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