llvm-project/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

//===-- ConstraintElimination.cpp - Eliminate conds using constraints. ----===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Eliminate conditions based on constraints collected from dominating
// conditions.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstraintSystem.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Transforms/Scalar.h"

using namespace llvm;
using namespace PatternMatch;

#define DEBUG_TYPE "constraint-elimination"

STATISTIC(NumCondsRemoved, "Number of instructions removed");
DEBUG_COUNTER(EliminatedCounter, "conds-eliminated",
              "Controls which conditions are eliminated");

static int64_t MaxConstraintValue = std::numeric_limits<int64_t>::max();

Optional<std::pair<int64_t, Value *>> decompose(Value *V) {
  if (auto *CI = dyn_cast<ConstantInt>(V)) {
    if (CI->isNegative() || CI->uge(MaxConstraintValue))
      return {};
    return {{CI->getSExtValue(), nullptr}};
  }
  auto *GEP = dyn_cast<GetElementPtrInst>(V);
  if (GEP && GEP->getNumOperands() == 2 &&
      isa<ConstantInt>(GEP->getOperand(GEP->getNumOperands() - 1))) {
    return {{cast<ConstantInt>(GEP->getOperand(GEP->getNumOperands() - 1))
                 ->getSExtValue(),
             GEP->getPointerOperand()}};
  }
  return {{0, V}};
}

/// Turn a condition \p CmpI into a constraint vector, using indices from \p
/// Value2Index. If \p ShouldAdd is true, new indices are added for values not
/// yet in \p Value2Index.
static SmallVector<int64_t, 8>
getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
              DenseMap<Value *, unsigned> &Value2Index, bool ShouldAdd) {
  Value *A, *B;

  int64_t Offset1 = 0;
  int64_t Offset2 = 0;

  auto TryToGetIndex = [ShouldAdd,
                        &Value2Index](Value *V) -> Optional<unsigned> {
    if (ShouldAdd) {
      Value2Index.insert({V, Value2Index.size() + 1});
      return Value2Index[V];
    }
    auto I = Value2Index.find(V);
    if (I == Value2Index.end())
      return None;
    return I->second;
  };

  if (Pred == CmpInst::ICMP_UGT || Pred == CmpInst::ICMP_UGE)
    return getConstraint(CmpInst::getSwappedPredicate(Pred), Op1, Op0,
                         Value2Index, ShouldAdd);

  if (Pred == CmpInst::ICMP_ULE || Pred == CmpInst::ICMP_ULT) {
    auto ADec = decompose(Op0);
    auto BDec = decompose(Op1);
    if (!ADec || !BDec)
      return {};
    std::tie(Offset1, A) = *ADec;
    std::tie(Offset2, B) = *BDec;
    Offset1 *= -1;

    if (!A && !B)
      return {};

    auto AIdx = A ? TryToGetIndex(A) : None;
    auto BIdx = B ? TryToGetIndex(B) : None;
    if ((A && !AIdx) || (B && !BIdx))
      return {};

    SmallVector<int64_t, 8> R(Value2Index.size() + 1, 0);
    if (AIdx)
      R[*AIdx] = 1;
    if (BIdx)
      R[*BIdx] = -1;
    R[0] = Offset1 + Offset2 + (Pred == CmpInst::ICMP_ULT ? -1 : 0);
    return R;
  }

  return {};
}

static SmallVector<int64_t, 8>
getConstraint(CmpInst *Cmp, DenseMap<Value *, unsigned> &Value2Index,
              bool ShouldAdd) {
  return getConstraint(Cmp->getPredicate(), Cmp->getOperand(0),
                       Cmp->getOperand(1), Value2Index, ShouldAdd);
}

/// Represents either a condition that holds on entry to a block or a basic
/// block, with their respective Dominator DFS in and out numbers.
struct ConstraintOrBlock {
  unsigned NumIn;
  unsigned NumOut;
  bool IsBlock;
  bool Not;
  union {
    BasicBlock *BB;
    CmpInst *Condition;
  };

  ConstraintOrBlock(DomTreeNode *DTN)
      : NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(true),
        BB(DTN->getBlock()) {}
  ConstraintOrBlock(DomTreeNode *DTN, CmpInst *Condition, bool Not)
      : NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(false),
        Not(Not), Condition(Condition) {}
};

struct StackEntry {
  unsigned NumIn;
  unsigned NumOut;
  CmpInst *Condition;
  bool IsNot;

  StackEntry(unsigned NumIn, unsigned NumOut, CmpInst *Condition, bool IsNot)
      : NumIn(NumIn), NumOut(NumOut), Condition(Condition), IsNot(IsNot) {}
};

static bool eliminateConstraints(Function &F, DominatorTree &DT) {
  bool Changed = false;
  DT.updateDFSNumbers();
  ConstraintSystem CS;

  SmallVector<ConstraintOrBlock, 64> WorkList;

  // First, collect conditions implied by branches and blocks with their
  // Dominator DFS in and out numbers.
  for (BasicBlock &BB : F) {
    if (!DT.getNode(&BB))
      continue;
    WorkList.emplace_back(DT.getNode(&BB));

    auto *Br = dyn_cast<BranchInst>(BB.getTerminator());
    if (!Br || !Br->isConditional())
      continue;
    auto *CmpI = dyn_cast<CmpInst>(Br->getCondition());
    if (!CmpI)
      continue;
    if (Br->getSuccessor(0)->getSinglePredecessor())
      WorkList.emplace_back(DT.getNode(Br->getSuccessor(0)), CmpI, false);
    if (Br->getSuccessor(1)->getSinglePredecessor())
      WorkList.emplace_back(DT.getNode(Br->getSuccessor(1)), CmpI, true);
  }

  // Next, sort worklist by dominance, so that dominating blocks and conditions
  // come before blocks and conditions dominated by them. If a block and a
  // condition have the same numbers, the condition comes before the block, as
  // it holds on entry to the block.
  sort(WorkList.begin(), WorkList.end(),
       [](const ConstraintOrBlock &A, const ConstraintOrBlock &B) {
         return std::tie(A.NumIn, A.IsBlock) < std::tie(B.NumIn, B.IsBlock);
       });

  // Finally, process ordered worklist and eliminate implied conditions.
  SmallVector<StackEntry, 16> DFSInStack;
  DenseMap<Value *, unsigned> Value2Index;
  for (ConstraintOrBlock &CB : WorkList) {
    // First, pop entries from the stack that are out-of-scope for CB. Remove
    // the corresponding entry from the constraint system.
    while (!DFSInStack.empty()) {
      auto &E = DFSInStack.back();
      LLVM_DEBUG(dbgs() << "Top of stack : " << E.NumIn << " " << E.NumOut
                        << "\n");
      LLVM_DEBUG(dbgs() << "CB: " << CB.NumIn << " " << CB.NumOut << "\n");
      bool IsDom = CB.NumIn >= E.NumIn && CB.NumOut <= E.NumOut;
      if (IsDom)
        break;
      LLVM_DEBUG(dbgs() << "Removing " << *E.Condition << " " << E.IsNot
                        << "\n");
      DFSInStack.pop_back();
      CS.popLastConstraint();
    }

    LLVM_DEBUG({
      dbgs() << "Processing ";
      if (CB.IsBlock)
        dbgs() << *CB.BB;
      else
        dbgs() << *CB.Condition;
      dbgs() << "\n";
    });

    // For a block, check if any CmpInsts become known based on the current set
    // of constraints.
    if (CB.IsBlock) {
      for (Instruction &I : *CB.BB) {
        auto *Cmp = dyn_cast<CmpInst>(&I);
        if (!Cmp)
          continue;
        auto R = getConstraint(Cmp, Value2Index, false);
        if (R.empty())
          continue;
        if (CS.isConditionImplied(R)) {
          if (!DebugCounter::shouldExecute(EliminatedCounter))
            continue;

          LLVM_DEBUG(dbgs() << "Condition " << *Cmp
                            << " implied by dominating constraints\n");
          LLVM_DEBUG({
            for (auto &E : reverse(DFSInStack))
              dbgs() << "   C " << *E.Condition << " " << E.IsNot << "\n";
          });
          Cmp->replaceAllUsesWith(
              ConstantInt::getTrue(F.getParent()->getContext()));
          NumCondsRemoved++;
          Changed = true;
        }
        if (CS.isConditionImplied(ConstraintSystem::negate(R))) {
          if (!DebugCounter::shouldExecute(EliminatedCounter))
            continue;

          LLVM_DEBUG(dbgs() << "Condition !" << *Cmp
                            << " implied by dominating constraints\n");
          LLVM_DEBUG({
            for (auto &E : reverse(DFSInStack))
              dbgs() << "   C " << *E.Condition << " " << E.IsNot << "\n";
          });
          Cmp->replaceAllUsesWith(
              ConstantInt::getFalse(F.getParent()->getContext()));
          NumCondsRemoved++;
          Changed = true;
        }
      }
      continue;
    }

    // Otherwise, add the condition to the system and stack, if we can transform
    // it into a constraint.
    auto R = getConstraint(CB.Condition, Value2Index, true);
    if (R.empty())
      continue;

    LLVM_DEBUG(dbgs() << "Adding " << *CB.Condition << " " << CB.Not << "\n");
    if (CB.Not)
      R = ConstraintSystem::negate(R);

    CS.addVariableRowFill(R);
    DFSInStack.emplace_back(CB.NumIn, CB.NumOut, CB.Condition, CB.Not);
  }

  return Changed;
}

namespace {

class ConstraintElimination : public FunctionPass {
public:
  static char ID;

  ConstraintElimination() : FunctionPass(ID) {
    initializeConstraintEliminationPass(*PassRegistry::getPassRegistry());
  }

  bool runOnFunction(Function &F) override {
    auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    return eliminateConstraints(F, DT);
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addPreserved<GlobalsAAWrapperPass>();
    AU.addPreserved<DominatorTreeWrapperPass>();
  }
};

} // end anonymous namespace

char ConstraintElimination::ID = 0;

INITIALIZE_PASS_BEGIN(ConstraintElimination, "constraint-elimination",
                      "Constraint Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
INITIALIZE_PASS_END(ConstraintElimination, "constraint-elimination",
                    "Constraint Elimination", false, false)

FunctionPass *llvm::createConstraintEliminationPass() {
  return new ConstraintElimination();
}