src/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp

   1 //===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass identifies expensive constants to hoist and coalesces them to
  11 // better prepare it for SelectionDAG-based code generation. This works around
  12 // the limitations of the basic-block-at-a-time approach.
  13 //
  14 // First it scans all instructions for integer constants and calculates its
  15 // cost. If the constant can be folded into the instruction (the cost is
  16 // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
  17 // consider it expensive and leave it alone. This is the default behavior and
  18 // the default implementation of getIntImmCost will always return TCC_Free.
  19 //
  20 // If the cost is more than TCC_BASIC, then the integer constant can't be folded
  21 // into the instruction and it might be beneficial to hoist the constant.
  22 // Similar constants are coalesced to reduce register pressure and
  23 // materialization code.
  24 //
  25 // When a constant is hoisted, it is also hidden behind a bitcast to force it to
  26 // be live-out of the basic block. Otherwise the constant would be just
  27 // duplicated and each basic block would have its own copy in the SelectionDAG.
  28 // The SelectionDAG recognizes such constants as opaque and doesn't perform
  29 // certain transformations on them, which would create a new expensive constant.
  30 //
  31 // This optimization is only applied to integer constants in instructions and
  32 // simple (this means not nested) constant cast expressions. For example:
  33 // %0 = load i64* inttoptr (i64 big_constant to i64*)
  34 //===----------------------------------------------------------------------===//
  35
  36 #include "llvm/Transforms/Scalar.h"
  37 #include "llvm/ADT/SmallSet.h"
  38 #include "llvm/ADT/SmallVector.h"
  39 #include "llvm/ADT/Statistic.h"
  40 #include "llvm/Analysis/TargetTransformInfo.h"
  41 #include "llvm/IR/Constants.h"
  42 #include "llvm/IR/Dominators.h"
  43 #include "llvm/IR/IntrinsicInst.h"
  44 #include "llvm/Pass.h"
  45 #include "llvm/Support/Debug.h"
  46 #include <tuple>
  47
  48 using namespace llvm;
  49
  50 #define DEBUG_TYPE "consthoist"
  51
  52 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
  53 STATISTIC(NumConstantsRebased, "Number of constants rebased");
  54
  55 namespace {
  56 struct ConstantUser;
  57 struct RebasedConstantInfo;
  58
  59 typedef SmallVector<ConstantUser, 8> ConstantUseListType;
  60 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
  61
  62 /// \brief Keeps track of the user of a constant and the operand index where the
  63 /// constant is used.
  64 struct ConstantUser {
  65   Instruction *Inst;
  66   unsigned OpndIdx;
  67
  68   ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { }
  69 };
  70
  71 /// \brief Keeps track of a constant candidate and its uses.
  72 struct ConstantCandidate {
  73   ConstantUseListType Uses;
  74   ConstantInt *ConstInt;
  75   unsigned CumulativeCost;
  76
  77   ConstantCandidate(ConstantInt *ConstInt)
  78     : ConstInt(ConstInt), CumulativeCost(0) { }
  79
  80   /// \brief Add the user to the use list and update the cost.
  81   void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
  82     CumulativeCost += Cost;
  83     Uses.push_back(ConstantUser(Inst, Idx));
  84   }
  85 };
  86
  87 /// \brief This represents a constant that has been rebased with respect to a
  88 /// base constant. The difference to the base constant is recorded in Offset.
  89 struct RebasedConstantInfo {
  90   ConstantUseListType Uses;
  91   Constant *Offset;
  92
  93   RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
  94     : Uses(std::move(Uses)), Offset(Offset) { }
  95 };
  96
  97 /// \brief A base constant and all its rebased constants.
  98 struct ConstantInfo {
  99   ConstantInt *BaseConstant;
 100   RebasedConstantListType RebasedConstants;
 101 };
 102
 103 /// \brief The constant hoisting pass.
 104 class ConstantHoisting : public FunctionPass {
 105   typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType;
 106   typedef std::vector<ConstantCandidate> ConstCandVecType;
 107
 108   const TargetTransformInfo *TTI;
 109   DominatorTree *DT;
 110   BasicBlock *Entry;
 111
 112   /// Keeps track of constant candidates found in the function.
 113   ConstCandVecType ConstCandVec;
 114
 115   /// Keep track of cast instructions we already cloned.
 116   SmallDenseMap<Instruction *, Instruction *> ClonedCastMap;
 117
 118   /// These are the final constants we decided to hoist.
 119   SmallVector<ConstantInfo, 8> ConstantVec;
 120 public:
 121   static char ID; // Pass identification, replacement for typeid
 122   ConstantHoisting() : FunctionPass(ID), TTI(nullptr), DT(nullptr),
 123                        Entry(nullptr) {
 124     initializeConstantHoistingPass(*PassRegistry::getPassRegistry());
 125   }
 126
 127   bool runOnFunction(Function &Fn) override;
 128
 129   const char *getPassName() const override { return "Constant Hoisting"; }
 130
 131   void getAnalysisUsage(AnalysisUsage &AU) const override {
 132     AU.setPreservesCFG();
 133     AU.addRequired<DominatorTreeWrapperPass>();
 134     AU.addRequired<TargetTransformInfo>();
 135   }
 136
 137 private:
 138   /// \brief Initialize the pass.
 139   void setup(Function &Fn) {
 140     DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 141     TTI = &getAnalysis<TargetTransformInfo>();
 142     Entry = &Fn.getEntryBlock();
 143   }
 144
 145   /// \brief Cleanup.
 146   void cleanup() {
 147     ConstantVec.clear();
 148     ClonedCastMap.clear();
 149     ConstCandVec.clear();
 150
 151     TTI = nullptr;
 152     DT = nullptr;
 153     Entry = nullptr;
 154   }
 155
 156   Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
 157   Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
 158   void collectConstantCandidates(ConstCandMapType &ConstCandMap,
 159                                  Instruction *Inst, unsigned Idx,
 160                                  ConstantInt *ConstInt);
 161   void collectConstantCandidates(ConstCandMapType &ConstCandMap,
 162                                  Instruction *Inst);
 163   void collectConstantCandidates(Function &Fn);
 164   void findAndMakeBaseConstant(ConstCandVecType::iterator S,
 165                                ConstCandVecType::iterator E);
 166   void findBaseConstants();
 167   void emitBaseConstants(Instruction *Base, Constant *Offset,
 168                          const ConstantUser &ConstUser);
 169   bool emitBaseConstants();
 170   void deleteDeadCastInst() const;
 171   bool optimizeConstants(Function &Fn);
 172 };
 173 }
 174
 175 char ConstantHoisting::ID = 0;
 176 INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting",
 177                       false, false)
 178 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 179 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
 180 INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting",
 181                     false, false)
 182
 183 FunctionPass *llvm::createConstantHoistingPass() {
 184   return new ConstantHoisting();
 185 }
 186
 187 /// \brief Perform the constant hoisting optimization for the given function.
 188 bool ConstantHoisting::runOnFunction(Function &Fn) {
 189   DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
 190   DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
 191
 192   setup(Fn);
 193
 194   bool MadeChange = optimizeConstants(Fn);
 195
 196   if (MadeChange) {
 197     DEBUG(dbgs() << "********** Function after Constant Hoisting: "
 198                  << Fn.getName() << '\n');
 199     DEBUG(dbgs() << Fn);
 200   }
 201   DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
 202
 203   cleanup();
 204
 205   return MadeChange;
 206 }
 207
 208
 209 /// \brief Find the constant materialization insertion point.
 210 Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
 211                                                unsigned Idx) const {
 212   // If the operand is a cast instruction, then we have to materialize the
 213   // constant before the cast instruction.
 214   if (Idx != ~0U) {
 215     Value *Opnd = Inst->getOperand(Idx);
 216     if (auto CastInst = dyn_cast<Instruction>(Opnd))
 217       if (CastInst->isCast())
 218         return CastInst;
 219   }
 220
 221   // The simple and common case. This also includes constant expressions.
 222   if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
 223     return Inst;
 224
 225   // We can't insert directly before a phi node or landing pad. Insert before
 226   // the terminator of the incoming or dominating block.
 227   assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
 228   if (Idx != ~0U && isa<PHINode>(Inst))
 229     return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
 230
 231   BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
 232   return IDom->getTerminator();
 233 }
 234
 235 /// \brief Find an insertion point that dominates all uses.
 236 Instruction *ConstantHoisting::
 237 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
 238   assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
 239   // Collect all basic blocks.
 240   SmallPtrSet<BasicBlock *, 8> BBs;
 241   for (auto const &RCI : ConstInfo.RebasedConstants)
 242     for (auto const &U : RCI.Uses)
 243       BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
 244
 245   if (BBs.count(Entry))
 246     return &Entry->front();
 247
 248   while (BBs.size() >= 2) {
 249     BasicBlock *BB, *BB1, *BB2;
 250     BB1 = *BBs.begin();
 251     BB2 = *std::next(BBs.begin());
 252     BB = DT->findNearestCommonDominator(BB1, BB2);
 253     if (BB == Entry)
 254       return &Entry->front();
 255     BBs.erase(BB1);
 256     BBs.erase(BB2);
 257     BBs.insert(BB);
 258   }
 259   assert((BBs.size() == 1) && "Expected only one element.");
 260   Instruction &FirstInst = (*BBs.begin())->front();
 261   return findMatInsertPt(&FirstInst);
 262 }
 263
 264
 265 /// \brief Record constant integer ConstInt for instruction Inst at operand
 266 /// index Idx.
 267 ///
 268 /// The operand at index Idx is not necessarily the constant integer itself. It
 269 /// could also be a cast instruction or a constant expression that uses the
 270 // constant integer.
 271 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
 272                                                  Instruction *Inst,
 273                                                  unsigned Idx,
 274                                                  ConstantInt *ConstInt) {
 275   unsigned Cost;
 276   // Ask the target about the cost of materializing the constant for the given
 277   // instruction and operand index.
 278   if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
 279     Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
 280                               ConstInt->getValue(), ConstInt->getType());
 281   else
 282     Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
 283                               ConstInt->getType());
 284
 285   // Ignore cheap integer constants.
 286   if (Cost > TargetTransformInfo::TCC_Basic) {
 287     ConstCandMapType::iterator Itr;
 288     bool Inserted;
 289     std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
 290     if (Inserted) {
 291       ConstCandVec.push_back(ConstantCandidate(ConstInt));
 292       Itr->second = ConstCandVec.size() - 1;
 293     }
 294     ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
 295     DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
 296             dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
 297                    << " with cost " << Cost << '\n';
 298           else
 299           dbgs() << "Collect constant " << *ConstInt << " indirectly from "
 300                  << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
 301                  << Cost << '\n';
 302     );
 303   }
 304 }
 305
 306 /// \brief Scan the instruction for expensive integer constants and record them
 307 /// in the constant candidate vector.
 308 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
 309                                                  Instruction *Inst) {
 310   // Skip all cast instructions. They are visited indirectly later on.
 311   if (Inst->isCast())
 312     return;
 313
 314   // Can't handle inline asm. Skip it.
 315   if (auto Call = dyn_cast<CallInst>(Inst))
 316     if (isa<InlineAsm>(Call->getCalledValue()))
 317       return;
 318
 319   // Scan all operands.
 320   for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
 321     Value *Opnd = Inst->getOperand(Idx);
 322
 323     // Visit constant integers.
 324     if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
 325       collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 326       continue;
 327     }
 328
 329     // Visit cast instructions that have constant integers.
 330     if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
 331       // Only visit cast instructions, which have been skipped. All other
 332       // instructions should have already been visited.
 333       if (!CastInst->isCast())
 334         continue;
 335
 336       if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
 337         // Pretend the constant is directly used by the instruction and ignore
 338         // the cast instruction.
 339         collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 340         continue;
 341       }
 342     }
 343
 344     // Visit constant expressions that have constant integers.
 345     if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
 346       // Only visit constant cast expressions.
 347       if (!ConstExpr->isCast())
 348         continue;
 349
 350       if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
 351         // Pretend the constant is directly used by the instruction and ignore
 352         // the constant expression.
 353         collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 354         continue;
 355       }
 356     }
 357   } // end of for all operands
 358 }
 359
 360 /// \brief Collect all integer constants in the function that cannot be folded
 361 /// into an instruction itself.
 362 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
 363   ConstCandMapType ConstCandMap;
 364   for (Function::iterator BB : Fn)
 365     for (BasicBlock::iterator Inst : *BB)
 366       collectConstantCandidates(ConstCandMap, Inst);
 367 }
 368
 369 /// \brief Find the base constant within the given range and rebase all other
 370 /// constants with respect to the base constant.
 371 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
 372                                                ConstCandVecType::iterator E) {
 373   auto MaxCostItr = S;
 374   unsigned NumUses = 0;
 375   // Use the constant that has the maximum cost as base constant.
 376   for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
 377     NumUses += ConstCand->Uses.size();
 378     if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
 379       MaxCostItr = ConstCand;
 380   }
 381
 382   // Don't hoist constants that have only one use.
 383   if (NumUses <= 1)
 384     return;
 385
 386   ConstantInfo ConstInfo;
 387   ConstInfo.BaseConstant = MaxCostItr->ConstInt;
 388   Type *Ty = ConstInfo.BaseConstant->getType();
 389
 390   // Rebase the constants with respect to the base constant.
 391   for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
 392     APInt Diff = ConstCand->ConstInt->getValue() -
 393                  ConstInfo.BaseConstant->getValue();
 394     Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
 395     ConstInfo.RebasedConstants.push_back(
 396       RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
 397   }
 398   ConstantVec.push_back(std::move(ConstInfo));
 399 }
 400
 401 /// \brief Finds and combines constant candidates that can be easily
 402 /// rematerialized with an add from a common base constant.
 403 void ConstantHoisting::findBaseConstants() {
 404   // Sort the constants by value and type. This invalidates the mapping!
 405   std::sort(ConstCandVec.begin(), ConstCandVec.end(),
 406             [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
 407     if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
 408       return LHS.ConstInt->getType()->getBitWidth() <
 409              RHS.ConstInt->getType()->getBitWidth();
 410     return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
 411   });
 412
 413   // Simple linear scan through the sorted constant candidate vector for viable
 414   // merge candidates.
 415   auto MinValItr = ConstCandVec.begin();
 416   for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
 417        CC != E; ++CC) {
 418     if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
 419       // Check if the constant is in range of an add with immediate.
 420       APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
 421       if ((Diff.getBitWidth() <= 64) &&
 422           TTI->isLegalAddImmediate(Diff.getSExtValue()))
 423         continue;
 424     }
 425     // We either have now a different constant type or the constant is not in
 426     // range of an add with immediate anymore.
 427     findAndMakeBaseConstant(MinValItr, CC);
 428     // Start a new base constant search.
 429     MinValItr = CC;
 430   }
 431   // Finalize the last base constant search.
 432   findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
 433 }
 434
 435 /// \brief Updates the operand at Idx in instruction Inst with the result of
 436 ///        instruction Mat. If the instruction is a PHI node then special
 437 ///        handling for duplicate values form the same incomming basic block is
 438 ///        required.
 439 /// \return The update will always succeed, but the return value indicated if
 440 ///         Mat was used for the update or not.
 441 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
 442   if (auto PHI = dyn_cast<PHINode>(Inst)) {
 443     // Check if any previous operand of the PHI node has the same incoming basic
 444     // block. This is a very odd case that happens when the incoming basic block
 445     // has a switch statement. In this case use the same value as the previous
 446     // operand(s), otherwise we will fail verification due to different values.
 447     // The values are actually the same, but the variable names are different
 448     // and the verifier doesn't like that.
 449     BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
 450     for (unsigned i = 0; i < Idx; ++i) {
 451       if (PHI->getIncomingBlock(i) == IncomingBB) {
 452         Value *IncomingVal = PHI->getIncomingValue(i);
 453         Inst->setOperand(Idx, IncomingVal);
 454         return false;
 455       }
 456     }
 457   }
 458
 459   Inst->setOperand(Idx, Mat);
 460   return true;
 461 }
 462
 463 /// \brief Emit materialization code for all rebased constants and update their
 464 /// users.
 465 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
 466                                          const ConstantUser &ConstUser) {
 467   Instruction *Mat = Base;
 468   if (Offset) {
 469     Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
 470                                                ConstUser.OpndIdx);
 471     Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
 472                                  "const_mat", InsertionPt);
 473
 474     DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
 475                  << " + " << *Offset << ") in BB "
 476                  << Mat->getParent()->getName() << '\n' << *Mat << '\n');
 477     Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
 478   }
 479   Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
 480
 481   // Visit constant integer.
 482   if (isa<ConstantInt>(Opnd)) {
 483     DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 484     if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
 485       Mat->eraseFromParent();
 486     DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 487     return;
 488   }
 489
 490   // Visit cast instruction.
 491   if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
 492     assert(CastInst->isCast() && "Expected an cast instruction!");
 493     // Check if we already have visited this cast instruction before to avoid
 494     // unnecessary cloning.
 495     Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
 496     if (!ClonedCastInst) {
 497       ClonedCastInst = CastInst->clone();
 498       ClonedCastInst->setOperand(0, Mat);
 499       ClonedCastInst->insertAfter(CastInst);
 500       // Use the same debug location as the original cast instruction.
 501       ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
 502       DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
 503                    << "To               : " << *ClonedCastInst << '\n');
 504     }
 505
 506     DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 507     updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
 508     DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 509     return;
 510   }
 511
 512   // Visit constant expression.
 513   if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
 514     Instruction *ConstExprInst = ConstExpr->getAsInstruction();
 515     ConstExprInst->setOperand(0, Mat);
 516     ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
 517                                                 ConstUser.OpndIdx));
 518
 519     // Use the same debug location as the instruction we are about to update.
 520     ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
 521
 522     DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
 523                  << "From              : " << *ConstExpr << '\n');
 524     DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 525     if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
 526       ConstExprInst->eraseFromParent();
 527       if (Offset)
 528         Mat->eraseFromParent();
 529     }
 530     DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 531     return;
 532   }
 533 }
 534
 535 /// \brief Hoist and hide the base constant behind a bitcast and emit
 536 /// materialization code for derived constants.
 537 bool ConstantHoisting::emitBaseConstants() {
 538   bool MadeChange = false;
 539   for (auto const &ConstInfo : ConstantVec) {
 540     // Hoist and hide the base constant behind a bitcast.
 541     Instruction *IP = findConstantInsertionPoint(ConstInfo);
 542     IntegerType *Ty = ConstInfo.BaseConstant->getType();
 543     Instruction *Base =
 544       new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
 545     DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
 546                  << IP->getParent()->getName() << '\n' << *Base << '\n');
 547     NumConstantsHoisted++;
 548
 549     // Emit materialization code for all rebased constants.
 550     for (auto const &RCI : ConstInfo.RebasedConstants) {
 551       NumConstantsRebased++;
 552       for (auto const &U : RCI.Uses)
 553         emitBaseConstants(Base, RCI.Offset, U);
 554     }
 555
 556     // Use the same debug location as the last user of the constant.
 557     assert(!Base->use_empty() && "The use list is empty!?");
 558     assert(isa<Instruction>(Base->user_back()) &&
 559            "All uses should be instructions.");
 560     Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
 561
 562     // Correct for base constant, which we counted above too.
 563     NumConstantsRebased--;
 564     MadeChange = true;
 565   }
 566   return MadeChange;
 567 }
 568
 569 /// \brief Check all cast instructions we made a copy of and remove them if they
 570 /// have no more users.
 571 void ConstantHoisting::deleteDeadCastInst() const {
 572   for (auto const &I : ClonedCastMap)
 573     if (I.first->use_empty())
 574       I.first->eraseFromParent();
 575 }
 576
 577 /// \brief Optimize expensive integer constants in the given function.
 578 bool ConstantHoisting::optimizeConstants(Function &Fn) {
 579   // Collect all constant candidates.
 580   collectConstantCandidates(Fn);
 581
 582   // There are no constant candidates to worry about.
 583   if (ConstCandVec.empty())
 584     return false;
 585
 586   // Combine constants that can be easily materialized with an add from a common
 587   // base constant.
 588   findBaseConstants();
 589
 590   // There are no constants to emit.
 591   if (ConstantVec.empty())
 592     return false;
 593
 594   // Finally hoist the base constant and emit materialization code for dependent
 595   // constants.
 596   bool MadeChange = emitBaseConstants();
 597
 598   // Cleanup dead instructions.
 599   deleteDeadCastInst();
 600
 601   return MadeChange;
 602 }