LLZKRedundantOperationEliminationPass.cpp

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//===-- LLZKRedundantOperationEliminationPass.cpp ---------------*- C++ -*-===//
//
// Part of the LLZK Project, under the Apache License v2.0.
// See LICENSE.txt for license information.
// Copyright 2025 Veridise Inc.
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
//===----------------------------------------------------------------------===//
 
#include "llzk/Analysis/CallGraphAnalyses.h"
#include "llzk/Dialect/Bool/IR/Ops.h"
#include "llzk/Dialect/Constrain/IR/Ops.h"
#include "llzk/Dialect/LLZK/IR/Ops.h"
#include "llzk/Dialect/Struct/IR/Dialect.h"
#include "llzk/Transforms/LLZKTransformationPasses.h"
#include "llzk/Util/EffectHelper.h"
#include "llzk/Util/SymbolHelper.h"
 
#include <mlir/Dialect/Arith/IR/Arith.h>
#include <mlir/IR/BuiltinOps.h>
#include <mlir/IR/Dominance.h>
#include <mlir/IR/OperationSupport.h>
 
#include <llvm/ADT/DenseMap.h>
#include <llvm/ADT/DenseSet.h>
#include <llvm/ADT/Hashing.h>
#include <llvm/ADT/PostOrderIterator.h>
#include <llvm/ADT/SmallVector.h>
 
#include <utility>
 
// Include the generated base pass class definitions.
namespace llzk {
#define GEN_PASS_DEF_REDUNDANTOPERATIONELIMINATIONPASS
#include "llzk/Transforms/LLZKTransformationPasses.h.inc"
} // namespace llzk
 
using namespace mlir;
using namespace llzk;
using namespace llzk::boolean;
using namespace llzk::component;
using namespace llzk::constrain;
using namespace llzk::function;
 
#define DEBUG_TYPE "llzk-duplicate-op-elim"
 
namespace {
 
// Maps original -> replacement value
using TranslationMap = DenseMap<Value, Value>;
 
static bool isDuplicateEliminationCandidate(Operation *op) {
  if (isa<NonDetOp>(op) || op->hasTrait<OpTrait::IsTerminator>() || op->getNumRegions() != 0 ||
      op->getNumSuccessors() != 0) {
    return false;
  }
 
  return isa<ConstraintOpInterface>(op) || isMemoryEffectFree(op);
}

class OperationComparator {
public:
  explicit OperationComparator(Operation *o) : op(o), operands(o->getOperands()) {}
 
  OperationComparator(Operation *o, const TranslationMap &m) : op(o) {
    for (Value operand : op->getOperands()) {
      if (auto it = m.find(operand); it != m.end()) {
        operands.push_back(it->second);
      } else {
        operands.push_back(operand);
      }
    }
  }
 
  Operation *getOp() const { return op; }
 
  const SmallVector<Value> &getOperands() const { return operands; }
 
  bool isCommutative() const { return op->hasTrait<OpTrait::IsCommutative>(); }
 
  friend bool operator==(const OperationComparator &lhs, const OperationComparator &rhs) {
    if (!OperationEquivalence::isEquivalentTo(
            lhs.op, rhs.op, OperationEquivalence::ignoreValueEquivalence,
            /*markEquivalent=*/nullptr, OperationEquivalence::IgnoreLocations
        )) {
      return false;
    }
 
    // Preserve the pass's existing commutative matching for binary operations.
    // For a future n-ary commutative op, exact operand order remains conservative.
    if (lhs.isCommutative() && lhs.operands.size() == 2) {
      return (lhs.operands[0] == rhs.operands[0] && lhs.operands[1] == rhs.operands[1]) ||
             (lhs.operands[0] == rhs.operands[1] && lhs.operands[1] == rhs.operands[0]);
    }
 
    return lhs.operands == rhs.operands;
  }
 
private:
  Operation *op;
  SmallVector<Value> operands;
};
 
} // namespace
 
namespace llvm {
 
template <> struct DenseMapInfo<OperationComparator> {
  static unsigned getHashValue(const OperationComparator &oc) {
    hash_code opHash = mlir::OperationEquivalence::computeHash(
        oc.getOp(), mlir::OperationEquivalence::ignoreHashValue,
        mlir::OperationEquivalence::ignoreHashValue, mlir::OperationEquivalence::IgnoreLocations
    );
 
    ArrayRef<Value> operands = oc.getOperands();
    hash_code operandHash;
    if (oc.isCommutative() && operands.size() == 2) {
      size_t lhsHash = hash_value(operands[0]);
      size_t rhsHash = hash_value(operands[1]);
      if (rhsHash < lhsHash) {
        std::swap(lhsHash, rhsHash);
      }
      operandHash = hash_combine(lhsHash, rhsHash);
    } else {
      operandHash = hash_combine_range(operands.begin(), operands.end());
    }
 
    return hash_combine(opHash, operandHash);
  }
  static bool isEqual(const OperationComparator &lhs, const OperationComparator &rhs) {
    return lhs == rhs;
  }
};
 
} // namespace llvm
 
namespace {
 
class PassImpl : public llzk::impl::RedundantOperationEliminationPassBase<PassImpl> {
  using Base = RedundantOperationEliminationPassBase<PassImpl>;
  using Base::Base;
 
  void runOnOperation() override {
    SymbolTableCollection symbolTables;
    // Traverse functions from the bottom of the call graph up.
    // This way, we may create empty constrain functions to which we can eliminate
    // calls.
    auto &cga = getAnalysis<CallGraphAnalysis>();
    const llzk::CallGraph *callGraph = &cga.getCallGraph();
    for (const llzk::CallGraphNode *node : llvm::post_order(callGraph)) {
      if (!node->isExternal()) {
        runOnFunc(symbolTables, node->getCalledFunction());
      }
    }
  }

  bool isPurposelessConstrainFunc(
      SymbolTableCollection &symbolTables, FuncDefOp fn, DenseSet<Operation *> &activeFunctions
  ) {
    if (!fn.isStructConstrain()) {
      return false;
    }
    // Calls to a constrain function are only removable when the callee cannot
    // contain witness-generation state mutations such as global.write or
    // ram.store. The WitnessGen verifier enforces that boundary unless the
    // callee is explicitly marked allow_witness.
    if (fn.hasAllowWitnessAttr()) {
      return false;
    }
    if (!activeFunctions.insert(fn.getOperation()).second) {
      // A recursive call path cannot be proven purposeless; retain the call
      // conservatively.
      return false;
    }
 
    bool res = true;
    fn.walk([&](Operation *op) {
      if (op == fn.getOperation()) {
        return WalkResult::advance();
      }
      if (isa<EmitEqualityOp, EmitContainmentOp, AssertOp>(op)) {
        res = false;
        return WalkResult::interrupt();
      } else if (auto callOp = dyn_cast<CallOp>(op)) {
        if (!callsPurposelessConstrainFunc(symbolTables, callOp, activeFunctions)) {
          res = false;
          return WalkResult::interrupt();
        }
        return WalkResult::advance();
      } else if (isMemoryEffectFree(op)) {
        return WalkResult::advance();
      }
 
      // Removing a call to a constrain function is only safe when the callee has
      // no unknown or mutating effects.
      if (hasUnknownOrNonReadEffect(op)) {
        res = false;
        return WalkResult::interrupt();
      }
      return WalkResult::advance();
    });
    activeFunctions.erase(fn.getOperation());
    return res;
  }

  bool callsPurposelessConstrainFunc(
      SymbolTableCollection &symbolTables, CallOp call, DenseSet<Operation *> &activeFunctions
  ) {
    auto callLookup = resolveCallable<FuncDefOp>(symbolTables, call);
    return succeeded(callLookup) &&
           isPurposelessConstrainFunc(symbolTables, callLookup->get(), activeFunctions);
  }
 
  void runOnFunc(SymbolTableCollection &symbolTables, CallableOpInterface callable) {
    TranslationMap map;
    SmallVector<Operation *> redundantOps;
    DenseSet<OperationComparator> uniqueOps;
    DominanceInfo domInfo(callable);
 
    auto unnecessaryOpCheck = [&](Operation *op) -> bool {
      if (auto emiteq = dyn_cast<EmitEqualityOp>(op);
          emiteq && emiteq.getLhs() == emiteq.getRhs()) {
        redundantOps.push_back(op);
        return true;
      }
 
      if (auto callOp = dyn_cast<CallOp>(op)) {
        DenseSet<Operation *> activeFunctions;
        if (callsPurposelessConstrainFunc(symbolTables, callOp, activeFunctions)) {
          redundantOps.push_back(op);
          return true;
        }
      }
      return false;
    };
 
    callable.walk([&](Operation *op) {
      if (op == callable.getOperation()) {
        return WalkResult::advance();
      }
 
      // Case 1: The operation itself is unnecessary.
      if (unnecessaryOpCheck(op)) {
        return WalkResult::advance();
      }
 
      // Case 2: An equivalent operation A has already been performed before
      // the current operation B and A dominates B.
      if (isDuplicateEliminationCandidate(op)) {
        OperationComparator comp(op, map);
        if (auto it = uniqueOps.find(comp);
            it != uniqueOps.end() && domInfo.dominates(it->getOp(), op)) {
          redundantOps.push_back(op);
          for (unsigned opNum = 0; opNum < op->getNumResults(); opNum++) {
            map[op->getResult(opNum)] = it->getOp()->getResult(opNum);
          }
        } else {
          uniqueOps.insert(comp);
        }
      }
      return WalkResult::advance();
    });
 
    DenseSet<Operation *> redundantOpSet;
    for (Operation *op : redundantOps) {
      redundantOpSet.insert(op);
    }
 
    SmallVector<Operation *> deadOpCandidates;
    DenseSet<Operation *> queuedDeadOps;
    auto enqueueDeadOpCandidate = [&](Value value) {
      Operation *definingOp = value.getDefiningOp();
      if (!definingOp || redundantOpSet.count(definingOp) ||
          !queuedDeadOps.insert(definingOp).second) {
        return;
      }
      deadOpCandidates.push_back(definingOp);
    };
 
    for (Operation *op : redundantOps) {
      LLVM_DEBUG(llvm::dbgs() << "Removing op: " << *op << '\n');
      for (Value result : op->getResults()) {
        if (!result.use_empty()) {
          auto it = map.find(result);
          ensure(
              it != map.end(), "failed to find a replacement value for redundant operation result"
          );
          LLVM_DEBUG(llvm::dbgs() << "Replacing " << it->first << " with " << it->second << '\n');
          result.replaceAllUsesWith(it->second);
        }
      }
 
      SmallVector<Value> operands(op->getOperands());
      op->erase();
      for (Value operand : operands) {
        enqueueDeadOpCandidate(operand);
      }
    }
 
    // Removing a redundant op may make its producers dead. Check whole
    // operations so effects and every result are considered before erasure.
    while (!deadOpCandidates.empty()) {
      Operation *op = deadOpCandidates.pop_back_val();
      queuedDeadOps.erase(op);
      if (!isOpTriviallyDead(op)) {
        continue;
      }
 
      SmallVector<Value> operands(op->getOperands());
      LLVM_DEBUG(llvm::dbgs() << "Removing dead producer: " << *op << '\n');
      op->erase();
      for (Value operand : operands) {
        enqueueDeadOpCandidate(operand);
      }
    }
  }
};
 
} // namespace