llvm/mlir/lib/Analysis/DataFlow/DenseAnalysis.cpp

//===- DenseAnalysis.cpp - Dense data-flow 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
//
//===----------------------------------------------------------------------===//

#include "mlir/Analysis/DataFlow/DenseAnalysis.h"
#include "mlir/Analysis/DataFlow/DeadCodeAnalysis.h"
#include "mlir/Interfaces/CallInterfaces.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"

using namespace mlir;
using namespace mlir::dataflow;

//===----------------------------------------------------------------------===//
// AbstractDenseDataFlowAnalysis
//===----------------------------------------------------------------------===//

LogicalResult AbstractDenseDataFlowAnalysis::initialize(Operation *top) {
  // Visit every operation and block.
  processOperation(top);
  for (Region &region : top->getRegions()) {
    for (Block &block : region) {
      visitBlock(&block);
      for (Operation &op : block)
        if (failed(initialize(&op)))
          return failure();
    }
  }
  return success();
}

LogicalResult AbstractDenseDataFlowAnalysis::visit(ProgramPoint point) {
  if (auto *op = llvm::dyn_cast_if_present<Operation *>(point))
    processOperation(op);
  else if (auto *block = llvm::dyn_cast_if_present<Block *>(point))
    visitBlock(block);
  else
    return failure();
  return success();
}

void AbstractDenseDataFlowAnalysis::visitCallOperation(
    CallOpInterface call, AbstractDenseLattice *after) {

  const auto *predecessors =
      getOrCreateFor<PredecessorState>(call.getOperation(), call);
  // If not all return sites are known, then conservatively assume we can't
  // reason about the data-flow.
  if (!predecessors->allPredecessorsKnown())
    return setToEntryState(after);

  for (Operation *predecessor : predecessors->getKnownPredecessors()) {
    // Get the lattices at callee return:
    //
    //   func.func @callee() {
    //     ...
    //     return  // predecessor
    //     // latticeAtCalleeReturn
    //   }
    //   func.func @caller() {
    //     ...
    //     call @callee
    //     // latticeAfterCall
    //     ...
    //   }
    AbstractDenseLattice *latticeAfterCall = after;
    const AbstractDenseLattice *latticeAtCalleeReturn =
        getLatticeFor(call.getOperation(), predecessor);
    visitCallControlFlowTransfer(call, CallControlFlowAction::ExitCallee,
                                 *latticeAtCalleeReturn, latticeAfterCall);
  }
}

void AbstractDenseDataFlowAnalysis::processOperation(Operation *op) {
  // If the containing block is not executable, bail out.
  if (!getOrCreateFor<Executable>(op, op->getBlock())->isLive())
    return;

  // Get the dense lattice to update.
  AbstractDenseLattice *after = getLattice(op);

  // Get the dense state before the execution of the op.
  const AbstractDenseLattice *before;
  if (Operation *prev = op->getPrevNode())
    before = getLatticeFor(op, prev);
  else
    before = getLatticeFor(op, op->getBlock());

  // If this op implements region control-flow, then control-flow dictates its
  // transfer function.
  if (auto branch = dyn_cast<RegionBranchOpInterface>(op))
    return visitRegionBranchOperation(op, branch, after);

  // If this is a call operation, then join its lattices across known return
  // sites.
  if (auto call = dyn_cast<CallOpInterface>(op))
    return visitCallOperation(call, after);

  // Invoke the operation transfer function.
  visitOperationImpl(op, *before, after);
}

void AbstractDenseDataFlowAnalysis::visitBlock(Block *block) {
  // If the block is not executable, bail out.
  if (!getOrCreateFor<Executable>(block, block)->isLive())
    return;

  // Get the dense lattice to update.
  AbstractDenseLattice *after = getLattice(block);

  // The dense lattices of entry blocks are set by region control-flow or the
  // callgraph.
  if (block->isEntryBlock()) {
    // Check if this block is the entry block of a callable region.
    auto callable = dyn_cast<CallableOpInterface>(block->getParentOp());
    if (callable && callable.getCallableRegion() == block->getParent()) {
      const auto *callsites = getOrCreateFor<PredecessorState>(block, callable);
      // If not all callsites are known, conservatively mark all lattices as
      // having reached their pessimistic fixpoints.
      if (!callsites->allPredecessorsKnown())
        return setToEntryState(after);
      for (Operation *callsite : callsites->getKnownPredecessors()) {
        // Get the dense lattice before the callsite.
        const AbstractDenseLattice *before;
        if (Operation *prev = callsite->getPrevNode())
          before = getLatticeFor(block, prev);
        else
          before = getLatticeFor(block, callsite->getBlock());

        visitCallControlFlowTransfer(cast<CallOpInterface>(callsite),
                                     CallControlFlowAction::EnterCallee,
                                     *before, after);
      }
      return;
    }

    // Check if we can reason about the control-flow.
    if (auto branch = dyn_cast<RegionBranchOpInterface>(block->getParentOp()))
      return visitRegionBranchOperation(block, branch, after);

    // Otherwise, we can't reason about the data-flow.
    return setToEntryState(after);
  }

  // Join the state with the state after the block's predecessors.
  for (Block::pred_iterator it = block->pred_begin(), e = block->pred_end();
       it != e; ++it) {
    // Skip control edges that aren't executable.
    Block *predecessor = *it;
    if (!getOrCreateFor<Executable>(
             block, getProgramPoint<CFGEdge>(predecessor, block))
             ->isLive())
      continue;

    // Merge in the state from the predecessor's terminator.
    join(after, *getLatticeFor(block, predecessor->getTerminator()));
  }
}

void AbstractDenseDataFlowAnalysis::visitRegionBranchOperation(
    ProgramPoint point, RegionBranchOpInterface branch,
    AbstractDenseLattice *after) {
  // Get the terminator predecessors.
  const auto *predecessors = getOrCreateFor<PredecessorState>(point, point);
  assert(predecessors->allPredecessorsKnown() &&
         "unexpected unresolved region successors");

  for (Operation *op : predecessors->getKnownPredecessors()) {
    const AbstractDenseLattice *before;
    // If the predecessor is the parent, get the state before the parent.
    if (op == branch) {
      if (Operation *prev = op->getPrevNode())
        before = getLatticeFor(point, prev);
      else
        before = getLatticeFor(point, op->getBlock());

      // Otherwise, get the state after the terminator.
    } else {
      before = getLatticeFor(point, op);
    }

    // This function is called in two cases:
    //   1. when visiting the block (point = block);
    //   2. when visiting the parent operation (point = parent op).
    // In both cases, we are looking for predecessor operations of the point,
    //   1. predecessor may be the terminator of another block from another
    //   region (assuming that the block does belong to another region via an
    //   assertion) or the parent (when parent can transfer control to this
    //   region);
    //   2. predecessor may be the terminator of a block that exits the
    //   region (when region transfers control to the parent) or the operation
    //   before the parent.
    // In the latter case, just perform the join as it isn't the control flow
    // affected by the region.
    std::optional<unsigned> regionFrom =
        op == branch ? std::optional<unsigned>()
                     : op->getBlock()->getParent()->getRegionNumber();
    if (auto *toBlock = point.dyn_cast<Block *>()) {
      assert(op == branch ||
             toBlock->getParent() != op->getBlock()->getParent());
      unsigned regionTo = toBlock->getParent()->getRegionNumber();
      visitRegionBranchControlFlowTransfer(branch, regionFrom, regionTo,
                                           *before, after);
    } else {
      assert(point.get<Operation *>() == branch &&
             "expected to be visiting the branch itself");
      // Only need to call the arc transfer when the predecessor is the region
      // or the op itself, not the previous op.
      if (op->getParentOp() == branch || op == branch) {
        visitRegionBranchControlFlowTransfer(
            branch, regionFrom, /*regionTo=*/std::nullopt, *before, after);
      } else {
        join(after, *before);
      }
    }
  }
}

const AbstractDenseLattice *
AbstractDenseDataFlowAnalysis::getLatticeFor(ProgramPoint dependent,
                                             ProgramPoint point) {
  AbstractDenseLattice *state = getLattice(point);
  addDependency(state, dependent);
  return state;
}

//===----------------------------------------------------------------------===//
// AbstractDenseBackwardDataFlowAnalysis
//===----------------------------------------------------------------------===//

LogicalResult
AbstractDenseBackwardDataFlowAnalysis::initialize(Operation *top) {
  // Visit every operation and block.
  processOperation(top);
  for (Region &region : top->getRegions()) {
    for (Block &block : region) {
      visitBlock(&block);
      for (Operation &op : llvm::reverse(block)) {
        if (failed(initialize(&op)))
          return failure();
      }
    }
  }
  return success();
}

LogicalResult AbstractDenseBackwardDataFlowAnalysis::visit(ProgramPoint point) {
  if (auto *op = llvm::dyn_cast_if_present<Operation *>(point))
    processOperation(op);
  else if (auto *block = llvm::dyn_cast_if_present<Block *>(point))
    visitBlock(block);
  else
    return failure();
  return success();
}

void AbstractDenseBackwardDataFlowAnalysis::visitCallOperation(
    CallOpInterface call, AbstractDenseLattice *before) {
  // Find the callee.
  Operation *callee = call.resolveCallable(&symbolTable);
  auto callable = dyn_cast_or_null<CallableOpInterface>(callee);
  if (!callable)
    return setToExitState(before);

  // No region means the callee is only declared in this module and we shouldn't
  // assume anything about it.
  Region *region = callable.getCallableRegion();
  if (!region || region->empty())
    return setToExitState(before);

  // Call-level control flow specifies the data flow here.
  //
  //   func.func @callee() {
  //     ^calleeEntryBlock:
  //     // latticeAtCalleeEntry
  //     ...
  //   }
  //   func.func @caller() {
  //     ...
  //     // latticeBeforeCall
  //     call @callee
  //     ...
  //   }
  Block *calleeEntryBlock = &region->front();
  ProgramPoint calleeEntry = calleeEntryBlock->empty()
                                 ? ProgramPoint(calleeEntryBlock)
                                 : &calleeEntryBlock->front();
  const AbstractDenseLattice &latticeAtCalleeEntry =
      *getLatticeFor(call.getOperation(), calleeEntry);
  AbstractDenseLattice *latticeBeforeCall = before;
  visitCallControlFlowTransfer(call, CallControlFlowAction::EnterCallee,
                               latticeAtCalleeEntry, latticeBeforeCall);
}

void AbstractDenseBackwardDataFlowAnalysis::processOperation(Operation *op) {
  // If the containing block is not executable, bail out.
  if (!getOrCreateFor<Executable>(op, op->getBlock())->isLive())
    return;

  // Get the dense lattice to update.
  AbstractDenseLattice *before = getLattice(op);

  // Get the dense state after execution of this op.
  const AbstractDenseLattice *after;
  if (Operation *next = op->getNextNode())
    after = getLatticeFor(op, next);
  else
    after = getLatticeFor(op, op->getBlock());

  // Special cases where control flow may dictate data flow.
  if (auto branch = dyn_cast<RegionBranchOpInterface>(op))
    return visitRegionBranchOperation(op, branch, std::nullopt, before);
  if (auto call = dyn_cast<CallOpInterface>(op))
    return visitCallOperation(call, before);

  // Invoke the operation transfer function.
  visitOperationImpl(op, *after, before);
}

void AbstractDenseBackwardDataFlowAnalysis::visitBlock(Block *block) {
  // If the block is not executable, bail out.
  if (!getOrCreateFor<Executable>(block, block)->isLive())
    return;

  AbstractDenseLattice *before = getLattice(block);

  // We need "exit" blocks, i.e. the blocks that may return control to the
  // parent operation.
  auto isExitBlock = [](Block *b) {
    // Treat empty and terminator-less blocks as exit blocks.
    if (b->empty() || !b->back().mightHaveTrait<OpTrait::IsTerminator>())
      return true;

    // There may be a weird case where a terminator may be transferring control
    // either to the parent or to another block, so exit blocks and successors
    // are not mutually exclusive.
    Operation *terminator = b->getTerminator();
    return terminator && (terminator->hasTrait<OpTrait::ReturnLike>() ||
                          isa<RegionBranchTerminatorOpInterface>(terminator));
  };
  if (isExitBlock(block)) {
    // If this block is exiting from a callable, the successors of exiting from
    // a callable are the successors of all call sites. And the call sites
    // themselves are predecessors of the callable.
    auto callable = dyn_cast<CallableOpInterface>(block->getParentOp());
    if (callable && callable.getCallableRegion() == block->getParent()) {
      const auto *callsites = getOrCreateFor<PredecessorState>(block, callable);
      // If not all call sites are known, conservative mark all lattices as
      // having reached their pessimistic fix points.
      if (!callsites->allPredecessorsKnown())
        return setToExitState(before);

      for (Operation *callsite : callsites->getKnownPredecessors()) {
        const AbstractDenseLattice *after;
        if (Operation *next = callsite->getNextNode())
          after = getLatticeFor(block, next);
        else
          after = getLatticeFor(block, callsite->getBlock());
        visitCallControlFlowTransfer(cast<CallOpInterface>(callsite),
                                     CallControlFlowAction::ExitCallee, *after,
                                     before);
      }
      return;
    }

    // If this block is exiting from an operation with region-based control
    // flow, propagate the lattice back along the control flow edge.
    if (auto branch = dyn_cast<RegionBranchOpInterface>(block->getParentOp())) {
      visitRegionBranchOperation(block, branch,
                                 block->getParent()->getRegionNumber(), before);
      return;
    }

    // Cannot reason about successors of an exit block, set the pessimistic
    // fixpoint.
    return setToExitState(before);
  }

  // Meet the state with the state before block's successors.
  for (Block *successor : block->getSuccessors()) {
    if (!getOrCreateFor<Executable>(block,
                                    getProgramPoint<CFGEdge>(block, successor))
             ->isLive())
      continue;

    // Merge in the state from the successor: either the first operation, or the
    // block itself when empty.
    if (successor->empty())
      meet(before, *getLatticeFor(block, successor));
    else
      meet(before, *getLatticeFor(block, &successor->front()));
  }
}

void AbstractDenseBackwardDataFlowAnalysis::visitRegionBranchOperation(
    ProgramPoint point, RegionBranchOpInterface branch,
    std::optional<unsigned> regionNo, AbstractDenseLattice *before) {

  // The successors of the operation may be either the first operation of the
  // entry block of each possible successor region, or the next operation when
  // the branch is a successor of itself.
  SmallVector<RegionSuccessor> successors;
  branch.getSuccessorRegions(regionNo, successors);
  for (const RegionSuccessor &successor : successors) {
    const AbstractDenseLattice *after;
    if (successor.isParent() || successor.getSuccessor()->empty()) {
      if (Operation *next = branch->getNextNode())
        after = getLatticeFor(point, next);
      else
        after = getLatticeFor(point, branch->getBlock());
    } else {
      Region *successorRegion = successor.getSuccessor();
      assert(!successorRegion->empty() && "unexpected empty successor region");
      Block *successorBlock = &successorRegion->front();

      if (!getOrCreateFor<Executable>(point, successorBlock)->isLive())
        continue;

      if (successorBlock->empty())
        after = getLatticeFor(point, successorBlock);
      else
        after = getLatticeFor(point, &successorBlock->front());
    }
    std::optional<unsigned> successorNo =
        successor.isParent() ? std::optional<unsigned>()
                             : successor.getSuccessor()->getRegionNumber();
    visitRegionBranchControlFlowTransfer(branch, regionNo, successorNo, *after,
                                         before);
  }
}

const AbstractDenseLattice *
AbstractDenseBackwardDataFlowAnalysis::getLatticeFor(ProgramPoint dependent,
                                                     ProgramPoint point) {
  AbstractDenseLattice *state = getLattice(point);
  addDependency(state, dependent);
  return state;
}