PPCBoolRetToInt.cpp   [plain text]

//===- PPCBoolRetToInt.cpp - Convert bool literals to i32 if they are returned ==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements converting i1 values to i32 if they could be more
// profitably allocated as GPRs rather than CRs. This pass will become totally
// unnecessary if Register Bank Allocation and Global Instruction Selection ever
// go upstream.
//
// Presently, the pass converts i1 Constants, and Arguments to i32 if the
// transitive closure of their uses includes only PHINodes, CallInsts, and
// ReturnInsts. The rational is that arguments are generally passed and returned
// in GPRs rather than CRs, so casting them to i32 at the LLVM IR level will
// actually save casts at the Machine Instruction level.
//
// It might be useful to expand this pass to add bit-wise operations to the list
// of safe transitive closure types. Also, we miss some opportunities when LLVM
// represents logical AND and OR operations with control flow rather than data
// flow. For example by lowering the expression: return (A && B && C)
//
// as: return A ? true : B && C.
//
// There's code in SimplifyCFG that code be used to turn control flow in data
// flow using SelectInsts. Selects are slow on some architectures (P7/P8), so
// this probably isn't good in general, but for the special case of i1, the
// Selects could be further lowered to bit operations that are fast everywhere.
//
//===----------------------------------------------------------------------===//

#include "PPC.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Pass.h"

using namespace llvm;

namespace {

#define DEBUG_TYPE "bool-ret-to-int"

STATISTIC(NumBoolRetPromotion,
          "Number of times a bool feeding a RetInst was promoted to an int");
STATISTIC(NumBoolCallPromotion,
          "Number of times a bool feeding a CallInst was promoted to an int");
STATISTIC(NumBoolToIntPromotion,
          "Total number of times a bool was promoted to an int");

class PPCBoolRetToInt : public FunctionPass {

  static SmallPtrSet<Value *, 8> findAllDefs(Value *V) {
    SmallPtrSet<Value *, 8> Defs;
    SmallVector<Value *, 8> WorkList;
    WorkList.push_back(V);
    Defs.insert(V);
    while (!WorkList.empty()) {
      Value *Curr = WorkList.back();
      WorkList.pop_back();
      if (User *CurrUser = dyn_cast<User>(Curr))
        for (auto &Op : CurrUser->operands())
          if (Defs.insert(Op).second)
            WorkList.push_back(Op);
    }
    return Defs;
  }

  // Translate a i1 value to an equivalent i32 value:
  static Value *translate(Value *V) {
    Type *Int32Ty = Type::getInt32Ty(V->getContext());
    if (Constant *C = dyn_cast<Constant>(V))
      return ConstantExpr::getZExt(C, Int32Ty);
    if (PHINode *P = dyn_cast<PHINode>(V)) {
      // Temporarily set the operands to 0. We'll fix this later in
      // runOnUse.
      Value *Zero = Constant::getNullValue(Int32Ty);
      PHINode *Q =
        PHINode::Create(Int32Ty, P->getNumIncomingValues(), P->getName(), P);
      for (unsigned i = 0; i < P->getNumOperands(); ++i)
        Q->addIncoming(Zero, P->getIncomingBlock(i));
      return Q;
    }

    Argument *A = dyn_cast<Argument>(V);
    Instruction *I = dyn_cast<Instruction>(V);
    assert((A || I) && "Unknown value type");

    auto InstPt =
      A ? &*A->getParent()->getEntryBlock().begin() : I->getNextNode();
    return new ZExtInst(V, Int32Ty, "", InstPt);
  }

  typedef SmallPtrSet<const PHINode *, 8> PHINodeSet;

  // A PHINode is Promotable if:
  // 1. Its type is i1 AND
  // 2. All of its uses are ReturnInt, CallInst, PHINode, or DbgInfoIntrinsic
  // AND
  // 3. All of its operands are Constant or Argument or
  //    CallInst or PHINode AND
  // 4. All of its PHINode uses are Promotable AND
  // 5. All of its PHINode operands are Promotable
  static PHINodeSet getPromotablePHINodes(const Function &F) {
    PHINodeSet Promotable;
    // Condition 1
    for (auto &BB : F)
      for (auto &I : BB)
        if (const PHINode *P = dyn_cast<PHINode>(&I))
          if (P->getType()->isIntegerTy(1))
            Promotable.insert(P);

    SmallVector<const PHINode *, 8> ToRemove;
    for (const auto &P : Promotable) {
      // Condition 2 and 3
      auto IsValidUser = [] (const Value *V) -> bool {
        return isa<ReturnInst>(V) || isa<CallInst>(V) || isa<PHINode>(V) ||
        isa<DbgInfoIntrinsic>(V);
      };
      auto IsValidOperand = [] (const Value *V) -> bool {
        return isa<Constant>(V) || isa<Argument>(V) || isa<CallInst>(V) ||
        isa<PHINode>(V);
      };
      const auto &Users = P->users();
      const auto &Operands = P->operands();
      if (!std::all_of(Users.begin(), Users.end(), IsValidUser) ||
          !std::all_of(Operands.begin(), Operands.end(), IsValidOperand))
        ToRemove.push_back(P);
    }

    // Iterate to convergence
    auto IsPromotable = [&Promotable] (const Value *V) -> bool {
      const PHINode *Phi = dyn_cast<PHINode>(V);
      return !Phi || Promotable.count(Phi);
    };
    while (!ToRemove.empty()) {
      for (auto &User : ToRemove)
        Promotable.erase(User);
      ToRemove.clear();

      for (const auto &P : Promotable) {
        // Condition 4 and 5
        const auto &Users = P->users();
        const auto &Operands = P->operands();
        if (!std::all_of(Users.begin(), Users.end(), IsPromotable) ||
            !std::all_of(Operands.begin(), Operands.end(), IsPromotable))
          ToRemove.push_back(P);
      }
    }

    return Promotable;
  }

  typedef DenseMap<Value *, Value *> B2IMap;

 public:
  static char ID;
  PPCBoolRetToInt() : FunctionPass(ID) {
    initializePPCBoolRetToIntPass(*PassRegistry::getPassRegistry());
  }

  bool runOnFunction(Function &F) {
    PHINodeSet PromotablePHINodes = getPromotablePHINodes(F);
    B2IMap Bool2IntMap;
    bool Changed = false;
    for (auto &BB : F) {
      for (auto &I : BB) {
        if (ReturnInst *R = dyn_cast<ReturnInst>(&I))
          if (F.getReturnType()->isIntegerTy(1))
            Changed |=
              runOnUse(R->getOperandUse(0), PromotablePHINodes, Bool2IntMap);

        if (CallInst *CI = dyn_cast<CallInst>(&I))
          for (auto &U : CI->operands())
            if (U->getType()->isIntegerTy(1))
              Changed |= runOnUse(U, PromotablePHINodes, Bool2IntMap);
      }
    }

    return Changed;
  }

  static bool runOnUse(Use &U, const PHINodeSet &PromotablePHINodes,
                       B2IMap &BoolToIntMap) {
    auto Defs = findAllDefs(U);

    // If the values are all Constants or Arguments, don't bother
    if (!std::any_of(Defs.begin(), Defs.end(), isa<Instruction, Value *>))
      return false;

    // Presently, we only know how to handle PHINode, Constant, and Arguments.
    // Potentially, bitwise operations (AND, OR, XOR, NOT) and sign extension
    // could also be handled in the future.
    for (const auto &V : Defs)
      if (!isa<PHINode>(V) && !isa<Constant>(V) && !isa<Argument>(V))
        return false;

    for (const auto &V : Defs)
      if (const PHINode *P = dyn_cast<PHINode>(V))
        if (!PromotablePHINodes.count(P))
          return false;

    if (isa<ReturnInst>(U.getUser()))
      ++NumBoolRetPromotion;
    if (isa<CallInst>(U.getUser()))
      ++NumBoolCallPromotion;
    ++NumBoolToIntPromotion;

    for (const auto &V : Defs)
      if (!BoolToIntMap.count(V))
        BoolToIntMap[V] = translate(V);

    // Replace the operands of the translated instructions. There were set to
    // zero in the translate function.
    for (auto &Pair : BoolToIntMap) {
      User *First = dyn_cast<User>(Pair.first);
      User *Second = dyn_cast<User>(Pair.second);
      assert((!First || Second) && "translated from user to non-user!?");
      if (First)
        for (unsigned i = 0; i < First->getNumOperands(); ++i)
          Second->setOperand(i, BoolToIntMap[First->getOperand(i)]);
    }

    Value *IntRetVal = BoolToIntMap[U];
    Type *Int1Ty = Type::getInt1Ty(U->getContext());
    Instruction *I = cast<Instruction>(U.getUser());
    Value *BackToBool = new TruncInst(IntRetVal, Int1Ty, "backToBool", I);
    U.set(BackToBool);

    return true;
  }

  void getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addPreserved<DominatorTreeWrapperPass>();
    FunctionPass::getAnalysisUsage(AU);
  }
};
}

char PPCBoolRetToInt::ID = 0;
INITIALIZE_PASS(PPCBoolRetToInt, "bool-ret-to-int",
                "Convert i1 constants to i32 if they are returned",
                false, false)

FunctionPass *llvm::createPPCBoolRetToIntPass() { return new PPCBoolRetToInt(); }