lib/Analysis/VectorUtils.cpp

   1 //===----------- VectorUtils.cpp - Vectorizer utility functions -----------===//
   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 file defines vectorizer utilities.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Analysis/VectorUtils.h"
  15 #include "llvm/ADT/EquivalenceClasses.h"
  16 #include "llvm/Analysis/DemandedBits.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 #include "llvm/Analysis/ScalarEvolution.h"
  19 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
  20 #include "llvm/Analysis/TargetTransformInfo.h"
  21 #include "llvm/Analysis/ValueTracking.h"
  22 #include "llvm/IR/Constants.h"
  23 #include "llvm/IR/GetElementPtrTypeIterator.h"
  24 #include "llvm/IR/IRBuilder.h"
  25 #include "llvm/IR/PatternMatch.h"
  26 #include "llvm/IR/Value.h"
  27
  28 using namespace llvm;
  29 using namespace llvm::PatternMatch;
  30
  31 /// Identify if the intrinsic is trivially vectorizable.
  32 /// This method returns true if the intrinsic's argument types are all
  33 /// scalars for the scalar form of the intrinsic and all vectors for
  34 /// the vector form of the intrinsic.
  35 bool llvm::isTriviallyVectorizable(Intrinsic::ID ID) {
  36   switch (ID) {
  37   case Intrinsic::sqrt:
  38   case Intrinsic::sin:
  39   case Intrinsic::cos:
  40   case Intrinsic::exp:
  41   case Intrinsic::exp2:
  42   case Intrinsic::log:
  43   case Intrinsic::log10:
  44   case Intrinsic::log2:
  45   case Intrinsic::fabs:
  46   case Intrinsic::minnum:
  47   case Intrinsic::maxnum:
  48   case Intrinsic::copysign:
  49   case Intrinsic::floor:
  50   case Intrinsic::ceil:
  51   case Intrinsic::trunc:
  52   case Intrinsic::rint:
  53   case Intrinsic::nearbyint:
  54   case Intrinsic::round:
  55   case Intrinsic::bswap:
  56   case Intrinsic::bitreverse:
  57   case Intrinsic::ctpop:
  58   case Intrinsic::pow:
  59   case Intrinsic::fma:
  60   case Intrinsic::fmuladd:
  61   case Intrinsic::ctlz:
  62   case Intrinsic::cttz:
  63   case Intrinsic::powi:
  64     return true;
  65   default:
  66     return false;
  67   }
  68 }
  69
  70 /// Identifies if the intrinsic has a scalar operand. It check for
  71 /// ctlz,cttz and powi special intrinsics whose argument is scalar.
  72 bool llvm::hasVectorInstrinsicScalarOpd(Intrinsic::ID ID,
  73                                         unsigned ScalarOpdIdx) {
  74   switch (ID) {
  75   case Intrinsic::ctlz:
  76   case Intrinsic::cttz:
  77   case Intrinsic::powi:
  78     return (ScalarOpdIdx == 1);
  79   default:
  80     return false;
  81   }
  82 }
  83
  84 /// Returns intrinsic ID for call.
  85 /// For the input call instruction it finds mapping intrinsic and returns
  86 /// its ID, in case it does not found it return not_intrinsic.
  87 Intrinsic::ID llvm::getVectorIntrinsicIDForCall(const CallInst *CI,
  88                                                 const TargetLibraryInfo *TLI) {
  89   Intrinsic::ID ID = getIntrinsicForCallSite(CI, TLI);
  90   if (ID == Intrinsic::not_intrinsic)
  91     return Intrinsic::not_intrinsic;
  92
  93   if (isTriviallyVectorizable(ID) || ID == Intrinsic::lifetime_start ||
  94       ID == Intrinsic::lifetime_end || ID == Intrinsic::assume ||
  95       ID == Intrinsic::sideeffect)
  96     return ID;
  97   return Intrinsic::not_intrinsic;
  98 }
  99
 100 /// Find the operand of the GEP that should be checked for consecutive
 101 /// stores. This ignores trailing indices that have no effect on the final
 102 /// pointer.
 103 unsigned llvm::getGEPInductionOperand(const GetElementPtrInst *Gep) {
 104   const DataLayout &DL = Gep->getModule()->getDataLayout();
 105   unsigned LastOperand = Gep->getNumOperands() - 1;
 106   unsigned GEPAllocSize = DL.getTypeAllocSize(Gep->getResultElementType());
 107
 108   // Walk backwards and try to peel off zeros.
 109   while (LastOperand > 1 && match(Gep->getOperand(LastOperand), m_Zero())) {
 110     // Find the type we're currently indexing into.
 111     gep_type_iterator GEPTI = gep_type_begin(Gep);
 112     std::advance(GEPTI, LastOperand - 2);
 113
 114     // If it's a type with the same allocation size as the result of the GEP we
 115     // can peel off the zero index.
 116     if (DL.getTypeAllocSize(GEPTI.getIndexedType()) != GEPAllocSize)
 117       break;
 118     --LastOperand;
 119   }
 120
 121   return LastOperand;
 122 }
 123
 124 /// If the argument is a GEP, then returns the operand identified by
 125 /// getGEPInductionOperand. However, if there is some other non-loop-invariant
 126 /// operand, it returns that instead.
 127 Value *llvm::stripGetElementPtr(Value *Ptr, ScalarEvolution *SE, Loop *Lp) {
 128   GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr);
 129   if (!GEP)
 130     return Ptr;
 131
 132   unsigned InductionOperand = getGEPInductionOperand(GEP);
 133
 134   // Check that all of the gep indices are uniform except for our induction
 135   // operand.
 136   for (unsigned i = 0, e = GEP->getNumOperands(); i != e; ++i)
 137     if (i != InductionOperand &&
 138         !SE->isLoopInvariant(SE->getSCEV(GEP->getOperand(i)), Lp))
 139       return Ptr;
 140   return GEP->getOperand(InductionOperand);
 141 }
 142
 143 /// If a value has only one user that is a CastInst, return it.
 144 Value *llvm::getUniqueCastUse(Value *Ptr, Loop *Lp, Type *Ty) {
 145   Value *UniqueCast = nullptr;
 146   for (User *U : Ptr->users()) {
 147     CastInst *CI = dyn_cast<CastInst>(U);
 148     if (CI && CI->getType() == Ty) {
 149       if (!UniqueCast)
 150         UniqueCast = CI;
 151       else
 152         return nullptr;
 153     }
 154   }
 155   return UniqueCast;
 156 }
 157
 158 /// Get the stride of a pointer access in a loop. Looks for symbolic
 159 /// strides "a[i*stride]". Returns the symbolic stride, or null otherwise.
 160 Value *llvm::getStrideFromPointer(Value *Ptr, ScalarEvolution *SE, Loop *Lp) {
 161   auto *PtrTy = dyn_cast<PointerType>(Ptr->getType());
 162   if (!PtrTy || PtrTy->isAggregateType())
 163     return nullptr;
 164
 165   // Try to remove a gep instruction to make the pointer (actually index at this
 166   // point) easier analyzable. If OrigPtr is equal to Ptr we are analyzing the
 167   // pointer, otherwise, we are analyzing the index.
 168   Value *OrigPtr = Ptr;
 169
 170   // The size of the pointer access.
 171   int64_t PtrAccessSize = 1;
 172
 173   Ptr = stripGetElementPtr(Ptr, SE, Lp);
 174   const SCEV *V = SE->getSCEV(Ptr);
 175
 176   if (Ptr != OrigPtr)
 177     // Strip off casts.
 178     while (const SCEVCastExpr *C = dyn_cast<SCEVCastExpr>(V))
 179       V = C->getOperand();
 180
 181   const SCEVAddRecExpr *S = dyn_cast<SCEVAddRecExpr>(V);
 182   if (!S)
 183     return nullptr;
 184
 185   V = S->getStepRecurrence(*SE);
 186   if (!V)
 187     return nullptr;
 188
 189   // Strip off the size of access multiplication if we are still analyzing the
 190   // pointer.
 191   if (OrigPtr == Ptr) {
 192     if (const SCEVMulExpr *M = dyn_cast<SCEVMulExpr>(V)) {
 193       if (M->getOperand(0)->getSCEVType() != scConstant)
 194         return nullptr;
 195
 196       const APInt &APStepVal = cast<SCEVConstant>(M->getOperand(0))->getAPInt();
 197
 198       // Huge step value - give up.
 199       if (APStepVal.getBitWidth() > 64)
 200         return nullptr;
 201
 202       int64_t StepVal = APStepVal.getSExtValue();
 203       if (PtrAccessSize != StepVal)
 204         return nullptr;
 205       V = M->getOperand(1);
 206     }
 207   }
 208
 209   // Strip off casts.
 210   Type *StripedOffRecurrenceCast = nullptr;
 211   if (const SCEVCastExpr *C = dyn_cast<SCEVCastExpr>(V)) {
 212     StripedOffRecurrenceCast = C->getType();
 213     V = C->getOperand();
 214   }
 215
 216   // Look for the loop invariant symbolic value.
 217   const SCEVUnknown *U = dyn_cast<SCEVUnknown>(V);
 218   if (!U)
 219     return nullptr;
 220
 221   Value *Stride = U->getValue();
 222   if (!Lp->isLoopInvariant(Stride))
 223     return nullptr;
 224
 225   // If we have stripped off the recurrence cast we have to make sure that we
 226   // return the value that is used in this loop so that we can replace it later.
 227   if (StripedOffRecurrenceCast)
 228     Stride = getUniqueCastUse(Stride, Lp, StripedOffRecurrenceCast);
 229
 230   return Stride;
 231 }
 232
 233 /// Given a vector and an element number, see if the scalar value is
 234 /// already around as a register, for example if it were inserted then extracted
 235 /// from the vector.
 236 Value *llvm::findScalarElement(Value *V, unsigned EltNo) {
 237   assert(V->getType()->isVectorTy() && "Not looking at a vector?");
 238   VectorType *VTy = cast<VectorType>(V->getType());
 239   unsigned Width = VTy->getNumElements();
 240   if (EltNo >= Width)  // Out of range access.
 241     return UndefValue::get(VTy->getElementType());
 242
 243   if (Constant *C = dyn_cast<Constant>(V))
 244     return C->getAggregateElement(EltNo);
 245
 246   if (InsertElementInst *III = dyn_cast<InsertElementInst>(V)) {
 247     // If this is an insert to a variable element, we don't know what it is.
 248     if (!isa<ConstantInt>(III->getOperand(2)))
 249       return nullptr;
 250     unsigned IIElt = cast<ConstantInt>(III->getOperand(2))->getZExtValue();
 251
 252     // If this is an insert to the element we are looking for, return the
 253     // inserted value.
 254     if (EltNo == IIElt)
 255       return III->getOperand(1);
 256
 257     // Otherwise, the insertelement doesn't modify the value, recurse on its
 258     // vector input.
 259     return findScalarElement(III->getOperand(0), EltNo);
 260   }
 261
 262   if (ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(V)) {
 263     unsigned LHSWidth = SVI->getOperand(0)->getType()->getVectorNumElements();
 264     int InEl = SVI->getMaskValue(EltNo);
 265     if (InEl < 0)
 266       return UndefValue::get(VTy->getElementType());
 267     if (InEl < (int)LHSWidth)
 268       return findScalarElement(SVI->getOperand(0), InEl);
 269     return findScalarElement(SVI->getOperand(1), InEl - LHSWidth);
 270   }
 271
 272   // Extract a value from a vector add operation with a constant zero.
 273   Value *Val = nullptr; Constant *Con = nullptr;
 274   if (match(V, m_Add(m_Value(Val), m_Constant(Con))))
 275     if (Constant *Elt = Con->getAggregateElement(EltNo))
 276       if (Elt->isNullValue())
 277         return findScalarElement(Val, EltNo);
 278
 279   // Otherwise, we don't know.
 280   return nullptr;
 281 }
 282
 283 /// Get splat value if the input is a splat vector or return nullptr.
 284 /// This function is not fully general. It checks only 2 cases:
 285 /// the input value is (1) a splat constants vector or (2) a sequence
 286 /// of instructions that broadcast a single value into a vector.
 287 ///
 288 const llvm::Value *llvm::getSplatValue(const Value *V) {
 289
 290   if (auto *C = dyn_cast<Constant>(V))
 291     if (isa<VectorType>(V->getType()))
 292       return C->getSplatValue();
 293
 294   auto *ShuffleInst = dyn_cast<ShuffleVectorInst>(V);
 295   if (!ShuffleInst)
 296     return nullptr;
 297   // All-zero (or undef) shuffle mask elements.
 298   for (int MaskElt : ShuffleInst->getShuffleMask())
 299     if (MaskElt != 0 && MaskElt != -1)
 300       return nullptr;
 301   // The first shuffle source is 'insertelement' with index 0.
 302   auto *InsertEltInst =
 303     dyn_cast<InsertElementInst>(ShuffleInst->getOperand(0));
 304   if (!InsertEltInst || !isa<ConstantInt>(InsertEltInst->getOperand(2)) ||
 305       !cast<ConstantInt>(InsertEltInst->getOperand(2))->isZero())
 306     return nullptr;
 307
 308   return InsertEltInst->getOperand(1);
 309 }
 310
 311 MapVector<Instruction *, uint64_t>
 312 llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
 313                                const TargetTransformInfo *TTI) {
 314
 315   // DemandedBits will give us every value's live-out bits. But we want
 316   // to ensure no extra casts would need to be inserted, so every DAG
 317   // of connected values must have the same minimum bitwidth.
 318   EquivalenceClasses<Value *> ECs;
 319   SmallVector<Value *, 16> Worklist;
 320   SmallPtrSet<Value *, 4> Roots;
 321   SmallPtrSet<Value *, 16> Visited;
 322   DenseMap<Value *, uint64_t> DBits;
 323   SmallPtrSet<Instruction *, 4> InstructionSet;
 324   MapVector<Instruction *, uint64_t> MinBWs;
 325
 326   // Determine the roots. We work bottom-up, from truncs or icmps.
 327   bool SeenExtFromIllegalType = false;
 328   for (auto *BB : Blocks)
 329     for (auto &I : *BB) {
 330       InstructionSet.insert(&I);
 331
 332       if (TTI && (isa<ZExtInst>(&I) || isa<SExtInst>(&I)) &&
 333           !TTI->isTypeLegal(I.getOperand(0)->getType()))
 334         SeenExtFromIllegalType = true;
 335
 336       // Only deal with non-vector integers up to 64-bits wide.
 337       if ((isa<TruncInst>(&I) || isa<ICmpInst>(&I)) &&
 338           !I.getType()->isVectorTy() &&
 339           I.getOperand(0)->getType()->getScalarSizeInBits() <= 64) {
 340         // Don't make work for ourselves. If we know the loaded type is legal,
 341         // don't add it to the worklist.
 342         if (TTI && isa<TruncInst>(&I) && TTI->isTypeLegal(I.getType()))
 343           continue;
 344
 345         Worklist.push_back(&I);
 346         Roots.insert(&I);
 347       }
 348     }
 349   // Early exit.
 350   if (Worklist.empty() || (TTI && !SeenExtFromIllegalType))
 351     return MinBWs;
 352
 353   // Now proceed breadth-first, unioning values together.
 354   while (!Worklist.empty()) {
 355     Value *Val = Worklist.pop_back_val();
 356     Value *Leader = ECs.getOrInsertLeaderValue(Val);
 357
 358     if (Visited.count(Val))
 359       continue;
 360     Visited.insert(Val);
 361
 362     // Non-instructions terminate a chain successfully.
 363     if (!isa<Instruction>(Val))
 364       continue;
 365     Instruction *I = cast<Instruction>(Val);
 366
 367     // If we encounter a type that is larger than 64 bits, we can't represent
 368     // it so bail out.
 369     if (DB.getDemandedBits(I).getBitWidth() > 64)
 370       return MapVector<Instruction *, uint64_t>();
 371
 372     uint64_t V = DB.getDemandedBits(I).getZExtValue();
 373     DBits[Leader] |= V;
 374     DBits[I] = V;
 375
 376     // Casts, loads and instructions outside of our range terminate a chain
 377     // successfully.
 378     if (isa<SExtInst>(I) || isa<ZExtInst>(I) || isa<LoadInst>(I) ||
 379         !InstructionSet.count(I))
 380       continue;
 381
 382     // Unsafe casts terminate a chain unsuccessfully. We can't do anything
 383     // useful with bitcasts, ptrtoints or inttoptrs and it'd be unsafe to
 384     // transform anything that relies on them.
 385     if (isa<BitCastInst>(I) || isa<PtrToIntInst>(I) || isa<IntToPtrInst>(I) ||
 386         !I->getType()->isIntegerTy()) {
 387       DBits[Leader] |= ~0ULL;
 388       continue;
 389     }
 390
 391     // We don't modify the types of PHIs. Reductions will already have been
 392     // truncated if possible, and inductions' sizes will have been chosen by
 393     // indvars.
 394     if (isa<PHINode>(I))
 395       continue;
 396
 397     if (DBits[Leader] == ~0ULL)
 398       // All bits demanded, no point continuing.
 399       continue;
 400
 401     for (Value *O : cast<User>(I)->operands()) {
 402       ECs.unionSets(Leader, O);
 403       Worklist.push_back(O);
 404     }
 405   }
 406
 407   // Now we've discovered all values, walk them to see if there are
 408   // any users we didn't see. If there are, we can't optimize that
 409   // chain.
 410   for (auto &I : DBits)
 411     for (auto *U : I.first->users())
 412       if (U->getType()->isIntegerTy() && DBits.count(U) == 0)
 413         DBits[ECs.getOrInsertLeaderValue(I.first)] |= ~0ULL;
 414
 415   for (auto I = ECs.begin(), E = ECs.end(); I != E; ++I) {
 416     uint64_t LeaderDemandedBits = 0;
 417     for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI)
 418       LeaderDemandedBits |= DBits[*MI];
 419
 420     uint64_t MinBW = (sizeof(LeaderDemandedBits) * 8) -
 421                      llvm::countLeadingZeros(LeaderDemandedBits);
 422     // Round up to a power of 2
 423     if (!isPowerOf2_64((uint64_t)MinBW))
 424       MinBW = NextPowerOf2(MinBW);
 425
 426     // We don't modify the types of PHIs. Reductions will already have been
 427     // truncated if possible, and inductions' sizes will have been chosen by
 428     // indvars.
 429     // If we are required to shrink a PHI, abandon this entire equivalence class.
 430     bool Abort = false;
 431     for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI)
 432       if (isa<PHINode>(*MI) && MinBW < (*MI)->getType()->getScalarSizeInBits()) {
 433         Abort = true;
 434         break;
 435       }
 436     if (Abort)
 437       continue;
 438
 439     for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI) {
 440       if (!isa<Instruction>(*MI))
 441         continue;
 442       Type *Ty = (*MI)->getType();
 443       if (Roots.count(*MI))
 444         Ty = cast<Instruction>(*MI)->getOperand(0)->getType();
 445       if (MinBW < Ty->getScalarSizeInBits())
 446         MinBWs[cast<Instruction>(*MI)] = MinBW;
 447     }
 448   }
 449
 450   return MinBWs;
 451 }
 452
 453 /// \returns \p I after propagating metadata from \p VL.
 454 Instruction *llvm::propagateMetadata(Instruction *Inst, ArrayRef<Value *> VL) {
 455   Instruction *I0 = cast<Instruction>(VL[0]);
 456   SmallVector<std::pair<unsigned, MDNode *>, 4> Metadata;
 457   I0->getAllMetadataOtherThanDebugLoc(Metadata);
 458
 459   for (auto Kind :
 460        {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
 461         LLVMContext::MD_noalias, LLVMContext::MD_fpmath,
 462         LLVMContext::MD_nontemporal, LLVMContext::MD_invariant_load}) {
 463     MDNode *MD = I0->getMetadata(Kind);
 464
 465     for (int J = 1, E = VL.size(); MD && J != E; ++J) {
 466       const Instruction *IJ = cast<Instruction>(VL[J]);
 467       MDNode *IMD = IJ->getMetadata(Kind);
 468       switch (Kind) {
 469       case LLVMContext::MD_tbaa:
 470         MD = MDNode::getMostGenericTBAA(MD, IMD);
 471         break;
 472       case LLVMContext::MD_alias_scope:
 473         MD = MDNode::getMostGenericAliasScope(MD, IMD);
 474         break;
 475       case LLVMContext::MD_fpmath:
 476         MD = MDNode::getMostGenericFPMath(MD, IMD);
 477         break;
 478       case LLVMContext::MD_noalias:
 479       case LLVMContext::MD_nontemporal:
 480       case LLVMContext::MD_invariant_load:
 481         MD = MDNode::intersect(MD, IMD);
 482         break;
 483       default:
 484         llvm_unreachable("unhandled metadata");
 485       }
 486     }
 487
 488     Inst->setMetadata(Kind, MD);
 489   }
 490
 491   return Inst;
 492 }
 493
 494 Constant *llvm::createInterleaveMask(IRBuilder<> &Builder, unsigned VF,
 495                                      unsigned NumVecs) {
 496   SmallVector<Constant *, 16> Mask;
 497   for (unsigned i = 0; i < VF; i++)
 498     for (unsigned j = 0; j < NumVecs; j++)
 499       Mask.push_back(Builder.getInt32(j * VF + i));
 500
 501   return ConstantVector::get(Mask);
 502 }
 503
 504 Constant *llvm::createStrideMask(IRBuilder<> &Builder, unsigned Start,
 505                                  unsigned Stride, unsigned VF) {
 506   SmallVector<Constant *, 16> Mask;
 507   for (unsigned i = 0; i < VF; i++)
 508     Mask.push_back(Builder.getInt32(Start + i * Stride));
 509
 510   return ConstantVector::get(Mask);
 511 }
 512
 513 Constant *llvm::createSequentialMask(IRBuilder<> &Builder, unsigned Start,
 514                                      unsigned NumInts, unsigned NumUndefs) {
 515   SmallVector<Constant *, 16> Mask;
 516   for (unsigned i = 0; i < NumInts; i++)
 517     Mask.push_back(Builder.getInt32(Start + i));
 518
 519   Constant *Undef = UndefValue::get(Builder.getInt32Ty());
 520   for (unsigned i = 0; i < NumUndefs; i++)
 521     Mask.push_back(Undef);
 522
 523   return ConstantVector::get(Mask);
 524 }
 525
 526 /// A helper function for concatenating vectors. This function concatenates two
 527 /// vectors having the same element type. If the second vector has fewer
 528 /// elements than the first, it is padded with undefs.
 529 static Value *concatenateTwoVectors(IRBuilder<> &Builder, Value *V1,
 530                                     Value *V2) {
 531   VectorType *VecTy1 = dyn_cast<VectorType>(V1->getType());
 532   VectorType *VecTy2 = dyn_cast<VectorType>(V2->getType());
 533   assert(VecTy1 && VecTy2 &&
 534          VecTy1->getScalarType() == VecTy2->getScalarType() &&
 535          "Expect two vectors with the same element type");
 536
 537   unsigned NumElts1 = VecTy1->getNumElements();
 538   unsigned NumElts2 = VecTy2->getNumElements();
 539   assert(NumElts1 >= NumElts2 && "Unexpect the first vector has less elements");
 540
 541   if (NumElts1 > NumElts2) {
 542     // Extend with UNDEFs.
 543     Constant *ExtMask =
 544         createSequentialMask(Builder, 0, NumElts2, NumElts1 - NumElts2);
 545     V2 = Builder.CreateShuffleVector(V2, UndefValue::get(VecTy2), ExtMask);
 546   }
 547
 548   Constant *Mask = createSequentialMask(Builder, 0, NumElts1 + NumElts2, 0);
 549   return Builder.CreateShuffleVector(V1, V2, Mask);
 550 }
 551
 552 Value *llvm::concatenateVectors(IRBuilder<> &Builder, ArrayRef<Value *> Vecs) {
 553   unsigned NumVecs = Vecs.size();
 554   assert(NumVecs > 1 && "Should be at least two vectors");
 555
 556   SmallVector<Value *, 8> ResList;
 557   ResList.append(Vecs.begin(), Vecs.end());
 558   do {
 559     SmallVector<Value *, 8> TmpList;
 560     for (unsigned i = 0; i < NumVecs - 1; i += 2) {
 561       Value *V0 = ResList[i], *V1 = ResList[i + 1];
 562       assert((V0->getType() == V1->getType() || i == NumVecs - 2) &&
 563              "Only the last vector may have a different type");
 564
 565       TmpList.push_back(concatenateTwoVectors(Builder, V0, V1));
 566     }
 567
 568     // Push the last vector if the total number of vectors is odd.
 569     if (NumVecs % 2 != 0)
 570       TmpList.push_back(ResList[NumVecs - 1]);
 571
 572     ResList = TmpList;
 573     NumVecs = ResList.size();
 574   } while (NumVecs > 1);
 575
 576   return ResList[0];
 577 }