Bufferization/Transforms/BufferViewFlowAnalysis.cpp

//======- BufferViewFlowAnalysis.cpp - Buffer alias analysis -*- C++ -*-======//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "mlir/Dialect/Bufferization/Transforms/BufferViewFlowAnalysis.h"

#include "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h"
#include "mlir/Interfaces/CallInterfaces.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/FunctionInterfaces.h"
#include "mlir/Interfaces/ViewLikeInterface.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"

using namespace mlir;
using namespace mlir::bufferization;

/// Constructs a new alias analysis using the op provided.
BufferViewFlowAnalysis::BufferViewFlowAnalysis(Operation *op) { build(op); }

/// Find all immediate and indirect dependent buffers this value could
/// potentially have. Note that the resulting set will also contain the value
/// provided as it is a dependent alias of itself.
BufferViewFlowAnalysis::ValueSetT
BufferViewFlowAnalysis::resolve(Value rootValue) const {
  ValueSetT result;
  SmallVector<Value, 8> queue;
  queue.push_back(rootValue);
  while (!queue.empty()) {
    Value currentValue = queue.pop_back_val();
    if (result.insert(currentValue).second) {
      auto it = dependencies.find(currentValue);
      if (it != dependencies.end()) {
        for (Value aliasValue : it->second)
          queue.push_back(aliasValue);
      }
    }
  }
  return result;
}

/// Removes the given values from all alias sets.
void BufferViewFlowAnalysis::remove(const SetVector<Value> &aliasValues) {
  for (auto &entry : dependencies)
    llvm::set_subtract(entry.second, aliasValues);
}

void BufferViewFlowAnalysis::rename(Value from, Value to) {
  dependencies[to] = dependencies[from];
  dependencies.erase(from);

  for (auto &[_, value] : dependencies) {
    if (value.contains(from)) {
      value.insert(to);
      value.erase(from);
    }
  }
}

/// This function constructs a mapping from values to its immediate
/// dependencies. It iterates over all blocks, gets their predecessors,
/// determines the values that will be passed to the corresponding block
/// arguments and inserts them into the underlying map. Furthermore, it wires
/// successor regions and branch-like return operations from nested regions.
void BufferViewFlowAnalysis::build(Operation *op) {
  // Registers all dependencies of the given values.
  auto registerDependencies = [&](ValueRange values, ValueRange dependencies) {
    for (auto [value, dep] : llvm::zip_equal(values, dependencies))
      this->dependencies[value].insert(dep);
  };

  // Mark all buffer results and buffer region entry block arguments of the
  // given op as terminals.
  auto populateTerminalValues = [&](Operation *op) {
    for (Value v : op->getResults())
      if (isa<BaseMemRefType>(v.getType()))
        this->terminals.insert(v);
    for (Region &r : op->getRegions())
      for (BlockArgument v : r.getArguments())
        if (isa<BaseMemRefType>(v.getType()))
          this->terminals.insert(v);
  };

  op->walk([&](Operation *op) {
    // Query BufferViewFlowOpInterface. If the op does not implement that
    // interface, try to infer the dependencies from other interfaces that the
    // op may implement.
    if (auto bufferViewFlowOp = dyn_cast<BufferViewFlowOpInterface>(op)) {
      bufferViewFlowOp.populateDependencies(registerDependencies);
      for (Value v : op->getResults())
        if (isa<BaseMemRefType>(v.getType()) &&
            bufferViewFlowOp.mayBeTerminalBuffer(v))
          this->terminals.insert(v);
      for (Region &r : op->getRegions())
        for (BlockArgument v : r.getArguments())
          if (isa<BaseMemRefType>(v.getType()) &&
              bufferViewFlowOp.mayBeTerminalBuffer(v))
            this->terminals.insert(v);
      return WalkResult::advance();
    }

    // Add additional dependencies created by view changes to the alias list.
    if (auto viewInterface = dyn_cast<ViewLikeOpInterface>(op)) {
      registerDependencies(viewInterface.getViewSource(),
                           viewInterface->getResult(0));
      return WalkResult::advance();
    }

    if (auto branchInterface = dyn_cast<BranchOpInterface>(op)) {
      // Query all branch interfaces to link block argument dependencies.
      Block *parentBlock = branchInterface->getBlock();
      for (auto it = parentBlock->succ_begin(), e = parentBlock->succ_end();
           it != e; ++it) {
        // Query the branch op interface to get the successor operands.
        auto successorOperands =
            branchInterface.getSuccessorOperands(it.getIndex());
        // Build the actual mapping of values to their immediate dependencies.
        registerDependencies(successorOperands.getForwardedOperands(),
                             (*it)->getArguments().drop_front(
                                 successorOperands.getProducedOperandCount()));
      }
      return WalkResult::advance();
    }

    if (auto regionInterface = dyn_cast<RegionBranchOpInterface>(op)) {
      // Query the RegionBranchOpInterface to find potential successor regions.
      // Extract all entry regions and wire all initial entry successor inputs.
      SmallVector<RegionSuccessor, 2> entrySuccessors;
      regionInterface.getSuccessorRegions(/*point=*/RegionBranchPoint::parent(),
                                          entrySuccessors);
      for (RegionSuccessor &entrySuccessor : entrySuccessors) {
        // Wire the entry region's successor arguments with the initial
        // successor inputs.
        registerDependencies(
            regionInterface.getEntrySuccessorOperands(entrySuccessor),
            entrySuccessor.getSuccessorInputs());
      }

      // Wire flow between regions and from region exits.
      for (Region &region : regionInterface->getRegions()) {
        // Iterate over all successor region entries that are reachable from the
        // current region.
        SmallVector<RegionSuccessor, 2> successorRegions;
        regionInterface.getSuccessorRegions(region, successorRegions);
        for (RegionSuccessor &successorRegion : successorRegions) {
          // Iterate over all immediate terminator operations and wire the
          // successor inputs with the successor operands of each terminator.
          for (Block &block : region)
            if (auto terminator = dyn_cast<RegionBranchTerminatorOpInterface>(
                    block.getTerminator()))
              registerDependencies(
                  terminator.getSuccessorOperands(successorRegion),
                  successorRegion.getSuccessorInputs());
        }
      }

      return WalkResult::advance();
    }

    // Region terminators are handled together with RegionBranchOpInterface.
    if (isa<RegionBranchTerminatorOpInterface>(op))
      return WalkResult::advance();

    if (isa<CallOpInterface>(op)) {
      // This is an intra-function analysis. We have no information about other
      // functions. Conservatively assume that each operand may alias with each
      // result. Also mark the results are terminals because the function could
      // return newly allocated buffers.
      populateTerminalValues(op);
      for (Value operand : op->getOperands())
        for (Value result : op->getResults())
          registerDependencies({operand}, {result});
      return WalkResult::advance();
    }

    // We have no information about unknown ops.
    populateTerminalValues(op);

    return WalkResult::advance();
  });
}

bool BufferViewFlowAnalysis::mayBeTerminalBuffer(Value value) const {
  assert(isa<BaseMemRefType>(value.getType()) && "expected memref");
  return terminals.contains(value);
}