llvm-project/mlir/lib/Dialect/Affine/Transforms/AffineScalarReplacement.cpp

//===- AffineScalarReplacement.cpp - Affine scalar replacement pass -------===//
//
// 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 a pass to forward affine memref stores to loads, thereby
// potentially getting rid of intermediate memrefs entirely. It also removes
// redundant loads.
// TODO: In the future, similar techniques could be used to eliminate
// dead memref store's and perform more complex forwarding when support for
// SSA scalars live out of 'affine.for'/'affine.if' statements is available.
//===----------------------------------------------------------------------===//

#include "PassDetail.h"
#include "mlir/Analysis/AffineAnalysis.h"
#include "mlir/Analysis/Utils.h"
#include "mlir/Dialect/Affine/IR/AffineOps.h"
#include "mlir/Dialect/Affine/Passes.h"
#include "mlir/Dialect/MemRef/IR/MemRef.h"
#include "mlir/Dialect/StandardOps/IR/Ops.h"
#include "mlir/IR/Dominance.h"
#include "mlir/Support/LogicalResult.h"
#include "llvm/ADT/SmallPtrSet.h"
#include <algorithm>

#define DEBUG_TYPE "memref-dataflow-opt"

using namespace mlir;

namespace {
// The store to load forwarding and load CSE rely on three conditions:
//
// 1) store/load providing a replacement value and load being replaced need to
// have mathematically equivalent affine access functions (checked after full
// composition of load/store operands); this implies that they access the same
// single memref element for all iterations of the common surrounding loop,
//
// 2) the store/load op should dominate the load op,
//
// 3) no operation that may write to memory read by the load being replaced can
// occur after executing the instruction (load or store) providing the
// replacement value and before the load being replaced (thus potentially
// allowing overwriting the memory read by the load).
//
// The above conditions are simple to check, sufficient, and powerful for most
// cases in practice - they are sufficient, but not necessary --- since they
// don't reason about loops that are guaranteed to execute at least once or
// multiple sources to forward from.
//
// TODO: more forwarding can be done when support for
// loop/conditional live-out SSA values is available.
// TODO: do general dead store elimination for memref's. This pass
// currently only eliminates the stores only if no other loads/uses (other
// than dealloc) remain.
//
struct AffineScalarReplacement
    : public AffineScalarReplacementBase<AffineScalarReplacement> {
  void runOnFunction() override;

  LogicalResult forwardStoreToLoad(AffineReadOpInterface loadOp,
                                   SmallVectorImpl<Operation *> &loadOpsToErase,
                                   SmallPtrSetImpl<Value> &memrefsToErase,
                                   DominanceInfo &domInfo);

  void loadCSE(AffineReadOpInterface loadOp,
               SmallVectorImpl<Operation *> &loadOpsToErase,
               DominanceInfo &domInfo);

  void findUnusedStore(AffineWriteOpInterface storeOp,
                       SmallVectorImpl<Operation *> &storeOpsToErase,
                       SmallPtrSetImpl<Value> &memrefsToErase,
                       PostDominanceInfo &postDominanceInfo);
};

} // end anonymous namespace

/// Creates a pass to perform optimizations relying on memref dataflow such as
/// store to load forwarding, elimination of dead stores, and dead allocs.
std::unique_ptr<OperationPass<FuncOp>>
mlir::createAffineScalarReplacementPass() {
  return std::make_unique<AffineScalarReplacement>();
}

/// Ensure that all operations that could be executed after `start`
/// (noninclusive) and prior to `memOp` (e.g. on a control flow/op path
/// between the operations) do not have the potential memory effect
/// `EffectType` on `memOp`. `memOp`  is an operation that reads or writes to
/// a memref. For example, if `EffectType` is MemoryEffects::Write, this method
/// will check if there is no write to the memory between `start` and `memOp`
/// that would change the read within `memOp`.
template <typename EffectType, typename T>
bool hasNoInterveningEffect(Operation *start, T memOp) {

  Value memref = memOp.getMemRef();
  bool isOriginalAllocation = memref.getDefiningOp<memref::AllocaOp>() ||
                              memref.getDefiningOp<memref::AllocOp>();

  // A boolean representing whether an intervening operation could have impacted
  // memOp.
  bool hasSideEffect = false;

  // Check whether the effect on memOp can be caused by a given operation op.
  std::function<void(Operation *)> checkOperation = [&](Operation *op) {
    // If the effect has alreay been found, early exit,
    if (hasSideEffect)
      return;

    if (auto memEffect = dyn_cast<MemoryEffectOpInterface>(op)) {
      SmallVector<MemoryEffects::EffectInstance, 1> effects;
      memEffect.getEffects(effects);

      bool opMayHaveEffect = false;
      for (auto effect : effects) {
        // If op causes EffectType on a potentially aliasing location for
        // memOp, mark as having the effect.
        if (isa<EffectType>(effect.getEffect())) {
          if (isOriginalAllocation && effect.getValue() &&
              (effect.getValue().getDefiningOp<memref::AllocaOp>() ||
               effect.getValue().getDefiningOp<memref::AllocOp>())) {
            if (effect.getValue() != memref)
              continue;
          }
          opMayHaveEffect = true;
          break;
        }
      }

      if (!opMayHaveEffect)
        return;

      // If the side effect comes from an affine read or write, try to
      // prove the side effecting `op` cannot reach `memOp`.
      if (isa<AffineReadOpInterface, AffineWriteOpInterface>(op)) {
        MemRefAccess srcAccess(op);
        MemRefAccess destAccess(memOp);
        // Dependence analysis is only correct if both ops operate on the same
        // memref.
        if (srcAccess.memref == destAccess.memref) {
          FlatAffineValueConstraints dependenceConstraints;

          // Number of loops containing the start op and the ending operation.
          unsigned minSurroundingLoops =
              getNumCommonSurroundingLoops(*start, *memOp);

          // Number of loops containing the operation `op` which has the
          // potential memory side effect and can occur on a path between
          // `start` and `memOp`.
          unsigned nsLoops = getNumCommonSurroundingLoops(*op, *memOp);

          // For ease, let's consider the case that `op` is a store and we're
          // looking for other potential stores (e.g `op`) that overwrite memory
          // after `start`, and before being read in `memOp`. In this case, we
          // only need to consider other potential stores with depth >
          // minSurrounding loops since `start` would overwrite any store with a
          // smaller number of surrounding loops before.
          unsigned d;
          for (d = nsLoops + 1; d > minSurroundingLoops; d--) {
            DependenceResult result = checkMemrefAccessDependence(
                srcAccess, destAccess, d, &dependenceConstraints,
                /*dependenceComponents=*/nullptr);
            if (hasDependence(result)) {
              hasSideEffect = true;
              return;
            }
          }

          // No side effect was seen, simply return.
          return;
        }
      }
      hasSideEffect = true;
      return;
    }

    if (op->hasTrait<OpTrait::HasRecursiveSideEffects>()) {
      // Recurse into the regions for this op and check whether the internal
      // operations may have the side effect `EffectType` on memOp.
      for (Region &region : op->getRegions())
        for (Block &block : region)
          for (Operation &op : block)
            checkOperation(&op);
      return;
    }

    // Otherwise, conservatively assume generic operations have the effect
    // on the operation
    hasSideEffect = true;
    return;
  };

  // Check all paths from ancestor op `parent` to the operation `to` for the
  // effect. It is known that `to` must be contained within `parent`.
  auto until = [&](Operation *parent, Operation *to) {
    // TODO check only the paths from `parent` to `to`.
    // Currently we fallback and check the entire parent op, rather than
    // just the paths from the parent path, stopping after reaching `to`.
    // This is conservatively correct, but could be made more aggressive.
    assert(parent->isAncestor(to));
    checkOperation(parent);
  };

  // Check for all paths from operation `from` to operation `untilOp` for the
  // given memory effect.
  std::function<void(Operation *, Operation *)> recur =
      [&](Operation *from, Operation *untilOp) {
        assert(
            from->getParentRegion()->isAncestor(untilOp->getParentRegion()) &&
            "Checking for side effect between two operations without a common "
            "ancestor");

        // If the operations are in different regions, recursively consider all
        // path from `from` to the parent of `to` and all paths from the parent
        // of `to` to `to`.
        if (from->getParentRegion() != untilOp->getParentRegion()) {
          recur(from, untilOp->getParentOp());
          until(untilOp->getParentOp(), untilOp);
          return;
        }

        // Now, assuming that `from` and `to` exist in the same region, perform
        // a CFG traversal to check all the relevant operations.

        // Additional blocks to consider.
        SmallVector<Block *, 2> todoBlocks;
        {
          // First consider the parent block of `from` an check all operations
          // after `from`.
          for (auto iter = ++from->getIterator(), end = from->getBlock()->end();
               iter != end && &*iter != untilOp; ++iter) {
            checkOperation(&*iter);
          }

          // If the parent of `from` doesn't contain `to`, add the successors
          // to the list of blocks to check.
          if (untilOp->getBlock() != from->getBlock())
            for (Block *succ : from->getBlock()->getSuccessors())
              todoBlocks.push_back(succ);
        }

        SmallPtrSet<Block *, 4> done;
        // Traverse the CFG until hitting `to`.
        while (todoBlocks.size()) {
          Block *blk = todoBlocks.pop_back_val();
          if (done.count(blk))
            continue;
          done.insert(blk);
          for (auto &op : *blk) {
            if (&op == untilOp)
              break;
            checkOperation(&op);
            if (&op == blk->getTerminator())
              for (Block *succ : blk->getSuccessors())
                todoBlocks.push_back(succ);
          }
        }
      };
  recur(start, memOp);
  return !hasSideEffect;
}

// This attempts to find stores which have no impact on the final result.
// A writing op writeA will be eliminated if there exists an op writeB if
// 1) writeA and writeB have mathematically equivalent affine access functions.
// 2) writeB postdominates writeA.
// 3) There is no potential read between writeA and writeB.
void AffineScalarReplacement::findUnusedStore(
    AffineWriteOpInterface writeA, SmallVectorImpl<Operation *> &opsToErase,
    SmallPtrSetImpl<Value> &memrefsToErase,
    PostDominanceInfo &postDominanceInfo) {

  for (Operation *user : writeA.getMemRef().getUsers()) {
    // Only consider writing operations.
    auto writeB = dyn_cast<AffineWriteOpInterface>(user);
    if (!writeB)
      continue;

    // The operations must be distinct.
    if (writeB == writeA)
      continue;

    // Both operations must lie in the same region.
    if (writeB->getParentRegion() != writeA->getParentRegion())
      continue;

    // Both operations must write to the same memory.
    MemRefAccess srcAccess(writeB);
    MemRefAccess destAccess(writeA);

    if (srcAccess != destAccess)
      continue;

    // writeB must postdominate writeA.
    if (!postDominanceInfo.postDominates(writeB, writeA))
      continue;

    // There cannot be an operation which reads from memory between
    // the two writes.
    if (!hasNoInterveningEffect<MemoryEffects::Read>(writeA, writeB))
      continue;

    opsToErase.push_back(writeA);
    break;
  }
}

/// Attempt to eliminate loadOp by replacing it with a value stored into memory
/// which the load is guaranteed to retrieve. This check involves three
/// components: 1) The store and load must be on the same location 2) The store
/// must dominate (and therefore must always occur prior to) the load 3) No
/// other operations will overwrite the memory loaded between the given load
/// and store.  If such a value exists, the replaced `loadOp` will be added to
/// `loadOpsToErase` and its memref will be added to `memrefsToErase`.
LogicalResult AffineScalarReplacement::forwardStoreToLoad(
    AffineReadOpInterface loadOp, SmallVectorImpl<Operation *> &loadOpsToErase,
    SmallPtrSetImpl<Value> &memrefsToErase, DominanceInfo &domInfo) {

  // The store op candidate for forwarding that satisfies all conditions
  // to replace the load, if any.
  Operation *lastWriteStoreOp = nullptr;

  for (auto *user : loadOp.getMemRef().getUsers()) {
    auto storeOp = dyn_cast<AffineWriteOpInterface>(user);
    if (!storeOp)
      continue;
    MemRefAccess srcAccess(storeOp);
    MemRefAccess destAccess(loadOp);

    // 1. Check if the store and the load have mathematically equivalent
    // affine access functions; this implies that they statically refer to the
    // same single memref element. As an example this filters out cases like:
    //     store %A[%i0 + 1]
    //     load %A[%i0]
    //     store %A[%M]
    //     load %A[%N]
    // Use the AffineValueMap difference based memref access equality checking.
    if (srcAccess != destAccess)
      continue;

    // 2. The store has to dominate the load op to be candidate.
    if (!domInfo.dominates(storeOp, loadOp))
      continue;

    // 3. Ensure there is no intermediate operation which could replace the
    // value in memory.
    if (!hasNoInterveningEffect<MemoryEffects::Write>(storeOp, loadOp))
      continue;

    // We now have a candidate for forwarding.
    assert(lastWriteStoreOp == nullptr &&
           "multiple simulataneous replacement stores");
    lastWriteStoreOp = storeOp;
  }

  if (!lastWriteStoreOp)
    return failure();

  // Perform the actual store to load forwarding.
  Value storeVal =
      cast<AffineWriteOpInterface>(lastWriteStoreOp).getValueToStore();
  // Check if 2 values have the same shape. This is needed for affine vector
  // loads and stores.
  if (storeVal.getType() != loadOp.getValue().getType())
    return failure();
  loadOp.getValue().replaceAllUsesWith(storeVal);
  // Record the memref for a later sweep to optimize away.
  memrefsToErase.insert(loadOp.getMemRef());
  // Record this to erase later.
  loadOpsToErase.push_back(loadOp);
  return success();
}

// The load to load forwarding / redundant load elimination is similar to the
// store to load forwarding.
// loadA will be be replaced with loadB if:
// 1) loadA and loadB have mathematically equivalent affine access functions.
// 2) loadB dominates loadA.
// 3) There is no write between loadA and loadB.
void AffineScalarReplacement::loadCSE(
    AffineReadOpInterface loadA, SmallVectorImpl<Operation *> &loadOpsToErase,
    DominanceInfo &domInfo) {
  SmallVector<AffineReadOpInterface, 4> loadCandidates;
  for (auto *user : loadA.getMemRef().getUsers()) {
    auto loadB = dyn_cast<AffineReadOpInterface>(user);
    if (!loadB || loadB == loadA)
      continue;

    MemRefAccess srcAccess(loadB);
    MemRefAccess destAccess(loadA);

    // 1. The accesses have to be to the same location.
    if (srcAccess != destAccess) {
      continue;
    }

    // 2. The store has to dominate the load op to be candidate.
    if (!domInfo.dominates(loadB, loadA))
      continue;

    // 3. There is no write between loadA and loadB.
    if (!hasNoInterveningEffect<MemoryEffects::Write>(loadB.getOperation(),
                                                      loadA))
      continue;

    // Check if two values have the same shape. This is needed for affine vector
    // loads.
    if (loadB.getValue().getType() != loadA.getValue().getType())
      continue;

    loadCandidates.push_back(loadB);
  }

  // Of the legal load candidates, use the one that dominates all others
  // to minimize the subsequent need to loadCSE
  Value loadB;
  for (AffineReadOpInterface option : loadCandidates) {
    if (llvm::all_of(loadCandidates, [&](AffineReadOpInterface depStore) {
          return depStore == option ||
                 domInfo.dominates(option.getOperation(),
                                   depStore.getOperation());
        })) {
      loadB = option.getValue();
      break;
    }
  }

  if (loadB) {
    loadA.getValue().replaceAllUsesWith(loadB);
    // Record this to erase later.
    loadOpsToErase.push_back(loadA);
  }
}

void AffineScalarReplacement::runOnFunction() {
  // Only supports single block functions at the moment.
  FuncOp f = getFunction();

  // Load op's whose results were replaced by those forwarded from stores.
  SmallVector<Operation *, 8> opsToErase;

  // A list of memref's that are potentially dead / could be eliminated.
  SmallPtrSet<Value, 4> memrefsToErase;

  auto &domInfo = getAnalysis<DominanceInfo>();
  auto &postDomInfo = getAnalysis<PostDominanceInfo>();

  // Walk all load's and perform store to load forwarding.
  f.walk([&](AffineReadOpInterface loadOp) {
    if (failed(
            forwardStoreToLoad(loadOp, opsToErase, memrefsToErase, domInfo))) {
      loadCSE(loadOp, opsToErase, domInfo);
    }
  });

  // Erase all load op's whose results were replaced with store fwd'ed ones.
  for (auto *op : opsToErase)
    op->erase();
  opsToErase.clear();

  // Walk all store's and perform unused store elimination
  f.walk([&](AffineWriteOpInterface storeOp) {
    findUnusedStore(storeOp, opsToErase, memrefsToErase, postDomInfo);
  });
  // Erase all store op's which don't impact the program
  for (auto *op : opsToErase)
    op->erase();

  // Check if the store fwd'ed memrefs are now left with only stores and can
  // thus be completely deleted. Note: the canonicalize pass should be able
  // to do this as well, but we'll do it here since we collected these anyway.
  for (auto memref : memrefsToErase) {
    // If the memref hasn't been alloc'ed in this function, skip.
    Operation *defOp = memref.getDefiningOp();
    if (!defOp || !isa<memref::AllocOp>(defOp))
      // TODO: if the memref was returned by a 'call' operation, we
      // could still erase it if the call had no side-effects.
      continue;
    if (llvm::any_of(memref.getUsers(), [&](Operation *ownerOp) {
          return !isa<AffineWriteOpInterface, memref::DeallocOp>(ownerOp);
        }))
      continue;

    // Erase all stores, the dealloc, and the alloc on the memref.
    for (auto *user : llvm::make_early_inc_range(memref.getUsers()))
      user->erase();
    defOp->erase();
  }
}