llvm/lib/Transforms/Scalar/ConstantHoisting.cpp

   1 //===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass identifies expensive constants to hoist and coalesces them to
  10 // better prepare it for SelectionDAG-based code generation. This works around
  11 // the limitations of the basic-block-at-a-time approach.
  12 //
  13 // First it scans all instructions for integer constants and calculates its
  14 // cost. If the constant can be folded into the instruction (the cost is
  15 // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
  16 // consider it expensive and leave it alone. This is the default behavior and
  17 // the default implementation of getIntImmCostInst will always return TCC_Free.
  18 //
  19 // If the cost is more than TCC_BASIC, then the integer constant can't be folded
  20 // into the instruction and it might be beneficial to hoist the constant.
  21 // Similar constants are coalesced to reduce register pressure and
  22 // materialization code.
  23 //
  24 // When a constant is hoisted, it is also hidden behind a bitcast to force it to
  25 // be live-out of the basic block. Otherwise the constant would be just
  26 // duplicated and each basic block would have its own copy in the SelectionDAG.
  27 // The SelectionDAG recognizes such constants as opaque and doesn't perform
  28 // certain transformations on them, which would create a new expensive constant.
  29 //
  30 // This optimization is only applied to integer constants in instructions and
  31 // simple (this means not nested) constant cast expressions. For example:
  32 // %0 = load i64* inttoptr (i64 big_constant to i64*)
  33 //===----------------------------------------------------------------------===//
  34
  35 #include "llvm/Transforms/Scalar/ConstantHoisting.h"
  36 #include "llvm/ADT/APInt.h"
  37 #include "llvm/ADT/DenseMap.h"
  38 #include "llvm/ADT/None.h"
  39 #include "llvm/ADT/Optional.h"
  40 #include "llvm/ADT/SmallPtrSet.h"
  41 #include "llvm/ADT/SmallVector.h"
  42 #include "llvm/ADT/Statistic.h"
  43 #include "llvm/Analysis/BlockFrequencyInfo.h"
  44 #include "llvm/Analysis/ProfileSummaryInfo.h"
  45 #include "llvm/Analysis/TargetTransformInfo.h"
  46 #include "llvm/IR/BasicBlock.h"
  47 #include "llvm/IR/Constants.h"
  48 #include "llvm/IR/DebugInfoMetadata.h"
  49 #include "llvm/IR/Dominators.h"
  50 #include "llvm/IR/Function.h"
  51 #include "llvm/IR/InstrTypes.h"
  52 #include "llvm/IR/Instruction.h"
  53 #include "llvm/IR/Instructions.h"
  54 #include "llvm/IR/IntrinsicInst.h"
  55 #include "llvm/IR/Value.h"
  56 #include "llvm/InitializePasses.h"
  57 #include "llvm/Pass.h"
  58 #include "llvm/Support/BlockFrequency.h"
  59 #include "llvm/Support/Casting.h"
  60 #include "llvm/Support/CommandLine.h"
  61 #include "llvm/Support/Debug.h"
  62 #include "llvm/Support/raw_ostream.h"
  63 #include "llvm/Transforms/Scalar.h"
  64 #include "llvm/Transforms/Utils/Local.h"
  65 #include "llvm/Transforms/Utils/SizeOpts.h"
  66 #include <algorithm>
  67 #include <cassert>
  68 #include <cstdint>
  69 #include <iterator>
  70 #include <tuple>
  71 #include <utility>
  72
  73 using namespace llvm;
  74 using namespace consthoist;
  75
  76 #define DEBUG_TYPE "consthoist"
  77
  78 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
  79 STATISTIC(NumConstantsRebased, "Number of constants rebased");
  80
  81 static cl::opt<bool> ConstHoistWithBlockFrequency(
  82     "consthoist-with-block-frequency", cl::init(true), cl::Hidden,
  83     cl::desc("Enable the use of the block frequency analysis to reduce the "
  84              "chance to execute const materialization more frequently than "
  85              "without hoisting."));
  86
  87 static cl::opt<bool> ConstHoistGEP(
  88     "consthoist-gep", cl::init(false), cl::Hidden,
  89     cl::desc("Try hoisting constant gep expressions"));
  90
  91 static cl::opt<unsigned>
  92 MinNumOfDependentToRebase("consthoist-min-num-to-rebase",
  93     cl::desc("Do not rebase if number of dependent constants of a Base is less "
  94              "than this number."),
  95     cl::init(0), cl::Hidden);
  96
  97 namespace {
  98
  99 /// The constant hoisting pass.
 100 class ConstantHoistingLegacyPass : public FunctionPass {
 101 public:
 102   static char ID; // Pass identification, replacement for typeid
 103
 104   ConstantHoistingLegacyPass() : FunctionPass(ID) {
 105     initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry());
 106   }
 107
 108   bool runOnFunction(Function &Fn) override;
 109
 110   StringRef getPassName() const override { return "Constant Hoisting"; }
 111
 112   void getAnalysisUsage(AnalysisUsage &AU) const override {
 113     AU.setPreservesCFG();
 114     if (ConstHoistWithBlockFrequency)
 115       AU.addRequired<BlockFrequencyInfoWrapperPass>();
 116     AU.addRequired<DominatorTreeWrapperPass>();
 117     AU.addRequired<ProfileSummaryInfoWrapperPass>();
 118     AU.addRequired<TargetTransformInfoWrapperPass>();
 119   }
 120
 121 private:
 122   ConstantHoistingPass Impl;
 123 };
 124
 125 } // end anonymous namespace
 126
 127 char ConstantHoistingLegacyPass::ID = 0;
 128
 129 INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist",
 130                       "Constant Hoisting", false, false)
 131 INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
 132 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 133 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
 134 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 135 INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist",
 136                     "Constant Hoisting", false, false)
 137
 138 FunctionPass *llvm::createConstantHoistingPass() {
 139   return new ConstantHoistingLegacyPass();
 140 }
 141
 142 /// Perform the constant hoisting optimization for the given function.
 143 bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) {
 144   if (skipFunction(Fn))
 145     return false;
 146
 147   LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
 148   LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
 149
 150   bool MadeChange =
 151       Impl.runImpl(Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn),
 152                    getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
 153                    ConstHoistWithBlockFrequency
 154                        ? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI()
 155                        : nullptr,
 156                    Fn.getEntryBlock(),
 157                    &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI());
 158
 159   if (MadeChange) {
 160     LLVM_DEBUG(dbgs() << "********** Function after Constant Hoisting: "
 161                       << Fn.getName() << '\n');
 162     LLVM_DEBUG(dbgs() << Fn);
 163   }
 164   LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
 165
 166   return MadeChange;
 167 }
 168
 169 /// Find the constant materialization insertion point.
 170 Instruction *ConstantHoistingPass::findMatInsertPt(Instruction *Inst,
 171                                                    unsigned Idx) const {
 172   // If the operand is a cast instruction, then we have to materialize the
 173   // constant before the cast instruction.
 174   if (Idx != ~0U) {
 175     Value *Opnd = Inst->getOperand(Idx);
 176     if (auto CastInst = dyn_cast<Instruction>(Opnd))
 177       if (CastInst->isCast())
 178         return CastInst;
 179   }
 180
 181   // The simple and common case. This also includes constant expressions.
 182   if (!isa<PHINode>(Inst) && !Inst->isEHPad())
 183     return Inst;
 184
 185   // We can't insert directly before a phi node or an eh pad. Insert before
 186   // the terminator of the incoming or dominating block.
 187   assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
 188   BasicBlock *InsertionBlock = nullptr;
 189   if (Idx != ~0U && isa<PHINode>(Inst)) {
 190     InsertionBlock = cast<PHINode>(Inst)->getIncomingBlock(Idx);
 191     if (!InsertionBlock->isEHPad()) {
 192       return InsertionBlock->getTerminator();
 193     }
 194   } else {
 195     InsertionBlock = Inst->getParent();
 196   }
 197
 198   // This must be an EH pad. Iterate over immediate dominators until we find a
 199   // non-EH pad. We need to skip over catchswitch blocks, which are both EH pads
 200   // and terminators.
 201   auto *IDom = DT->getNode(InsertionBlock)->getIDom();
 202   while (IDom->getBlock()->isEHPad()) {
 203     assert(Entry != IDom->getBlock() && "eh pad in entry block");
 204     IDom = IDom->getIDom();
 205   }
 206
 207   return IDom->getBlock()->getTerminator();
 208 }
 209
 210 /// Given \p BBs as input, find another set of BBs which collectively
 211 /// dominates \p BBs and have the minimal sum of frequencies. Return the BB
 212 /// set found in \p BBs.
 213 static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI,
 214                                  BasicBlock *Entry,
 215                                  SetVector<BasicBlock *> &BBs) {
 216   assert(!BBs.count(Entry) && "Assume Entry is not in BBs");
 217   // Nodes on the current path to the root.
 218   SmallPtrSet<BasicBlock *, 8> Path;
 219   // Candidates includes any block 'BB' in set 'BBs' that is not strictly
 220   // dominated by any other blocks in set 'BBs', and all nodes in the path
 221   // in the dominator tree from Entry to 'BB'.
 222   SmallPtrSet<BasicBlock *, 16> Candidates;
 223   for (auto BB : BBs) {
 224     // Ignore unreachable basic blocks.
 225     if (!DT.isReachableFromEntry(BB))
 226       continue;
 227     Path.clear();
 228     // Walk up the dominator tree until Entry or another BB in BBs
 229     // is reached. Insert the nodes on the way to the Path.
 230     BasicBlock *Node = BB;
 231     // The "Path" is a candidate path to be added into Candidates set.
 232     bool isCandidate = false;
 233     do {
 234       Path.insert(Node);
 235       if (Node == Entry || Candidates.count(Node)) {
 236         isCandidate = true;
 237         break;
 238       }
 239       assert(DT.getNode(Node)->getIDom() &&
 240              "Entry doens't dominate current Node");
 241       Node = DT.getNode(Node)->getIDom()->getBlock();
 242     } while (!BBs.count(Node));
 243
 244     // If isCandidate is false, Node is another Block in BBs dominating
 245     // current 'BB'. Drop the nodes on the Path.
 246     if (!isCandidate)
 247       continue;
 248
 249     // Add nodes on the Path into Candidates.
 250     Candidates.insert(Path.begin(), Path.end());
 251   }
 252
 253   // Sort the nodes in Candidates in top-down order and save the nodes
 254   // in Orders.
 255   unsigned Idx = 0;
 256   SmallVector<BasicBlock *, 16> Orders;
 257   Orders.push_back(Entry);
 258   while (Idx != Orders.size()) {
 259     BasicBlock *Node = Orders[Idx++];
 260     for (auto ChildDomNode : DT.getNode(Node)->children()) {
 261       if (Candidates.count(ChildDomNode->getBlock()))
 262         Orders.push_back(ChildDomNode->getBlock());
 263     }
 264   }
 265
 266   // Visit Orders in bottom-up order.
 267   using InsertPtsCostPair =
 268       std::pair<SetVector<BasicBlock *>, BlockFrequency>;
 269
 270   // InsertPtsMap is a map from a BB to the best insertion points for the
 271   // subtree of BB (subtree not including the BB itself).
 272   DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap;
 273   InsertPtsMap.reserve(Orders.size() + 1);
 274   for (auto RIt = Orders.rbegin(); RIt != Orders.rend(); RIt++) {
 275     BasicBlock *Node = *RIt;
 276     bool NodeInBBs = BBs.count(Node);
 277     auto &InsertPts = InsertPtsMap[Node].first;
 278     BlockFrequency &InsertPtsFreq = InsertPtsMap[Node].second;
 279
 280     // Return the optimal insert points in BBs.
 281     if (Node == Entry) {
 282       BBs.clear();
 283       if (InsertPtsFreq > BFI.getBlockFreq(Node) ||
 284           (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1))
 285         BBs.insert(Entry);
 286       else
 287         BBs.insert(InsertPts.begin(), InsertPts.end());
 288       break;
 289     }
 290
 291     BasicBlock *Parent = DT.getNode(Node)->getIDom()->getBlock();
 292     // Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child
 293     // will update its parent's ParentInsertPts and ParentPtsFreq.
 294     auto &ParentInsertPts = InsertPtsMap[Parent].first;
 295     BlockFrequency &ParentPtsFreq = InsertPtsMap[Parent].second;
 296     // Choose to insert in Node or in subtree of Node.
 297     // Don't hoist to EHPad because we may not find a proper place to insert
 298     // in EHPad.
 299     // If the total frequency of InsertPts is the same as the frequency of the
 300     // target Node, and InsertPts contains more than one nodes, choose hoisting
 301     // to reduce code size.
 302     if (NodeInBBs ||
 303         (!Node->isEHPad() &&
 304          (InsertPtsFreq > BFI.getBlockFreq(Node) ||
 305           (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)))) {
 306       ParentInsertPts.insert(Node);
 307       ParentPtsFreq += BFI.getBlockFreq(Node);
 308     } else {
 309       ParentInsertPts.insert(InsertPts.begin(), InsertPts.end());
 310       ParentPtsFreq += InsertPtsFreq;
 311     }
 312   }
 313 }
 314
 315 /// Find an insertion point that dominates all uses.
 316 SetVector<Instruction *> ConstantHoistingPass::findConstantInsertionPoint(
 317     const ConstantInfo &ConstInfo) const {
 318   assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
 319   // Collect all basic blocks.
 320   SetVector<BasicBlock *> BBs;
 321   SetVector<Instruction *> InsertPts;
 322   for (auto const &RCI : ConstInfo.RebasedConstants)
 323     for (auto const &U : RCI.Uses)
 324       BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
 325
 326   if (BBs.count(Entry)) {
 327     InsertPts.insert(&Entry->front());
 328     return InsertPts;
 329   }
 330
 331   if (BFI) {
 332     findBestInsertionSet(*DT, *BFI, Entry, BBs);
 333     for (auto BB : BBs) {
 334       BasicBlock::iterator InsertPt = BB->begin();
 335       for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 336         ;
 337       InsertPts.insert(&*InsertPt);
 338     }
 339     return InsertPts;
 340   }
 341
 342   while (BBs.size() >= 2) {
 343     BasicBlock *BB, *BB1, *BB2;
 344     BB1 = BBs.pop_back_val();
 345     BB2 = BBs.pop_back_val();
 346     BB = DT->findNearestCommonDominator(BB1, BB2);
 347     if (BB == Entry) {
 348       InsertPts.insert(&Entry->front());
 349       return InsertPts;
 350     }
 351     BBs.insert(BB);
 352   }
 353   assert((BBs.size() == 1) && "Expected only one element.");
 354   Instruction &FirstInst = (*BBs.begin())->front();
 355   InsertPts.insert(findMatInsertPt(&FirstInst));
 356   return InsertPts;
 357 }
 358
 359 /// Record constant integer ConstInt for instruction Inst at operand
 360 /// index Idx.
 361 ///
 362 /// The operand at index Idx is not necessarily the constant integer itself. It
 363 /// could also be a cast instruction or a constant expression that uses the
 364 /// constant integer.
 365 void ConstantHoistingPass::collectConstantCandidates(
 366     ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
 367     ConstantInt *ConstInt) {
 368   InstructionCost Cost;
 369   // Ask the target about the cost of materializing the constant for the given
 370   // instruction and operand index.
 371   if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
 372     Cost = TTI->getIntImmCostIntrin(IntrInst->getIntrinsicID(), Idx,
 373                                     ConstInt->getValue(), ConstInt->getType(),
 374                                     TargetTransformInfo::TCK_SizeAndLatency);
 375   else
 376     Cost = TTI->getIntImmCostInst(
 377         Inst->getOpcode(), Idx, ConstInt->getValue(), ConstInt->getType(),
 378         TargetTransformInfo::TCK_SizeAndLatency, Inst);
 379
 380   // Ignore cheap integer constants.
 381   if (Cost > TargetTransformInfo::TCC_Basic) {
 382     ConstCandMapType::iterator Itr;
 383     bool Inserted;
 384     ConstPtrUnionType Cand = ConstInt;
 385     std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
 386     if (Inserted) {
 387       ConstIntCandVec.push_back(ConstantCandidate(ConstInt));
 388       Itr->second = ConstIntCandVec.size() - 1;
 389     }
 390     ConstIntCandVec[Itr->second].addUser(Inst, Idx, *Cost.getValue());
 391     LLVM_DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs()
 392                    << "Collect constant " << *ConstInt << " from " << *Inst
 393                    << " with cost " << Cost << '\n';
 394                else dbgs() << "Collect constant " << *ConstInt
 395                            << " indirectly from " << *Inst << " via "
 396                            << *Inst->getOperand(Idx) << " with cost " << Cost
 397                            << '\n';);
 398   }
 399 }
 400
 401 /// Record constant GEP expression for instruction Inst at operand index Idx.
 402 void ConstantHoistingPass::collectConstantCandidates(
 403     ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
 404     ConstantExpr *ConstExpr) {
 405   // TODO: Handle vector GEPs
 406   if (ConstExpr->getType()->isVectorTy())
 407     return;
 408
 409   GlobalVariable *BaseGV = dyn_cast<GlobalVariable>(ConstExpr->getOperand(0));
 410   if (!BaseGV)
 411     return;
 412
 413   // Get offset from the base GV.
 414   PointerType *GVPtrTy = cast<PointerType>(BaseGV->getType());
 415   IntegerType *PtrIntTy = DL->getIntPtrType(*Ctx, GVPtrTy->getAddressSpace());
 416   APInt Offset(DL->getTypeSizeInBits(PtrIntTy), /*val*/0, /*isSigned*/true);
 417   auto *GEPO = cast<GEPOperator>(ConstExpr);
 418   if (!GEPO->accumulateConstantOffset(*DL, Offset))
 419     return;
 420
 421   if (!Offset.isIntN(32))
 422     return;
 423
 424   // A constant GEP expression that has a GlobalVariable as base pointer is
 425   // usually lowered to a load from constant pool. Such operation is unlikely
 426   // to be cheaper than compute it by <Base + Offset>, which can be lowered to
 427   // an ADD instruction or folded into Load/Store instruction.
 428   InstructionCost Cost =
 429       TTI->getIntImmCostInst(Instruction::Add, 1, Offset, PtrIntTy,
 430                              TargetTransformInfo::TCK_SizeAndLatency, Inst);
 431   ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV];
 432   ConstCandMapType::iterator Itr;
 433   bool Inserted;
 434   ConstPtrUnionType Cand = ConstExpr;
 435   std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
 436   if (Inserted) {
 437     ExprCandVec.push_back(ConstantCandidate(
 438         ConstantInt::get(Type::getInt32Ty(*Ctx), Offset.getLimitedValue()),
 439         ConstExpr));
 440     Itr->second = ExprCandVec.size() - 1;
 441   }
 442   ExprCandVec[Itr->second].addUser(Inst, Idx, *Cost.getValue());
 443 }
 444
 445 /// Check the operand for instruction Inst at index Idx.
 446 void ConstantHoistingPass::collectConstantCandidates(
 447     ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) {
 448   Value *Opnd = Inst->getOperand(Idx);
 449
 450   // Visit constant integers.
 451   if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
 452     collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 453     return;
 454   }
 455
 456   // Visit cast instructions that have constant integers.
 457   if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
 458     // Only visit cast instructions, which have been skipped. All other
 459     // instructions should have already been visited.
 460     if (!CastInst->isCast())
 461       return;
 462
 463     if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
 464       // Pretend the constant is directly used by the instruction and ignore
 465       // the cast instruction.
 466       collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 467       return;
 468     }
 469   }
 470
 471   // Visit constant expressions that have constant integers.
 472   if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
 473     // Handle constant gep expressions.
 474     if (ConstHoistGEP && ConstExpr->isGEPWithNoNotionalOverIndexing())
 475       collectConstantCandidates(ConstCandMap, Inst, Idx, ConstExpr);
 476
 477     // Only visit constant cast expressions.
 478     if (!ConstExpr->isCast())
 479       return;
 480
 481     if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
 482       // Pretend the constant is directly used by the instruction and ignore
 483       // the constant expression.
 484       collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
 485       return;
 486     }
 487   }
 488 }
 489
 490 /// Scan the instruction for expensive integer constants and record them
 491 /// in the constant candidate vector.
 492 void ConstantHoistingPass::collectConstantCandidates(
 493     ConstCandMapType &ConstCandMap, Instruction *Inst) {
 494   // Skip all cast instructions. They are visited indirectly later on.
 495   if (Inst->isCast())
 496     return;
 497
 498   // Scan all operands.
 499   for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
 500     // The cost of materializing the constants (defined in
 501     // `TargetTransformInfo::getIntImmCostInst`) for instructions which only
 502     // take constant variables is lower than `TargetTransformInfo::TCC_Basic`.
 503     // So it's safe for us to collect constant candidates from all
 504     // IntrinsicInsts.
 505     if (canReplaceOperandWithVariable(Inst, Idx)) {
 506       collectConstantCandidates(ConstCandMap, Inst, Idx);
 507     }
 508   } // end of for all operands
 509 }
 510
 511 /// Collect all integer constants in the function that cannot be folded
 512 /// into an instruction itself.
 513 void ConstantHoistingPass::collectConstantCandidates(Function &Fn) {
 514   ConstCandMapType ConstCandMap;
 515   for (BasicBlock &BB : Fn) {
 516     // Ignore unreachable basic blocks.
 517     if (!DT->isReachableFromEntry(&BB))
 518       continue;
 519     for (Instruction &Inst : BB)
 520       collectConstantCandidates(ConstCandMap, &Inst);
 521   }
 522 }
 523
 524 // This helper function is necessary to deal with values that have different
 525 // bit widths (APInt Operator- does not like that). If the value cannot be
 526 // represented in uint64 we return an "empty" APInt. This is then interpreted
 527 // as the value is not in range.
 528 static Optional<APInt> calculateOffsetDiff(const APInt &V1, const APInt &V2) {
 529   Optional<APInt> Res = None;
 530   unsigned BW = V1.getBitWidth() > V2.getBitWidth() ?
 531                 V1.getBitWidth() : V2.getBitWidth();
 532   uint64_t LimVal1 = V1.getLimitedValue();
 533   uint64_t LimVal2 = V2.getLimitedValue();
 534
 535   if (LimVal1 == ~0ULL || LimVal2 == ~0ULL)
 536     return Res;
 537
 538   uint64_t Diff = LimVal1 - LimVal2;
 539   return APInt(BW, Diff, true);
 540 }
 541
 542 // From a list of constants, one needs to picked as the base and the other
 543 // constants will be transformed into an offset from that base constant. The
 544 // question is which we can pick best? For example, consider these constants
 545 // and their number of uses:
 546 //
 547 //  Constants| 2 | 4 | 12 | 42 |
 548 //  NumUses  | 3 | 2 |  8 |  7 |
 549 //
 550 // Selecting constant 12 because it has the most uses will generate negative
 551 // offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative
 552 // offsets lead to less optimal code generation, then there might be better
 553 // solutions. Suppose immediates in the range of 0..35 are most optimally
 554 // supported by the architecture, then selecting constant 2 is most optimal
 555 // because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in
 556 // range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would
 557 // have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in
 558 // selecting the base constant the range of the offsets is a very important
 559 // factor too that we take into account here. This algorithm calculates a total
 560 // costs for selecting a constant as the base and substract the costs if
 561 // immediates are out of range. It has quadratic complexity, so we call this
 562 // function only when we're optimising for size and there are less than 100
 563 // constants, we fall back to the straightforward algorithm otherwise
 564 // which does not do all the offset calculations.
 565 unsigned
 566 ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S,
 567                                            ConstCandVecType::iterator E,
 568                                            ConstCandVecType::iterator &MaxCostItr) {
 569   unsigned NumUses = 0;
 570
 571   bool OptForSize = Entry->getParent()->hasOptSize() ||
 572                     llvm::shouldOptimizeForSize(Entry->getParent(), PSI, BFI,
 573                                                 PGSOQueryType::IRPass);
 574   if (!OptForSize || std::distance(S,E) > 100) {
 575     for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
 576       NumUses += ConstCand->Uses.size();
 577       if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
 578         MaxCostItr = ConstCand;
 579     }
 580     return NumUses;
 581   }
 582
 583   LLVM_DEBUG(dbgs() << "== Maximize constants in range ==\n");
 584   InstructionCost MaxCost = -1;
 585   for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
 586     auto Value = ConstCand->ConstInt->getValue();
 587     Type *Ty = ConstCand->ConstInt->getType();
 588     InstructionCost Cost = 0;
 589     NumUses += ConstCand->Uses.size();
 590     LLVM_DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue()
 591                       << "\n");
 592
 593     for (auto User : ConstCand->Uses) {
 594       unsigned Opcode = User.Inst->getOpcode();
 595       unsigned OpndIdx = User.OpndIdx;
 596       Cost += TTI->getIntImmCostInst(Opcode, OpndIdx, Value, Ty,
 597                                      TargetTransformInfo::TCK_SizeAndLatency);
 598       LLVM_DEBUG(dbgs() << "Cost: " << Cost << "\n");
 599
 600       for (auto C2 = S; C2 != E; ++C2) {
 601         Optional<APInt> Diff = calculateOffsetDiff(
 602                                    C2->ConstInt->getValue(),
 603                                    ConstCand->ConstInt->getValue());
 604         if (Diff) {
 605           const InstructionCost ImmCosts =
 606               TTI->getIntImmCodeSizeCost(Opcode, OpndIdx, Diff.getValue(), Ty);
 607           Cost -= ImmCosts;
 608           LLVM_DEBUG(dbgs() << "Offset " << Diff.getValue() << " "
 609                             << "has penalty: " << ImmCosts << "\n"
 610                             << "Adjusted cost: " << Cost << "\n");
 611         }
 612       }
 613     }
 614     LLVM_DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n");
 615     if (Cost > MaxCost) {
 616       MaxCost = Cost;
 617       MaxCostItr = ConstCand;
 618       LLVM_DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue()
 619                         << "\n");
 620     }
 621   }
 622   return NumUses;
 623 }
 624
 625 /// Find the base constant within the given range and rebase all other
 626 /// constants with respect to the base constant.
 627 void ConstantHoistingPass::findAndMakeBaseConstant(
 628     ConstCandVecType::iterator S, ConstCandVecType::iterator E,
 629     SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec) {
 630   auto MaxCostItr = S;
 631   unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr);
 632
 633   // Don't hoist constants that have only one use.
 634   if (NumUses <= 1)
 635     return;
 636
 637   ConstantInt *ConstInt = MaxCostItr->ConstInt;
 638   ConstantExpr *ConstExpr = MaxCostItr->ConstExpr;
 639   ConstantInfo ConstInfo;
 640   ConstInfo.BaseInt = ConstInt;
 641   ConstInfo.BaseExpr = ConstExpr;
 642   Type *Ty = ConstInt->getType();
 643
 644   // Rebase the constants with respect to the base constant.
 645   for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
 646     APInt Diff = ConstCand->ConstInt->getValue() - ConstInt->getValue();
 647     Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
 648     Type *ConstTy =
 649         ConstCand->ConstExpr ? ConstCand->ConstExpr->getType() : nullptr;
 650     ConstInfo.RebasedConstants.push_back(
 651       RebasedConstantInfo(std::move(ConstCand->Uses), Offset, ConstTy));
 652   }
 653   ConstInfoVec.push_back(std::move(ConstInfo));
 654 }
 655
 656 /// Finds and combines constant candidates that can be easily
 657 /// rematerialized with an add from a common base constant.
 658 void ConstantHoistingPass::findBaseConstants(GlobalVariable *BaseGV) {
 659   // If BaseGV is nullptr, find base among candidate constant integers;
 660   // Otherwise find base among constant GEPs that share the same BaseGV.
 661   ConstCandVecType &ConstCandVec = BaseGV ?
 662       ConstGEPCandMap[BaseGV] : ConstIntCandVec;
 663   ConstInfoVecType &ConstInfoVec = BaseGV ?
 664       ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
 665
 666   // Sort the constants by value and type. This invalidates the mapping!
 667   llvm::stable_sort(ConstCandVec, [](const ConstantCandidate &LHS,
 668                                      const ConstantCandidate &RHS) {
 669     if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
 670       return LHS.ConstInt->getType()->getBitWidth() <
 671              RHS.ConstInt->getType()->getBitWidth();
 672     return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
 673   });
 674
 675   // Simple linear scan through the sorted constant candidate vector for viable
 676   // merge candidates.
 677   auto MinValItr = ConstCandVec.begin();
 678   for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
 679        CC != E; ++CC) {
 680     if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
 681       Type *MemUseValTy = nullptr;
 682       for (auto &U : CC->Uses) {
 683         auto *UI = U.Inst;
 684         if (LoadInst *LI = dyn_cast<LoadInst>(UI)) {
 685           MemUseValTy = LI->getType();
 686           break;
 687         } else if (StoreInst *SI = dyn_cast<StoreInst>(UI)) {
 688           // Make sure the constant is used as pointer operand of the StoreInst.
 689           if (SI->getPointerOperand() == SI->getOperand(U.OpndIdx)) {
 690             MemUseValTy = SI->getValueOperand()->getType();
 691             break;
 692           }
 693         }
 694       }
 695
 696       // Check if the constant is in range of an add with immediate.
 697       APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
 698       if ((Diff.getBitWidth() <= 64) &&
 699           TTI->isLegalAddImmediate(Diff.getSExtValue()) &&
 700           // Check if Diff can be used as offset in addressing mode of the user
 701           // memory instruction.
 702           (!MemUseValTy || TTI->isLegalAddressingMode(MemUseValTy,
 703            /*BaseGV*/nullptr, /*BaseOffset*/Diff.getSExtValue(),
 704            /*HasBaseReg*/true, /*Scale*/0)))
 705         continue;
 706     }
 707     // We either have now a different constant type or the constant is not in
 708     // range of an add with immediate anymore.
 709     findAndMakeBaseConstant(MinValItr, CC, ConstInfoVec);
 710     // Start a new base constant search.
 711     MinValItr = CC;
 712   }
 713   // Finalize the last base constant search.
 714   findAndMakeBaseConstant(MinValItr, ConstCandVec.end(), ConstInfoVec);
 715 }
 716
 717 /// Updates the operand at Idx in instruction Inst with the result of
 718 ///        instruction Mat. If the instruction is a PHI node then special
 719 ///        handling for duplicate values form the same incoming basic block is
 720 ///        required.
 721 /// \return The update will always succeed, but the return value indicated if
 722 ///         Mat was used for the update or not.
 723 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
 724   if (auto PHI = dyn_cast<PHINode>(Inst)) {
 725     // Check if any previous operand of the PHI node has the same incoming basic
 726     // block. This is a very odd case that happens when the incoming basic block
 727     // has a switch statement. In this case use the same value as the previous
 728     // operand(s), otherwise we will fail verification due to different values.
 729     // The values are actually the same, but the variable names are different
 730     // and the verifier doesn't like that.
 731     BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
 732     for (unsigned i = 0; i < Idx; ++i) {
 733       if (PHI->getIncomingBlock(i) == IncomingBB) {
 734         Value *IncomingVal = PHI->getIncomingValue(i);
 735         Inst->setOperand(Idx, IncomingVal);
 736         return false;
 737       }
 738     }
 739   }
 740
 741   Inst->setOperand(Idx, Mat);
 742   return true;
 743 }
 744
 745 /// Emit materialization code for all rebased constants and update their
 746 /// users.
 747 void ConstantHoistingPass::emitBaseConstants(Instruction *Base,
 748                                              Constant *Offset,
 749                                              Type *Ty,
 750                                              const ConstantUser &ConstUser) {
 751   Instruction *Mat = Base;
 752
 753   // The same offset can be dereferenced to different types in nested struct.
 754   if (!Offset && Ty && Ty != Base->getType())
 755     Offset = ConstantInt::get(Type::getInt32Ty(*Ctx), 0);
 756
 757   if (Offset) {
 758     Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
 759                                                ConstUser.OpndIdx);
 760     if (Ty) {
 761       // Constant being rebased is a ConstantExpr.
 762       PointerType *Int8PtrTy = Type::getInt8PtrTy(*Ctx,
 763           cast<PointerType>(Ty)->getAddressSpace());
 764       Base = new BitCastInst(Base, Int8PtrTy, "base_bitcast", InsertionPt);
 765       Mat = GetElementPtrInst::Create(Int8PtrTy->getElementType(), Base,
 766           Offset, "mat_gep", InsertionPt);
 767       Mat = new BitCastInst(Mat, Ty, "mat_bitcast", InsertionPt);
 768     } else
 769       // Constant being rebased is a ConstantInt.
 770       Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
 771                                  "const_mat", InsertionPt);
 772
 773     LLVM_DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
 774                       << " + " << *Offset << ") in BB "
 775                       << Mat->getParent()->getName() << '\n'
 776                       << *Mat << '\n');
 777     Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
 778   }
 779   Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
 780
 781   // Visit constant integer.
 782   if (isa<ConstantInt>(Opnd)) {
 783     LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 784     if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
 785       Mat->eraseFromParent();
 786     LLVM_DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 787     return;
 788   }
 789
 790   // Visit cast instruction.
 791   if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
 792     assert(CastInst->isCast() && "Expected an cast instruction!");
 793     // Check if we already have visited this cast instruction before to avoid
 794     // unnecessary cloning.
 795     Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
 796     if (!ClonedCastInst) {
 797       ClonedCastInst = CastInst->clone();
 798       ClonedCastInst->setOperand(0, Mat);
 799       ClonedCastInst->insertAfter(CastInst);
 800       // Use the same debug location as the original cast instruction.
 801       ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
 802       LLVM_DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
 803                         << "To               : " << *ClonedCastInst << '\n');
 804     }
 805
 806     LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 807     updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
 808     LLVM_DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 809     return;
 810   }
 811
 812   // Visit constant expression.
 813   if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
 814     if (ConstExpr->isGEPWithNoNotionalOverIndexing()) {
 815       // Operand is a ConstantGEP, replace it.
 816       updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat);
 817       return;
 818     }
 819
 820     // Aside from constant GEPs, only constant cast expressions are collected.
 821     assert(ConstExpr->isCast() && "ConstExpr should be a cast");
 822     Instruction *ConstExprInst = ConstExpr->getAsInstruction();
 823     ConstExprInst->setOperand(0, Mat);
 824     ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
 825                                                 ConstUser.OpndIdx));
 826
 827     // Use the same debug location as the instruction we are about to update.
 828     ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
 829
 830     LLVM_DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
 831                       << "From              : " << *ConstExpr << '\n');
 832     LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
 833     if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
 834       ConstExprInst->eraseFromParent();
 835       if (Offset)
 836         Mat->eraseFromParent();
 837     }
 838     LLVM_DEBUG(dbgs() << "To    : " << *ConstUser.Inst << '\n');
 839     return;
 840   }
 841 }
 842
 843 /// Hoist and hide the base constant behind a bitcast and emit
 844 /// materialization code for derived constants.
 845 bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {
 846   bool MadeChange = false;
 847   SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec =
 848       BaseGV ? ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
 849   for (auto const &ConstInfo : ConstInfoVec) {
 850     SetVector<Instruction *> IPSet = findConstantInsertionPoint(ConstInfo);
 851     // We can have an empty set if the function contains unreachable blocks.
 852     if (IPSet.empty())
 853       continue;
 854
 855     unsigned UsesNum = 0;
 856     unsigned ReBasesNum = 0;
 857     unsigned NotRebasedNum = 0;
 858     for (Instruction *IP : IPSet) {
 859       // First, collect constants depending on this IP of the base.
 860       unsigned Uses = 0;
 861       using RebasedUse = std::tuple<Constant *, Type *, ConstantUser>;
 862       SmallVector<RebasedUse, 4> ToBeRebased;
 863       for (auto const &RCI : ConstInfo.RebasedConstants) {
 864         for (auto const &U : RCI.Uses) {
 865           Uses++;
 866           BasicBlock *OrigMatInsertBB =
 867               findMatInsertPt(U.Inst, U.OpndIdx)->getParent();
 868           // If Base constant is to be inserted in multiple places,
 869           // generate rebase for U using the Base dominating U.
 870           if (IPSet.size() == 1 ||
 871               DT->dominates(IP->getParent(), OrigMatInsertBB))
 872             ToBeRebased.push_back(RebasedUse(RCI.Offset, RCI.Ty, U));
 873         }
 874       }
 875       UsesNum = Uses;
 876
 877       // If only few constants depend on this IP of base, skip rebasing,
 878       // assuming the base and the rebased have the same materialization cost.
 879       if (ToBeRebased.size() < MinNumOfDependentToRebase) {
 880         NotRebasedNum += ToBeRebased.size();
 881         continue;
 882       }
 883
 884       // Emit an instance of the base at this IP.
 885       Instruction *Base = nullptr;
 886       // Hoist and hide the base constant behind a bitcast.
 887       if (ConstInfo.BaseExpr) {
 888         assert(BaseGV && "A base constant expression must have an base GV");
 889         Type *Ty = ConstInfo.BaseExpr->getType();
 890         Base = new BitCastInst(ConstInfo.BaseExpr, Ty, "const", IP);
 891       } else {
 892         IntegerType *Ty = ConstInfo.BaseInt->getType();
 893         Base = new BitCastInst(ConstInfo.BaseInt, Ty, "const", IP);
 894       }
 895
 896       Base->setDebugLoc(IP->getDebugLoc());
 897
 898       LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseInt
 899                         << ") to BB " << IP->getParent()->getName() << '\n'
 900                         << *Base << '\n');
 901
 902       // Emit materialization code for rebased constants depending on this IP.
 903       for (auto const &R : ToBeRebased) {
 904         Constant *Off = std::get<0>(R);
 905         Type *Ty = std::get<1>(R);
 906         ConstantUser U = std::get<2>(R);
 907         emitBaseConstants(Base, Off, Ty, U);
 908         ReBasesNum++;
 909         // Use the same debug location as the last user of the constant.
 910         Base->setDebugLoc(DILocation::getMergedLocation(
 911             Base->getDebugLoc(), U.Inst->getDebugLoc()));
 912       }
 913       assert(!Base->use_empty() && "The use list is empty!?");
 914       assert(isa<Instruction>(Base->user_back()) &&
 915              "All uses should be instructions.");
 916     }
 917     (void)UsesNum;
 918     (void)ReBasesNum;
 919     (void)NotRebasedNum;
 920     // Expect all uses are rebased after rebase is done.
 921     assert(UsesNum == (ReBasesNum + NotRebasedNum) &&
 922            "Not all uses are rebased");
 923
 924     NumConstantsHoisted++;
 925
 926     // Base constant is also included in ConstInfo.RebasedConstants, so
 927     // deduct 1 from ConstInfo.RebasedConstants.size().
 928     NumConstantsRebased += ConstInfo.RebasedConstants.size() - 1;
 929
 930     MadeChange = true;
 931   }
 932   return MadeChange;
 933 }
 934
 935 /// Check all cast instructions we made a copy of and remove them if they
 936 /// have no more users.
 937 void ConstantHoistingPass::deleteDeadCastInst() const {
 938   for (auto const &I : ClonedCastMap)
 939     if (I.first->use_empty())
 940       I.first->eraseFromParent();
 941 }
 942
 943 /// Optimize expensive integer constants in the given function.
 944 bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI,
 945                                    DominatorTree &DT, BlockFrequencyInfo *BFI,
 946                                    BasicBlock &Entry, ProfileSummaryInfo *PSI) {
 947   this->TTI = &TTI;
 948   this->DT = &DT;
 949   this->BFI = BFI;
 950   this->DL = &Fn.getParent()->getDataLayout();
 951   this->Ctx = &Fn.getContext();
 952   this->Entry = &Entry;
 953   this->PSI = PSI;
 954   // Collect all constant candidates.
 955   collectConstantCandidates(Fn);
 956
 957   // Combine constants that can be easily materialized with an add from a common
 958   // base constant.
 959   if (!ConstIntCandVec.empty())
 960     findBaseConstants(nullptr);
 961   for (const auto &MapEntry : ConstGEPCandMap)
 962     if (!MapEntry.second.empty())
 963       findBaseConstants(MapEntry.first);
 964
 965   // Finally hoist the base constant and emit materialization code for dependent
 966   // constants.
 967   bool MadeChange = false;
 968   if (!ConstIntInfoVec.empty())
 969     MadeChange = emitBaseConstants(nullptr);
 970   for (const auto &MapEntry : ConstGEPInfoMap)
 971     if (!MapEntry.second.empty())
 972       MadeChange |= emitBaseConstants(MapEntry.first);
 973
 974
 975   // Cleanup dead instructions.
 976   deleteDeadCastInst();
 977
 978   cleanup();
 979
 980   return MadeChange;
 981 }
 982
 983 PreservedAnalyses ConstantHoistingPass::run(Function &F,
 984                                             FunctionAnalysisManager &AM) {
 985   auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
 986   auto &TTI = AM.getResult<TargetIRAnalysis>(F);
 987   auto BFI = ConstHoistWithBlockFrequency
 988                  ? &AM.getResult<BlockFrequencyAnalysis>(F)
 989                  : nullptr;
 990   auto &MAMProxy = AM.getResult<ModuleAnalysisManagerFunctionProxy>(F);
 991   auto *PSI = MAMProxy.getCachedResult<ProfileSummaryAnalysis>(*F.getParent());
 992   if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock(), PSI))
 993     return PreservedAnalyses::all();
 994
 995   PreservedAnalyses PA;
 996   PA.preserveSet<CFGAnalyses>();
 997   return PA;
 998 }