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/ConstantHoisting.h" 37#include "llvm/ADT/SmallSet.h" 38#include "llvm/ADT/SmallVector.h" 39#include "llvm/ADT/Statistic.h" 40#include "llvm/IR/Constants.h" 41#include "llvm/IR/IntrinsicInst.h" 42#include "llvm/Pass.h" 43#include "llvm/Support/Debug.h" 44#include "llvm/Support/raw_ostream.h" 45#include "llvm/Transforms/Scalar.h" 46#include <tuple> 47 48using namespace llvm; 49using namespace consthoist; 50 51#define DEBUG_TYPE "consthoist" 52 53STATISTIC(NumConstantsHoisted, "Number of constants hoisted"); 54STATISTIC(NumConstantsRebased, "Number of constants rebased"); 55 56namespace { 57/// \brief The constant hoisting pass. 58class ConstantHoistingLegacyPass : public FunctionPass { 59public: 60 static char ID; // Pass identification, replacement for typeid 61 ConstantHoistingLegacyPass() : FunctionPass(ID) { 62 initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry()); 63 } 64 65 bool runOnFunction(Function &Fn) override; 66 67 const char *getPassName() const override { return "Constant Hoisting"; } 68 69 void getAnalysisUsage(AnalysisUsage &AU) const override { 70 AU.setPreservesCFG(); 71 AU.addRequired<DominatorTreeWrapperPass>(); 72 AU.addRequired<TargetTransformInfoWrapperPass>(); 73 } 74 75 void releaseMemory() override { Impl.releaseMemory(); } 76 77private: 78 ConstantHoistingPass Impl; 79}; 80} 81 82char ConstantHoistingLegacyPass::ID = 0; 83INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist", 84 "Constant Hoisting", false, false) 85INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 86INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 87INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist", 88 "Constant Hoisting", false, false) 89 90FunctionPass *llvm::createConstantHoistingPass() { 91 return new ConstantHoistingLegacyPass(); 92} 93 94/// \brief Perform the constant hoisting optimization for the given function. 95bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) { 96 if (skipFunction(Fn)) 97 return false; 98 99 DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n"); 100 DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n'); 101 102 bool MadeChange = Impl.runImpl( 103 Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn), 104 getAnalysis<DominatorTreeWrapperPass>().getDomTree(), Fn.getEntryBlock()); 105 106 if (MadeChange) { 107 DEBUG(dbgs() << "********** Function after Constant Hoisting: " 108 << Fn.getName() << '\n'); 109 DEBUG(dbgs() << Fn); 110 } 111 DEBUG(dbgs() << "********** End Constant Hoisting **********\n"); 112 113 return MadeChange; 114} 115 116 117/// \brief Find the constant materialization insertion point. 118Instruction *ConstantHoistingPass::findMatInsertPt(Instruction *Inst, 119 unsigned Idx) const { 120 // If the operand is a cast instruction, then we have to materialize the 121 // constant before the cast instruction. 122 if (Idx != ~0U) { 123 Value *Opnd = Inst->getOperand(Idx); 124 if (auto CastInst = dyn_cast<Instruction>(Opnd)) 125 if (CastInst->isCast()) 126 return CastInst; 127 } 128 129 // The simple and common case. This also includes constant expressions. 130 if (!isa<PHINode>(Inst) && !Inst->isEHPad()) 131 return Inst; 132 133 // We can't insert directly before a phi node or an eh pad. Insert before 134 // the terminator of the incoming or dominating block. 135 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!"); 136 if (Idx != ~0U && isa<PHINode>(Inst)) 137 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator(); 138 139 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock(); 140 return IDom->getTerminator(); 141} 142 143/// \brief Find an insertion point that dominates all uses. 144Instruction *ConstantHoistingPass::findConstantInsertionPoint( 145 const ConstantInfo &ConstInfo) const { 146 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry."); 147 // Collect all basic blocks. 148 SmallPtrSet<BasicBlock *, 8> BBs; 149 for (auto const &RCI : ConstInfo.RebasedConstants) 150 for (auto const &U : RCI.Uses) 151 BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent()); 152 153 if (BBs.count(Entry)) 154 return &Entry->front(); 155 156 while (BBs.size() >= 2) { 157 BasicBlock *BB, *BB1, *BB2; 158 BB1 = *BBs.begin(); 159 BB2 = *std::next(BBs.begin()); 160 BB = DT->findNearestCommonDominator(BB1, BB2); 161 if (BB == Entry) 162 return &Entry->front(); 163 BBs.erase(BB1); 164 BBs.erase(BB2); 165 BBs.insert(BB); 166 } 167 assert((BBs.size() == 1) && "Expected only one element."); 168 Instruction &FirstInst = (*BBs.begin())->front(); 169 return findMatInsertPt(&FirstInst); 170} 171 172 173/// \brief Record constant integer ConstInt for instruction Inst at operand 174/// index Idx. 175/// 176/// The operand at index Idx is not necessarily the constant integer itself. It 177/// could also be a cast instruction or a constant expression that uses the 178// constant integer. 179void ConstantHoistingPass::collectConstantCandidates( 180 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx, 181 ConstantInt *ConstInt) { 182 unsigned Cost; 183 // Ask the target about the cost of materializing the constant for the given 184 // instruction and operand index. 185 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst)) 186 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx, 187 ConstInt->getValue(), ConstInt->getType()); 188 else 189 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(), 190 ConstInt->getType()); 191 192 // Ignore cheap integer constants. 193 if (Cost > TargetTransformInfo::TCC_Basic) { 194 ConstCandMapType::iterator Itr; 195 bool Inserted; 196 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0)); 197 if (Inserted) { 198 ConstCandVec.push_back(ConstantCandidate(ConstInt)); 199 Itr->second = ConstCandVec.size() - 1; 200 } 201 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost); 202 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) 203 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst 204 << " with cost " << Cost << '\n'; 205 else 206 dbgs() << "Collect constant " << *ConstInt << " indirectly from " 207 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost " 208 << Cost << '\n'; 209 ); 210 } 211} 212 213/// \brief Scan the instruction for expensive integer constants and record them 214/// in the constant candidate vector. 215void ConstantHoistingPass::collectConstantCandidates( 216 ConstCandMapType &ConstCandMap, Instruction *Inst) { 217 // Skip all cast instructions. They are visited indirectly later on. 218 if (Inst->isCast()) 219 return; 220 221 // Can't handle inline asm. Skip it. 222 if (auto Call = dyn_cast<CallInst>(Inst)) 223 if (isa<InlineAsm>(Call->getCalledValue())) 224 return; 225 226 // Switch cases must remain constant, and if the value being tested is 227 // constant the entire thing should disappear. 228 if (isa<SwitchInst>(Inst)) 229 return; 230 231 // Static allocas (constant size in the entry block) are handled by 232 // prologue/epilogue insertion so they're free anyway. We definitely don't 233 // want to make them non-constant. 234 auto AI = dyn_cast<AllocaInst>(Inst); 235 if (AI && AI->isStaticAlloca()) 236 return; 237 238 // Scan all operands. 239 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) { 240 Value *Opnd = Inst->getOperand(Idx); 241 242 // Visit constant integers. 243 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) { 244 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); 245 continue; 246 } 247 248 // Visit cast instructions that have constant integers. 249 if (auto CastInst = dyn_cast<Instruction>(Opnd)) { 250 // Only visit cast instructions, which have been skipped. All other 251 // instructions should have already been visited. 252 if (!CastInst->isCast()) 253 continue; 254 255 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) { 256 // Pretend the constant is directly used by the instruction and ignore 257 // the cast instruction. 258 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); 259 continue; 260 } 261 } 262 263 // Visit constant expressions that have constant integers. 264 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { 265 // Only visit constant cast expressions. 266 if (!ConstExpr->isCast()) 267 continue; 268 269 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) { 270 // Pretend the constant is directly used by the instruction and ignore 271 // the constant expression. 272 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); 273 continue; 274 } 275 } 276 } // end of for all operands 277} 278 279/// \brief Collect all integer constants in the function that cannot be folded 280/// into an instruction itself. 281void ConstantHoistingPass::collectConstantCandidates(Function &Fn) { 282 ConstCandMapType ConstCandMap; 283 for (BasicBlock &BB : Fn) 284 for (Instruction &Inst : BB) 285 collectConstantCandidates(ConstCandMap, &Inst); 286} 287 288// This helper function is necessary to deal with values that have different 289// bit widths (APInt Operator- does not like that). If the value cannot be 290// represented in uint64 we return an "empty" APInt. This is then interpreted 291// as the value is not in range. 292static llvm::Optional<APInt> calculateOffsetDiff(APInt V1, APInt V2) 293{ 294 llvm::Optional<APInt> Res = None; 295 unsigned BW = V1.getBitWidth() > V2.getBitWidth() ? 296 V1.getBitWidth() : V2.getBitWidth(); 297 uint64_t LimVal1 = V1.getLimitedValue(); 298 uint64_t LimVal2 = V2.getLimitedValue(); 299 300 if (LimVal1 == ~0ULL || LimVal2 == ~0ULL) 301 return Res; 302 303 uint64_t Diff = LimVal1 - LimVal2; 304 return APInt(BW, Diff, true); 305} 306 307// From a list of constants, one needs to picked as the base and the other 308// constants will be transformed into an offset from that base constant. The 309// question is which we can pick best? For example, consider these constants 310// and their number of uses: 311// 312// Constants| 2 | 4 | 12 | 42 | 313// NumUses | 3 | 2 | 8 | 7 | 314// 315// Selecting constant 12 because it has the most uses will generate negative 316// offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative 317// offsets lead to less optimal code generation, then there might be better 318// solutions. Suppose immediates in the range of 0..35 are most optimally 319// supported by the architecture, then selecting constant 2 is most optimal 320// because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in 321// range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would 322// have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in 323// selecting the base constant the range of the offsets is a very important 324// factor too that we take into account here. This algorithm calculates a total 325// costs for selecting a constant as the base and substract the costs if 326// immediates are out of range. It has quadratic complexity, so we call this 327// function only when we're optimising for size and there are less than 100 328// constants, we fall back to the straightforward algorithm otherwise 329// which does not do all the offset calculations. 330unsigned 331ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S, 332 ConstCandVecType::iterator E, 333 ConstCandVecType::iterator &MaxCostItr) { 334 unsigned NumUses = 0; 335 336 if(!Entry->getParent()->optForSize() || std::distance(S,E) > 100) { 337 for (auto ConstCand = S; ConstCand != E; ++ConstCand) { 338 NumUses += ConstCand->Uses.size(); 339 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost) 340 MaxCostItr = ConstCand; 341 } 342 return NumUses; 343 } 344 345 DEBUG(dbgs() << "== Maximize constants in range ==\n"); 346 int MaxCost = -1; 347 for (auto ConstCand = S; ConstCand != E; ++ConstCand) { 348 auto Value = ConstCand->ConstInt->getValue(); 349 Type *Ty = ConstCand->ConstInt->getType(); 350 int Cost = 0; 351 NumUses += ConstCand->Uses.size(); 352 DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue() << "\n"); 353 354 for (auto User : ConstCand->Uses) { 355 unsigned Opcode = User.Inst->getOpcode(); 356 unsigned OpndIdx = User.OpndIdx; 357 Cost += TTI->getIntImmCost(Opcode, OpndIdx, Value, Ty); 358 DEBUG(dbgs() << "Cost: " << Cost << "\n"); 359 360 for (auto C2 = S; C2 != E; ++C2) { 361 llvm::Optional<APInt> Diff = calculateOffsetDiff( 362 C2->ConstInt->getValue(), 363 ConstCand->ConstInt->getValue()); 364 if (Diff) { 365 const int ImmCosts = 366 TTI->getIntImmCodeSizeCost(Opcode, OpndIdx, Diff.getValue(), Ty); 367 Cost -= ImmCosts; 368 DEBUG(dbgs() << "Offset " << Diff.getValue() << " " 369 << "has penalty: " << ImmCosts << "\n" 370 << "Adjusted cost: " << Cost << "\n"); 371 } 372 } 373 } 374 DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n"); 375 if (Cost > MaxCost) { 376 MaxCost = Cost; 377 MaxCostItr = ConstCand; 378 DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue() 379 << "\n"); 380 } 381 } 382 return NumUses; 383} 384 385/// \brief Find the base constant within the given range and rebase all other 386/// constants with respect to the base constant. 387void ConstantHoistingPass::findAndMakeBaseConstant( 388 ConstCandVecType::iterator S, ConstCandVecType::iterator E) { 389 auto MaxCostItr = S; 390 unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr); 391 392 // Don't hoist constants that have only one use. 393 if (NumUses <= 1) 394 return; 395 396 ConstantInfo ConstInfo; 397 ConstInfo.BaseConstant = MaxCostItr->ConstInt; 398 Type *Ty = ConstInfo.BaseConstant->getType(); 399 400 // Rebase the constants with respect to the base constant. 401 for (auto ConstCand = S; ConstCand != E; ++ConstCand) { 402 APInt Diff = ConstCand->ConstInt->getValue() - 403 ConstInfo.BaseConstant->getValue(); 404 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff); 405 ConstInfo.RebasedConstants.push_back( 406 RebasedConstantInfo(std::move(ConstCand->Uses), Offset)); 407 } 408 ConstantVec.push_back(std::move(ConstInfo)); 409} 410 411/// \brief Finds and combines constant candidates that can be easily 412/// rematerialized with an add from a common base constant. 413void ConstantHoistingPass::findBaseConstants() { 414 // Sort the constants by value and type. This invalidates the mapping! 415 std::sort(ConstCandVec.begin(), ConstCandVec.end(), 416 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) { 417 if (LHS.ConstInt->getType() != RHS.ConstInt->getType()) 418 return LHS.ConstInt->getType()->getBitWidth() < 419 RHS.ConstInt->getType()->getBitWidth(); 420 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue()); 421 }); 422 423 // Simple linear scan through the sorted constant candidate vector for viable 424 // merge candidates. 425 auto MinValItr = ConstCandVec.begin(); 426 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end(); 427 CC != E; ++CC) { 428 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) { 429 // Check if the constant is in range of an add with immediate. 430 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue(); 431 if ((Diff.getBitWidth() <= 64) && 432 TTI->isLegalAddImmediate(Diff.getSExtValue())) 433 continue; 434 } 435 // We either have now a different constant type or the constant is not in 436 // range of an add with immediate anymore. 437 findAndMakeBaseConstant(MinValItr, CC); 438 // Start a new base constant search. 439 MinValItr = CC; 440 } 441 // Finalize the last base constant search. 442 findAndMakeBaseConstant(MinValItr, ConstCandVec.end()); 443} 444 445/// \brief Updates the operand at Idx in instruction Inst with the result of 446/// instruction Mat. If the instruction is a PHI node then special 447/// handling for duplicate values form the same incomming basic block is 448/// required. 449/// \return The update will always succeed, but the return value indicated if 450/// Mat was used for the update or not. 451static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) { 452 if (auto PHI = dyn_cast<PHINode>(Inst)) { 453 // Check if any previous operand of the PHI node has the same incoming basic 454 // block. This is a very odd case that happens when the incoming basic block 455 // has a switch statement. In this case use the same value as the previous 456 // operand(s), otherwise we will fail verification due to different values. 457 // The values are actually the same, but the variable names are different 458 // and the verifier doesn't like that. 459 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx); 460 for (unsigned i = 0; i < Idx; ++i) { 461 if (PHI->getIncomingBlock(i) == IncomingBB) { 462 Value *IncomingVal = PHI->getIncomingValue(i); 463 Inst->setOperand(Idx, IncomingVal); 464 return false; 465 } 466 } 467 } 468 469 Inst->setOperand(Idx, Mat); 470 return true; 471} 472 473/// \brief Emit materialization code for all rebased constants and update their 474/// users. 475void ConstantHoistingPass::emitBaseConstants(Instruction *Base, 476 Constant *Offset, 477 const ConstantUser &ConstUser) { 478 Instruction *Mat = Base; 479 if (Offset) { 480 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst, 481 ConstUser.OpndIdx); 482 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset, 483 "const_mat", InsertionPt); 484 485 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0) 486 << " + " << *Offset << ") in BB " 487 << Mat->getParent()->getName() << '\n' << *Mat << '\n'); 488 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc()); 489 } 490 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx); 491 492 // Visit constant integer. 493 if (isa<ConstantInt>(Opnd)) { 494 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); 495 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset) 496 Mat->eraseFromParent(); 497 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); 498 return; 499 } 500 501 // Visit cast instruction. 502 if (auto CastInst = dyn_cast<Instruction>(Opnd)) { 503 assert(CastInst->isCast() && "Expected an cast instruction!"); 504 // Check if we already have visited this cast instruction before to avoid 505 // unnecessary cloning. 506 Instruction *&ClonedCastInst = ClonedCastMap[CastInst]; 507 if (!ClonedCastInst) { 508 ClonedCastInst = CastInst->clone(); 509 ClonedCastInst->setOperand(0, Mat); 510 ClonedCastInst->insertAfter(CastInst); 511 // Use the same debug location as the original cast instruction. 512 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc()); 513 DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n' 514 << "To : " << *ClonedCastInst << '\n'); 515 } 516 517 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); 518 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst); 519 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); 520 return; 521 } 522 523 // Visit constant expression. 524 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { 525 Instruction *ConstExprInst = ConstExpr->getAsInstruction(); 526 ConstExprInst->setOperand(0, Mat); 527 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst, 528 ConstUser.OpndIdx)); 529 530 // Use the same debug location as the instruction we are about to update. 531 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc()); 532 533 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n' 534 << "From : " << *ConstExpr << '\n'); 535 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); 536 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) { 537 ConstExprInst->eraseFromParent(); 538 if (Offset) 539 Mat->eraseFromParent(); 540 } 541 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); 542 return; 543 } 544} 545 546/// \brief Hoist and hide the base constant behind a bitcast and emit 547/// materialization code for derived constants. 548bool ConstantHoistingPass::emitBaseConstants() { 549 bool MadeChange = false; 550 for (auto const &ConstInfo : ConstantVec) { 551 // Hoist and hide the base constant behind a bitcast. 552 Instruction *IP = findConstantInsertionPoint(ConstInfo); 553 IntegerType *Ty = ConstInfo.BaseConstant->getType(); 554 Instruction *Base = 555 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP); 556 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB " 557 << IP->getParent()->getName() << '\n' << *Base << '\n'); 558 NumConstantsHoisted++; 559 560 // Emit materialization code for all rebased constants. 561 for (auto const &RCI : ConstInfo.RebasedConstants) { 562 NumConstantsRebased++; 563 for (auto const &U : RCI.Uses) 564 emitBaseConstants(Base, RCI.Offset, U); 565 } 566 567 // Use the same debug location as the last user of the constant. 568 assert(!Base->use_empty() && "The use list is empty!?"); 569 assert(isa<Instruction>(Base->user_back()) && 570 "All uses should be instructions."); 571 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc()); 572 573 // Correct for base constant, which we counted above too. 574 NumConstantsRebased--; 575 MadeChange = true; 576 } 577 return MadeChange; 578} 579 580/// \brief Check all cast instructions we made a copy of and remove them if they 581/// have no more users. 582void ConstantHoistingPass::deleteDeadCastInst() const { 583 for (auto const &I : ClonedCastMap) 584 if (I.first->use_empty()) 585 I.first->eraseFromParent(); 586} 587 588/// \brief Optimize expensive integer constants in the given function. 589bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI, 590 DominatorTree &DT, BasicBlock &Entry) { 591 this->TTI = &TTI; 592 this->DT = &DT; 593 this->Entry = &Entry; 594 // Collect all constant candidates. 595 collectConstantCandidates(Fn); 596 597 // There are no constant candidates to worry about. 598 if (ConstCandVec.empty()) 599 return false; 600 601 // Combine constants that can be easily materialized with an add from a common 602 // base constant. 603 findBaseConstants(); 604 605 // There are no constants to emit. 606 if (ConstantVec.empty()) 607 return false; 608 609 // Finally hoist the base constant and emit materialization code for dependent 610 // constants. 611 bool MadeChange = emitBaseConstants(); 612 613 // Cleanup dead instructions. 614 deleteDeadCastInst(); 615 616 return MadeChange; 617} 618 619PreservedAnalyses ConstantHoistingPass::run(Function &F, 620 FunctionAnalysisManager &AM) { 621 auto &DT = AM.getResult<DominatorTreeAnalysis>(F); 622 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 623 if (!runImpl(F, TTI, DT, F.getEntryBlock())) 624 return PreservedAnalyses::all(); 625 626 // FIXME: This should also 'preserve the CFG'. 627 return PreservedAnalyses::none(); 628} 629