RegionUtils.cpp 26.6 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705
//===- RegionUtils.cpp - Region-related transformation utilities ----------===//
//
// 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 "mlir/Transforms/RegionUtils.h"
#include "mlir/IR/Block.h"
#include "mlir/IR/Operation.h"
#include "mlir/IR/RegionGraphTraits.h"
#include "mlir/IR/Value.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"

#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallSet.h"

using namespace mlir;

void mlir::replaceAllUsesInRegionWith(Value orig, Value replacement,
                                      Region &region) {
  for (auto &use : llvm::make_early_inc_range(orig.getUses())) {
    if (region.isAncestor(use.getOwner()->getParentRegion()))
      use.set(replacement);
  }
}

void mlir::visitUsedValuesDefinedAbove(
    Region &region, Region &limit, function_ref<void(OpOperand *)> callback) {
  assert(limit.isAncestor(&region) &&
         "expected isolation limit to be an ancestor of the given region");

  // Collect proper ancestors of `limit` upfront to avoid traversing the region
  // tree for every value.
  SmallPtrSet<Region *, 4> properAncestors;
  for (auto *reg = limit.getParentRegion(); reg != nullptr;
       reg = reg->getParentRegion()) {
    properAncestors.insert(reg);
  }

  region.walk([callback, &properAncestors](Operation *op) {
    for (OpOperand &operand : op->getOpOperands())
      // Callback on values defined in a proper ancestor of region.
      if (properAncestors.count(operand.get().getParentRegion()))
        callback(&operand);
  });
}

void mlir::visitUsedValuesDefinedAbove(
    MutableArrayRef<Region> regions, function_ref<void(OpOperand *)> callback) {
  for (Region &region : regions)
    visitUsedValuesDefinedAbove(region, region, callback);
}

void mlir::getUsedValuesDefinedAbove(Region &region, Region &limit,
                                     llvm::SetVector<Value> &values) {
  visitUsedValuesDefinedAbove(region, limit, [&](OpOperand *operand) {
    values.insert(operand->get());
  });
}

void mlir::getUsedValuesDefinedAbove(MutableArrayRef<Region> regions,
                                     llvm::SetVector<Value> &values) {
  for (Region &region : regions)
    getUsedValuesDefinedAbove(region, region, values);
}

//===----------------------------------------------------------------------===//
// Unreachable Block Elimination
//===----------------------------------------------------------------------===//

/// Erase the unreachable blocks within the provided regions. Returns success
/// if any blocks were erased, failure otherwise.
// TODO: We could likely merge this with the DCE algorithm below.
static LogicalResult eraseUnreachableBlocks(MutableArrayRef<Region> regions) {
  // Set of blocks found to be reachable within a given region.
  llvm::df_iterator_default_set<Block *, 16> reachable;
  // If any blocks were found to be dead.
  bool erasedDeadBlocks = false;

  SmallVector<Region *, 1> worklist;
  worklist.reserve(regions.size());
  for (Region &region : regions)
    worklist.push_back(&region);
  while (!worklist.empty()) {
    Region *region = worklist.pop_back_val();
    if (region->empty())
      continue;

    // If this is a single block region, just collect the nested regions.
    if (std::next(region->begin()) == region->end()) {
      for (Operation &op : region->front())
        for (Region &region : op.getRegions())
          worklist.push_back(&region);
      continue;
    }

    // Mark all reachable blocks.
    reachable.clear();
    for (Block *block : depth_first_ext(&region->front(), reachable))
      (void)block /* Mark all reachable blocks */;

    // Collect all of the dead blocks and push the live regions onto the
    // worklist.
    for (Block &block : llvm::make_early_inc_range(*region)) {
      if (!reachable.count(&block)) {
        block.dropAllDefinedValueUses();
        block.erase();
        erasedDeadBlocks = true;
        continue;
      }

      // Walk any regions within this block.
      for (Operation &op : block)
        for (Region &region : op.getRegions())
          worklist.push_back(&region);
    }
  }

  return success(erasedDeadBlocks);
}

//===----------------------------------------------------------------------===//
// Dead Code Elimination
//===----------------------------------------------------------------------===//

namespace {
/// Data structure used to track which values have already been proved live.
///
/// Because Operation's can have multiple results, this data structure tracks
/// liveness for both Value's and Operation's to avoid having to look through
/// all Operation results when analyzing a use.
///
/// This data structure essentially tracks the dataflow lattice.
/// The set of values/ops proved live increases monotonically to a fixed-point.
class LiveMap {
public:
  /// Value methods.
  bool wasProvenLive(Value value) { return liveValues.count(value); }
  void setProvedLive(Value value) {
    changed |= liveValues.insert(value).second;
  }

  /// Operation methods.
  bool wasProvenLive(Operation *op) { return liveOps.count(op); }
  void setProvedLive(Operation *op) { changed |= liveOps.insert(op).second; }

  /// Methods for tracking if we have reached a fixed-point.
  void resetChanged() { changed = false; }
  bool hasChanged() { return changed; }

private:
  bool changed = false;
  DenseSet<Value> liveValues;
  DenseSet<Operation *> liveOps;
};
} // namespace

static bool isUseSpeciallyKnownDead(OpOperand &use, LiveMap &liveMap) {
  Operation *owner = use.getOwner();
  unsigned operandIndex = use.getOperandNumber();
  // This pass generally treats all uses of an op as live if the op itself is
  // considered live. However, for successor operands to terminators we need a
  // finer-grained notion where we deduce liveness for operands individually.
  // The reason for this is easiest to think about in terms of a classical phi
  // node based SSA IR, where each successor operand is really an operand to a
  // *separate* phi node, rather than all operands to the branch itself as with
  // the block argument representation that MLIR uses.
  //
  // And similarly, because each successor operand is really an operand to a phi
  // node, rather than to the terminator op itself, a terminator op can't e.g.
  // "print" the value of a successor operand.
  if (owner->isKnownTerminator()) {
    if (BranchOpInterface branchInterface = dyn_cast<BranchOpInterface>(owner))
      if (auto arg = branchInterface.getSuccessorBlockArgument(operandIndex))
        return !liveMap.wasProvenLive(*arg);
    return false;
  }
  return false;
}

static void processValue(Value value, LiveMap &liveMap) {
  bool provedLive = llvm::any_of(value.getUses(), [&](OpOperand &use) {
    if (isUseSpeciallyKnownDead(use, liveMap))
      return false;
    return liveMap.wasProvenLive(use.getOwner());
  });
  if (provedLive)
    liveMap.setProvedLive(value);
}

static bool isOpIntrinsicallyLive(Operation *op) {
  // This pass doesn't modify the CFG, so terminators are never deleted.
  if (!op->isKnownNonTerminator())
    return true;
  // If the op has a side effect, we treat it as live.
  // TODO: Properly handle region side effects.
  return !MemoryEffectOpInterface::hasNoEffect(op) || op->getNumRegions() != 0;
}

static void propagateLiveness(Region &region, LiveMap &liveMap);

static void propagateTerminatorLiveness(Operation *op, LiveMap &liveMap) {
  // Terminators are always live.
  liveMap.setProvedLive(op);

  // Check to see if we can reason about the successor operands and mutate them.
  BranchOpInterface branchInterface = dyn_cast<BranchOpInterface>(op);
  if (!branchInterface) {
    for (Block *successor : op->getSuccessors())
      for (BlockArgument arg : successor->getArguments())
        liveMap.setProvedLive(arg);
    return;
  }

  // If we can't reason about the operands to a successor, conservatively mark
  // all arguments as live.
  for (unsigned i = 0, e = op->getNumSuccessors(); i != e; ++i) {
    if (!branchInterface.getMutableSuccessorOperands(i))
      for (BlockArgument arg : op->getSuccessor(i)->getArguments())
        liveMap.setProvedLive(arg);
  }
}

static void propagateLiveness(Operation *op, LiveMap &liveMap) {
  // All Value's are either a block argument or an op result.
  // We call processValue on those cases.

  // Recurse on any regions the op has.
  for (Region &region : op->getRegions())
    propagateLiveness(region, liveMap);

  // Process terminator operations.
  if (op->isKnownTerminator())
    return propagateTerminatorLiveness(op, liveMap);

  // Process the op itself.
  if (isOpIntrinsicallyLive(op)) {
    liveMap.setProvedLive(op);
    return;
  }
  for (Value value : op->getResults())
    processValue(value, liveMap);
  bool provedLive = llvm::any_of(op->getResults(), [&](Value value) {
    return liveMap.wasProvenLive(value);
  });
  if (provedLive)
    liveMap.setProvedLive(op);
}

static void propagateLiveness(Region &region, LiveMap &liveMap) {
  if (region.empty())
    return;

  for (Block *block : llvm::post_order(&region.front())) {
    // We process block arguments after the ops in the block, to promote
    // faster convergence to a fixed point (we try to visit uses before defs).
    for (Operation &op : llvm::reverse(block->getOperations()))
      propagateLiveness(&op, liveMap);
    for (Value value : block->getArguments())
      processValue(value, liveMap);
  }
}

static void eraseTerminatorSuccessorOperands(Operation *terminator,
                                             LiveMap &liveMap) {
  BranchOpInterface branchOp = dyn_cast<BranchOpInterface>(terminator);
  if (!branchOp)
    return;

  for (unsigned succI = 0, succE = terminator->getNumSuccessors();
       succI < succE; succI++) {
    // Iterating successors in reverse is not strictly needed, since we
    // aren't erasing any successors. But it is slightly more efficient
    // since it will promote later operands of the terminator being erased
    // first, reducing the quadratic-ness.
    unsigned succ = succE - succI - 1;
    Optional<MutableOperandRange> succOperands =
        branchOp.getMutableSuccessorOperands(succ);
    if (!succOperands)
      continue;
    Block *successor = terminator->getSuccessor(succ);

    for (unsigned argI = 0, argE = succOperands->size(); argI < argE; ++argI) {
      // Iterating args in reverse is needed for correctness, to avoid
      // shifting later args when earlier args are erased.
      unsigned arg = argE - argI - 1;
      if (!liveMap.wasProvenLive(successor->getArgument(arg)))
        succOperands->erase(arg);
    }
  }
}

static LogicalResult deleteDeadness(MutableArrayRef<Region> regions,
                                    LiveMap &liveMap) {
  bool erasedAnything = false;
  for (Region &region : regions) {
    if (region.empty())
      continue;

    // We do the deletion in an order that deletes all uses before deleting
    // defs.
    // MLIR's SSA structural invariants guarantee that except for block
    // arguments, the use-def graph is acyclic, so this is possible with a
    // single walk of ops and then a final pass to clean up block arguments.
    //
    // To do this, we visit ops in an order that visits domtree children
    // before domtree parents. A CFG post-order (with reverse iteration with a
    // block) satisfies that without needing an explicit domtree calculation.
    for (Block *block : llvm::post_order(&region.front())) {
      eraseTerminatorSuccessorOperands(block->getTerminator(), liveMap);
      for (Operation &childOp :
           llvm::make_early_inc_range(llvm::reverse(block->getOperations()))) {
        erasedAnything |=
            succeeded(deleteDeadness(childOp.getRegions(), liveMap));
        if (!liveMap.wasProvenLive(&childOp)) {
          erasedAnything = true;
          childOp.erase();
        }
      }
    }
    // Delete block arguments.
    // The entry block has an unknown contract with their enclosing block, so
    // skip it.
    for (Block &block : llvm::drop_begin(region.getBlocks(), 1)) {
      // Iterate in reverse to avoid shifting later arguments when deleting
      // earlier arguments.
      for (unsigned i = 0, e = block.getNumArguments(); i < e; i++)
        if (!liveMap.wasProvenLive(block.getArgument(e - i - 1))) {
          block.eraseArgument(e - i - 1);
          erasedAnything = true;
        }
    }
  }
  return success(erasedAnything);
}

// This function performs a simple dead code elimination algorithm over the
// given regions.
//
// The overall goal is to prove that Values are dead, which allows deleting ops
// and block arguments.
//
// This uses an optimistic algorithm that assumes everything is dead until
// proved otherwise, allowing it to delete recursively dead cycles.
//
// This is a simple fixed-point dataflow analysis algorithm on a lattice
// {Dead,Alive}. Because liveness flows backward, we generally try to
// iterate everything backward to speed up convergence to the fixed-point. This
// allows for being able to delete recursively dead cycles of the use-def graph,
// including block arguments.
//
// This function returns success if any operations or arguments were deleted,
// failure otherwise.
static LogicalResult runRegionDCE(MutableArrayRef<Region> regions) {
  LiveMap liveMap;
  do {
    liveMap.resetChanged();

    for (Region &region : regions)
      propagateLiveness(region, liveMap);
  } while (liveMap.hasChanged());

  return deleteDeadness(regions, liveMap);
}

//===----------------------------------------------------------------------===//
// Block Merging
//===----------------------------------------------------------------------===//

//===----------------------------------------------------------------------===//
// BlockEquivalenceData

namespace {
/// This class contains the information for comparing the equivalencies of two
/// blocks. Blocks are considered equivalent if they contain the same operations
/// in the same order. The only allowed divergence is for operands that come
/// from sources outside of the parent block, i.e. the uses of values produced
/// within the block must be equivalent.
///   e.g.,
/// Equivalent:
///  ^bb1(%arg0: i32)
///    return %arg0, %foo : i32, i32
///  ^bb2(%arg1: i32)
///    return %arg1, %bar : i32, i32
/// Not Equivalent:
///  ^bb1(%arg0: i32)
///    return %foo, %arg0 : i32, i32
///  ^bb2(%arg1: i32)
///    return %arg1, %bar : i32, i32
struct BlockEquivalenceData {
  BlockEquivalenceData(Block *block);

  /// Return the order index for the given value that is within the block of
  /// this data.
  unsigned getOrderOf(Value value) const;

  /// The block this data refers to.
  Block *block;
  /// A hash value for this block.
  llvm::hash_code hash;
  /// A map of result producing operations to their relative orders within this
  /// block. The order of an operation is the number of defined values that are
  /// produced within the block before this operation.
  DenseMap<Operation *, unsigned> opOrderIndex;
};
} // end anonymous namespace

BlockEquivalenceData::BlockEquivalenceData(Block *block)
    : block(block), hash(0) {
  unsigned orderIt = block->getNumArguments();
  for (Operation &op : *block) {
    if (unsigned numResults = op.getNumResults()) {
      opOrderIndex.try_emplace(&op, orderIt);
      orderIt += numResults;
    }
    auto opHash = OperationEquivalence::computeHash(
        &op, OperationEquivalence::Flags::IgnoreOperands);
    hash = llvm::hash_combine(hash, opHash);
  }
}

unsigned BlockEquivalenceData::getOrderOf(Value value) const {
  assert(value.getParentBlock() == block && "expected value of this block");

  // Arguments use the argument number as the order index.
  if (BlockArgument arg = value.dyn_cast<BlockArgument>())
    return arg.getArgNumber();

  // Otherwise, the result order is offset from the parent op's order.
  OpResult result = value.cast<OpResult>();
  auto opOrderIt = opOrderIndex.find(result.getDefiningOp());
  assert(opOrderIt != opOrderIndex.end() && "expected op to have an order");
  return opOrderIt->second + result.getResultNumber();
}

//===----------------------------------------------------------------------===//
// BlockMergeCluster

namespace {
/// This class represents a cluster of blocks to be merged together.
class BlockMergeCluster {
public:
  BlockMergeCluster(BlockEquivalenceData &&leaderData)
      : leaderData(std::move(leaderData)) {}

  /// Attempt to add the given block to this cluster. Returns success if the
  /// block was merged, failure otherwise.
  LogicalResult addToCluster(BlockEquivalenceData &blockData);

  /// Try to merge all of the blocks within this cluster into the leader block.
  LogicalResult merge();

private:
  /// The equivalence data for the leader of the cluster.
  BlockEquivalenceData leaderData;

  /// The set of blocks that can be merged into the leader.
  llvm::SmallSetVector<Block *, 1> blocksToMerge;

  /// A set of operand+index pairs that correspond to operands that need to be
  /// replaced by arguments when the cluster gets merged.
  std::set<std::pair<int, int>> operandsToMerge;

  /// A map of operations with external uses to a replacement within the leader
  /// block.
  DenseMap<Operation *, Operation *> opsToReplace;
};
} // end anonymous namespace

LogicalResult BlockMergeCluster::addToCluster(BlockEquivalenceData &blockData) {
  if (leaderData.hash != blockData.hash)
    return failure();
  Block *leaderBlock = leaderData.block, *mergeBlock = blockData.block;
  if (leaderBlock->getArgumentTypes() != mergeBlock->getArgumentTypes())
    return failure();

  // A set of operands that mismatch between the leader and the new block.
  SmallVector<std::pair<int, int>, 8> mismatchedOperands;
  SmallVector<std::pair<Operation *, Operation *>, 2> newOpsToReplace;
  auto lhsIt = leaderBlock->begin(), lhsE = leaderBlock->end();
  auto rhsIt = blockData.block->begin(), rhsE = blockData.block->end();
  for (int opI = 0; lhsIt != lhsE && rhsIt != rhsE; ++lhsIt, ++rhsIt, ++opI) {
    // Check that the operations are equivalent.
    if (!OperationEquivalence::isEquivalentTo(
            &*lhsIt, &*rhsIt, OperationEquivalence::Flags::IgnoreOperands))
      return failure();

    // Compare the operands of the two operations. If the operand is within
    // the block, it must refer to the same operation.
    auto lhsOperands = lhsIt->getOperands(), rhsOperands = rhsIt->getOperands();
    for (int operand : llvm::seq<int>(0, lhsIt->getNumOperands())) {
      Value lhsOperand = lhsOperands[operand];
      Value rhsOperand = rhsOperands[operand];
      if (lhsOperand == rhsOperand)
        continue;
      // Check that the types of the operands match.
      if (lhsOperand.getType() != rhsOperand.getType())
        return failure();

      // Check that these uses are both external, or both internal.
      bool lhsIsInBlock = lhsOperand.getParentBlock() == leaderBlock;
      bool rhsIsInBlock = rhsOperand.getParentBlock() == mergeBlock;
      if (lhsIsInBlock != rhsIsInBlock)
        return failure();
      // Let the operands differ if they are defined in a different block. These
      // will become new arguments if the blocks get merged.
      if (!lhsIsInBlock) {
        mismatchedOperands.emplace_back(opI, operand);
        continue;
      }

      // Otherwise, these operands must have the same logical order within the
      // parent block.
      if (leaderData.getOrderOf(lhsOperand) != blockData.getOrderOf(rhsOperand))
        return failure();
    }

    // If the rhs has external uses, it will need to be replaced.
    if (rhsIt->isUsedOutsideOfBlock(mergeBlock))
      newOpsToReplace.emplace_back(&*rhsIt, &*lhsIt);
  }
  // Make sure that the block sizes are equivalent.
  if (lhsIt != lhsE || rhsIt != rhsE)
    return failure();

  // If we get here, the blocks are equivalent and can be merged.
  operandsToMerge.insert(mismatchedOperands.begin(), mismatchedOperands.end());
  opsToReplace.insert(newOpsToReplace.begin(), newOpsToReplace.end());
  blocksToMerge.insert(blockData.block);
  return success();
}

/// Returns true if the predecessor terminators of the given block can not have
/// their operands updated.
static bool ableToUpdatePredOperands(Block *block) {
  for (auto it = block->pred_begin(), e = block->pred_end(); it != e; ++it) {
    auto branch = dyn_cast<BranchOpInterface>((*it)->getTerminator());
    if (!branch || !branch.getMutableSuccessorOperands(it.getSuccessorIndex()))
      return false;
  }
  return true;
}

LogicalResult BlockMergeCluster::merge() {
  // Don't consider clusters that don't have blocks to merge.
  if (blocksToMerge.empty())
    return failure();

  Block *leaderBlock = leaderData.block;
  if (!operandsToMerge.empty()) {
    // If the cluster has operands to merge, verify that the predecessor
    // terminators of each of the blocks can have their successor operands
    // updated.
    // TODO: We could try and sub-partition this cluster if only some blocks
    // cause the mismatch.
    if (!ableToUpdatePredOperands(leaderBlock) ||
        !llvm::all_of(blocksToMerge, ableToUpdatePredOperands))
      return failure();

    // Replace any necessary operations.
    for (std::pair<Operation *, Operation *> &it : opsToReplace)
      it.first->replaceAllUsesWith(it.second);

    // Collect the iterators for each of the blocks to merge. We will walk all
    // of the iterators at once to avoid operand index invalidation.
    SmallVector<Block::iterator, 2> blockIterators;
    blockIterators.reserve(blocksToMerge.size() + 1);
    blockIterators.push_back(leaderBlock->begin());
    for (Block *mergeBlock : blocksToMerge)
      blockIterators.push_back(mergeBlock->begin());

    // Update each of the predecessor terminators with the new arguments.
    SmallVector<SmallVector<Value, 8>, 2> newArguments(
        1 + blocksToMerge.size(),
        SmallVector<Value, 8>(operandsToMerge.size()));
    unsigned curOpIndex = 0;
    for (auto it : llvm::enumerate(operandsToMerge)) {
      unsigned nextOpOffset = it.value().first - curOpIndex;
      curOpIndex = it.value().first;

      // Process the operand for each of the block iterators.
      for (unsigned i = 0, e = blockIterators.size(); i != e; ++i) {
        Block::iterator &blockIter = blockIterators[i];
        std::advance(blockIter, nextOpOffset);
        auto &operand = blockIter->getOpOperand(it.value().second);
        newArguments[i][it.index()] = operand.get();

        // Update the operand and insert an argument if this is the leader.
        if (i == 0)
          operand.set(leaderBlock->addArgument(operand.get().getType()));
      }
    }
    // Update the predecessors for each of the blocks.
    auto updatePredecessors = [&](Block *block, unsigned clusterIndex) {
      for (auto predIt = block->pred_begin(), predE = block->pred_end();
           predIt != predE; ++predIt) {
        auto branch = cast<BranchOpInterface>((*predIt)->getTerminator());
        unsigned succIndex = predIt.getSuccessorIndex();
        branch.getMutableSuccessorOperands(succIndex)->append(
            newArguments[clusterIndex]);
      }
    };
    updatePredecessors(leaderBlock, /*clusterIndex=*/0);
    for (unsigned i = 0, e = blocksToMerge.size(); i != e; ++i)
      updatePredecessors(blocksToMerge[i], /*clusterIndex=*/i + 1);
  }

  // Replace all uses of the merged blocks with the leader and erase them.
  for (Block *block : blocksToMerge) {
    block->replaceAllUsesWith(leaderBlock);
    block->erase();
  }
  return success();
}

/// Identify identical blocks within the given region and merge them, inserting
/// new block arguments as necessary. Returns success if any blocks were merged,
/// failure otherwise.
static LogicalResult mergeIdenticalBlocks(Region &region) {
  if (region.empty() || llvm::hasSingleElement(region))
    return failure();

  // Identify sets of blocks, other than the entry block, that branch to the
  // same successors. We will use these groups to create clusters of equivalent
  // blocks.
  DenseMap<SuccessorRange, SmallVector<Block *, 1>> matchingSuccessors;
  for (Block &block : llvm::drop_begin(region, 1))
    matchingSuccessors[block.getSuccessors()].push_back(&block);

  bool mergedAnyBlocks = false;
  for (ArrayRef<Block *> blocks : llvm::make_second_range(matchingSuccessors)) {
    if (blocks.size() == 1)
      continue;

    SmallVector<BlockMergeCluster, 1> clusters;
    for (Block *block : blocks) {
      BlockEquivalenceData data(block);

      // Don't allow merging if this block has any regions.
      // TODO: Add support for regions if necessary.
      bool hasNonEmptyRegion = llvm::any_of(*block, [](Operation &op) {
        return llvm::any_of(op.getRegions(),
                            [](Region &region) { return !region.empty(); });
      });
      if (hasNonEmptyRegion)
        continue;

      // Try to add this block to an existing cluster.
      bool addedToCluster = false;
      for (auto &cluster : clusters)
        if ((addedToCluster = succeeded(cluster.addToCluster(data))))
          break;
      if (!addedToCluster)
        clusters.emplace_back(std::move(data));
    }
    for (auto &cluster : clusters)
      mergedAnyBlocks |= succeeded(cluster.merge());
  }

  return success(mergedAnyBlocks);
}

/// Identify identical blocks within the given regions and merge them, inserting
/// new block arguments as necessary.
static LogicalResult mergeIdenticalBlocks(MutableArrayRef<Region> regions) {
  llvm::SmallSetVector<Region *, 1> worklist;
  for (auto &region : regions)
    worklist.insert(&region);
  bool anyChanged = false;
  while (!worklist.empty()) {
    Region *region = worklist.pop_back_val();
    if (succeeded(mergeIdenticalBlocks(*region))) {
      worklist.insert(region);
      anyChanged = true;
    }

    // Add any nested regions to the worklist.
    for (Block &block : *region)
      for (auto &op : block)
        for (auto &nestedRegion : op.getRegions())
          worklist.insert(&nestedRegion);
  }

  return success(anyChanged);
}

//===----------------------------------------------------------------------===//
// Region Simplification
//===----------------------------------------------------------------------===//

/// Run a set of structural simplifications over the given regions. This
/// includes transformations like unreachable block elimination, dead argument
/// elimination, as well as some other DCE. This function returns success if any
/// of the regions were simplified, failure otherwise.
LogicalResult mlir::simplifyRegions(MutableArrayRef<Region> regions) {
  bool eliminatedBlocks = succeeded(eraseUnreachableBlocks(regions));
  bool eliminatedOpsOrArgs = succeeded(runRegionDCE(regions));
  bool mergedIdenticalBlocks = succeeded(mergeIdenticalBlocks(regions));
  return success(eliminatedBlocks || eliminatedOpsOrArgs ||
                 mergedIdenticalBlocks);
}