mlir/lib/Transforms/Utils/FoldUtils.cpp

//===- FoldUtils.cpp ---- Fold 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
//
//===----------------------------------------------------------------------===//
//
// This file defines various operation fold utilities. These utilities are
// intended to be used by passes to unify and simply their logic.
//
//===----------------------------------------------------------------------===//

#include "mlir/Transforms/FoldUtils.h"

#include "mlir/IR/Builders.h"
#include "mlir/IR/Matchers.h"
#include "mlir/IR/Operation.h"
#include "llvm/Support/DebugLog.h"

using namespace mlir;

/// Given an operation, find the parent region that folded constants should be
/// inserted into.
static Region *
getInsertionRegion(DialectInterfaceCollection<DialectFoldInterface> &interfaces,
                   Block *insertionBlock) {
  while (Region *region = insertionBlock->getParent()) {
    // Insert in this region for any of the following scenarios:
    //  * The parent is unregistered, or is known to be isolated from above.
    //  * The parent is a top-level operation.
    auto *parentOp = region->getParentOp();
    if (parentOp->mightHaveTrait<OpTrait::IsIsolatedFromAbove>() ||
        !parentOp->getBlock())
      return region;

    // Otherwise, check if this region is a desired insertion region.
    auto *interface = interfaces.getInterfaceFor(parentOp);
    if (LLVM_UNLIKELY(interface && interface->shouldMaterializeInto(region)))
      return region;

    // Traverse up the parent looking for an insertion region.
    insertionBlock = parentOp->getBlock();
  }
  llvm_unreachable("expected valid insertion region");
}

/// A utility function used to materialize a constant for a given attribute and
/// type. On success, a valid constant value is returned. Otherwise, null is
/// returned
static Operation *materializeConstant(Dialect *dialect, OpBuilder &builder,
                                      Attribute value, Type type,
                                      Location loc) {
  auto insertPt = builder.getInsertionPoint();
  (void)insertPt;

  // Ask the dialect to materialize a constant operation for this value.
  if (auto *constOp = dialect->materializeConstant(builder, value, type, loc)) {
    assert(insertPt == builder.getInsertionPoint());
    assert(matchPattern(constOp, m_Constant()));
    return constOp;
  }

  return nullptr;
}

//===----------------------------------------------------------------------===//
// OperationFolder
//===----------------------------------------------------------------------===//

LogicalResult OperationFolder::tryToFold(Operation *op, bool *inPlaceUpdate,
                                         int maxIterations) {
  if (inPlaceUpdate)
    *inPlaceUpdate = false;

  // If this is a unique'd constant, return failure as we know that it has
  // already been folded.
  if (isFolderOwnedConstant(op)) {
    // Check to see if we should rehoist, i.e. if a non-constant operation was
    // inserted before this one.
    Block *opBlock = op->getBlock();
    if (&opBlock->front() != op && !isFolderOwnedConstant(op->getPrevNode())) {
      op->moveBefore(&opBlock->front());
      op->setLoc(erasedFoldedLocation);
    }
    return failure();
  }

  // Try to fold the operation.
  SmallVector<Value, 8> results;
  if (failed(tryToFold(op, results, maxIterations)))
    return failure();

  // Check to see if the operation was just updated in place.
  if (results.empty()) {
    if (inPlaceUpdate)
      *inPlaceUpdate = true;
    if (auto *rewriteListener = dyn_cast_if_present<RewriterBase::Listener>(
            rewriter.getListener())) {
      // Folding API does not notify listeners, so we have to notify manually.
      rewriteListener->notifyOperationModified(op);
    }
    return success();
  }

  // Constant folding succeeded. Replace all of the result values and erase the
  // operation.
  notifyRemoval(op);
  rewriter.replaceOp(op, results);
  return success();
}

bool OperationFolder::insertKnownConstant(Operation *op, Attribute constValue) {
  Block *opBlock = op->getBlock();

  // If this is a constant we unique'd, we don't need to insert, but we can
  // check to see if we should rehoist it.
  if (isFolderOwnedConstant(op)) {
    if (&opBlock->front() != op && !isFolderOwnedConstant(op->getPrevNode())) {
      op->moveBefore(&opBlock->front());
      op->setLoc(erasedFoldedLocation);
    }
    return true;
  }

  // Get the constant value of the op if necessary.
  if (!constValue) {
    matchPattern(op, m_Constant(&constValue));
    assert(constValue && "expected `op` to be a constant");
  } else {
    // Ensure that the provided constant was actually correct.
#ifndef NDEBUG
    Attribute expectedValue;
    matchPattern(op, m_Constant(&expectedValue));
    assert(
        expectedValue == constValue &&
        "provided constant value was not the expected value of the constant");
#endif
  }

  // Check for an existing constant operation for the attribute value.
  Region *insertRegion = getInsertionRegion(interfaces, opBlock);
  auto &uniquedConstants = foldScopes[insertRegion];
  Operation *&folderConstOp = uniquedConstants[std::make_tuple(
      op->getDialect(), constValue, *op->result_type_begin())];

  // If there is an existing constant, replace `op`.
  if (folderConstOp) {
    notifyRemoval(op);
    rewriter.replaceOp(op, folderConstOp->getResults());
    folderConstOp->setLoc(erasedFoldedLocation);
    return false;
  }

  // Otherwise, we insert `op`. If `op` is in the insertion block and is either
  // already at the front of the block, or the previous operation is already a
  // constant we unique'd (i.e. one we inserted), then we don't need to do
  // anything. Otherwise, we move the constant to the insertion block.
  // The location info is erased if the constant is moved to a different block.
  Block *insertBlock = &insertRegion->front();
  if (opBlock != insertBlock) {
    op->moveBefore(&insertBlock->front());
    op->setLoc(erasedFoldedLocation);
  } else if (&insertBlock->front() != op &&
             !isFolderOwnedConstant(op->getPrevNode())) {
    op->moveBefore(&insertBlock->front());
  }

  folderConstOp = op;
  referencedDialects[op].push_back(op->getDialect());
  return true;
}

/// Notifies that the given constant `op` should be remove from this
/// OperationFolder's internal bookkeeping.
void OperationFolder::notifyRemoval(Operation *op) {
  // Check to see if this operation is uniqued within the folder.
  auto it = referencedDialects.find(op);
  if (it == referencedDialects.end())
    return;

  // Get the constant value for this operation, this is the value that was used
  // to unique the operation internally.
  Attribute constValue;
  matchPattern(op, m_Constant(&constValue));
  assert(constValue);

  // Get the constant map that this operation was uniqued in.
  auto &uniquedConstants =
      foldScopes[getInsertionRegion(interfaces, op->getBlock())];

  // Erase all of the references to this operation.
  auto type = op->getResult(0).getType();
  for (auto *dialect : it->second)
    uniquedConstants.erase(std::make_tuple(dialect, constValue, type));
  referencedDialects.erase(it);
}

/// Clear out any constants cached inside of the folder.
void OperationFolder::clear() {
  foldScopes.clear();
  referencedDialects.clear();
}

/// Get or create a constant using the given builder. On success this returns
/// the constant operation, nullptr otherwise.
Value OperationFolder::getOrCreateConstant(Block *block, Dialect *dialect,
                                           Attribute value, Type type) {
  // Find an insertion point for the constant.
  auto *insertRegion = getInsertionRegion(interfaces, block);
  auto &entry = insertRegion->front();
  rewriter.setInsertionPointToStart(&entry);

  // Get the constant map for the insertion region of this operation.
  // Use erased location since the op is being built at the front of block.
  auto &uniquedConstants = foldScopes[insertRegion];
  Operation *constOp = tryGetOrCreateConstant(uniquedConstants, dialect, value,
                                              type, erasedFoldedLocation);
  return constOp ? constOp->getResult(0) : Value();
}

bool OperationFolder::isFolderOwnedConstant(Operation *op) const {
  return referencedDialects.count(op);
}

/// Tries to perform folding on the given `op`. If successful, populates
/// `results` with the results of the folding.
LogicalResult OperationFolder::tryToFold(Operation *op,
                                         SmallVectorImpl<Value> &results,
                                         int maxIterations) {
  SmallVector<OpFoldResult, 8> foldResults;
  if (failed(op->fold(foldResults)))
    return failure();
  int count = 1;
  do {
    LDBG() << "Folded in place #" << count
           << " times: " << OpWithFlags(op, OpPrintingFlags().skipRegions());
  } while (count++ < maxIterations && foldResults.empty() &&
           succeeded(op->fold(foldResults)));

  if (failed(processFoldResults(op, results, foldResults)))
    return failure();
  return success();
}

LogicalResult
OperationFolder::processFoldResults(Operation *op,
                                    SmallVectorImpl<Value> &results,
                                    ArrayRef<OpFoldResult> foldResults) {
  // Check to see if the operation was just updated in place.
  if (foldResults.empty())
    return success();
  assert(foldResults.size() == op->getNumResults());

  // Create a builder to insert new operations into the entry block of the
  // insertion region.
  auto *insertRegion = getInsertionRegion(interfaces, op->getBlock());
  auto &entry = insertRegion->front();
  rewriter.setInsertionPointToStart(&entry);

  // Get the constant map for the insertion region of this operation.
  auto &uniquedConstants = foldScopes[insertRegion];

  // Create the result constants and replace the results.
  auto *dialect = op->getDialect();
  for (unsigned i = 0, e = op->getNumResults(); i != e; ++i) {
    assert(!foldResults[i].isNull() && "expected valid OpFoldResult");

    // Check if the result was an SSA value.
    if (auto repl = llvm::dyn_cast_if_present<Value>(foldResults[i])) {
      results.emplace_back(repl);
      continue;
    }

    // Check to see if there is a canonicalized version of this constant.
    auto res = op->getResult(i);
    Attribute attrRepl = cast<Attribute>(foldResults[i]);
    if (auto *constOp =
            tryGetOrCreateConstant(uniquedConstants, dialect, attrRepl,
                                   res.getType(), erasedFoldedLocation)) {
      // Ensure that this constant dominates the operation we are replacing it
      // with. This may not automatically happen if the operation being folded
      // was inserted before the constant within the insertion block.
      Block *opBlock = op->getBlock();
      if (opBlock == constOp->getBlock() && &opBlock->front() != constOp)
        constOp->moveBefore(&opBlock->front());

      results.push_back(constOp->getResult(0));
      continue;
    }
    // If materialization fails, cleanup any operations generated for the
    // previous results and return failure.
    for (Operation &op : llvm::make_early_inc_range(
             llvm::make_range(entry.begin(), rewriter.getInsertionPoint()))) {
      notifyRemoval(&op);
      rewriter.eraseOp(&op);
    }

    results.clear();
    return failure();
  }

  return success();
}

/// Try to get or create a new constant entry. On success this returns the
/// constant operation value, nullptr otherwise.
Operation *
OperationFolder::tryGetOrCreateConstant(ConstantMap &uniquedConstants,
                                        Dialect *dialect, Attribute value,
                                        Type type, Location loc) {
  // Check if an existing mapping already exists.
  auto constKey = std::make_tuple(dialect, value, type);
  Operation *&constOp = uniquedConstants[constKey];
  if (constOp) {
    if (loc != constOp->getLoc())
      constOp->setLoc(erasedFoldedLocation);
    return constOp;
  }

  // If one doesn't exist, try to materialize one.
  if (!(constOp = materializeConstant(dialect, rewriter, value, type, loc)))
    return nullptr;

  // Check to see if the generated constant is in the expected dialect.
  auto *newDialect = constOp->getDialect();
  if (newDialect == dialect) {
    referencedDialects[constOp].push_back(dialect);
    return constOp;
  }

  // If it isn't, then we also need to make sure that the mapping for the new
  // dialect is valid.
  auto newKey = std::make_tuple(newDialect, value, type);

  // If an existing operation in the new dialect already exists, delete the
  // materialized operation in favor of the existing one.
  if (auto *existingOp = uniquedConstants.lookup(newKey)) {
    notifyRemoval(constOp);
    rewriter.eraseOp(constOp);
    referencedDialects[existingOp].push_back(dialect);
    if (loc != existingOp->getLoc())
      existingOp->setLoc(erasedFoldedLocation);
    return constOp = existingOp;
  }

  // Otherwise, update the new dialect to the materialized operation.
  referencedDialects[constOp].assign({dialect, newDialect});
  auto newIt = uniquedConstants.insert({newKey, constOp});
  return newIt.first->second;
}