OneShotAnalysis.cpp

Go to the documentation of this file.

//===- OneShotAnalysis.cpp - One-Shot (Single Pass) Analysis --------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// One-Shot Analysis analyzes function bodies. By default, function boundaries
// (FuncOp bbArgs, CallOps, ReturnOps) are treated as "unknown" ops.
// OneShotModuleBufferization.cpp is an extension of One-Shot Analysis for
// simple call graphs without loops.
//
// One-Shot Bufferize consists of three phases.
//
// 1. Analyze ops to decide which OpOperands can bufferize inplace, i.e.,
//    without inserting buffer copies. The analysis queries op bufferization
//    semantics via `BufferizableOpInterface`.
// 2. Insert copies for OpOperands that were decided to bufferize out-of-place
//    in tensor land during `TensorCopyInsertion`.
// 3. Bufferize ops by calling `BufferizableOpInterface::bufferize`.
//
// This file contains only the analysis. For convenience, this file also
// contains a helper function `runOneShotBufferize` that analyzes an op (and its
// nested ops) and then bufferizes it.
//
// Inplace bufferization decisions are passed from the analysis to the
// `TensorCopyInsertion` phase via `AnalysisState`. They can be printed for
// debugging purposes with `testAnalysisOnly`.
//
// Ops that do not implement `BufferizableOpInterface` can be analyzed but are
// treated conservatively. E.g., the analysis has to assume that their tensor
// OpOperands bufferize to memory writes. While such ops can be analyzed, they
// are not bufferized and remain in the IR. to_tensor and to_buffer ops are
// inserted at the bufferization boundary.
//
// This analysis caters to high-performance codegen where buffer reuse is deemed
// critical: the analysis should fail if the bufferized form of the function
// needs to return a buffer, unless `allowReturnAllocs` is enabled.
 
#include "mlir/Dialect/Bufferization/Transforms/OneShotAnalysis.h"
 
#include <random>
#include <tuple>
 
#include "mlir/Dialect/Bufferization/IR/BufferizableOpInterface.h"
#include "mlir/Dialect/Bufferization/IR/Bufferization.h"
#include "mlir/Dialect/Bufferization/Transforms/Bufferize.h"
#include "mlir/Dialect/Bufferization/Transforms/Transforms.h"
#include "mlir/Dialect/MemRef/IR/MemRef.h"
#include "mlir/IR/AsmState.h"
#include "mlir/IR/Dominance.h"
#include "mlir/IR/Iterators.h"
#include "mlir/IR/Operation.h"
#include "mlir/IR/TypeUtilities.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/SubsetOpInterface.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/DebugLog.h"
 
MLIR_DEFINE_EXPLICIT_TYPE_ID(mlir::bufferization::OneShotAnalysisState)
 
// Run mlir-opt with `-debug-only="one-shot-analysis"` for detailed debug
// output.
#define DEBUG_TYPE "one-shot-analysis"
 
using namespace mlir;
using namespace mlir::bufferization;
 
static bool isaTensor(Type t) { return isa<TensorLikeType>(t); }
 
//===----------------------------------------------------------------------===//
// Bufferization-specific attribute manipulation.
// These are for testing and debugging only. Bufferization information is stored
// in OneShotBufferizationState. When run with `testAnalysisOnly`, the IR is
// annotated with the results of the analysis, so that they can be checked in
// tests.
//===----------------------------------------------------------------------===//
 
/// Attribute marker to specify op operands that bufferize in-place.
constexpr StringLiteral kInPlaceOperandsAttrName = "__inplace_operands_attr__";
 
constexpr StringLiteral kOpResultAliasSetAttrName =
    "__opresult_alias_set_attr__";
 
constexpr StringLiteral kBbArgAliasSetAttrName = "__bbarg_alias_set_attr__";
 
/// Mark whether OpOperand will be bufferized inplace.
static void setInPlaceOpOperand(OpOperand &opOperand, bool inPlace) {
  Operation *op = opOperand.getOwner();
  SmallVector<StringRef> inPlaceVector;
  if (auto attr = op->getDiscardableAttr(kInPlaceOperandsAttrName)) {
    inPlaceVector = SmallVector<StringRef>(llvm::to_vector<4>(
        cast<ArrayAttr>(attr).getAsValueRange<StringAttr>()));
    // The existing attribute may have fewer entries than the current operand
    // count (e.g., when user-provided annotations are inconsistent with the
    // op's actual operand count). Resize to avoid an out-of-bounds access.
    if (inPlaceVector.size() < op->getNumOperands())
      inPlaceVector.resize(op->getNumOperands(), "none");
  } else {
    inPlaceVector = SmallVector<StringRef>(op->getNumOperands(), "none");
    for (OpOperand &opOperand : op->getOpOperands())
      if (isa<TensorLikeType>(opOperand.get().getType()))
        inPlaceVector[opOperand.getOperandNumber()] = "false";
  }
  inPlaceVector[opOperand.getOperandNumber()] = inPlace ? "true" : "false";
  op->setDiscardableAttr(kInPlaceOperandsAttrName,
                         OpBuilder(op).getStrArrayAttr(inPlaceVector));
}
 
//===----------------------------------------------------------------------===//
// OneShotAnalysisState
//===----------------------------------------------------------------------===//
 
/// A region with more than one block is unstructured control flow. Single-block
/// regions, including structured loops, are not.
static bool detectUnstructuredControlFlow(Operation *op) {
  WalkResult walkRes = op->walk([&](Operation *nested) {
    for (Region &region : nested->getRegions()) {
      if (region.getBlocks().size() > 1) {
        return WalkResult::interrupt();
      }
    }
    return WalkResult::advance();
  });
  return walkRes.wasInterrupted();
}
 
/// Return "true" if any allowed op has a parallel region.
static bool detectParallelRegions(Operation *op,
                                  const BufferizationOptions &options) {
  WalkResult walkRes =
      op->walk([&](BufferizableOpInterface bufferizableOp) -> WalkResult {
        if (!options.isOpAllowed(bufferizableOp))
          return WalkResult::skip();
        for (Region &region : bufferizableOp->getRegions()) {
          if (bufferizableOp.isParallelRegion(region.getRegionNumber()))
            return WalkResult::interrupt();
        }
        return WalkResult::advance();
      });
  return walkRes.wasInterrupted();
}
 
OneShotAnalysisState::OneShotAnalysisState(
    Operation *op, const OneShotBufferizationOptions &options)
    : AnalysisState(options, TypeID::get<OneShotAnalysisState>()) {
  mayHaveUnstructuredCF = options.mayHaveUnstructuredControlFlow.value_or(
      detectUnstructuredControlFlow(op));
  if (!options.mayHaveParallelRegions.has_value()) {
    mayHaveParallelRegionsFlag = detectParallelRegions(op, options);
  } else {
    mayHaveParallelRegionsFlag = *options.mayHaveParallelRegions;
  }
 
  // Set up alias sets.
  op->walk([&](Operation *op) {
    for (Value v : op->getResults())
      if (isa<TensorLikeType>(v.getType()))
        createAliasInfoEntry(v);
    for (Region &r : op->getRegions())
      for (Block &b : r.getBlocks())
        for (auto bbArg : b.getArguments())
          if (isa<TensorLikeType>(bbArg.getType()))
            createAliasInfoEntry(bbArg);
  });
 
  // Mark OpOperands in-place that must bufferize in-place.
  op->walk([&](BufferizableOpInterface bufferizableOp) {
    if (!options.isOpAllowed(bufferizableOp))
      return WalkResult::skip();
 
    for (OpOperand &opOperand : bufferizableOp->getOpOperands())
      if (isa<TensorLikeType>(opOperand.get().getType()))
        if (bufferizableOp.mustBufferizeInPlace(opOperand, *this))
          bufferizeInPlace(opOperand);
    return WalkResult::advance();
  });
}
 
void OneShotAnalysisState::applyOnEquivalenceClass(
    Value v, function_ref<void(Value)> fun) const {
  auto leaderIt = equivalentInfo.findLeader(v);
  for (auto mit = leaderIt, meit = equivalentInfo.member_end(); mit != meit;
       ++mit) {
    fun(*mit);
  }
}
 
void OneShotAnalysisState::applyOnAliases(Value v,
                                          function_ref<void(Value)> fun) const {
  auto leaderIt = aliasInfo.findLeader(v);
  for (auto mit = leaderIt, meit = aliasInfo.member_end(); mit != meit; ++mit) {
    fun(*mit);
  }
}
 
bool OneShotAnalysisState::areEquivalentBufferizedValues(Value v1,
                                                         Value v2) const {
  return equivalentInfo.isEquivalent(v1, v2);
}
 
bool OneShotAnalysisState::areAliasingBufferizedValues(Value v1,
                                                       Value v2) const {
  return aliasInfo.isEquivalent(v1, v2);
}
 
void OneShotAnalysisState::bufferizeInPlace(OpOperand &operand) {
  if (inplaceBufferized.contains(&operand))
    return;
  inplaceBufferized.insert(&operand);
  for (AliasingValue alias : getAliasingValues(operand))
    aliasInfo.unionSets(alias.value, operand.get());
  ++statNumTensorInPlace;
}
 
void OneShotAnalysisState::bufferizeOutOfPlace(OpOperand &operand) {
  assert(!inplaceBufferized.contains(&operand) &&
         "OpOperand was already decided to bufferize inplace");
  ++statNumTensorOutOfPlace;
}
 
void OneShotAnalysisState::createAliasInfoEntry(Value v) {
  aliasInfo.insert(v);
  equivalentInfo.insert(v);
}
 
void OneShotAnalysisState::gatherUndefinedTensorUses(Operation *op) {
  op->walk([&](Operation *op) {
    // Skip unknown ops.
    auto bufferizableOp = getOptions().dynCastBufferizableOp(op);
    if (!bufferizableOp)
      return WalkResult::skip();
 
    // Check all tensor OpResults.
    for (OpResult opResult : op->getOpResults()) {
      if (!isa<TensorLikeType>(opResult.getType()))
        continue;
 
      // If there is no preceding definition, the tensor contents are
      // undefined.
      if (opResult.getUses().empty())
        continue;
      // It does not really matter which use to take to search about
      // the value's definitions.
      OpOperand *opOperand = &(*opResult.getUses().begin());
      if (findDefinitionsCached(opOperand).empty())
        for (OpOperand &use : opResult.getUses())
          undefinedTensorUses.insert(&use);
    }
 
    return WalkResult::advance();
  });
}
 
bool OneShotAnalysisState::hasUndefinedContents(OpOperand *opOperand) const {
  return undefinedTensorUses.contains(opOperand);
}
 
bool OneShotAnalysisState::isInPlace(OpOperand &opOperand) const {
  return inplaceBufferized.contains(&opOperand);
}
 
bool OneShotAnalysisState::isValueWritten(Value value) const {
  bool isWritten = false;
  applyOnAliases(value, [&](Value val) {
    for (OpOperand &use : val.getUses())
      if (isInPlace(use) && bufferizesToMemoryWrite(use))
        isWritten = true;
  });
  return isWritten;
}
 
bool OneShotAnalysisState::isWritable(Value value) const {
  // TODO: Out-of-place bufferized value could be considered writable.
  // Query BufferizableOpInterface to see if the BlockArgument is writable.
  if (auto bufferizableOp =
          getOptions().dynCastBufferizableOp(getOwnerOfValue(value)))
    return bufferizableOp.isWritable(value, *this);
 
  // Not a bufferizable op: The conservative answer is "not writable".
  return false;
}
 
void OneShotAnalysisState::unionAliasSets(Value v1, Value v2) {
  aliasInfo.unionSets(v1, v2);
}
 
void OneShotAnalysisState::unionEquivalenceClasses(Value v1, Value v2) {
  equivalentInfo.unionSets(v1, v2);
}
 
OneShotAnalysisState::Extension::~Extension() = default;
 
//===----------------------------------------------------------------------===//
// Bufferization-specific alias analysis.
//===----------------------------------------------------------------------===//
 
/// Return true if opOperand has been decided to bufferize in-place.
static bool isInplaceMemoryWrite(OpOperand &opOperand,
                                 const OneShotAnalysisState &state) {
  // OpOperands that do not bufferize to a memory write do not write in-place.
  if (!state.bufferizesToMemoryWrite(opOperand))
    return false;
  // Check current bufferization decisions.
  return state.isInPlace(opOperand);
}
 
/// Cached CFG reachability to avoid repeated linear BFS traversals from
/// `Block->isReachable`.
class OneShotAnalysisState::CFGReachabilityCache {
public:
  bool isReachable(Block *from, Block *to,
                   const SmallPtrSetImpl<Block *> *barriers = nullptr) {
    assert(from->getParent() == to->getParent() && "expected same region");
 
    SmallPtrSet<Block *, 16> except;
    BarrierKey sortedBarriers;
    if (barriers && !barriers->empty()) {
      except.insert(barriers->begin(), barriers->end());
      sortedBarriers.append(barriers->begin(), barriers->end());
      llvm::sort(sortedBarriers);
    }
 
    ReachableByBarriers &inner = cached[from];
    if (auto it = inner.find(sortedBarriers); it != inner.end())
      return it->second.contains(to);
 
    // Same traversal as `Block::isReachable`: `except` is both the barrier set
    // and the visited set.
    ReachableSet reachable;
    SmallVector<Block *> worklist(from->succ_begin(), from->succ_end());
    while (!worklist.empty()) {
      Block *next = worklist.pop_back_val();
      if (!except.insert(next).second)
        continue;
      reachable.insert(next);
      worklist.append(next->succ_begin(), next->succ_end());
    }
    bool result = reachable.contains(to);
    inner.try_emplace(std::move(sortedBarriers), std::move(reachable));
    return result;
  }
 
  void clear() { cached.clear(); }
 
private:
  using ReachableSet = SmallPtrSet<Block *, 16>;
  /// Sorted extra-barrier blocks for one query. Empty means no extra barriers.
  using BarrierKey = SmallVector<Block *, 8>;
  /// Sorted barrier list -> blocks reachable from `from` without crossing those
  /// barriers.
  using ReachableByBarriers = DenseMap<BarrierKey, ReachableSet>;
  /// `from` block -> reachable-set keyed by sorted extra-barrier blocks.
  DenseMap<Block *, ReachableByBarriers> cached;
};
 
/// `canUseOpDominanceDueToBlocks` walks enclosing regions and queries CFG
/// reachability, but the answer is a property of blocks: every op in a block
/// has the same parent chain, and a definition contributes only its block as
/// a reachability barrier. One entry per (read block, write block, def blocks)
/// therefore covers every operand pair in those blocks.
class OneShotAnalysisState::OpDominanceBlockCache {
public:
  bool getOrCompute(Block *readBlock, Block *writeBlock,
                    ArrayRef<Block *> sortedDefBlocks,
                    function_ref<bool()> compute) {
    if (sortedDefBlocks.size() == 1) {
      auto key =
          std::make_tuple(readBlock, writeBlock, sortedDefBlocks.front());
      if (auto it = singleDef.find(key); it != singleDef.end())
        return it->second;
      bool result = compute();
      singleDef.try_emplace(key, result);
      return result;
    }
 
    DefKey key(sortedDefBlocks.begin(), sortedDefBlocks.end());
    auto &byDefs = manyDefs[std::make_pair(readBlock, writeBlock)];
    if (auto it = byDefs.find(key); it != byDefs.end())
      return it->second;
    bool result = compute();
    byDefs.try_emplace(std::move(key), result);
    return result;
  }
 
  void clear() {
    singleDef.clear();
    manyDefs.clear();
  }
 
private:
  using DefKey = SmallVector<Block *, 4>;
  /// Common case: a single definition block.
  DenseMap<std::tuple<Block *, Block *, Block *>, bool> singleDef;
  /// Sorted unique definition blocks, for reads with several definitions.
  DenseMap<std::pair<Block *, Block *>, DenseMap<DefKey, bool>> manyDefs;
};
 
OneShotAnalysisState::~OneShotAnalysisState() = default;
 
/// Return true if `a` cannot happen after `b`.
/// True if `a` properly dominates `b` and `b` is not inside `a`, or if they
/// are in the same region with no CFG path from `b` to `a`.
/// Dominance is sufficient but not necessary. E.g.:
///
/// ^bb0:
///   cf.cond_br %c, ^bb1, ^bb2
/// ^bb1:
///   "op_a"(%t)
///   cf.br ^bb3
/// ^bb2:
///   "op_b"(%t)
///   cf.br ^bb3
/// ^bb3:
///
/// `op_a` does not dominate `op_b`, but there is no CFG path from `op_b` to
/// `op_a`, so `op_a` cannot happen after `op_b`.
///
/// `extraBarriers` are extra blocks a CFG path from `b` to `a` must not cross.
static bool cannotHappenAfter(Operation *a, Operation *b,
                              const DominanceInfo &domInfo,
                              OneShotAnalysisState &state,
                              SmallPtrSet<Block *, 16> extraBarriers = {}) {
  do {
    // TODO: Instead of isProperAncestor + properlyDominates, we should use
    // properlyDominatesImpl(a, b, /*enclosingOpOk=*/false)
    if (a->isProperAncestor(b))
      return false;
    // Dominance is a stronger condition than reachability. Prefer using it
    // since it is cached and works within a single block.
    if (domInfo.properlyDominates(a, b))
      return true;
    // Distinct blocks in the same region only exist with unstructured control
    // flow. Dominance is a complete ordering otherwise.
    if (state.mayHaveUnstructuredControlFlow()) {
      Block *aBlock = a->getBlock();
      Block *bBlock = b->getBlock();
      if (aBlock != bBlock && aBlock->getParent() == bBlock->getParent() &&
          !state.isReachableCached(bBlock, aBlock, &extraBarriers))
        return true;
    }
  } while ((a = a->getParentOp()));
  return false;
}
 
/// Return `true` if op dominance can be used to rule out a read-after-write
/// conflicts based on the ordering of ops. Returns `false` if op dominance
/// cannot be used to due region-based loops.
///
/// Generalized op dominance can often be used to rule out potential conflicts
/// due to "read happens before write". E.g., the following IR is not a RaW
/// conflict because the read happens *before* the write.
///
/// Example 1:
/// %0 = ... : tensor<?xf32>                                // DEF
/// "reading_op"(%0) : tensor<?xf32>                        // READ
/// %1 = "writing_op"(%0) : tensor<?xf32> -> tensor<?xf32>  // WRITE
///
/// This is no longer true inside loops (or repetitive regions). In such cases,
/// there may not be a meaningful `cannotHappenAfter` relationship because ops
/// could be executed multiple times. E.g.:
///
/// Example 2:
/// %0 = ... : tensor<?xf32>                                  // DEF
/// scf.for ... {
///   "reading_op"(%0) : tensor<?xf32>                        // READ
///   %1 = "writing_op"(%0) : tensor<?xf32> -> tensor<?xf32>  // WRITE
///   ...
/// }
///
/// In the above example, reading_op happens before writing_op according to
/// op dominance. However, both ops may happen multiple times; in
/// particular, the second execution of reading_op happens after the first
/// execution of writing_op. This is problematic because the tensor %0 they
/// operate on (i.e., the "definition") is defined outside of the loop.
///
/// On a high-level, there is a potential RaW in a program if there exists a
/// possible program execution such that there is a sequence of DEF, followed
/// by WRITE, followed by READ. Each additional DEF resets the sequence.
///
/// E.g.:
/// No conflict:        DEF, WRITE, DEF, READ
/// Potential conflict: DEF, READ, WRITE, READ, WRITE
///
/// Example 1 has no conflict:          DEF, READ, WRITE
/// Example 2 has a potential conflict: DEF, (READ, WRITE)*
//
/// Example 3:
/// scf.for ... {
///   %0 = ... : tensor<?xf32>
///   "reading_op"(%0) : tensor<?xf32>
///   %1 = "writing_op"(%0) : tensor<?xf32> -> tensor<?xf32>
///   ...
/// }
/// This has no conflict: (DEF, READ, WRITE)*
///
/// Example 4:
/// %0 = ... : tensor<?xf32>
/// scf.for ... {
///   scf.for ... { "reading_op"(%0) }
///   %1 = "writing_op"(%0)
/// }
/// This has a potential conflict: DEF, ((READ)*, WRITE)*
///
/// Example 5:
/// %0 = ... : tensor<?xf32>
/// scf.for ... { %1 = "writing_op"(%0) }
/// scf.for ... { "reading_op"(%0) }
/// This has a potential conflict: DEF, WRITE*, READ*
///
/// The following rules are used to rule out RaW conflicts via ordering of ops:
///
/// 1. If the closest enclosing repetitive region of DEF is a proper ancestor of
///    a repetitive region that enclosing both READ and WRITE, we cannot rule
///    out RaW conflict due to the ordering of ops.
/// 2. Otherwise: There are no loops that interfere with our analysis; for
///    analysis purposes, we can assume that there are no loops/repetitive
///    regions. I.e., we can rule out a RaW conflict if READ cannot happen after
///    WRITE or WRITE cannot happen after DEF. (Checked in
///    `hasReadAfterWriteInterference`.)
///
static bool canUseOpDominanceDueToRegions(OpOperand *uRead, OpOperand *uWrite,
                                          const SetVector<Value> &definitions,
                                          AnalysisState &state) {
  const BufferizationOptions &options = state.getOptions();
  for (Value def : definitions) {
    Region *rRead =
        state.getEnclosingRepetitiveRegion(uRead->getOwner(), options);
    Region *rDef = state.getEnclosingRepetitiveRegion(def, options);
 
    // READ and DEF are in the same repetitive region. `cannotHappenAfter` can
    // be used to rule out RaW conflicts due to op ordering.
    if (rRead == rDef)
      continue;
 
    // Find the enclosing repetitive region of READ that is closest to DEF but
    // not the repetitive region of DEF itself.
    while (true) {
      Region *nextRegion = getNextEnclosingRepetitiveRegion(rRead, options);
      if (nextRegion == rDef)
        break;
      assert(nextRegion && "expected to find another repetitive region");
      rRead = nextRegion;
    }
 
    // We cannot use op dominance if WRITE is inside the same repetitive region.
    if (rRead->getParentOp()->isAncestor(uWrite->getOwner()))
      return false;
  }
 
  return true;
}
 
/// Return `true` if op dominance can be used to rule out a read-after-write
/// conflicts based on the ordering of ops. Returns `false` if op dominance
/// cannot be used to due block-based loops within a region.
///
/// Refer to the `canUseOpDominanceDueToRegions` documentation for details on
/// how op domiance is used during RaW conflict detection.
///
/// On a high-level, there is a potential RaW in a program if there exists a
/// possible program execution such that there is a sequence of DEF, followed
/// by WRITE, followed by READ. Each additional DEF resets the sequence.
///
/// Op dominance cannot be used if there is a path from block(READ) to
/// block(WRITE) and a path from block(WRITE) to block(READ). block(DEF) should
/// not appear on that path. Walk from the uses up through enclosing regions
/// and stop at the definition region(s): SSA dominance means a cycle that
/// skips DEF cannot live above DEF.
static bool
computeCanUseOpDominanceDueToBlocks(OpOperand *uRead, OpOperand *uWrite,
                                    const SetVector<Value> &definitions,
                                    OneShotAnalysisState &state) {
  // Outermost region that contains a definition. SSA dominance means a cycle
  // that skips DEF cannot live above this.
  assert(!definitions.empty() && "expected at least one definition");
  Region *outermostDefRegion = nullptr;
  for (Value def : definitions) {
    Region *defRegion = def.getParentRegion();
    if (!outermostDefRegion || defRegion->isAncestor(outermostDefRegion))
      outermostDefRegion = defRegion;
  }
  assert(outermostDefRegion && "expected a definition region");
 
  for (Operation *readOp = uRead->getOwner(); readOp != nullptr;
       readOp = readOp->getParentOp()) {
    Region *region = readOp->getParentRegion();
    if (!region)
      continue;
 
    Operation *writeOp = region->findAncestorOpInRegion(*uWrite->getOwner());
    if (!writeOp)
      continue;
 
    Block *readBlock = readOp->getBlock();
    Block *writeBlock = writeOp->getBlock();
    for (Value def : definitions) {
      SmallPtrSet<Block *, 16> except = {def.getParentBlock()};
      if (state.isReachableCached(readBlock, writeBlock, &except) &&
          state.isReachableCached(writeBlock, readBlock, &except))
        return false;
    }
    if (region == outermostDefRegion)
      break;
  }
  return true;
}
 
static bool canUseOpDominanceDueToBlocks(OpOperand *uRead, OpOperand *uWrite,
                                         const SetVector<Value> &definitions,
                                         OneShotAnalysisState &state) {
  // No multi-block region means no block-based cycle for dominance to miss.
  if (!state.mayHaveUnstructuredControlFlow())
    return true;
 
  assert(!definitions.empty() && "expected at least one definition");
  Block *readBlock = uRead->getOwner()->getBlock();
  Block *writeBlock = uWrite->getOwner()->getBlock();
 
  SmallVector<Block *> defBlocks;
  defBlocks.reserve(definitions.size());
  for (Value def : definitions)
    defBlocks.push_back(def.getParentBlock());
  llvm::sort(defBlocks);
  defBlocks.erase(llvm::unique(defBlocks), defBlocks.end());
 
  return state.canUseOpDominanceDueToBlocksCached(
      readBlock, writeBlock, defBlocks, [&] {
        return computeCanUseOpDominanceDueToBlocks(uRead, uWrite, definitions,
                                                   state);
      });
}
 
static bool canUseOpDominance(OpOperand *uRead, OpOperand *uWrite,
                              const SetVector<Value> &definitions,
                              OneShotAnalysisState &state) {
  return canUseOpDominanceDueToRegions(uRead, uWrite, definitions, state) &&
         canUseOpDominanceDueToBlocks(uRead, uWrite, definitions, state);
}
 
/// Annotate IR with details about the detected RaW conflict.
static void annotateConflict(OpOperand *uRead, OpOperand *uConflictingWrite,
                             Value definition) {
  static uint64_t counter = 0;
  Operation *readingOp = uRead->getOwner();
  Operation *conflictingWritingOp = uConflictingWrite->getOwner();
 
  OpBuilder b(conflictingWritingOp->getContext());
  std::string id = "C_" + std::to_string(counter++);
 
  std::string conflictingWriteAttr =
      id +
      "[CONFL-WRITE: " + std::to_string(uConflictingWrite->getOperandNumber()) +
      "]";
  conflictingWritingOp->setDiscardableAttr(conflictingWriteAttr,
                                           b.getUnitAttr());
 
  std::string readAttr =
      id + "[READ: " + std::to_string(uRead->getOperandNumber()) + "]";
  readingOp->setDiscardableAttr(readAttr, b.getUnitAttr());
 
  if (auto opResult = dyn_cast<OpResult>(definition)) {
    std::string defAttr =
        id + "[DEF: result " + std::to_string(opResult.getResultNumber()) + "]";
    opResult.getDefiningOp()->setDiscardableAttr(defAttr, b.getUnitAttr());
  } else {
    auto bbArg = cast<BlockArgument>(definition);
    std::string defAttr =
        id + "[DEF: bbArg " + std::to_string(bbArg.getArgNumber()) + "]";
    bbArg.getOwner()->getParentOp()->setDiscardableAttr(defAttr,
                                                        b.getUnitAttr());
  }
}
 
/// Return 'true' if a tensor that is equivalent to `other` can be found in the
/// reverse use-def chain of `start`. Note: If an OpOperand bufferizes out of
/// place along that use-def chain, the two tensors may not materialize as
/// equivalent buffers (but separate allocations).
///
/// Note: This function also requires that the two tensors have equivalent
/// indexing. I.e., the tensor types do not change along the use-def chain,
/// apart from static <-> dynamic dim casts.
static bool hasEquivalentValueInReverseUseDefChain(AnalysisState &state,
                                                   OpOperand *start,
                                                   Value other) {
  TraversalConfig config;
  config.followEquivalentOnly = true;
  config.alwaysIncludeLeaves = false;
  config.followSameTypeOrCastsOnly = true;
  return !state
              .findValueInReverseUseDefChain(
                  start, [&](Value v) { return v == other; }, config)
              .empty();
}
 
/// Return "true" if the given operand's value is originating from a subset
/// that is equivalent to the subset that `subsetOp` inserts into.
static bool matchesInsertDestination(const AnalysisState &state,
                                     OpOperand *opOperand,
                                     SubsetInsertionOpInterface subsetOp) {
  auto matchingSubset = [&](Value val) {
    if (auto opResult = dyn_cast<OpResult>(val))
      if (subsetOp.isEquivalentSubset(opResult, [&](Value v1, Value v2) {
            return state.areEquivalentBufferizedValues(v1, v2);
          }))
        return true;
    return false;
  };
  // There may be multiple leaves at which the reverse SSA use-def chain lookup
  // terminates. All of them must be equivalent subsets.
  SetVector<Value> backwardSlice =
      state.findValueInReverseUseDefChain(opOperand, matchingSubset);
  return llvm::all_of(backwardSlice, matchingSubset);
}
 
/// Return "true" if the given "read" and potentially conflicting "write" are
/// not conflicting due to their subset relationship. The comments in this
/// function are expressed in terms of tensor.extract_slice/tensor.insert_slice
/// pairs, but apply to any subset ops that implement the
/// `SubsetInsertionOpInterface`.
static bool areNonConflictingSubsets(OpOperand *uRead,
                                     OpOperand *uConflictingWrite,
                                     const AnalysisState &state) {
  Operation *readingOp = uRead->getOwner();
  Operation *conflictingWritingOp = uConflictingWrite->getOwner();
 
  // Special rules for matching ExtractSliceOp/InsertSliceOp pairs. If
  // uRead is an InsertSliceOp...
  if (auto subsetOp = dyn_cast<SubsetInsertionOpInterface>(readingOp)) {
    // As an example, consider the following IR.
    //
    // %0 = tensor.extract_slice %t[%a, %b][%c, %d][1, 1] {inplace = [true] }
    // %1 = linalg.fill %cst, %0 {inplace= [true] }
    // %2 = tensor.insert_slice %1 into %t[%a, %b][%c, %d][1, 1]
    //     {inplace= [true] }
 
    if (uRead == &subsetOp.getDestinationOperand() &&
        matchesInsertDestination(state, uConflictingWrite, subsetOp))
      // Case 1: The main insight is that InsertSliceOp reads only part of
      // the destination tensor. The overwritten area is not read. If
      // uConflictingWrite writes into exactly the memory location that is
      // being read by uRead, this is not a conflict.
      //
      // In the above example:
      // uRead             = OpOperand 1 (%t) of tensor.insert_slice
      // uConflictingWrite = OpOperand 1 (%0) of linalg.fill
      //
      // The read of %t does not conflict with the write of the FillOp
      // (same aliases!) because the area that the FillOp operates on is
      // exactly the one that is *not* read via %t.
      return true;
 
    if (uRead == &subsetOp.getSourceOperand() &&
        uConflictingWrite == &subsetOp.getDestinationOperand() &&
        matchesInsertDestination(state, uRead, subsetOp))
      // Case 2: The read of the source tensor and the write to the dest
      // tensor via an InsertSliceOp is not a conflict if the read is
      // reading exactly that part of an equivalent tensor that the
      // InsertSliceOp is writing.
      //
      // In the above example:
      // uRead             = OpOperand 0 (%1) of tensor.insert_slice
      // uConflictingWrite = OpOperand 1 (%t) of tensor.insert_slice
      return true;
  }
 
  // If uConflictingWrite is an InsertSliceOp...
  if (auto subsetOp =
          dyn_cast<SubsetInsertionOpInterface>(conflictingWritingOp)) {
    if (uConflictingWrite == &subsetOp.getDestinationOperand()) {
      auto writtenSubset = cast<SubsetOpInterface>(conflictingWritingOp);
      auto isDisjointSubset = [&](SubsetOpInterface readSubset) {
        return readSubset.operatesOnDisjointSubset(
            writtenSubset, [&](Value v1, Value v2) {
              return state.areEquivalentBufferizedValues(v1, v2);
            });
      };
      auto isDisjointExtraction = [&](Value value) {
        auto extraction = value.getDefiningOp<SubsetExtractionOpInterface>();
        return extraction && isDisjointSubset(cast<SubsetOpInterface>(
                                 extraction.getOperation()));
      };
 
      // Example:
      //
      // %0 = tensor.insert_slice %s into %t[0][4][1]
      // %1 = vector.transfer_read %t[%c4], %cst
      //
      // A read from a subset does not conflict with a write to a disjoint
      // subset of an equivalent tensor. Check the operand roles explicitly
      // because not every subset extraction bufferizes to a memory read.
      if (auto extraction = dyn_cast<SubsetExtractionOpInterface>(readingOp)) {
        if (uRead == &extraction.getSourceOperand() &&
            isDisjointSubset(cast<SubsetOpInterface>(readingOp)))
          return true;
      }
 
      // Example:
      //
      // %0 = tensor.insert_slice %s into %t[0][4][1]
      // %1 = tensor.extract_slice %t[4][4][1]
      // return %0, %1
      //
      // The actual read may be further down the aliasing use-def chain. E.g.,
      // tensor.extract_slice is an alias-only op and the read is attributed to
      // a return or another consumer of its result. Trace such reads back to
      // their subset extractions. Every origin must be a disjoint subset; a
      // non-subset leaf or an extraction that may overlap keeps the analysis
      // conservative.
      SetVector<Value> readOrigins =
          state.findValueInReverseUseDefChain(uRead, isDisjointExtraction);
      if (!readOrigins.empty() &&
          llvm::all_of(readOrigins, isDisjointExtraction))
        return true;
    }
 
    // As an example, consider the following IR.
    //
    // %0 = tensor.extract_slice %t[%a, %b][%c, %d][1, 1] {inplace = [true] }
    // %1 = linalg.fill %cst, %0 {inplace= [true] }
    // %2 = tensor.insert_slice %1 into %t[%a, %b][%c, %d][1, 1]
    //     {inplace= [true] }
    // %3 = vector.transfer_read %1, %cst
    //
    // In the above example:
    // uRead             = OpOperand 0 (%1) of vector.transfer_read
    // uConflictingWrite = OpOperand 1 (%t) of tensor.insert_slice
    // definition        = %1
    //
    // This is not a conflict because the InsertSliceOp overwrites the
    // memory segment of %1 with the exact same data. (Effectively, there
    // is no memory write here.)
    if (uConflictingWrite == &subsetOp.getDestinationOperand() &&
        state.areEquivalentBufferizedValues(
            uRead->get(), subsetOp.getSourceOperand().get()) &&
        matchesInsertDestination(state, &subsetOp.getSourceOperand(), subsetOp))
      return true;
  }
 
  return false;
}
 
/// Given sets of uses and writes, return true if there is a RaW conflict under
/// the assumption that all given reads/writes alias the same buffer and that
/// all given writes bufferize inplace.
///
/// A conflict is: According to SSA use-def chains, a read R is supposed to read
/// the result of a definition W1. But because of bufferization decisions, R
/// actually reads another definition W2.
static bool
hasReadAfterWriteInterference(const DenseSet<OpOperand *> &usesRead,
                              const DenseSet<OpOperand *> &usesWrite,
                              const DominanceInfo &domInfo,
                              OneShotAnalysisState &state) {
  const BufferizationOptions &options = state.getOptions();
 
  // Before going through the main RaW analysis, find cases where a buffer must
  // be privatized due to parallelism. If the result of a write is never read,
  // privatization is not necessary (and large parts of the IR are likely dead).
  if (state.mayHaveParallelRegions() && !usesRead.empty()) {
    for (OpOperand *uConflictingWrite : usesWrite) {
      // Find the allocation point or last write (definition) of the buffer.
      // Note: In contrast to `findDefinitions`, this also returns results of
      // ops that do not bufferize to memory write when no other definition
      // could be found. E.g., "bufferization.alloc_tensor" would be included,
      // even though that op just bufferizes to an allocation but does define
      // the contents of the buffer.
      SetVector<Value> definitionsOrLeaves =
          state.findValueInReverseUseDefChain(uConflictingWrite, [&](Value v) {
            return state.bufferizesToMemoryWrite(v);
          });
      assert(!definitionsOrLeaves.empty() &&
             "expected at least one definition or leaf");
 
      // The writing op must bufferize out-of-place if the definition is in a
      // different parallel region than this write.
      Region *writeParallelRegion = getParallelRegion(
          uConflictingWrite->getOwner()->getParentRegion(), options);
      for (Value def : definitionsOrLeaves) {
        if (getParallelRegion(def.getParentRegion(), options) !=
            writeParallelRegion) {
          LDBG() << "\n- bufferizes out-of-place due to parallel region:\n"
                 << "  unConflictingWrite = operand "
                 << uConflictingWrite->getOperandNumber() << " of "
                 << OpWithFlags(uConflictingWrite->getOwner(),
                                OpPrintingFlags().skipRegions());
          return true;
        }
      }
    }
  }
 
  for (OpOperand *uRead : usesRead) {
    Operation *readingOp = uRead->getOwner();
    LDBG() << "\n- check conflict:\n"
           << "  uRead = operand " << uRead->getOperandNumber() << " of "
           << OpWithFlags(readingOp, OpPrintingFlags().skipRegions());
 
    // Find the definition of uRead by following the SSA use-def chain.
    // E.g.:
    //
    // %0 = "writing_op"(%t) : tensor<?x32> -> tensor<?xf32>
    // %1 = "aliasing_op"(%0) : tensor<?x32> -> tensor<?xf32>
    // %2 = "reading_op"(%1) : : tensor<?x32> -> not_a_tensor_type
    //
    // In the above example, if uRead is the OpOperand of reading_op, the
    // definition is %0. Note that operations that create an alias but do not
    // bufferize to a memory write (such as ExtractSliceOp) are skipped.
    const SetVector<Value> &definitions = state.findDefinitionsCached(uRead);
    if (definitions.empty()) {
      // Fast path: No conflict if there are no definitions.
      LDBG() << "  no conflict: read value has no definitions";
      continue;
    }
 
    // If `useDominance` is true below, then for CFG regions we know that we can
    // rule out blocks containing DEFs of block args for the reachability check
    // in `cannotHappenAfter`. See the conditions in
    // `canUseOpDominanceDueToBlocks`. E.g.:
    //
    // ^h(%t: tensor<?xf32>):            // DEF
    //   cf.cond_br %c, ^exit, ^body
    // ^body:
    //   %w = "writing_op"(%t)           // WRITE
    //   cf.br ^h(%w)
    // ^exit:
    //   "reading_op"(%t)                // READ
    //
    // The only CFG path from WRITE to READ is ^body -> ^h -> ^exit, so READ
    // cannot happen after WRITE without going through the DEF in ^h, and
    // therefore there is no conflict.
    SmallPtrSet<Block *, 16> bbArgDefBlocks;
    for (Value def : definitions)
      if (isa<BlockArgument>(def))
        bbArgDefBlocks.insert(def.getParentBlock());
 
    // Look for conflicting memory writes. Potential conflicts are writes to an
    // alias that have been decided to bufferize inplace.
    for (OpOperand *uConflictingWrite : usesWrite) {
      LDBG() << "  unConflictingWrite = operand "
             << uConflictingWrite->getOperandNumber() << " of "
             << OpWithFlags(uConflictingWrite->getOwner(),
                            OpPrintingFlags().skipRegions());
 
      // Check if op dominance can be used to rule out read-after-write
      // conflicts.
      bool useDominance =
          canUseOpDominance(uRead, uConflictingWrite, definitions, state);
      LDBG() << "\n- useDominance = " << useDominance;
 
      // Throughout this loop, check for multiple requirements that have to be
      // met for uConflictingWrite to be an actual conflict.
      Operation *conflictingWritingOp = uConflictingWrite->getOwner();
 
      // Inside of repetitive regions, ops may be executed multiple times and op
      // dominance cannot be used to rule out conflicts.
      if (useDominance) {
        // No conflict if the readingOp cannot happen after
        // conflictingWritingOp, i.e., the write is not visible when reading.
        //
        // Note: If ops are executed multiple times (e.g., because they are
        //       inside a loop), there may be no meaningful `cannotHappenAfter`
        //       relationship.
        if (cannotHappenAfter(readingOp, conflictingWritingOp, domInfo, state,
                              bbArgDefBlocks)) {
          LDBG() << "  no conflict: read cannot happen after write";
          continue;
        }
 
        // No conflict if the reading use equals the use of the conflicting
        // write. A use cannot conflict with itself.
        //
        // Note: Just being the same op is not enough. It has to be the same
        //       use.
        // Note: If the op is executed multiple times (e.g., because it is
        //       inside a loop), it may be conflicting with itself.
        if (uConflictingWrite == uRead) {
          LDBG() << "  no conflict: read and write are same use";
          continue;
        }
 
        // Ops are not conflicting if they are in mutually exclusive regions.
        //
        // Note: If ops are executed multiple times (e.g., because they are
        //       inside a loop), mutually exclusive regions may be executed
        //       multiple times.
        if (state.insideMutuallyExclusiveRegions(readingOp,
                                                 conflictingWritingOp)) {
          LDBG() << "  no conflict: read and write are in "
                    "mutually exclusive regions";
          continue;
        }
 
        // Two equivalent operands of the same op are not conflicting if the op
        // bufferizes to element-wise access. I.e., all loads at a position
        // happen before all stores to the same position.
        if (conflictingWritingOp == readingOp) {
          if (auto bufferizableOp = options.dynCastBufferizableOp(readingOp)) {
            if (bufferizableOp.bufferizesToElementwiseAccess(
                    state, {uRead, uConflictingWrite})) {
              if (hasEquivalentValueInReverseUseDefChain(
                      state, uRead, uConflictingWrite->get()) ||
                  hasEquivalentValueInReverseUseDefChain(
                      state, uConflictingWrite, uRead->get())) {
                LDBG() << "  no conflict: op bufferizes to element-wise access";
                continue;
              }
            }
          }
        }
      }
 
      // No conflict if the operands are non-conflicting subsets.
      if (state.areNonConflictingSubsetsCached(uRead, uConflictingWrite)) {
        LDBG() << "  no conflict: non-conflicting subsets";
        continue;
      }
 
      // No conflict if the op interface says so.
      if (auto bufferizableOp = options.dynCastBufferizableOp(readingOp)) {
        if (bufferizableOp.isNotConflicting(uRead, uConflictingWrite, state)) {
          LDBG() << "  no conflict: op interace of reading op says 'no'";
          continue;
        }
      }
 
      if (conflictingWritingOp != readingOp) {
        if (auto bufferizableOp =
                options.dynCastBufferizableOp(conflictingWritingOp)) {
          if (bufferizableOp.isNotConflicting(uRead, uConflictingWrite,
                                              state)) {
            LDBG() << "  no conflict: op interace of writing op says 'no'";
            continue;
          }
        }
      }
 
      // Check all possible definitions.
      for (Value definition : definitions) {
        LDBG() << "  * definition = " << definition;
 
        // No conflict if the conflicting write cannot happen after the
        // definition.
        if (Operation *defOp = definition.getDefiningOp()) {
          if (cannotHappenAfter(conflictingWritingOp, defOp, domInfo, state)) {
            LDBG() << "    no conflict: write cannot happen after definition";
            continue;
          }
          // No conflict if conflictingWritingOp is contained in defOp.
          if (defOp->isProperAncestor(conflictingWritingOp)) {
            LDBG() << "    no conflict: write is contained in definition";
            continue;
          }
        } else {
          // A bbArg is defined at block entry. The write happens after that
          // only if it is in the same block or in a reachable block (e.g. a
          // successor).
          Block *defBlock = cast<BlockArgument>(definition).getOwner();
          Operation *writeOp = defBlock->getParent()->findAncestorOpInRegion(
              *conflictingWritingOp);
          if (!writeOp ||
              (writeOp->getBlock() != defBlock &&
               !state.isReachableCached(defBlock, writeOp->getBlock()))) {
            LDBG() << "    no conflict: definition is bbArg and write cannot "
                      "happen after def";
            continue;
          }
        }
 
        // No conflict if the conflicting write and the definition are the same
        // use.
        AliasingValueList aliases = state.getAliasingValues(*uConflictingWrite);
        if (aliases.getNumAliases() == 1 &&
            aliases.getAliases()[0].value == definition) {
          LDBG() << "    no conflict: definition and write are same";
          continue;
        }
 
        // All requirements are met. Conflict found!
 
        if (options.printConflicts)
          annotateConflict(uRead, uConflictingWrite, definition);
        LDBG() << "  => RaW CONFLICT FOUND";
        return true;
      }
    }
  }
 
  return false;
}
 
// Helper function to iterate on aliases of `root` and capture the writes.
static void getAliasingInplaceWrites(DenseSet<OpOperand *> &res, Value root,
                                     const OneShotAnalysisState &state) {
  state.applyOnAliases(root, [&](Value alias) {
    for (auto &use : alias.getUses())
      // Inplace write to a value that aliases root.
      if (isInplaceMemoryWrite(use, state))
        res.insert(&use);
  });
}
 
// Helper function to iterate on aliases of `root` and capture the reads.
static void getAliasingReads(DenseSet<OpOperand *> &res, Value root,
                             const OneShotAnalysisState &state) {
  state.applyOnAliases(root, [&](Value alias) {
    for (auto &use : alias.getUses()) {
      // Read of a value that aliases root.
      if (state.bufferizesToMemoryRead(use)) {
        res.insert(&use);
        continue;
      }
 
      // Read of a dependent value in the SSA use-def chain. E.g.:
      //
      // %0 = ...
      // %1 = tensor.extract_slice %0 {not_analyzed_yet}
      // "read"(%1)
      //
      // In the above example, getAliasingReads(%0) includes the first OpOperand
      // of the tensor.extract_slice op. The extract_slice itself does not read
      // but its aliasing result is eventually fed into an op that does.
      //
      // Note: This is considered a "read" only if the use does not bufferize to
      // a memory write. (We already ruled out memory reads. In case of a memory
      // write, the buffer would be entirely overwritten; in the above example
      // there would then be no flow of data from the extract_slice operand to
      // its result's uses.)
      if (!state.bufferizesToMemoryWrite(use)) {
        AliasingValueList aliases = state.getAliasingValues(use);
        if (llvm::any_of(aliases, [&](AliasingValue a) {
              return state.isValueRead(a.value);
            }))
          res.insert(&use);
      }
    }
  });
}
 
/// Return true if bufferizing `operand` inplace would create a conflict. A read
/// R and a write W of the same alias set is a conflict if inplace bufferization
/// of W changes the value read by R to a value different from the one that
/// would be expected by tracing back R's origin through SSA use-def chains.
/// A conflict can only be introduced by a new alias and/or an inplace
/// bufferization decision.
///
/// Example:
/// %0 = tensor.extract_slice %t[...][...][1, 1] {inplace?}
/// %1 = vector.transfer_write %v1, %t {inplace} : vector<5xf32>, tensor<?xf32>
/// %e = tensor.extract_slice %1
/// %2 = vector.transfer_write %v2, %0 {inplace} : vector<6xf32>, tensor<?xf32>
/// %3 = vector.transfer_read %e, %cst : tensor<?xf32>, vector<7xf32>
///
/// In the above example, the two TransferWriteOps have already been decided to
/// bufferize inplace. Bufferizing the ExtractSliceOp inplace would create a
/// conflict because:
/// * According to SSA use-def chains, we expect to read the result of %1.
/// * However, adding an alias {%0, %t} would mean that the second
///   TransferWriteOp overwrites the result of the first one. Therefore, the
///   TransferReadOp would no longer be reading the result of %1.
///
/// If `checkConsistencyOnly` is true, this function checks if there is a
/// read-after-write conflict without bufferizing `operand` inplace. This would
/// indicate a problem with the current inplace bufferization decisions.
///
/// Note: If `checkConsistencyOnly`, this function may be called with a null
/// OpResult. In that case, only the consistency of bufferization decisions
/// involving aliases of the given OpOperand are checked.
static bool wouldCreateReadAfterWriteInterference(
    OpOperand &operand, const DominanceInfo &domInfo,
    OneShotAnalysisState &state, bool checkConsistencyOnly = false) {
  // Collect reads and writes of all aliases of OpOperand and OpResult.
  // Collect writes before reads: without a write, there can be no conflict.
  DenseSet<OpOperand *> usesWrite;
  getAliasingInplaceWrites(usesWrite, operand.get(), state);
  AliasingValueList aliases = state.getAliasingValues(operand);
  for (AliasingValue alias : aliases)
    getAliasingInplaceWrites(usesWrite, alias.value, state);
  if (!checkConsistencyOnly && state.bufferizesToMemoryWrite(operand))
    usesWrite.insert(&operand);
  if (usesWrite.empty())
    return false;
 
  DenseSet<OpOperand *> usesRead;
  getAliasingReads(usesRead, operand.get(), state);
  for (AliasingValue alias : aliases)
    getAliasingReads(usesRead, alias.value, state);
 
  return hasReadAfterWriteInterference(usesRead, usesWrite, domInfo, state);
}
 
/// Annotate IR with details about the detected non-writability conflict.
static void annotateNonWritableTensor(Value value) {
  static int64_t counter = 0;
  OpBuilder b(value.getContext());
  std::string id = "W_" + std::to_string(counter++);
  if (auto opResult = dyn_cast<OpResult>(value)) {
    std::string attr = id + "[NOT-WRITABLE: result " +
                       std::to_string(opResult.getResultNumber()) + "]";
    opResult.getDefiningOp()->setDiscardableAttr(attr, b.getUnitAttr());
  } else {
    auto bbArg = cast<BlockArgument>(value);
    std::string attr = id + "[NOT-WRITABLE: bbArg " +
                       std::to_string(bbArg.getArgNumber()) + "]";
    bbArg.getOwner()->getParentOp()->setDiscardableAttr(attr, b.getUnitAttr());
  }
}
 
/// Return true if bufferizing `operand` inplace would create a write to a
/// non-writable buffer.
static bool
wouldCreateWriteToNonWritableBuffer(OpOperand &operand,
                                    OneShotAnalysisState &state,
                                    bool checkConsistencyOnly = false) {
  bool foundWrite =
      !checkConsistencyOnly && state.bufferizesToMemoryWrite(operand);
 
  if (!foundWrite) {
    // Collect writes of all aliases of OpOperand and OpResult.
    DenseSet<OpOperand *> usesWrite;
    getAliasingInplaceWrites(usesWrite, operand.get(), state);
    for (AliasingValue alias : state.getAliasingValues(operand))
      getAliasingInplaceWrites(usesWrite, alias.value, state);
    foundWrite = !usesWrite.empty();
  }
 
  if (!foundWrite)
    return false;
 
  // Look for a read-only tensor among all aliases.
  bool foundReadOnly = false;
  auto checkReadOnly = [&](Value v) {
    if (!state.isWritable(v)) {
      foundReadOnly = true;
      if (state.getOptions().printConflicts)
        annotateNonWritableTensor(v);
    }
  };
  state.applyOnAliases(operand.get(), checkReadOnly);
  for (AliasingValue alias : state.getAliasingValues(operand))
    state.applyOnAliases(alias.value, checkReadOnly);
  if (foundReadOnly) {
    LDBG() << "=> NOT WRITABLE";
    return true;
  }
 
  return false;
}
 
//===----------------------------------------------------------------------===//
// Bufferization analyses.
//===----------------------------------------------------------------------===//
 
// Find the values that define the contents of the given operand's value.
const llvm::SetVector<Value> &
OneShotAnalysisState::findDefinitionsCached(OpOperand *opOperand) {
  Value value = opOperand->get();
  if (!cachedDefinitions.count(value))
    cachedDefinitions[value] = findDefinitions(opOperand);
  return cachedDefinitions[value];
}
 
bool OneShotAnalysisState::isReachableCached(
    Block *from, Block *to, const SmallPtrSetImpl<Block *> *barriers) {
  if (!cfgReachabilityCache)
    cfgReachabilityCache = std::make_unique<CFGReachabilityCache>();
  return cfgReachabilityCache->isReachable(from, to, barriers);
}
 
bool OneShotAnalysisState::canUseOpDominanceDueToBlocksCached(
    Block *readBlock, Block *writeBlock, ArrayRef<Block *> defBlocks,
    function_ref<bool()> compute) {
  if (!opDominanceBlockCache)
    opDominanceBlockCache = std::make_unique<OpDominanceBlockCache>();
  return opDominanceBlockCache->getOrCompute(readBlock, writeBlock, defBlocks,
                                             compute);
}
 
bool OneShotAnalysisState::areNonConflictingSubsetsCached(
    OpOperand *uRead, OpOperand *uConflictingWrite) {
  auto key = std::make_pair(uRead, uConflictingWrite);
  auto [it, inserted] = nonConflictingSubsetCache.try_emplace(key, false);
  if (inserted)
    it->second = areNonConflictingSubsets(uRead, uConflictingWrite, *this);
  return it->second;
}
 
void OneShotAnalysisState::resetCache() {
  AnalysisState::resetCache();
  cachedDefinitions.clear();
  nonConflictingSubsetCache.clear();
  if (cfgReachabilityCache)
    cfgReachabilityCache->clear();
  if (opDominanceBlockCache)
    opDominanceBlockCache->clear();
}
 
/// Determine if `operand` can be bufferized in-place.
static LogicalResult
bufferizableInPlaceAnalysisImpl(OpOperand &operand, OneShotAnalysisState &state,
                                const DominanceInfo &domInfo) {
  LDBG() << "//===-------------------------------------------===//\n"
         << "Analyzing operand #" << operand.getOperandNumber() << " of "
         << OpWithFlags(operand.getOwner(), OpPrintingFlags().skipRegions());
 
  bool foundInterference =
      wouldCreateWriteToNonWritableBuffer(operand, state) ||
      wouldCreateReadAfterWriteInterference(operand, domInfo, state);
 
  if (foundInterference)
    state.bufferizeOutOfPlace(operand);
  else
    state.bufferizeInPlace(operand);
 
  LDBG() << "//===-------------------------------------------===//";
  return success();
}
 
LogicalResult
OneShotAnalysisState::analyzeSingleOp(Operation *op,
                                      const DominanceInfo &domInfo) {
  for (OpOperand &opOperand : op->getOpOperands())
    if (isa<TensorLikeType>(opOperand.get().getType()))
      if (failed(bufferizableInPlaceAnalysisImpl(opOperand, *this, domInfo)))
        return failure();
  return success();
}
 
/// Analyze equivalence of tied OpResult/OpOperand pairs of the given ops.
static void equivalenceAnalysis(SmallVector<Operation *> &ops,
                                OneShotAnalysisState &state) {
  for (Operation *op : ops) {
    if (auto bufferizableOp = state.getOptions().dynCastBufferizableOp(op)) {
      for (OpResult opResult : op->getOpResults()) {
        if (!isa<TensorLikeType>(opResult.getType()))
          continue;
        AliasingOpOperandList aliases = state.getAliasingOpOperands(opResult);
        if (aliases.getNumAliases() == 0)
          // Nothing to do if there are no aliasing OpOperands.
          continue;
 
        Value firstOperand = aliases.begin()->opOperand->get();
        bool allEquivalent = true;
        for (AliasingOpOperand alias : aliases) {
          bool isEquiv = alias.relation == BufferRelation::Equivalent;
          bool isInPlace = state.isInPlace(*alias.opOperand);
          Value operand = alias.opOperand->get();
          if (isEquiv && isInPlace && alias.isDefinite) {
            // Found a definite, equivalent alias. Merge equivalence sets.
            // There can only be one definite alias, so we can stop here.
            state.unionEquivalenceClasses(opResult, operand);
            allEquivalent = false;
            break;
          }
          if (!isEquiv || !isInPlace)
            allEquivalent = false;
          if (!state.areEquivalentBufferizedValues(operand, firstOperand))
            allEquivalent = false;
        }
 
        // If all "maybe" aliases are equivalent and the OpResult is not a new
        // allocation, it is a definite, equivalent alias. E.g.:
        //
        // aliasingOpOperands(%r) = {(%t0, EQUIV, MAYBE), (%t1, EQUIV, MAYBE)}
        // aliasingValues(%t0) = {(%r, EQUIV, MAYBE)}
        // aliasingValues(%t1) = {(%r, EQUIV, MAYBE)}
        // %r = arith.select %c, %t0, %t1 : tensor<?xf32>
        //
        // If %t0 and %t1 are equivalent, it is safe to union the equivalence
        // classes of %r, %t0 and %t1.
        if (allEquivalent && !bufferizableOp.bufferizesToAllocation(opResult))
          state.unionEquivalenceClasses(opResult, firstOperand);
      }
    }
  }
}
 
/// Analyze equivalence of tied OpResult/OpOperand pairs of all ops contained
/// in `op`.
static void equivalenceAnalysis(Operation *op, OneShotAnalysisState &state) {
  // Traverse ops in PostOrder: Nested ops first, then enclosing ops.
  SmallVector<Operation *> ops;
  op->walk<WalkOrder::PostOrder>([&](Operation *op) {
    // No tensors => no buffers.
    if (none_of(op->getResultTypes(), isaTensor))
      return;
    ops.push_back(op);
  });
 
  equivalenceAnalysis(ops, state);
}
 
/// "Bottom-up from terminators" heuristic.
static SmallVector<Operation *>
bottomUpFromTerminatorsHeuristic(Operation *op,
                                 const OneShotAnalysisState &state) {
  SetVector<Operation *> traversedOps;
 
  // Find region terminators.
  op->walk<WalkOrder::PostOrder>([&](RegionBranchTerminatorOpInterface term) {
    if (!traversedOps.insert(term))
      return;
    // Follow the reverse SSA use-def chain from each yielded value as long as
    // we stay within the same region.
    SmallVector<OpResult> worklist;
    for (Value v : term->getOperands()) {
      if (!isa<TensorLikeType>(v.getType()))
        continue;
      auto opResult = dyn_cast<OpResult>(v);
      if (!opResult)
        continue;
      worklist.push_back(opResult);
    }
    while (!worklist.empty()) {
      OpResult opResult = worklist.pop_back_val();
      Operation *defOp = opResult.getDefiningOp();
      if (!traversedOps.insert(defOp))
        continue;
      if (!term->getParentRegion()->findAncestorOpInRegion(*defOp))
        continue;
      AliasingOpOperandList aliases = state.getAliasingOpOperands(opResult);
      for (auto alias : aliases) {
        Value v = alias.opOperand->get();
        if (!isa<TensorLikeType>(v.getType()))
          continue;
        auto opResult = dyn_cast<OpResult>(v);
        if (!opResult)
          continue;
        worklist.push_back(opResult);
      }
    }
  });
 
  // Analyze traversed ops, then all remaining ops.
  SmallVector<Operation *> result(traversedOps.begin(), traversedOps.end());
  op->walk<WalkOrder::PostOrder, ReverseIterator>([&](Operation *op) {
    if (!traversedOps.contains(op) && hasTensorSemantics(op))
      result.push_back(op);
  });
  return result;
}
 
LogicalResult OneShotAnalysisState::analyzeOp(Operation *op,
                                              const DominanceInfo &domInfo) {
  OneShotBufferizationOptions::AnalysisHeuristic heuristic =
      getOptions().analysisHeuristic;
 
  SmallVector<Operation *> orderedOps;
  if (heuristic ==
      OneShotBufferizationOptions::AnalysisHeuristic::BottomUpFromTerminators) {
    orderedOps = bottomUpFromTerminatorsHeuristic(op, *this);
  } else {
    op->walk([&](Operation *op) {
      // No tensors => no buffers.
      if (!hasTensorSemantics(op))
        return;
      orderedOps.push_back(op);
    });
    switch (heuristic) {
    case OneShotBufferizationOptions::AnalysisHeuristic::BottomUp: {
      // Default: Walk ops in reverse for better interference analysis.
      std::reverse(orderedOps.begin(), orderedOps.end());
      break;
    }
    case OneShotBufferizationOptions::AnalysisHeuristic::TopDown: {
      // Ops are already sorted top-down in `orderedOps`.
      break;
    }
    case OneShotBufferizationOptions::AnalysisHeuristic::Fuzzer: {
      assert(getOptions().analysisFuzzerSeed &&
             "expected that fuzzer seed it set");
      // This is a fuzzer. For testing purposes only. Randomize the order in
      // which operations are analyzed. The bufferization quality is likely
      // worse, but we want to make sure that no assertions are triggered
      // anywhere.
      std::mt19937 g(getOptions().analysisFuzzerSeed);
      llvm::shuffle(orderedOps.begin(), orderedOps.end(), g);
      break;
    }
    default: {
      llvm_unreachable("unsupported heuristic");
    }
    }
  }
 
  // Analyze ops in the computed order.
  for (Operation *op : orderedOps)
    if (failed(analyzeSingleOp(op, domInfo)))
      return failure();
 
  equivalenceAnalysis(op, *this);
  return success();
}
 
LogicalResult bufferization::checkPreBufferizationAssumptions(
    Operation *op, const DominanceInfo &domInfo, OneShotAnalysisState &state) {
  const BufferizationOptions &options = state.getOptions();
 
  // Note: This walk cannot be combined with the one below because interface
  // methods of invalid/unsupported ops may be called during the second walk.
  // (On ops different from `op`.)
  WalkResult walkResult = op->walk([&](BufferizableOpInterface op) {
    // Skip ops that are not in the filter.
    if (!options.isOpAllowed(op.getOperation()))
      return WalkResult::advance();
 
    // Check for unsupported unstructured control flow.
    if (!op.supportsUnstructuredControlFlow()) {
      for (Region &r : op->getRegions()) {
        if (r.getBlocks().size() > 1) {
          op->emitOpError("op or BufferizableOpInterface implementation does "
                          "not support unstructured control flow, but at least "
                          "one region has multiple blocks");
          return WalkResult::interrupt();
        }
      }
    }
 
    return WalkResult::advance();
  });
  if (walkResult.wasInterrupted())
    return failure();
 
  walkResult = op->walk([&](BufferizableOpInterface op) {
    // Skip ops that are not in the filter.
    if (!options.isOpAllowed(op.getOperation()))
      return WalkResult::advance();
 
    // Input IR may not contain any ToTensorOps without the "restrict"
    // attribute. Such tensors may alias any other tensor, which is currently
    // not handled in the analysis.
    if (auto toTensorOp = dyn_cast<ToTensorOp>(op.getOperation())) {
      if (!toTensorOp.getRestrict() && !toTensorOp->getUses().empty()) {
        op->emitOpError("to_tensor ops without `restrict` are not supported by "
                        "One-Shot Analysis");
        return WalkResult::interrupt();
      }
    }
 
    for (OpOperand &opOperand : op->getOpOperands()) {
      if (isa<TensorLikeType>(opOperand.get().getType())) {
        if (wouldCreateReadAfterWriteInterference(
                opOperand, domInfo, state,
                /*checkConsistencyOnly=*/true)) {
          // This error can happen if certain "mustBufferizeInPlace" interface
          // methods are implemented incorrectly, such that the IR already has
          // a RaW conflict before making any bufferization decisions. It can
          // also happen if the bufferization.materialize_in_destination is used
          // in such a way that a RaW conflict is not avoidable.
          op->emitOpError("not bufferizable under the given constraints: "
                          "cannot avoid RaW conflict");
          return WalkResult::interrupt();
        }
 
        if (state.isInPlace(opOperand) &&
            wouldCreateWriteToNonWritableBuffer(
                opOperand, state, /*checkConsistencyOnly=*/true)) {
          op->emitOpError("not bufferizable under the given constraints: would "
                          "write to read-only buffer");
          return WalkResult::interrupt();
        }
      }
    }
 
    return WalkResult::advance();
  });
 
  return success(!walkResult.wasInterrupted());
}
 
/// Annotate the IR with the result of the analysis. For testing/debugging only.
static void
annotateOpsWithBufferizationMarkers(Operation *op,
                                    const OneShotAnalysisState &state) {
  // Add __inplace_operands_attr__.
  op->walk([&](Operation *op) {
    for (OpOperand &opOperand : op->getOpOperands())
      if (isa<TensorLikeType>(opOperand.get().getType()))
        setInPlaceOpOperand(opOperand, state.isInPlace(opOperand));
  });
}
 
static void annotateOpsWithAliasSets(Operation *op,
                                     const OneShotAnalysisState &state) {
  AsmState asmState(op);
  Builder b(op->getContext());
  // Helper function to build an array attribute of aliasing SSA value strings.
  auto buildAliasesArray = [&](Value v) {
    SmallVector<Attribute> aliases;
    state.applyOnAliases(v, [&](Value alias) {
      std::string buffer;
      llvm::raw_string_ostream stream(buffer);
      alias.printAsOperand(stream, asmState);
      aliases.push_back(b.getStringAttr(buffer));
    });
    return b.getArrayAttr(aliases);
  };
 
  op->walk([&](Operation *op) {
    // Build alias set array for every OpResult.
    SmallVector<Attribute> opResultAliasSets;
    for (OpResult opResult : op->getOpResults()) {
      if (llvm::isa<TensorLikeType>(opResult.getType())) {
        opResultAliasSets.push_back(buildAliasesArray(opResult));
      }
    }
    if (!opResultAliasSets.empty())
      op->setDiscardableAttr(kOpResultAliasSetAttrName,
                             b.getArrayAttr(opResultAliasSets));
 
    // Build alias set array for every BlockArgument.
    SmallVector<Attribute> regionAliasSets;
    bool hasTensorBbArg = false;
    for (Region &r : op->getRegions()) {
      SmallVector<Attribute> blockAliasSets;
      for (Block &block : r.getBlocks()) {
        SmallVector<Attribute> bbArgAliasSets;
        for (BlockArgument bbArg : block.getArguments()) {
          if (llvm::isa<TensorLikeType>(bbArg.getType())) {
            bbArgAliasSets.push_back(buildAliasesArray(bbArg));
            hasTensorBbArg = true;
          }
        }
        blockAliasSets.push_back(b.getArrayAttr(bbArgAliasSets));
      }
      regionAliasSets.push_back(b.getArrayAttr(blockAliasSets));
    }
    if (hasTensorBbArg)
      op->setDiscardableAttr(kBbArgAliasSetAttrName,
                             b.getArrayAttr(regionAliasSets));
  });
}
 
LogicalResult bufferization::analyzeOp(Operation *op,
                                       OneShotAnalysisState &state,
                                       BufferizationStatistics *statistics) {
  DominanceInfo domInfo(op);
  const OneShotBufferizationOptions &options = state.getOptions();
 
  if (failed(checkPreBufferizationAssumptions(op, domInfo, state)))
    return failure();
 
  // If the analysis fails, just return.
  if (failed(state.analyzeOp(op, domInfo)))
    return failure();
 
  if (statistics) {
    statistics->numTensorInPlace = state.getStatNumTensorInPlace();
    statistics->numTensorOutOfPlace = state.getStatNumTensorOutOfPlace();
  }
 
  bool failedAnalysis = false;
 
  // Gather some extra analysis data.
  state.gatherUndefinedTensorUses(op);
 
  // Analysis verification: After setting up alias/equivalence sets, each op
  // can check for expected invariants/limitations and fail the analysis if
  // necessary.
  op->walk([&](Operation *op) {
    if (BufferizableOpInterface bufferizableOp =
            options.dynCastBufferizableOp(op))
      failedAnalysis |= failed(bufferizableOp.verifyAnalysis(state));
  });
 
  // Annotate operations if we only want to report the analysis.
  if (options.testAnalysisOnly)
    annotateOpsWithBufferizationMarkers(op, state);
  if (options.dumpAliasSets)
    annotateOpsWithAliasSets(op, state);
 
  return success(!failedAnalysis);
}
 
LogicalResult bufferization::runOneShotBufferize(
    Operation *op, const OneShotBufferizationOptions &options,
    BufferizationState &state, BufferizationStatistics *statistics) {
  // copy-before-write deactivates the analysis. It cannot be used together with
  // test-analysis-only.
  assert(!(options.copyBeforeWrite && options.testAnalysisOnly) &&
         "invalid combination of bufferization flags");
 
  if (options.copyBeforeWrite) {
    // Copy buffer before each write. No analysis is needed.
  } else {
    // Run One-Shot Analysis and insert buffer copies (on the tensor level)
    // only where needed. This is the default and much more efficient than
    // copy-before-write.
    if (failed(insertTensorCopies(op, options, state, statistics)))
      return failure();
 
    // If test-analysis-only is set, the IR was annotated with RaW conflict
    // markers (attributes) during One-Shot Analysis.
    if (options.testAnalysisOnly)
      return success();
  }
 
  // Bufferize the op and its nested ops. If options.copyBeforeWrite is set,
  // a new buffer copy is allocated every time a buffer is written to.
  return bufferizeOp(op, options, state, statistics);
}