BufferOptimizations.cpp

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//===- BufferOptimizations.cpp - pre-pass optimizations for bufferization -===//
//
// 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 logic for three optimization passes. The first two
// passes try to move alloc nodes out of blocks to reduce the number of
// allocations and copies during buffer deallocation. The third pass tries to
// convert heap-based allocations to stack-based allocations, if possible.
 
#include "mlir/Dialect/Bufferization/Transforms/Passes.h"
 
#include "mlir/Dialect/Bufferization/IR/AllocationOpInterface.h"
#include "mlir/Dialect/Bufferization/Transforms/BufferUtils.h"
#include "mlir/Dialect/Bufferization/Transforms/Transforms.h"
#include "mlir/Dialect/Func/IR/FuncOps.h"
#include "mlir/Dialect/MemRef/IR/MemRef.h"
#include "mlir/IR/Operation.h"
#include "mlir/Interfaces/LoopLikeInterface.h"
#include "mlir/Pass/Pass.h"
 
namespace mlir {
namespace bufferization {
#define GEN_PASS_DEF_BUFFERHOISTINGPASS
#define GEN_PASS_DEF_BUFFERLOOPHOISTINGPASS
#define GEN_PASS_DEF_PROMOTEBUFFERSTOSTACKPASS
#include "mlir/Dialect/Bufferization/Transforms/Passes.h.inc"
} // namespace bufferization
} // namespace mlir
 
using namespace mlir;
using namespace mlir::bufferization;
 
/// Returns true if the given operation implements a known high-level region-
/// based control-flow interface.
static bool isKnownControlFlowInterface(Operation *op) {
  return isa<LoopLikeOpInterface, RegionBranchOpInterface>(op);
}
 
/// Returns true if the given operation represents a loop by testing whether it
/// implements the `LoopLikeOpInterface` or the `RegionBranchOpInterface`. In
/// the case of a `RegionBranchOpInterface`, it checks all region-based control-
/// flow edges for cycles.
static bool isLoop(Operation *op) {
  // If the operation implements the `LoopLikeOpInterface` it can be considered
  // a loop.
  if (isa<LoopLikeOpInterface>(op))
    return true;
 
  // If the operation does not implement the `RegionBranchOpInterface`, it is
  // (currently) not possible to detect a loop.
  auto regionInterface = dyn_cast<RegionBranchOpInterface>(op);
  if (!regionInterface)
    return false;
 
  return regionInterface.hasLoop();
}
 
/// Return whether the given operation is a loop with sequential execution
/// semantics.
static bool isSequentialLoop(Operation *op) {
  return !op->hasTrait<OpTrait::HasParallelRegion>() && isLoop(op);
}
 
/// Returns true if the given operation implements the AllocationOpInterface
/// and it supports the dominate block hoisting.
static bool allowAllocDominateBlockHoisting(Operation *op) {
  auto allocOp = dyn_cast<AllocationOpInterface>(op);
  return allocOp &&
         static_cast<uint8_t>(allocOp.getHoistingKind() & HoistingKind::Block);
}
 
/// Returns true if the given operation implements the AllocationOpInterface
/// and it supports the loop hoisting.
static bool allowAllocLoopHoisting(Operation *op) {
  auto allocOp = dyn_cast<AllocationOpInterface>(op);
  return allocOp &&
         static_cast<uint8_t>(allocOp.getHoistingKind() & HoistingKind::Loop);
}
 
/// Check if the size of the allocation is less than the given size. The
/// transformation is only applied to small buffers since large buffers could
/// exceed the stack space.
static bool defaultIsSmallAlloc(Value alloc, unsigned maximumSizeInBytes,
                                unsigned maxRankOfAllocatedMemRef) {
  auto type = dyn_cast<ShapedType>(alloc.getType());
  if (!type || !alloc.getDefiningOp<memref::AllocOp>())
    return false;
  if (!type.hasStaticShape()) {
    // Check if the dynamic shape dimension of the alloc is produced by
    // `memref.rank`. If this is the case, it is likely to be small.
    // Furthermore, the dimension is limited to the maximum rank of the
    // allocated memref to avoid large values by multiplying several small
    // values.
    if (type.getRank() <= maxRankOfAllocatedMemRef) {
      return llvm::all_of(alloc.getDefiningOp()->getOperands(),
                          [&](Value operand) {
                            return operand.getDefiningOp<memref::RankOp>();
                          });
    }
    return false;
  }
  unsigned bitwidth = mlir::DataLayout::closest(alloc.getDefiningOp())
                          .getTypeSizeInBits(type.getElementType());
  // Use tryGetNumElements to avoid an assertion on integer overflow (e.g. for
  // very large statically-shaped memrefs).  If the element count overflows
  // int64_t the allocation is certainly not "small", so return false.
  std::optional<int64_t> numElements = type.tryGetNumElements();
  if (!numElements)
    return false;
  // Guard against overflow in the size computation as well.
  if (bitwidth != 0 &&
      *numElements > static_cast<int64_t>(maximumSizeInBytes * 8ULL / bitwidth))
    return false;
  return *numElements * bitwidth <= maximumSizeInBytes * 8;
}
 
/// Checks whether the given aliases leave the allocation scope.
static bool
leavesAllocationScope(Region *parentRegion,
                      const BufferViewFlowAnalysis::ValueSetT &aliases) {
  for (Value alias : aliases) {
    for (auto *use : alias.getUsers()) {
      // If there is at least one alias that leaves the parent region, we know
      // that this alias escapes the whole region and hence the associated
      // allocation leaves allocation scope.
      if (isa<RegionBranchTerminatorOpInterface>(use) &&
          use->getParentRegion() == parentRegion)
        return true;
    }
  }
  return false;
}
 
/// Checks, if an automated allocation scope for a given alloc value exists.
static bool hasAllocationScope(Value alloc,
                               const BufferViewFlowAnalysis &aliasAnalysis) {
  Region *region = alloc.getParentRegion();
  do {
    if (Operation *parentOp = region->getParentOp()) {
      // Check if the operation is an automatic allocation scope and whether an
      // alias leaves the scope. This means, an allocation yields out of
      // this scope and can not be transformed in a stack-based allocation.
      if (parentOp->hasTrait<OpTrait::AutomaticAllocationScope>() &&
          !leavesAllocationScope(region, aliasAnalysis.resolve(alloc)))
        return true;
      // Check if the operation is a known control flow interface and break the
      // loop to avoid transformation in loops. Furthermore skip transformation
      // if the operation does not implement a RegionBeanchOpInterface.
      if (isLoop(parentOp) || !isKnownControlFlowInterface(parentOp))
        break;
    }
  } while ((region = region->getParentRegion()));
  return false;
}
 
namespace {
 
//===----------------------------------------------------------------------===//
// BufferAllocationHoisting
//===----------------------------------------------------------------------===//
 
/// A base implementation compatible with the `BufferAllocationHoisting` class.
struct BufferAllocationHoistingStateBase {
  /// A pointer to the current dominance info.
  DominanceInfo *dominators;
 
  /// The current allocation value.
  Value allocValue;
 
  /// The current placement block (if any).
  Block *placementBlock;
 
  /// Initializes the state base.
  BufferAllocationHoistingStateBase(DominanceInfo *dominators, Value allocValue,
                                    Block *placementBlock)
      : dominators(dominators), allocValue(allocValue),
        placementBlock(placementBlock) {}
};
 
/// Implements the actual hoisting logic for allocation nodes.
template <typename StateT>
class BufferAllocationHoisting : public BufferPlacementTransformationBase {
public:
  BufferAllocationHoisting(Operation *op)
      : BufferPlacementTransformationBase(op), dominators(op),
        postDominators(op), scopeOp(op) {}
 
  /// Moves allocations upwards.
  void hoist() {
    SmallVector<Value> allocsAndAllocas;
    for (BufferPlacementAllocs::AllocEntry &entry : allocs)
      allocsAndAllocas.push_back(std::get<0>(entry));
    scopeOp->walk([&](memref::AllocaOp op) {
      allocsAndAllocas.push_back(op.getMemref());
    });
 
    for (auto allocValue : allocsAndAllocas) {
      if (!StateT::shouldHoistOpType(allocValue.getDefiningOp()))
        continue;
      Operation *definingOp = allocValue.getDefiningOp();
      assert(definingOp && "No defining op");
      // Skip allocations in blocks that are not reachable from the function
      // entry. Such blocks are dead code and the dominator tree analysis may
      // not have nodes for them, which would cause crashes below.
      if (!dominators.isReachableFromEntry(allocValue.getParentBlock()))
        continue;
      auto operands = definingOp->getOperands();
      auto resultAliases = aliases.resolve(allocValue);
      // Determine the common dominator block of all aliases.
      Block *dominatorBlock =
          findCommonDominator(allocValue, resultAliases, dominators);
      // Init the initial hoisting state.
      StateT state(&dominators, allocValue, allocValue.getParentBlock());
      // Check for additional allocation dependencies to compute an upper bound
      // for hoisting.
      Block *dependencyBlock = nullptr;
      // If this node has dependencies, check all dependent nodes. This ensures
      // that all dependency values have been computed before allocating the
      // buffer.
      for (Value depValue : operands) {
        Block *depBlock = depValue.getParentBlock();
        if (!dependencyBlock || dominators.dominates(dependencyBlock, depBlock))
          dependencyBlock = depBlock;
      }
 
      // Find the actual placement block and determine the start operation using
      // an upper placement-block boundary. The idea is that placement block
      // cannot be moved any further upwards than the given upper bound.
      Block *placementBlock = findPlacementBlock(
          state, state.computeUpperBound(dominatorBlock, dependencyBlock));
      Operation *startOperation = BufferPlacementAllocs::getStartOperation(
          allocValue, placementBlock, liveness);
 
      // Move the alloc in front of the start operation.
      Operation *allocOperation = allocValue.getDefiningOp();
      allocOperation->moveBefore(startOperation);
    }
  }
 
private:
  /// Finds a valid placement block by walking upwards in the CFG until we
  /// either cannot continue our walk due to constraints (given by the StateT
  /// implementation) or we have reached the upper-most dominator block.
  Block *findPlacementBlock(StateT &state, Block *upperBound) {
    Block *currentBlock = state.placementBlock;
    // Walk from the innermost regions/loops to the outermost regions/loops and
    // find an appropriate placement block that satisfies the constraint of the
    // current StateT implementation. Walk until we reach the upperBound block
    // (if any).
 
    // If we are not able to find a valid parent operation or an associated
    // parent block, break the walk loop.
    Operation *parentOp;
    Block *parentBlock;
    while ((parentOp = currentBlock->getParentOp()) &&
           (parentBlock = parentOp->getBlock()) &&
           (!upperBound ||
            dominators.properlyDominates(upperBound, currentBlock))) {
      // Try to find an immediate dominator and check whether the parent block
      // is above the immediate dominator (if any).
      DominanceInfoNode *idom = nullptr;
 
      // DominanceInfo doesn't support getNode queries for single-block regions.
      if (!currentBlock->isEntryBlock())
        idom = dominators.getNode(currentBlock)->getIDom();
 
      if (idom && dominators.properlyDominates(parentBlock, idom->getBlock())) {
        // If the current immediate dominator is below the placement block, move
        // to the immediate dominator block.
        currentBlock = idom->getBlock();
        state.recordMoveToDominator(currentBlock);
      } else {
        // We have to move to our parent block since an immediate dominator does
        // either not exist or is above our parent block. If we cannot move to
        // our parent operation due to constraints given by the StateT
        // implementation, break the walk loop. Furthermore, we should not move
        // allocations out of unknown region-based control-flow operations.
        if (!isKnownControlFlowInterface(parentOp) ||
            !state.isLegalPlacement(parentOp))
          break;
        // Move to our parent block by notifying the current StateT
        // implementation.
        currentBlock = parentBlock;
        state.recordMoveToParent(currentBlock);
      }
    }
    // Return the finally determined placement block.
    return state.placementBlock;
  }
 
  /// The dominator info to find the appropriate start operation to move the
  /// allocs.
  DominanceInfo dominators;
 
  /// The post dominator info to move the dependent allocs in the right
  /// position.
  PostDominanceInfo postDominators;
 
  /// The map storing the final placement blocks of a given alloc value.
  llvm::DenseMap<Value, Block *> placementBlocks;
 
  /// The operation that this transformation is working on. It is used to also
  /// gather allocas.
  Operation *scopeOp;
};
 
/// A state implementation compatible with the `BufferAllocationHoisting` class
/// that hoists allocations into dominator blocks while keeping them inside of
/// loops.
struct BufferAllocationHoistingState : BufferAllocationHoistingStateBase {
  using BufferAllocationHoistingStateBase::BufferAllocationHoistingStateBase;
 
  /// Computes the upper bound for the placement block search.
  Block *computeUpperBound(Block *dominatorBlock, Block *dependencyBlock) {
    // If we do not have a dependency block, the upper bound is given by the
    // dominator block.
    if (!dependencyBlock)
      return dominatorBlock;
 
    // Find the "lower" block of the dominator and the dependency block to
    // ensure that we do not move allocations above this block.
    return dominators->properlyDominates(dominatorBlock, dependencyBlock)
               ? dependencyBlock
               : dominatorBlock;
  }
 
  /// Returns true if the given operation does not represent a loop.
  bool isLegalPlacement(Operation *op) { return !isLoop(op); }
 
  /// Returns true if the given operation should be considered for hoisting.
  static bool shouldHoistOpType(Operation *op) {
    return allowAllocDominateBlockHoisting(op);
  }
 
  /// Sets the current placement block to the given block.
  void recordMoveToDominator(Block *block) { placementBlock = block; }
 
  /// Sets the current placement block to the given block.
  void recordMoveToParent(Block *block) { recordMoveToDominator(block); }
};
 
/// A state implementation compatible with the `BufferAllocationHoisting` class
/// that hoists allocations out of loops.
struct BufferAllocationLoopHoistingState : BufferAllocationHoistingStateBase {
  using BufferAllocationHoistingStateBase::BufferAllocationHoistingStateBase;
 
  /// Remembers the dominator block of all aliases.
  Block *aliasDominatorBlock = nullptr;
 
  /// Computes the upper bound for the placement block search.
  Block *computeUpperBound(Block *dominatorBlock, Block *dependencyBlock) {
    aliasDominatorBlock = dominatorBlock;
    // If there is a dependency block, we have to use this block as an upper
    // bound to satisfy all allocation value dependencies.
    return dependencyBlock ? dependencyBlock : nullptr;
  }
 
  /// Returns true if the given operation represents a loop with sequential
  /// execution semantics and one of the aliases caused the
  /// `aliasDominatorBlock` to be "above" the block of the given loop operation.
  /// If this is the case, it indicates that the allocation is passed via a back
  /// edge.
  bool isLegalPlacement(Operation *op) {
    return isSequentialLoop(op) &&
           !dominators->dominates(aliasDominatorBlock, op->getBlock());
  }
 
  /// Returns true if the given operation should be considered for hoisting.
  static bool shouldHoistOpType(Operation *op) {
    return allowAllocLoopHoisting(op);
  }
 
  /// Does not change the internal placement block, as we want to move
  /// operations out of loops only.
  void recordMoveToDominator(Block *block) {}
 
  /// Sets the current placement block to the given block.
  void recordMoveToParent(Block *block) { placementBlock = block; }
};
 
//===----------------------------------------------------------------------===//
// BufferPlacementPromotion
//===----------------------------------------------------------------------===//
 
/// Promotes heap-based allocations to stack-based allocations (if possible).
class BufferPlacementPromotion : BufferPlacementTransformationBase {
public:
  BufferPlacementPromotion(Operation *op)
      : BufferPlacementTransformationBase(op) {}
 
  /// Promote buffers to stack-based allocations.
  void promote(function_ref<bool(Value)> isSmallAlloc) {
    for (BufferPlacementAllocs::AllocEntry &entry : allocs) {
      Value alloc = std::get<0>(entry);
      Operation *dealloc = std::get<1>(entry);
      // Checking several requirements to transform an AllocOp into an AllocaOp.
      // The transformation is done if the allocation is limited to a given
      // size. Furthermore, a deallocation must not be defined for this
      // allocation entry and a parent allocation scope must exist.
      if (!isSmallAlloc(alloc) || dealloc ||
          !hasAllocationScope(alloc, aliases))
        continue;
 
      Operation *startOperation = BufferPlacementAllocs::getStartOperation(
          alloc, alloc.getParentBlock(), liveness);
      // Build a new alloca that is associated with its parent
      // `AutomaticAllocationScope` determined during the initialization phase.
      OpBuilder builder(startOperation);
      Operation *allocOp = alloc.getDefiningOp();
      if (auto allocInterface = dyn_cast<AllocationOpInterface>(allocOp)) {
        std::optional<Operation *> alloca =
            allocInterface.buildPromotedAlloc(builder, alloc);
        if (!alloca)
          continue;
        // Replace the original alloc by a newly created alloca.
        allocOp->replaceAllUsesWith(alloca.value());
        allocOp->erase();
      }
    }
  }
};
 
//===----------------------------------------------------------------------===//
// BufferOptimizationPasses
//===----------------------------------------------------------------------===//
 
/// The buffer hoisting pass that hoists allocation nodes into dominating
/// blocks.
struct BufferHoistingPass
    : public bufferization::impl::BufferHoistingPassBase<BufferHoistingPass> {
 
  void runOnOperation() override {
    // Hoist all allocations into dominator blocks.
    BufferAllocationHoisting<BufferAllocationHoistingState> optimizer(
        getOperation());
    optimizer.hoist();
  }
};
 
/// The buffer loop hoisting pass that hoists allocation nodes out of loops.
struct BufferLoopHoistingPass
    : public bufferization::impl::BufferLoopHoistingPassBase<
          BufferLoopHoistingPass> {
 
  void runOnOperation() override {
    // Hoist all allocations out of loops.
    hoistBuffersFromLoops(getOperation());
  }
};
 
/// The promote buffer to stack pass that tries to convert alloc nodes into
/// alloca nodes.
class PromoteBuffersToStackPass
    : public bufferization::impl::PromoteBuffersToStackPassBase<
          PromoteBuffersToStackPass> {
  using Base::Base;
 
public:
  explicit PromoteBuffersToStackPass(std::function<bool(Value)> isSmallAlloc)
      : isSmallAlloc(std::move(isSmallAlloc)) {}
 
  LogicalResult initialize(MLIRContext *context) override {
    if (isSmallAlloc == nullptr) {
      isSmallAlloc = [=](Value alloc) {
        return defaultIsSmallAlloc(alloc, maxAllocSizeInBytes,
                                   maxRankOfAllocatedMemRef);
      };
    }
    return success();
  }
 
  void runOnOperation() override {
    // Move all allocation nodes and convert candidates into allocas.
    BufferPlacementPromotion optimizer(getOperation());
    optimizer.promote(isSmallAlloc);
  }
 
private:
  std::function<bool(Value)> isSmallAlloc;
};
 
} // namespace
 
void mlir::bufferization::hoistBuffersFromLoops(Operation *op) {
  BufferAllocationHoisting<BufferAllocationLoopHoistingState> optimizer(op);
  optimizer.hoist();
}
 
std::unique_ptr<Pass> mlir::bufferization::createPromoteBuffersToStackPass(
    std::function<bool(Value)> isSmallAlloc) {
  return std::make_unique<PromoteBuffersToStackPass>(std::move(isSmallAlloc));
}