift = d * step;
    }
    // Augment the list of operations that get into the current open interval.
    instGroupQueue.push_back({d, sortedInstGroups[d]});
  }

  // Those operations groups left in the queue now need to be processed (FIFO)
  // and their loops completed.
  for (unsigned i = 0, e = instGroupQueue.size(); i < e; ++i) {
    uint64_t ubShift = (instGroupQueue[i].first + tripCount) * step;
    epilogue = generateLoop(b.getShiftedAffineMap(origLbMap, lbShift),
                            b.getShiftedAffineMap(origLbMap, ubShift),
                            instGroupQueue, i, forOp, &b);
    lbShift = ubShift;
    if (!prologue)
      prologue = epilogue;
  }

  // Erase the original for op.
  forOp.erase();

  if (unrollPrologueEpilogue && prologue)
    loopUnrollFull(prologue);
  if (unrollPrologueEpilogue && !epilogue &&
      epilogue.getOperation() != prologue.getOperation())
    loopUnrollFull(epilogue);

  return success();
}

/// Get perfectly nested sequence of loops starting at root of loop nest
/// (the first op being another AffineFor, and the second op - a terminator).
/// A loop is perfectly nested iff: the first op in the loop's body is another
/// AffineForOp, and the second op is a terminator).
void mlir::getPerfectlyNestedLoops(SmallVectorImpl<AffineForOp> &nestedLoops,
                                   AffineForOp root) {
  AffineForOp curr = root;
  nestedLoops.push_back(curr);
  auto *currBody = curr.getBody();
  while (currBody->begin() == std::prev(currBody->end(), 2) &&
         (curr = curr.getBody()->front().dyn_cast<AffineForOp>())) {
    nestedLoops.push_back(curr);
    currBody = curr.getBody();
  }
}

/// Unrolls this loop completely.
LogicalResult mlir::loopUnrollFull(AffineForOp forOp) {
  Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(forOp);
  if (mayBeConstantTripCount.hasValue()) {
    uint64_t tripCount = mayBeConstantTripCount.getValue();
    if (tripCount == 1) {
      return promoteIfSingleIteration(forOp);
    }
    return loopUnrollByFactor(forOp, tripCount);
  }
  return failure();
}

/// Unrolls and jams this loop by the specified factor or by the trip count (if
/// constant) whichever is lower.
LogicalResult mlir::loopUnrollUpToFactor(AffineForOp forOp,
                                         uint64_t unrollFactor) {
  Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(forOp);

  if (mayBeConstantTripCount.hasValue() &&
      mayBeConstantTripCount.getValue() < unrollFactor)
    return loopUnrollByFactor(forOp, mayBeConstantTripCount.getValue());
  return loopUnrollByFactor(forOp, unrollFactor);
}

/// Unrolls this loop by the specified factor. Returns success if the loop
/// is successfully unrolled.
LogicalResult mlir::loopUnrollByFactor(AffineForOp forOp,
                                       uint64_t unrollFactor) {
  assert(unrollFactor >= 1 && "unroll factor should be >= 1");

  if (unrollFactor == 1)
    return promoteIfSingleIteration(forOp);

  if (forOp.getBody()->empty() ||
      forOp.getBody()->begin() == std::prev(forOp.getBody()->end()))
    return failure();

  // Loops where the lower bound is a max expression isn't supported for
  // unrolling since the trip count can be expressed as an affine function when
  // both the lower bound and the upper bound are multi-result maps. However,
  // one meaningful way to do such unrolling would be to specialize the loop for
  // the 'hotspot' case and unroll that hotspot.
  if (forOp.getLowerBoundMap().getNumResults() != 1)
    return failure();

  // If the trip count is lower than the unroll factor, no unrolled body.
  // TODO(bondhugula): option to specify cleanup loop unrolling.
  Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(forOp);
  if (mayBeConstantTripCount.hasValue() &&
      mayBeConstantTripCount.getValue() < unrollFactor)
    return failure();

  // Generate the cleanup loop if trip count isn't a multiple of unrollFactor.
  Operation *op = forOp.getOperation();
  if (getLargestDivisorOfTripCount(forOp) % unrollFactor != 0) {
    FuncBuilder builder(op->getBlock(), ++Block::iterator(op));
    auto cleanupForInst = builder.clone(*op)->cast<AffineForOp>();
    AffineMap cleanupMap;
    SmallVector<Value *, 4> cleanupOperands;
    getCleanupLoopLowerBound(forOp, unrollFactor, &cleanupMap, &cleanupOperands,
                             &builder);
    assert(cleanupMap &&
           "cleanup loop lower bound map for single result lower bound maps "
           "can always be determined");
    cleanupForInst.setLowerBound(cleanupOperands, cleanupMap);
    // Promote the loop body up if this has turned into a single iteration loop.
    promoteIfSingleIteration(cleanupForInst);

    // Adjust upper bound of the original loop; this is the same as the lower
    // bound of the cleanup loop.
    forOp.setUpperBound(cleanupOperands, cleanupMap);
  }

  // Scale the step of loop being unrolled by unroll factor.
  int64_t step = forOp.getStep();
  forOp.setStep(step * unrollFactor);

  // Builder to insert unrolled bodies just before the terminator of the body of
  // 'forOp'.
  FuncBuilder builder = forOp.getBodyBuilder();

  // Keep a pointer to the last non-terminator operation in the original block
  // so that we know what to clone (since we are doing this in-place).
  Block::iterator srcBlockEnd = std::prev(forOp.getBody()->end(), 2);

  // Unroll the contents of 'forOp' (append unrollFactor-1 additional copies).
  auto *forOpIV = forOp.getInductionVar();
  for (unsigned i = 1; i < unrollFactor; i++) {
    BlockAndValueMapping operandMap;

    // If the induction variable is used, create a remapping to the value for
    // this unrolled instance.
    if (!forOpIV->use_empty()) {
      // iv' = iv + 1/2/3...unrollFactor-1;
      auto d0 = builder.getAffineDimExpr(0);
      auto bumpMap = builder.getAffineMap(1, 0, {d0 + i * step}, {});
      auto ivUnroll =
          builder.create<AffineApplyOp>(forOp.getLoc(), bumpMap, forOpIV);
      operandMap.map(forOpIV, ivUnroll);
    }

    // Clone the original body of 'forOp'.
    for (auto it = forOp.getBody()->begin(); it != std::next(srcBlockEnd);
         it++) {
      builder.clone(*it, operandMap);
    }
  }

  // Promote the loop body up if this has turned into a single iteration loop.
  promoteIfSingleIteration(forOp);
  return success();
}

/// Performs loop interchange on 'forOpA' and 'forOpB', where 'forOpB' is
/// nested within 'forOpA' as the only non-terminator operation in its block.
void mlir::interchangeLoops(AffineForOp forOpA, AffineForOp forOpB) {
  auto *forOpAInst = forOpA.getOperation();

  assert(&*forOpA.getBody()->begin() == forOpB.getOperation());
  auto &forOpABody = forOpA.getBody()->getOperations();
  auto &forOpBBody = forOpB.getBody()->getOperations();

  // 1) Splice forOpA's non-terminator operations (which is just forOpB) just
  // before forOpA (in ForOpA's parent's block) this should leave 'forOpA's
  // body containing only the terminator.
  forOpAInst->getBlock()->getOperations().splice(Block::iterator(forOpAInst),
                                                 forOpABody, forOpABody.begin(),
                                                 std::prev(forOpABody.end()));
  // 2) Splice forOpB's non-terminator operations into the beginning of forOpA's
  // body (this leaves forOpB's body containing only the terminator).
  forOpABody.splice(forOpABody.begin(), forOpBBody, forOpBBody.begin(),
                    std::prev(forOpBBody.end()));
  // 3) Splice forOpA into the beginning of forOpB's body.
  forOpBBody.splice(forOpBBody.begin(), forOpAInst->getBlock()->getOperations(),
                    Block::iterator(forOpAInst));
}

/// Performs a series of loop interchanges to sink 'forOp' 'loopDepth' levels
/// deeper in the loop nest.
void mlir::sinkLoop(AffineForOp forOp, unsigned loopDepth) {
  for (unsigned i = 0; i < loopDepth; ++i) {
    assert(forOp.getBody()->front().isa<AffineForOp>());
    AffineForOp nextForOp = forOp.getBody()->front().cast<AffineForOp>();
    interchangeLoops(forOp, nextForOp);
  }
}

// Factors out common behavior to add a new `iv` (resp. `iv` + `offset`) to the
// lower (resp. upper) loop bound. When called for both the lower and upper
// bounds, the resulting IR resembles:
//
// ```mlir
//    affine.for %i = max (`iv, ...) to min (`iv` + `offset`) {
//      ...
//    }
// ```
static void augmentMapAndBounds(FuncBuilder *b, Value *iv, AffineMap *map,
                                SmallVector<Value *, 4> *operands,
                                int64_t offset = 0) {
  auto bounds = llvm::to_vector<4>(map->getResults());
  bounds.push_back(b->getAffineDimExpr(map->getNumDims()) + offset);
  operands->insert(operands->begin() + map->getNumDims(), iv);
  *map =
      b->getAffineMap(map->getNumDims() + 1, map->getNumSymbols(), bounds, {});
  canonicalizeMapAndOperands(map, operands);
}

// Clone the original body of `forOp` into the body of `newForOp` while
// substituting `oldIv` in place of
// `forOp.getInductionVariable()` and ignoring the terminator.
// Note: `newForOp` may be nested under `forOp`.
static void cloneLoopBodyInto(AffineForOp forOp, Value *oldIv,
                              AffineForOp newForOp) {
  BlockAndValueMapping map;
  map.map(oldIv, newForOp.getInductionVar());
  FuncBuilder b = newForOp.getBodyBuilder();
  for (auto &op : *forOp.getBody()) {
    // Step over newForOp in case it is nested under forOp.
    if (&op == newForOp.getOperation()) {
      continue;
    }
    if (op.isa<AffineTerminatorOp>()) {
      continue;
    }
    auto *instClone = b.clone(op, map);
    unsigned idx = 0;
    for (auto r : op.getResults()) {
      // Since we do a forward pass over the body, we iteratively augment
      // the `map` with everything we clone.
      map.map(r, instClone->getResult(idx++));
    }
  }
}

// Stripmines `forOp` by `factor` and sinks it under each of the `targets`.
// Stripmine-sink is a primitive building block for generalized tiling of
// imperfectly nested loops.
// This transformation is purely mechanical and does not check legality,
// profitability or even structural correctness. It is the user's
// responsibility to specify `targets` that are dominated by `forOp`.
// Returns the new AffineForOps, one per `targets`, nested immediately under
// each of the `targets`.
static SmallVector<AffineForOp, 8>
stripmineSink(AffineForOp forOp, uint64_t factor,
              ArrayRef<AffineForOp> targets) {
  // TODO(ntv): Use cheap structural assertions that targets are nested under
  // forOp and that targets are not nested under each other when DominanceInfo
  // exposes the capability. It seems overkill to construct a whole function
  // dominance tree at this point.
  auto originalStep = forOp.getStep();
  auto scaledStep = originalStep * factor;
  forOp.setStep(scaledStep);

  auto *op = forOp.getOperation();
  FuncBuilder b(op->getBlock(), ++Block::iterator(op));

  // Lower-bound map creation.
  auto lbMap = forOp.getLowerBoundMap();
  SmallVector<Value *, 4> lbOperands(forOp.getLowerBoundOperands());
  augmentMapAndBounds(&b, forOp.getInductionVar(), &lbMap, &lbOperands);

  // Upper-bound map creation.
  auto ubMap = forOp.getUpperBoundMap();
  SmallVector<Value *, 4> ubOperands(forOp.getUpperBoundOperands());
  augmentMapAndBounds(&b, forOp.getInductionVar(), &ubMap, &ubOperands,
                      /*offset=*/scaledStep);

  SmallVector<AffineForOp, 8> innerLoops;
  for (auto t : targets) {
    // Insert newForOp before the terminator of `t`.
    FuncBuilder b = t.getBodyBuilder();
    auto newForOp = b.create<AffineForOp>(t.getLoc(), lbOperands, lbMap,
                                          ubOperands, ubMap, originalStep);
    cloneLoopBodyInto(t, forOp.getInductionVar(), newForOp);
    // Remove all operations from `t` except `newForOp`.
    auto rit = ++newForOp.getOperation()->getReverseIterator();
    auto re = t.getBody()->rend();
    for (auto &op : llvm::make_early_inc_range(llvm::make_range(rit, re))) {
      op.erase();
    }
    innerLoops.push_back(newForOp);
  }

  return innerLoops;
}

// Stripmines a `forOp` by `factor` and sinks it under a single `target`.
// Returns the new AffineForOps, nested immediately under `target`.
AffineForOp stripmineSink(AffineForOp forOp, uint64_t factor,
                          AffineForOp target) {
  auto res = stripmineSink(forOp, factor, ArrayRef<AffineForOp>{target});
  assert(res.size() == 1 && "Expected 1 inner forOp");
  return res[0];
}

SmallVector<SmallVector<AffineForOp, 8>, 8>
mlir::tile(ArrayRef<AffineForOp> forOps, ArrayRef<uint64_t> sizes,
           ArrayRef<AffineForOp> targets) {
  SmallVector<SmallVector<AffineForOp, 8>, 8> res;
  SmallVector<AffineForOp, 8> currentTargets(targets.begin(), targets.end());
  for (auto it : llvm::zip(forOps, sizes)) {
    auto step = stripmineSink(std::get<0>(it), std::get<1>(it), currentTargets);
    res.push_back(step);
    currentTargets = step;
  }
  return res;
}

SmallVector<AffineForOp, 8> mlir::tile(ArrayRef<AffineForOp> forOps,
                                       ArrayRef<uint64_t> sizes,
                                       AffineForOp target) {
  return tile(forOps, sizes, ArrayRef<AffineForOp>{target})[0];
}