RemoveDeadValues.cpp

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//===- RemoveDeadValues.cpp - Remove Dead Values --------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The goal of this pass is optimization (reducing runtime) by removing
// unnecessary instructions. Unlike other passes that rely on local information
// gathered from patterns to accomplish optimization, this pass uses a full
// analysis of the IR, specifically, liveness analysis, and is thus more
// powerful.
//
// Currently, this pass performs the following optimizations:
// (A) Removes function arguments that are not live,
// (B) Removes function return values that are not live across all callers of
// the function,
// (C) Removes unneccesary operands, results, region arguments, and region
// terminator operands of region branch ops, and,
// (D) Removes simple and region branch ops that have all non-live results and
// don't affect memory in any way,
//
// iff
//
// the IR doesn't have any non-function symbol ops, non-call symbol user ops and
// branch ops.
//
// Here, a "simple op" refers to an op that isn't a symbol op, symbol-user op,
// region branch op, branch op, region branch terminator op, or return-like.
//
//===----------------------------------------------------------------------===//
 
#include "mlir/Analysis/DataFlow/DeadCodeAnalysis.h"
#include "mlir/Analysis/DataFlow/LivenessAnalysis.h"
#include "mlir/Dialect/UB/IR/UBOps.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/BuiltinAttributes.h"
#include "mlir/IR/Dialect.h"
#include "mlir/IR/Operation.h"
#include "mlir/IR/OperationSupport.h"
#include "mlir/IR/SymbolTable.h"
#include "mlir/IR/Value.h"
#include "mlir/IR/ValueRange.h"
#include "mlir/IR/Visitors.h"
#include "mlir/Interfaces/CallInterfaces.h"
#include "mlir/Interfaces/ControlFlowInterfaces.h"
#include "mlir/Interfaces/FunctionInterfaces.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"
#include "mlir/Pass/Pass.h"
#include "mlir/Support/LLVM.h"
#include "mlir/Transforms/FoldUtils.h"
#include "mlir/Transforms/Passes.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugLog.h"
#include <cassert>
#include <cstddef>
#include <memory>
#include <optional>
#include <vector>
 
#define DEBUG_TYPE "remove-dead-values"
 
namespace mlir {
#define GEN_PASS_DEF_REMOVEDEADVALUES
#include "mlir/Transforms/Passes.h.inc"
} // namespace mlir
 
using namespace mlir;
using namespace mlir::dataflow;
 
//===----------------------------------------------------------------------===//
// RemoveDeadValues Pass
//===----------------------------------------------------------------------===//
 
namespace {
 
// Set of structures below to be filled with operations and arguments to erase.
// This is done to separate analysis and tree modification phases,
// otherwise analysis is operating on half-deleted tree which is incorrect.
 
struct FunctionToCleanUp {
  FunctionOpInterface funcOp;
  BitVector nonLiveArgs;
  BitVector nonLiveRets;
};
 
struct ResultsToCleanup {
  Operation *op;
  BitVector nonLive;
};
 
struct OperandsToCleanup {
  Operation *op;
  BitVector nonLive;
  Operation *callee =
      nullptr; // Optional: For CallOpInterface ops, stores the callee function
};
 
struct BlockArgsToCleanup {
  Block *b;
  BitVector nonLiveArgs;
};
 
struct SuccessorOperandsToCleanup {
  BranchOpInterface branch;
  unsigned successorIndex;
  BitVector nonLiveOperands;
};
 
struct RDVFinalCleanupList {
  SmallVector<Operation *> operations;
  SmallVector<FunctionToCleanUp> functions;
  SmallVector<OperandsToCleanup> operands;
  SmallVector<ResultsToCleanup> results;
  SmallVector<BlockArgsToCleanup> blocks;
  SmallVector<SuccessorOperandsToCleanup> successorOperands;
};
 
// Some helper functions...
 
/// Return true iff at least one value in `values` is live, given the liveness
/// information in `la`.
static bool hasLive(ValueRange values, const DenseSet<Value> &nonLiveSet,
                    RunLivenessAnalysis &la) {
  for (Value value : values) {
    if (nonLiveSet.contains(value)) {
      LDBG() << "Value " << value << " is already marked non-live (dead)";
      continue;
    }
 
    const Liveness *liveness = la.getLiveness(value);
    if (!liveness) {
      LDBG() << "Value " << value
             << " has no liveness info, conservatively considered live";
      return true;
    }
    if (liveness->isLive) {
      LDBG() << "Value " << value << " is live according to liveness analysis";
      return true;
    } else {
      LDBG() << "Value " << value << " is dead according to liveness analysis";
    }
  }
  return false;
}
 
/// Return a BitVector of size `values.size()` where its i-th bit is 1 iff the
/// i-th value in `values` is live, given the liveness information in `la`.
static BitVector markLives(ValueRange values, const DenseSet<Value> &nonLiveSet,
                           RunLivenessAnalysis &la) {
  BitVector lives(values.size(), true);
 
  for (auto [index, value] : llvm::enumerate(values)) {
    if (nonLiveSet.contains(value)) {
      lives.reset(index);
      LDBG() << "Value " << value
             << " is already marked non-live (dead) at index " << index;
      continue;
    }
 
    const Liveness *liveness = la.getLiveness(value);
    // It is important to note that when `liveness` is null, we can't tell if
    // `value` is live or not. So, the safe option is to consider it live. Also,
    // the execution of this pass might create new SSA values when erasing some
    // of the results of an op and we know that these new values are live
    // (because they weren't erased) and also their liveness is null because
    // liveness analysis ran before their creation.
    if (!liveness) {
      LDBG() << "Value " << value << " at index " << index
             << " has no liveness info, conservatively considered live";
      continue;
    }
    if (!liveness->isLive) {
      lives.reset(index);
      LDBG() << "Value " << value << " at index " << index
             << " is dead according to liveness analysis";
    } else {
      LDBG() << "Value " << value << " at index " << index
             << " is live according to liveness analysis";
    }
  }
 
  return lives;
}
 
/// Collects values marked as "non-live" in the provided range and inserts them
/// into the nonLiveSet. A value is considered "non-live" if the corresponding
/// index in the `nonLive` bit vector is set.
static void collectNonLiveValues(DenseSet<Value> &nonLiveSet, ValueRange range,
                                 const BitVector &nonLive) {
  for (auto [index, result] : llvm::enumerate(range)) {
    if (!nonLive[index])
      continue;
    nonLiveSet.insert(result);
    LDBG() << "Marking value " << result << " as non-live (dead) at index "
           << index;
  }
}
 
/// Drop the uses of the i-th result of `op` and then erase it iff toErase[i]
/// is 1.
static void dropUsesAndEraseResults(Operation *op, BitVector toErase) {
  assert(op->getNumResults() == toErase.size() &&
         "expected the number of results in `op` and the size of `toErase` to "
         "be the same");
  for (auto idx : toErase.set_bits())
    op->getResult(idx).dropAllUses();
  IRRewriter rewriter(op);
  rewriter.eraseOpResults(op, toErase);
}
 
/// Convert a list of `Operand`s to a list of `OpOperand`s.
static SmallVector<OpOperand *> operandsToOpOperands(OperandRange operands) {
  OpOperand *values = operands.getBase();
  SmallVector<OpOperand *> opOperands;
  for (unsigned i = 0, e = operands.size(); i < e; i++)
    opOperands.push_back(&values[i]);
  return opOperands;
}
 
/// Process a simple operation `op` using the liveness analysis `la`.
/// If the operation has no memory effects and none of its results are live:
///   1. Add the operation to a list for future removal, and
///   2. Mark all its results as non-live values
///
/// The operation `op` is assumed to be simple. A simple operation is one that
/// is NOT:
///   - Function-like
///   - Call-like
///   - A region branch operation
///   - A branch operation
///   - A region branch terminator
///   - Return-like
static void processSimpleOp(Operation *op, RunLivenessAnalysis &la,
                            DenseSet<Value> &nonLiveSet,
                            RDVFinalCleanupList &cl) {
  // Operations that have dead operands can be erased regardless of their
  // side effects. The liveness analysis would not have marked an SSA value as
  // "dead" if it had a side-effecting user that is reachable.
  bool hasDeadOperand =
      markLives(op->getOperands(), nonLiveSet, la).flip().any();
  if (hasDeadOperand) {
    LDBG() << "Simple op has dead operands, so the op must be dead: "
           << OpWithFlags(op,
                          OpPrintingFlags().skipRegions().printGenericOpForm());
    assert(!hasLive(op->getResults(), nonLiveSet, la) &&
           "expected the op to have no live results");
    cl.operations.push_back(op);
    collectNonLiveValues(nonLiveSet, op->getResults(),
                         BitVector(op->getNumResults(), true));
    return;
  }
 
  if (!isMemoryEffectFree(op) || hasLive(op->getResults(), nonLiveSet, la)) {
    LDBG() << "Simple op is not memory effect free or has live results, "
              "preserving it: "
           << OpWithFlags(op,
                          OpPrintingFlags().skipRegions().printGenericOpForm());
    return;
  }
 
  LDBG()
      << "Simple op has all dead results and is memory effect free, scheduling "
         "for removal: "
      << OpWithFlags(op, OpPrintingFlags().skipRegions().printGenericOpForm());
  cl.operations.push_back(op);
  collectNonLiveValues(nonLiveSet, op->getResults(),
                       BitVector(op->getNumResults(), true));
}
 
/// Process a function-like operation `funcOp` using the liveness analysis `la`
/// and the IR in `module`. If it is not public or external:
///   (1) Adding its non-live arguments to a list for future removal.
///   (2) Marking their corresponding operands in its callers for removal.
///   (3) Identifying and enqueueing unnecessary terminator operands
///       (return values that are non-live across all callers) for removal.
///   (4) Enqueueing the non-live arguments and return values for removal.
///   (5) Collecting the uses of these return values in its callers for future
///       removal.
///   (6) Marking all its results as non-live values.
static void processFuncOp(FunctionOpInterface funcOp, Operation *module,
                          RunLivenessAnalysis &la, DenseSet<Value> &nonLiveSet,
                          RDVFinalCleanupList &cl) {
  LDBG() << "Processing function op: "
         << OpWithFlags(funcOp,
                        OpPrintingFlags().skipRegions().printGenericOpForm());
  if (funcOp.isPublic() || funcOp.isExternal()) {
    LDBG() << "Function is public or external, skipping: "
           << funcOp.getOperation()->getName();
    return;
  }
 
  // Get the list of unnecessary (non-live) arguments in `nonLiveArgs`.
  SmallVector<Value> arguments(funcOp.getArguments());
  BitVector nonLiveArgs = markLives(arguments, nonLiveSet, la);
  nonLiveArgs = nonLiveArgs.flip();
 
  // Do (1).
  for (auto [index, arg] : llvm::enumerate(arguments))
    if (arg && nonLiveArgs[index])
      nonLiveSet.insert(arg);
 
  // Do (2). (Skip creating generic operand cleanup entries for call ops.
  // Call arguments will be removed in the call-site specific segment-aware
  // cleanup, avoiding generic eraseOperands bitvector mechanics.)
  SymbolTable::UseRange uses = *funcOp.getSymbolUses(module);
  for (SymbolTable::SymbolUse use : uses) {
    Operation *callOp = use.getUser();
    assert(isa<CallOpInterface>(callOp) && "expected a call-like user");
    // Push an empty operand cleanup entry so that call-site specific logic in
    // cleanUpDeadVals runs (it keys off CallOpInterface). The BitVector is
    // intentionally all false to avoid generic erasure.
    // Store the funcOp as the callee to avoid expensive symbol lookup later.
    cl.operands.push_back({callOp, BitVector(callOp->getNumOperands(), false),
                           funcOp.getOperation()});
  }
 
  // Do (3).
  // Get the list of unnecessary terminator operands (return values that are
  // non-live across all callers) in `nonLiveRets`. There is a very important
  // subtlety here. Unnecessary terminator operands are NOT the operands of the
  // terminator that are non-live. Instead, these are the return values of the
  // callers such that a given return value is non-live across all callers. Such
  // corresponding operands in the terminator could be live. An example to
  // demonstrate this:
  //  func.func private @f(%arg0: memref<i32>) -> (i32, i32) {
  //    %c0_i32 = arith.constant 0 : i32
  //    %0 = arith.addi %c0_i32, %c0_i32 : i32
  //    memref.store %0, %arg0[] : memref<i32>
  //    return %c0_i32, %0 : i32, i32
  //  }
  //  func.func @main(%arg0: i32, %arg1: memref<i32>) -> (i32) {
  //    %1:2 = call @f(%arg1) : (memref<i32>) -> i32
  //    return %1#0 : i32
  //  }
  // Here, we can see that %1#1 is never used. It is non-live. Thus, @f doesn't
  // need to return %0. But, %0 is live. And, still, we want to stop it from
  // being returned, in order to optimize our IR. So, this demonstrates how we
  // can make our optimization strong by even removing a live return value (%0),
  // since it forwards only to non-live value(s) (%1#1).
  size_t numReturns = funcOp.getNumResults();
  BitVector nonLiveRets(numReturns, true);
  for (SymbolTable::SymbolUse use : uses) {
    Operation *callOp = use.getUser();
    assert(isa<CallOpInterface>(callOp) && "expected a call-like user");
    BitVector liveCallRets = markLives(callOp->getResults(), nonLiveSet, la);
    nonLiveRets &= liveCallRets.flip();
  }
 
  // Note that in the absence of control flow ops forcing the control to go from
  // the entry (first) block to the other blocks, the control never reaches any
  // block other than the entry block, because every block has a terminator.
  for (Block &block : funcOp.getBlocks()) {
    Operation *returnOp = block.getTerminator();
    if (!returnOp->hasTrait<OpTrait::ReturnLike>())
      continue;
    if (returnOp && returnOp->getNumOperands() == numReturns)
      cl.operands.push_back({returnOp, nonLiveRets});
  }
 
  // Do (4).
  cl.functions.push_back({funcOp, nonLiveArgs, nonLiveRets});
 
  // Do (5) and (6).
  if (numReturns == 0)
    return;
  for (SymbolTable::SymbolUse use : uses) {
    Operation *callOp = use.getUser();
    assert(isa<CallOpInterface>(callOp) && "expected a call-like user");
    cl.results.push_back({callOp, nonLiveRets});
    collectNonLiveValues(nonLiveSet, callOp->getResults(), nonLiveRets);
  }
}
 
/// Process a region branch operation `regionBranchOp` using the liveness
/// information in `la`. The processing involves two scenarios:
///
/// Scenario 1: If the operation has no memory effects and none of its results
/// are live:
///   (1') Enqueue all its uses for deletion.
///   (2') Enqueue the branch itself for deletion.
///
/// Scenario 2: Otherwise:
///   (1) Collect its unnecessary operands (operands forwarded to unnecessary
///       results or arguments).
///   (2) Process each of its regions.
///   (3) Collect the uses of its unnecessary results (results forwarded from
///       unnecessary operands
///       or terminator operands).
///   (4) Add these results to the deletion list.
///
/// Processing a region includes:
///   (a) Collecting the uses of its unnecessary arguments (arguments forwarded
///       from unnecessary operands
///       or terminator operands).
///   (b) Collecting these unnecessary arguments.
///   (c) Collecting its unnecessary terminator operands (terminator operands
///       forwarded to unnecessary results
///       or arguments).
///
/// Value Flow Note: In this operation, values flow as follows:
/// - From operands and terminator operands (successor operands)
/// - To arguments and results (successor inputs).
static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
                                  RunLivenessAnalysis &la,
                                  DenseSet<Value> &nonLiveSet,
                                  RDVFinalCleanupList &cl) {
  LDBG() << "Processing region branch op: "
         << OpWithFlags(regionBranchOp,
                        OpPrintingFlags().skipRegions().printGenericOpForm());
 
  // Scenario 1. This is the only case where the entire `regionBranchOp`
  // is removed. It will not happen in any other scenario. Note that in this
  // case, a non-forwarded operand of `regionBranchOp` could be live/non-live.
  // It could never be live because of this op but its liveness could have been
  // attributed to something else.
  // Do (1') and (2').
  if (isMemoryEffectFree(regionBranchOp.getOperation()) &&
      !hasLive(regionBranchOp->getResults(), nonLiveSet, la)) {
    cl.operations.push_back(regionBranchOp.getOperation());
    return;
  }
 
  // Mark live results of `regionBranchOp` in `liveResults`.
  auto markLiveResults = [&](BitVector &liveResults) {
    liveResults = markLives(regionBranchOp->getResults(), nonLiveSet, la);
  };
 
  // Mark live arguments in the regions of `regionBranchOp` in `liveArgs`.
  auto markLiveArgs = [&](DenseMap<Region *, BitVector> &liveArgs) {
    for (Region &region : regionBranchOp->getRegions()) {
      if (region.empty())
        continue;
      SmallVector<Value> arguments(region.front().getArguments());
      BitVector regionLiveArgs = markLives(arguments, nonLiveSet, la);
      liveArgs[&region] = regionLiveArgs;
    }
  };
 
  // Return the successors of `region` if the latter is not null. Else return
  // the successors of `regionBranchOp`.
  auto getSuccessors = [&](RegionBranchPoint point) {
    SmallVector<RegionSuccessor> successors;
    regionBranchOp.getSuccessorRegions(point, successors);
    return successors;
  };
 
  // Return the operands of `terminator` that are forwarded to `successor` if
  // the former is not null. Else return the operands of `regionBranchOp`
  // forwarded to `successor`.
  auto getForwardedOpOperands = [&](RegionBranchPoint src,
                                    const RegionSuccessor &successor) {
    SmallVector<OpOperand *> opOperands = operandsToOpOperands(
        regionBranchOp.getSuccessorOperands(src, successor));
    return opOperands;
  };
 
  // Mark the non-forwarded operands of `regionBranchOp` in
  // `nonForwardedOperands`.
  auto markNonForwardedOperands = [&](BitVector &nonForwardedOperands) {
    nonForwardedOperands.resize(regionBranchOp->getNumOperands(), true);
    for (const RegionSuccessor &successor :
         getSuccessors(RegionBranchPoint::parent())) {
      for (OpOperand *opOperand :
           getForwardedOpOperands(RegionBranchPoint::parent(), successor))
        nonForwardedOperands.reset(opOperand->getOperandNumber());
    }
  };
 
  // Mark the non-forwarded terminator operands of the various regions of
  // `regionBranchOp` in `nonForwardedRets`.
  auto markNonForwardedReturnValues =
      [&](DenseMap<Operation *, BitVector> &nonForwardedRets) {
        for (Region &region : regionBranchOp->getRegions()) {
          if (region.empty())
            continue;
          // TODO: this isn't correct in face of multiple terminators.
          auto terminator = cast<RegionBranchTerminatorOpInterface>(
              region.front().getTerminator());
          nonForwardedRets[terminator] =
              BitVector(terminator->getNumOperands(), true);
          for (const RegionSuccessor &successor : getSuccessors(terminator)) {
            for (OpOperand *opOperand : getForwardedOpOperands(
                     RegionBranchPoint(terminator), successor))
              nonForwardedRets[terminator].reset(opOperand->getOperandNumber());
          }
        }
      };
 
  // Update `valuesToKeep` (which is expected to correspond to operands or
  // terminator operands) based on `resultsToKeep` and `argsToKeep`, given
  // `region`. When `valuesToKeep` correspond to operands, `region` is null.
  // Else, `region` is the parent region of the terminator.
  auto updateOperandsOrTerminatorOperandsToKeep =
      [&](BitVector &valuesToKeep, BitVector &resultsToKeep,
          DenseMap<Region *, BitVector> &argsToKeep, Region *region = nullptr) {
        Operation *terminator =
            region ? region->front().getTerminator() : nullptr;
        RegionBranchPoint point =
            terminator
                ? RegionBranchPoint(
                      cast<RegionBranchTerminatorOpInterface>(terminator))
                : RegionBranchPoint::parent();
 
        for (const RegionSuccessor &successor : getSuccessors(point)) {
          Region *successorRegion = successor.getSuccessor();
          for (auto [opOperand, input] :
               llvm::zip(getForwardedOpOperands(point, successor),
                         successor.getSuccessorInputs())) {
            size_t operandNum = opOperand->getOperandNumber();
            bool updateBasedOn =
                successorRegion
                    ? argsToKeep[successorRegion]
                                [cast<BlockArgument>(input).getArgNumber()]
                    : resultsToKeep[cast<OpResult>(input).getResultNumber()];
            valuesToKeep[operandNum] = valuesToKeep[operandNum] | updateBasedOn;
          }
        }
      };
 
  // Recompute `resultsToKeep` and `argsToKeep` based on `operandsToKeep` and
  // `terminatorOperandsToKeep`. Store true in `resultsOrArgsToKeepChanged` if a
  // value is modified, else, false.
  auto recomputeResultsAndArgsToKeep =
      [&](BitVector &resultsToKeep, DenseMap<Region *, BitVector> &argsToKeep,
          BitVector &operandsToKeep,
          DenseMap<Operation *, BitVector> &terminatorOperandsToKeep,
          bool &resultsOrArgsToKeepChanged) {
        resultsOrArgsToKeepChanged = false;
 
        // Recompute `resultsToKeep` and `argsToKeep` based on `operandsToKeep`.
        for (const RegionSuccessor &successor :
             getSuccessors(RegionBranchPoint::parent())) {
          Region *successorRegion = successor.getSuccessor();
          for (auto [opOperand, input] :
               llvm::zip(getForwardedOpOperands(RegionBranchPoint::parent(),
                                                successor),
                         successor.getSuccessorInputs())) {
            bool recomputeBasedOn =
                operandsToKeep[opOperand->getOperandNumber()];
            bool toRecompute =
                successorRegion
                    ? argsToKeep[successorRegion]
                                [cast<BlockArgument>(input).getArgNumber()]
                    : resultsToKeep[cast<OpResult>(input).getResultNumber()];
            if (!toRecompute && recomputeBasedOn)
              resultsOrArgsToKeepChanged = true;
            if (successorRegion) {
              argsToKeep[successorRegion][cast<BlockArgument>(input)
                                              .getArgNumber()] =
                  argsToKeep[successorRegion]
                            [cast<BlockArgument>(input).getArgNumber()] |
                  recomputeBasedOn;
            } else {
              resultsToKeep[cast<OpResult>(input).getResultNumber()] =
                  resultsToKeep[cast<OpResult>(input).getResultNumber()] |
                  recomputeBasedOn;
            }
          }
        }
 
        // Recompute `resultsToKeep` and `argsToKeep` based on
        // `terminatorOperandsToKeep`.
        for (Region &region : regionBranchOp->getRegions()) {
          if (region.empty())
            continue;
          auto terminator = cast<RegionBranchTerminatorOpInterface>(
              region.front().getTerminator());
          for (const RegionSuccessor &successor : getSuccessors(terminator)) {
            Region *successorRegion = successor.getSuccessor();
            for (auto [opOperand, input] :
                 llvm::zip(getForwardedOpOperands(RegionBranchPoint(terminator),
                                                  successor),
                           successor.getSuccessorInputs())) {
              bool recomputeBasedOn =
                  terminatorOperandsToKeep[region.back().getTerminator()]
                                          [opOperand->getOperandNumber()];
              bool toRecompute =
                  successorRegion
                      ? argsToKeep[successorRegion]
                                  [cast<BlockArgument>(input).getArgNumber()]
                      : resultsToKeep[cast<OpResult>(input).getResultNumber()];
              if (!toRecompute && recomputeBasedOn)
                resultsOrArgsToKeepChanged = true;
              if (successorRegion) {
                argsToKeep[successorRegion][cast<BlockArgument>(input)
                                                .getArgNumber()] =
                    argsToKeep[successorRegion]
                              [cast<BlockArgument>(input).getArgNumber()] |
                    recomputeBasedOn;
              } else {
                resultsToKeep[cast<OpResult>(input).getResultNumber()] =
                    resultsToKeep[cast<OpResult>(input).getResultNumber()] |
                    recomputeBasedOn;
              }
            }
          }
        }
      };
 
  // Mark the values that we want to keep in `resultsToKeep`, `argsToKeep`,
  // `operandsToKeep`, and `terminatorOperandsToKeep`.
  auto markValuesToKeep =
      [&](BitVector &resultsToKeep, DenseMap<Region *, BitVector> &argsToKeep,
          BitVector &operandsToKeep,
          DenseMap<Operation *, BitVector> &terminatorOperandsToKeep) {
        bool resultsOrArgsToKeepChanged = true;
        // We keep updating and recomputing the values until we reach a point
        // where they stop changing.
        while (resultsOrArgsToKeepChanged) {
          // Update the operands that need to be kept.
          updateOperandsOrTerminatorOperandsToKeep(operandsToKeep,
                                                   resultsToKeep, argsToKeep);
 
          // Update the terminator operands that need to be kept.
          for (Region &region : regionBranchOp->getRegions()) {
            if (region.empty())
              continue;
            updateOperandsOrTerminatorOperandsToKeep(
                terminatorOperandsToKeep[region.back().getTerminator()],
                resultsToKeep, argsToKeep, &region);
          }
 
          // Recompute the results and arguments that need to be kept.
          recomputeResultsAndArgsToKeep(
              resultsToKeep, argsToKeep, operandsToKeep,
              terminatorOperandsToKeep, resultsOrArgsToKeepChanged);
        }
      };
 
  // Scenario 2.
  // At this point, we know that every non-forwarded operand of `regionBranchOp`
  // is live.
 
  // Stores the results of `regionBranchOp` that we want to keep.
  BitVector resultsToKeep;
  // Stores the mapping from regions of `regionBranchOp` to their arguments that
  // we want to keep.
  DenseMap<Region *, BitVector> argsToKeep;
  // Stores the operands of `regionBranchOp` that we want to keep.
  BitVector operandsToKeep;
  // Stores the mapping from region terminators in `regionBranchOp` to their
  // operands that we want to keep.
  DenseMap<Operation *, BitVector> terminatorOperandsToKeep;
 
  // Initializing the above variables...
 
  // The live results of `regionBranchOp` definitely need to be kept.
  markLiveResults(resultsToKeep);
  // Similarly, the live arguments of the regions in `regionBranchOp` definitely
  // need to be kept.
  markLiveArgs(argsToKeep);
  // The non-forwarded operands of `regionBranchOp` definitely need to be kept.
  // A live forwarded operand can be removed but no non-forwarded operand can be
  // removed since it "controls" the flow of data in this control flow op.
  markNonForwardedOperands(operandsToKeep);
  // Similarly, the non-forwarded terminator operands of the regions in
  // `regionBranchOp` definitely need to be kept.
  markNonForwardedReturnValues(terminatorOperandsToKeep);
 
  // Mark the values (results, arguments, operands, and terminator operands)
  // that we want to keep.
  markValuesToKeep(resultsToKeep, argsToKeep, operandsToKeep,
                   terminatorOperandsToKeep);
 
  // Do (1).
  cl.operands.push_back({regionBranchOp, operandsToKeep.flip()});
 
  // Do (2.a) and (2.b).
  for (Region &region : regionBranchOp->getRegions()) {
    if (region.empty())
      continue;
    BitVector argsToRemove = argsToKeep[&region].flip();
    cl.blocks.push_back({&region.front(), argsToRemove});
    collectNonLiveValues(nonLiveSet, region.front().getArguments(),
                         argsToRemove);
  }
 
  // Do (2.c).
  for (Region &region : regionBranchOp->getRegions()) {
    if (region.empty())
      continue;
    Operation *terminator = region.front().getTerminator();
    cl.operands.push_back(
        {terminator, terminatorOperandsToKeep[terminator].flip()});
  }
 
  // Do (3) and (4).
  BitVector resultsToRemove = resultsToKeep.flip();
  collectNonLiveValues(nonLiveSet, regionBranchOp.getOperation()->getResults(),
                       resultsToRemove);
  cl.results.push_back({regionBranchOp.getOperation(), resultsToRemove});
}
 
/// Steps to process a `BranchOpInterface` operation:
///
/// When a non-forwarded operand is dead (e.g., the condition value of a
/// conditional branch op), the entire operation is dead.
///
/// Otherwise, iterate through each successor block of `branchOp`.
/// (1) For each successor block, gather all operands from all successors.
/// (2) Fetch their associated liveness analysis data and collect for future
///     removal.
/// (3) Identify and collect the dead operands from the successor block
///     as well as their corresponding arguments.
 
static void processBranchOp(BranchOpInterface branchOp, RunLivenessAnalysis &la,
                            DenseSet<Value> &nonLiveSet,
                            RDVFinalCleanupList &cl) {
  LDBG() << "Processing branch op: " << *branchOp;
 
  // Check for dead non-forwarded operands.
  BitVector deadNonForwardedOperands =
      markLives(branchOp->getOperands(), nonLiveSet, la).flip();
  unsigned numSuccessors = branchOp->getNumSuccessors();
  for (unsigned succIdx = 0; succIdx < numSuccessors; ++succIdx) {
    SuccessorOperands successorOperands =
        branchOp.getSuccessorOperands(succIdx);
    // Remove all non-forwarded operands from the bit vector.
    for (OpOperand &opOperand : successorOperands.getMutableForwardedOperands())
      deadNonForwardedOperands[opOperand.getOperandNumber()] = false;
  }
  if (deadNonForwardedOperands.any()) {
    cl.operations.push_back(branchOp.getOperation());
    return;
  }
 
  for (unsigned succIdx = 0; succIdx < numSuccessors; ++succIdx) {
    Block *successorBlock = branchOp->getSuccessor(succIdx);
 
    // Do (1)
    SuccessorOperands successorOperands =
        branchOp.getSuccessorOperands(succIdx);
    SmallVector<Value> operandValues;
    for (unsigned operandIdx = 0; operandIdx < successorOperands.size();
         ++operandIdx) {
      operandValues.push_back(successorOperands[operandIdx]);
    }
 
    // Do (2)
    BitVector successorNonLive =
        markLives(operandValues, nonLiveSet, la).flip();
    collectNonLiveValues(nonLiveSet, successorBlock->getArguments(),
                         successorNonLive);
 
    // Do (3)
    cl.blocks.push_back({successorBlock, successorNonLive});
    cl.successorOperands.push_back({branchOp, succIdx, successorNonLive});
  }
}
 
/// Removes dead values collected in RDVFinalCleanupList.
/// To be run once when all dead values have been collected.
static void cleanUpDeadVals(RDVFinalCleanupList &list) {
  LDBG() << "Starting cleanup of dead values...";
 
  // 1. Blocks, We must remove the block arguments and successor operands before
  // deleting the operation, as they may reside in the region operation.
  LDBG() << "Cleaning up " << list.blocks.size() << " block argument lists";
  for (auto &b : list.blocks) {
    // blocks that are accessed via multiple codepaths processed once
    if (b.b->getNumArguments() != b.nonLiveArgs.size())
      continue;
    LDBG_OS([&](raw_ostream &os) {
      os << "Erasing non-live arguments [";
      llvm::interleaveComma(b.nonLiveArgs.set_bits(), os);
      os << "] from block #" << b.b->computeBlockNumber() << " in region #"
         << b.b->getParent()->getRegionNumber() << " of operation "
         << OpWithFlags(b.b->getParent()->getParentOp(),
                        OpPrintingFlags().skipRegions().printGenericOpForm());
    });
    // Note: Iterate from the end to make sure that that indices of not yet
    // processes arguments do not change.
    for (int i = b.nonLiveArgs.size() - 1; i >= 0; --i) {
      if (!b.nonLiveArgs[i])
        continue;
      b.b->getArgument(i).dropAllUses();
      b.b->eraseArgument(i);
    }
  }
 
  // 2. Successor Operands
  LDBG() << "Cleaning up " << list.successorOperands.size()
         << " successor operand lists";
  for (auto &op : list.successorOperands) {
    SuccessorOperands successorOperands =
        op.branch.getSuccessorOperands(op.successorIndex);
    // blocks that are accessed via multiple codepaths processed once
    if (successorOperands.size() != op.nonLiveOperands.size())
      continue;
    LDBG_OS([&](raw_ostream &os) {
      os << "Erasing non-live successor operands [";
      llvm::interleaveComma(op.nonLiveOperands.set_bits(), os);
      os << "] from successor " << op.successorIndex << " of branch: "
         << OpWithFlags(op.branch.getOperation(),
                        OpPrintingFlags().skipRegions().printGenericOpForm());
    });
    // it iterates backwards because erase invalidates all successor indexes
    for (int i = successorOperands.size() - 1; i >= 0; --i) {
      if (!op.nonLiveOperands[i])
        continue;
      successorOperands.erase(i);
    }
  }
 
  // 3. Operations
  LDBG() << "Cleaning up " << list.operations.size() << " operations";
  for (Operation *op : list.operations) {
    LDBG() << "Erasing operation: "
           << OpWithFlags(op,
                          OpPrintingFlags().skipRegions().printGenericOpForm());
    if (op->hasTrait<OpTrait::IsTerminator>()) {
      // When erasing a terminator, insert an unreachable op in its place.
      OpBuilder b(op);
      ub::UnreachableOp::create(b, op->getLoc());
    }
    op->dropAllUses();
    op->erase();
  }
 
  // 4. Functions
  LDBG() << "Cleaning up " << list.functions.size() << " functions";
  // Record which function arguments were erased so we can shrink call-site
  // argument segments for CallOpInterface operations (e.g. ops using
  // AttrSizedOperandSegments) in the next phase.
  DenseMap<Operation *, BitVector> erasedFuncArgs;
  for (auto &f : list.functions) {
    LDBG() << "Cleaning up function: " << f.funcOp.getOperation()->getName()
           << " (" << f.funcOp.getOperation() << ")";
    LDBG_OS([&](raw_ostream &os) {
      os << "  Erasing non-live arguments [";
      llvm::interleaveComma(f.nonLiveArgs.set_bits(), os);
      os << "]\n";
      os << "  Erasing non-live return values [";
      llvm::interleaveComma(f.nonLiveRets.set_bits(), os);
      os << "]";
    });
    // Drop all uses of the dead arguments.
    for (auto deadIdx : f.nonLiveArgs.set_bits())
      f.funcOp.getArgument(deadIdx).dropAllUses();
    // Some functions may not allow erasing arguments or results. These calls
    // return failure in such cases without modifying the function, so it's okay
    // to proceed.
    if (succeeded(f.funcOp.eraseArguments(f.nonLiveArgs))) {
      // Record only if we actually erased something.
      if (f.nonLiveArgs.any())
        erasedFuncArgs.try_emplace(f.funcOp.getOperation(), f.nonLiveArgs);
    }
    (void)f.funcOp.eraseResults(f.nonLiveRets);
  }
 
  // 5. Operands
  LDBG() << "Cleaning up " << list.operands.size() << " operand lists";
  for (OperandsToCleanup &o : list.operands) {
    // Handle call-specific cleanup only when we have a cached callee reference.
    // This avoids expensive symbol lookup and is defensive against future
    // changes.
    bool handledAsCall = false;
    if (o.callee && isa<CallOpInterface>(o.op)) {
      auto call = cast<CallOpInterface>(o.op);
      auto it = erasedFuncArgs.find(o.callee);
      if (it != erasedFuncArgs.end()) {
        const BitVector &deadArgIdxs = it->second;
        MutableOperandRange args = call.getArgOperandsMutable();
        // First, erase the call arguments corresponding to erased callee
        // args. We iterate backwards to preserve indices.
        for (unsigned argIdx : llvm::reverse(deadArgIdxs.set_bits()))
          args.erase(argIdx);
        // If this operand cleanup entry also has a generic nonLive bitvector,
        // clear bits for call arguments we already erased above to avoid
        // double-erasing (which could impact other segments of ops with
        // AttrSizedOperandSegments).
        if (o.nonLive.any()) {
          // Map the argument logical index to the operand number(s) recorded.
          int operandOffset = call.getArgOperands().getBeginOperandIndex();
          for (int argIdx : deadArgIdxs.set_bits()) {
            int operandNumber = operandOffset + argIdx;
            if (operandNumber < static_cast<int>(o.nonLive.size()))
              o.nonLive.reset(operandNumber);
          }
        }
        handledAsCall = true;
      }
    }
    // Perform generic operand erasure for:
    // - Non-call operations
    // - Call operations without cached callee (where handledAsCall is false)
    // But skip call operations that were already handled via segment-aware path
    if (!handledAsCall && o.nonLive.any()) {
      LDBG_OS([&](raw_ostream &os) {
        os << "Erasing non-live operands [";
        llvm::interleaveComma(o.nonLive.set_bits(), os);
        os << "] from operation: "
           << OpWithFlags(o.op,
                          OpPrintingFlags().skipRegions().printGenericOpForm());
      });
      o.op->eraseOperands(o.nonLive);
    }
  }
 
  // 6. Results
  LDBG() << "Cleaning up " << list.results.size() << " result lists";
  for (auto &r : list.results) {
    LDBG_OS([&](raw_ostream &os) {
      os << "Erasing non-live results [";
      llvm::interleaveComma(r.nonLive.set_bits(), os);
      os << "] from operation: "
         << OpWithFlags(r.op,
                        OpPrintingFlags().skipRegions().printGenericOpForm());
    });
    dropUsesAndEraseResults(r.op, r.nonLive);
  }
  LDBG() << "Finished cleanup of dead values";
}
 
struct RemoveDeadValues : public impl::RemoveDeadValuesBase<RemoveDeadValues> {
  void runOnOperation() override;
};
} // namespace
 
void RemoveDeadValues::runOnOperation() {
  auto &la = getAnalysis<RunLivenessAnalysis>();
  Operation *module = getOperation();
 
  // Tracks values eligible for erasure - complements liveness analysis to
  // identify "droppable" values.
  DenseSet<Value> deadVals;
 
  // Maintains a list of Ops, values, branches, etc., slated for cleanup at the
  // end of this pass.
  RDVFinalCleanupList finalCleanupList;
 
  module->walk([&](Operation *op) {
    if (auto funcOp = dyn_cast<FunctionOpInterface>(op)) {
      processFuncOp(funcOp, module, la, deadVals, finalCleanupList);
    } else if (auto regionBranchOp = dyn_cast<RegionBranchOpInterface>(op)) {
      processRegionBranchOp(regionBranchOp, la, deadVals, finalCleanupList);
    } else if (auto branchOp = dyn_cast<BranchOpInterface>(op)) {
      processBranchOp(branchOp, la, deadVals, finalCleanupList);
    } else if (op->hasTrait<::mlir::OpTrait::IsTerminator>()) {
      // Nothing to do here because this is a terminator op and it should be
      // honored with respect to its parent
    } else if (isa<CallOpInterface>(op)) {
      // Nothing to do because this op is associated with a function op and gets
      // cleaned when the latter is cleaned.
    } else {
      processSimpleOp(op, la, deadVals, finalCleanupList);
    }
  });
 
  cleanUpDeadVals(finalCleanupList);
}
 
std::unique_ptr<Pass> mlir::createRemoveDeadValuesPass() {
  return std::make_unique<RemoveDeadValues>();
}