EmptyTensorElimination.cpp

Go to the documentation of this file.

//===- EmptyTensorElimination.cpp - tensor.empty op elimination -----------===//
//
// 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/Dialect/Bufferization/Transforms/Passes.h"
 
#include "mlir/Dialect/Bufferization/IR/BufferizableOpInterface.h"
#include "mlir/Dialect/Bufferization/IR/Bufferization.h"
#include "mlir/Dialect/Bufferization/Transforms/OneShotAnalysis.h"
#include "mlir/Dialect/Bufferization/Transforms/OneShotModuleBufferize.h"
#include "mlir/Dialect/Bufferization/Transforms/Transforms.h"
#include "mlir/Dialect/Tensor/IR/Tensor.h"
#include "mlir/IR/Dominance.h"
#include "mlir/Interfaces/SubsetOpInterface.h"
#include "mlir/Transforms/RegionUtils.h"
 
namespace mlir {
namespace bufferization {
#define GEN_PASS_DEF_EMPTYTENSORELIMINATIONPASS
#include "mlir/Dialect/Bufferization/Transforms/Passes.h.inc"
} // namespace bufferization
} // namespace mlir
 
using namespace mlir;
using namespace mlir::bufferization;
 
/// Return true if all `neededValues` are in scope at the given
/// `insertionPoint`.
static bool
neededValuesDominateInsertionPoint(const DominanceInfo &domInfo,
                                   Operation *insertionPoint,
                                   const SmallVector<Value> &neededValues) {
  for (Value val : neededValues) {
    if (auto bbArg = dyn_cast<BlockArgument>(val)) {
      Block *owner = bbArg.getOwner();
      if (!owner->findAncestorOpInBlock(*insertionPoint))
        return false;
    } else {
      auto opResult = cast<OpResult>(val);
      if (!domInfo.properlyDominates(opResult.getOwner(), insertionPoint))
        return false;
    }
  }
  return true;
}
 
/// Find a valid insertion point for a replacement of `emptyTensorOp`'s
/// use of `user` operation, assuming that the replacement may use any
/// value from `neededValues`.
static Operation *
findValidInsertionPoint(Operation *emptyTensorOp, Operation *user,
                        const SmallVector<Value> &neededValues) {
  DominanceInfo domInfo;
  Operation *candidateInsertionPoint = emptyTensorOp;
 
  // Gather all possible insertion points: the location of
  // `candidateInsertionPoint` and right after the definition of each value in
  // `neededValues`.
  SmallVector<Operation *> insertionPointCandidates;
  insertionPointCandidates.push_back(candidateInsertionPoint);
  for (Value val : neededValues) {
    // Note: The anchor op is using all of `neededValues`, so:
    // * in case of a block argument: There must be at least one op in the block
    //                                (the anchor op or one of its parents).
    // * in case of an OpResult: There must be at least one op right after the
    //                           defining op (the anchor op or one of its
    //                           parents).
    if (auto bbArg = dyn_cast<BlockArgument>(val)) {
      insertionPointCandidates.push_back(
          &bbArg.getOwner()->getOperations().front());
    } else {
      insertionPointCandidates.push_back(val.getDefiningOp()->getNextNode());
    }
  }
 
  // Select first matching insertion point.
  for (Operation *insertionPoint : insertionPointCandidates) {
    // Check if all needed values are in scope.
    if (!neededValuesDominateInsertionPoint(domInfo, insertionPoint,
                                            neededValues))
      continue;
    // Check if the insertion point is before the use to be replaced.
    if (!domInfo.dominates(insertionPoint, user))
      continue;
    return insertionPoint;
  }
 
  // No suitable insertion point was found.
  return nullptr;
}
 
Value mlir::bufferization::buildSubsetExtraction(RewriterBase &rewriter,
                                                 SubsetInsertionOpInterface op,
                                                 tensor::EmptyOp emptyTensorOp,
                                                 Operation *user) {
 
  mlir::OpBuilder::InsertionGuard guard(rewriter);
  // All values that are needed to create the replacement op.
  SmallVector<Value> neededValues = op.getValuesNeededToBuildSubsetExtraction();
  // Find a suitable insertion point. If no suitable insertion point
  // for the replacement can be found, return an empty value to skip
  // this replacement.
  Operation *insertionPoint =
      findValidInsertionPoint(emptyTensorOp, user, neededValues);
  if (!insertionPoint) {
    // If no already suitable insertion point was found, attempt to move all
    // needed values before the user.
    if (failed(moveValueDefinitions(rewriter, neededValues, user)))
      return {};
    insertionPoint = user;
  }
 
  rewriter.setInsertionPoint(insertionPoint);
  Value replacement =
      op.buildSubsetExtraction(rewriter, emptyTensorOp->getLoc());
  return replacement;
}
 
LogicalResult mlir::bufferization::eliminateEmptyTensors(
    RewriterBase &rewriter, Operation *op, OneShotAnalysisState &state,
    ControlBuildSubsetExtractionFn subsetsExtractionFn) {
  OpBuilder::InsertionGuard g(rewriter);
  llvm::DenseSet<OpOperand *> visitedOpOperands;
  op->walk([&](SubsetInsertionOpInterface op) {
    visitedOpOperands.clear();
    OpOperand &source = op.getSourceOperand();
    // Skip operands that do not bufferize inplace. "tensor.empty" could still
    // be replaced, but the transformation may not be beneficial.
    if (!state.isInPlace(source))
      return WalkResult::skip();
 
    // Find tensor.empty ops on the reverse SSA use-def chain. Only follow
    // equivalent tensors. I.e., stop when there are ops such as extract_slice
    // on the path.
    TraversalConfig config;
    config.followEquivalentOnly = true;
    config.alwaysIncludeLeaves = false;
    // Replace only if the types match or are static <-> dynamic casts. We do
    // not support slices or reshapes.
    // TODO: This could be extended to support IR such as:
    // %0 = tensor.empty() : tensor<128xf32>
    // %1 = "some_op"(%0) : (tensor<128xf32>) -> (tensor<128xf32>)
    // %2 = tensor.expand_shape %1 ...
    // %3 = tensor.insert_slice %2 into ...
    config.followSameTypeOrCastsOnly = true;
    SetVector<Value> emptyTensors = state.findValueInReverseUseDefChain(
        &source, /*condition=*/
        [&](Value val) { return val.getDefiningOp<tensor::EmptyOp>(); }, config,
        &visitedOpOperands);
 
    for (Value v : emptyTensors) {
      auto emptyTensorOp = v.getDefiningOp<tensor::EmptyOp>();
      assert(emptyTensorOp && "expected tensor.empty op");
      // Find the use to be replaced from the use-def chain.
      auto iter = llvm::find_if(
          visitedOpOperands, [&emptyTensorOp](OpOperand *opOperand) {
            return llvm::count(emptyTensorOp->getUses(), *opOperand);
          });
 
      assert(iter != visitedOpOperands.end() && "could not find use");
      OpOperand *useToBeReplaced = *iter;
      Operation *user = useToBeReplaced->getOwner();
      auto replacement = subsetsExtractionFn(rewriter, op, emptyTensorOp, user);
      if (!replacement)
        continue;
      if (emptyTensorOp == replacement.getDefiningOp())
        continue;
      if (replacement.getType() != v.getType()) {
        if (cast<ShapedType>(replacement.getType()).getElementType() !=
            cast<ShapedType>(v.getType()).getElementType())
          continue;
        rewriter.setInsertionPointAfterValue(replacement);
        replacement = tensor::CastOp::create(rewriter, v.getLoc(), v.getType(),
                                             replacement);
      }
      // Replace the specific use of the tensor::EmptyOp.
      rewriter.modifyOpInPlace(user, [&]() {
        user->setOperand(useToBeReplaced->getOperandNumber(), replacement);
      });
      state.resetCache();
    }
 
    return WalkResult::advance();
  });
 
  return success();
}
 
namespace {
struct EmptyTensorElimination
    : public bufferization::impl::EmptyTensorEliminationPassBase<
          EmptyTensorElimination> {
  using Base::Base;
 
  void runOnOperation() override;
 
  void getDependentDialects(DialectRegistry &registry) const override {
    registry
        .insert<bufferization::BufferizationDialect, tensor::TensorDialect>();
  }
};
} // namespace
 
LogicalResult mlir::bufferization::eliminateEmptyTensors(RewriterBase &rewriter,
                                                         Operation *op) {
  auto moduleOp = dyn_cast<ModuleOp>(op);
  OneShotBufferizationOptions options;
  options.allowReturnAllocsFromLoops = true;
  if (moduleOp)
    options.bufferizeFunctionBoundaries = true;
  OneShotAnalysisState state(op, options);
  if (moduleOp) {
    // Module analysis takes into account function boundaries.
    if (failed(analyzeModuleOp(moduleOp, state)))
      return failure();
  } else {
    // Regular One-Shot Bufferize ignores func.func block arguments, func.call,
    // func.return.
    if (failed(analyzeOp(op, state)))
      return failure();
  }
 
  return bufferization::eliminateEmptyTensors(rewriter, op, state);
}
 
void EmptyTensorElimination::runOnOperation() {
  IRRewriter rewriter(getOperation()->getContext());
  if (failed(bufferization::eliminateEmptyTensors(rewriter, getOperation())))
    signalPassFailure();
}