1//===- InstCombineSelect.cpp ----------------------------------------------===//
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 implements the visitSelect function.
11//
12//===----------------------------------------------------------------------===//
13
14#include "InstCombine.h"
15#include "llvm/Analysis/ConstantFolding.h"
16#include "llvm/Analysis/InstructionSimplify.h"
17#include "llvm/IR/PatternMatch.h"
18using namespace llvm;
19using namespace PatternMatch;
20
21#define DEBUG_TYPE "instcombine"
22
23/// MatchSelectPattern - Pattern match integer [SU]MIN, [SU]MAX, and ABS idioms,
24/// returning the kind and providing the out parameter results if we
25/// successfully match.
26static SelectPatternFlavor
27MatchSelectPattern(Value *V, Value *&LHS, Value *&RHS) {
28  SelectInst *SI = dyn_cast<SelectInst>(V);
29  if (!SI) return SPF_UNKNOWN;
30
31  ICmpInst *ICI = dyn_cast<ICmpInst>(SI->getCondition());
32  if (!ICI) return SPF_UNKNOWN;
33
34  ICmpInst::Predicate Pred = ICI->getPredicate();
35  Value *CmpLHS = ICI->getOperand(0);
36  Value *CmpRHS = ICI->getOperand(1);
37  Value *TrueVal = SI->getTrueValue();
38  Value *FalseVal = SI->getFalseValue();
39
40  LHS = CmpLHS;
41  RHS = CmpRHS;
42
43  // (icmp X, Y) ? X : Y
44  if (TrueVal == CmpLHS && FalseVal == CmpRHS) {
45    switch (Pred) {
46    default: return SPF_UNKNOWN; // Equality.
47    case ICmpInst::ICMP_UGT:
48    case ICmpInst::ICMP_UGE: return SPF_UMAX;
49    case ICmpInst::ICMP_SGT:
50    case ICmpInst::ICMP_SGE: return SPF_SMAX;
51    case ICmpInst::ICMP_ULT:
52    case ICmpInst::ICMP_ULE: return SPF_UMIN;
53    case ICmpInst::ICMP_SLT:
54    case ICmpInst::ICMP_SLE: return SPF_SMIN;
55    }
56  }
57
58  // (icmp X, Y) ? Y : X
59  if (TrueVal == CmpRHS && FalseVal == CmpLHS) {
60    switch (Pred) {
61    default: return SPF_UNKNOWN; // Equality.
62    case ICmpInst::ICMP_UGT:
63    case ICmpInst::ICMP_UGE: return SPF_UMIN;
64    case ICmpInst::ICMP_SGT:
65    case ICmpInst::ICMP_SGE: return SPF_SMIN;
66    case ICmpInst::ICMP_ULT:
67    case ICmpInst::ICMP_ULE: return SPF_UMAX;
68    case ICmpInst::ICMP_SLT:
69    case ICmpInst::ICMP_SLE: return SPF_SMAX;
70    }
71  }
72
73  if (ConstantInt *C1 = dyn_cast<ConstantInt>(CmpRHS)) {
74    if ((CmpLHS == TrueVal && match(FalseVal, m_Neg(m_Specific(CmpLHS)))) ||
75        (CmpLHS == FalseVal && match(TrueVal, m_Neg(m_Specific(CmpLHS))))) {
76
77      // ABS(X) ==> (X >s 0) ? X : -X and (X >s -1) ? X : -X
78      // NABS(X) ==> (X >s 0) ? -X : X and (X >s -1) ? -X : X
79      if (Pred == ICmpInst::ICMP_SGT && (C1->isZero() || C1->isMinusOne())) {
80        return (CmpLHS == TrueVal) ? SPF_ABS : SPF_NABS;
81      }
82
83      // ABS(X) ==> (X <s 0) ? -X : X and (X <s 1) ? -X : X
84      // NABS(X) ==> (X <s 0) ? X : -X and (X <s 1) ? X : -X
85      if (Pred == ICmpInst::ICMP_SLT && (C1->isZero() || C1->isOne())) {
86        return (CmpLHS == FalseVal) ? SPF_ABS : SPF_NABS;
87      }
88    }
89  }
90
91  // TODO: (X > 4) ? X : 5   -->  (X >= 5) ? X : 5  -->  MAX(X, 5)
92
93  return SPF_UNKNOWN;
94}
95
96
97/// GetSelectFoldableOperands - We want to turn code that looks like this:
98///   %C = or %A, %B
99///   %D = select %cond, %C, %A
100/// into:
101///   %C = select %cond, %B, 0
102///   %D = or %A, %C
103///
104/// Assuming that the specified instruction is an operand to the select, return
105/// a bitmask indicating which operands of this instruction are foldable if they
106/// equal the other incoming value of the select.
107///
108static unsigned GetSelectFoldableOperands(Instruction *I) {
109  switch (I->getOpcode()) {
110  case Instruction::Add:
111  case Instruction::Mul:
112  case Instruction::And:
113  case Instruction::Or:
114  case Instruction::Xor:
115    return 3;              // Can fold through either operand.
116  case Instruction::Sub:   // Can only fold on the amount subtracted.
117  case Instruction::Shl:   // Can only fold on the shift amount.
118  case Instruction::LShr:
119  case Instruction::AShr:
120    return 1;
121  default:
122    return 0;              // Cannot fold
123  }
124}
125
126/// GetSelectFoldableConstant - For the same transformation as the previous
127/// function, return the identity constant that goes into the select.
128static Constant *GetSelectFoldableConstant(Instruction *I) {
129  switch (I->getOpcode()) {
130  default: llvm_unreachable("This cannot happen!");
131  case Instruction::Add:
132  case Instruction::Sub:
133  case Instruction::Or:
134  case Instruction::Xor:
135  case Instruction::Shl:
136  case Instruction::LShr:
137  case Instruction::AShr:
138    return Constant::getNullValue(I->getType());
139  case Instruction::And:
140    return Constant::getAllOnesValue(I->getType());
141  case Instruction::Mul:
142    return ConstantInt::get(I->getType(), 1);
143  }
144}
145
146/// FoldSelectOpOp - Here we have (select c, TI, FI), and we know that TI and FI
147/// have the same opcode and only one use each.  Try to simplify this.
148Instruction *InstCombiner::FoldSelectOpOp(SelectInst &SI, Instruction *TI,
149                                          Instruction *FI) {
150  if (TI->getNumOperands() == 1) {
151    // If this is a non-volatile load or a cast from the same type,
152    // merge.
153    if (TI->isCast()) {
154      Type *FIOpndTy = FI->getOperand(0)->getType();
155      if (TI->getOperand(0)->getType() != FIOpndTy)
156        return nullptr;
157      // The select condition may be a vector. We may only change the operand
158      // type if the vector width remains the same (and matches the condition).
159      Type *CondTy = SI.getCondition()->getType();
160      if (CondTy->isVectorTy() && (!FIOpndTy->isVectorTy() ||
161          CondTy->getVectorNumElements() != FIOpndTy->getVectorNumElements()))
162        return nullptr;
163    } else {
164      return nullptr;  // unknown unary op.
165    }
166
167    // Fold this by inserting a select from the input values.
168    Value *NewSI = Builder->CreateSelect(SI.getCondition(), TI->getOperand(0),
169                                         FI->getOperand(0), SI.getName()+".v");
170    return CastInst::Create(Instruction::CastOps(TI->getOpcode()), NewSI,
171                            TI->getType());
172  }
173
174  // Only handle binary operators here.
175  if (!isa<BinaryOperator>(TI))
176    return nullptr;
177
178  // Figure out if the operations have any operands in common.
179  Value *MatchOp, *OtherOpT, *OtherOpF;
180  bool MatchIsOpZero;
181  if (TI->getOperand(0) == FI->getOperand(0)) {
182    MatchOp  = TI->getOperand(0);
183    OtherOpT = TI->getOperand(1);
184    OtherOpF = FI->getOperand(1);
185    MatchIsOpZero = true;
186  } else if (TI->getOperand(1) == FI->getOperand(1)) {
187    MatchOp  = TI->getOperand(1);
188    OtherOpT = TI->getOperand(0);
189    OtherOpF = FI->getOperand(0);
190    MatchIsOpZero = false;
191  } else if (!TI->isCommutative()) {
192    return nullptr;
193  } else if (TI->getOperand(0) == FI->getOperand(1)) {
194    MatchOp  = TI->getOperand(0);
195    OtherOpT = TI->getOperand(1);
196    OtherOpF = FI->getOperand(0);
197    MatchIsOpZero = true;
198  } else if (TI->getOperand(1) == FI->getOperand(0)) {
199    MatchOp  = TI->getOperand(1);
200    OtherOpT = TI->getOperand(0);
201    OtherOpF = FI->getOperand(1);
202    MatchIsOpZero = true;
203  } else {
204    return nullptr;
205  }
206
207  // If we reach here, they do have operations in common.
208  Value *NewSI = Builder->CreateSelect(SI.getCondition(), OtherOpT,
209                                       OtherOpF, SI.getName()+".v");
210
211  if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TI)) {
212    if (MatchIsOpZero)
213      return BinaryOperator::Create(BO->getOpcode(), MatchOp, NewSI);
214    else
215      return BinaryOperator::Create(BO->getOpcode(), NewSI, MatchOp);
216  }
217  llvm_unreachable("Shouldn't get here");
218}
219
220static bool isSelect01(Constant *C1, Constant *C2) {
221  ConstantInt *C1I = dyn_cast<ConstantInt>(C1);
222  if (!C1I)
223    return false;
224  ConstantInt *C2I = dyn_cast<ConstantInt>(C2);
225  if (!C2I)
226    return false;
227  if (!C1I->isZero() && !C2I->isZero()) // One side must be zero.
228    return false;
229  return C1I->isOne() || C1I->isAllOnesValue() ||
230         C2I->isOne() || C2I->isAllOnesValue();
231}
232
233/// FoldSelectIntoOp - Try fold the select into one of the operands to
234/// facilitate further optimization.
235Instruction *InstCombiner::FoldSelectIntoOp(SelectInst &SI, Value *TrueVal,
236                                            Value *FalseVal) {
237  // See the comment above GetSelectFoldableOperands for a description of the
238  // transformation we are doing here.
239  if (Instruction *TVI = dyn_cast<Instruction>(TrueVal)) {
240    if (TVI->hasOneUse() && TVI->getNumOperands() == 2 &&
241        !isa<Constant>(FalseVal)) {
242      if (unsigned SFO = GetSelectFoldableOperands(TVI)) {
243        unsigned OpToFold = 0;
244        if ((SFO & 1) && FalseVal == TVI->getOperand(0)) {
245          OpToFold = 1;
246        } else if ((SFO & 2) && FalseVal == TVI->getOperand(1)) {
247          OpToFold = 2;
248        }
249
250        if (OpToFold) {
251          Constant *C = GetSelectFoldableConstant(TVI);
252          Value *OOp = TVI->getOperand(2-OpToFold);
253          // Avoid creating select between 2 constants unless it's selecting
254          // between 0, 1 and -1.
255          if (!isa<Constant>(OOp) || isSelect01(C, cast<Constant>(OOp))) {
256            Value *NewSel = Builder->CreateSelect(SI.getCondition(), OOp, C);
257            NewSel->takeName(TVI);
258            BinaryOperator *TVI_BO = cast<BinaryOperator>(TVI);
259            BinaryOperator *BO = BinaryOperator::Create(TVI_BO->getOpcode(),
260                                                        FalseVal, NewSel);
261            if (isa<PossiblyExactOperator>(BO))
262              BO->setIsExact(TVI_BO->isExact());
263            if (isa<OverflowingBinaryOperator>(BO)) {
264              BO->setHasNoUnsignedWrap(TVI_BO->hasNoUnsignedWrap());
265              BO->setHasNoSignedWrap(TVI_BO->hasNoSignedWrap());
266            }
267            return BO;
268          }
269        }
270      }
271    }
272  }
273
274  if (Instruction *FVI = dyn_cast<Instruction>(FalseVal)) {
275    if (FVI->hasOneUse() && FVI->getNumOperands() == 2 &&
276        !isa<Constant>(TrueVal)) {
277      if (unsigned SFO = GetSelectFoldableOperands(FVI)) {
278        unsigned OpToFold = 0;
279        if ((SFO & 1) && TrueVal == FVI->getOperand(0)) {
280          OpToFold = 1;
281        } else if ((SFO & 2) && TrueVal == FVI->getOperand(1)) {
282          OpToFold = 2;
283        }
284
285        if (OpToFold) {
286          Constant *C = GetSelectFoldableConstant(FVI);
287          Value *OOp = FVI->getOperand(2-OpToFold);
288          // Avoid creating select between 2 constants unless it's selecting
289          // between 0, 1 and -1.
290          if (!isa<Constant>(OOp) || isSelect01(C, cast<Constant>(OOp))) {
291            Value *NewSel = Builder->CreateSelect(SI.getCondition(), C, OOp);
292            NewSel->takeName(FVI);
293            BinaryOperator *FVI_BO = cast<BinaryOperator>(FVI);
294            BinaryOperator *BO = BinaryOperator::Create(FVI_BO->getOpcode(),
295                                                        TrueVal, NewSel);
296            if (isa<PossiblyExactOperator>(BO))
297              BO->setIsExact(FVI_BO->isExact());
298            if (isa<OverflowingBinaryOperator>(BO)) {
299              BO->setHasNoUnsignedWrap(FVI_BO->hasNoUnsignedWrap());
300              BO->setHasNoSignedWrap(FVI_BO->hasNoSignedWrap());
301            }
302            return BO;
303          }
304        }
305      }
306    }
307  }
308
309  return nullptr;
310}
311
312/// SimplifyWithOpReplaced - See if V simplifies when its operand Op is
313/// replaced with RepOp.
314static Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
315                                     const DataLayout *TD,
316                                     const TargetLibraryInfo *TLI) {
317  // Trivial replacement.
318  if (V == Op)
319    return RepOp;
320
321  Instruction *I = dyn_cast<Instruction>(V);
322  if (!I)
323    return nullptr;
324
325  // If this is a binary operator, try to simplify it with the replaced op.
326  if (BinaryOperator *B = dyn_cast<BinaryOperator>(I)) {
327    if (B->getOperand(0) == Op)
328      return SimplifyBinOp(B->getOpcode(), RepOp, B->getOperand(1), TD, TLI);
329    if (B->getOperand(1) == Op)
330      return SimplifyBinOp(B->getOpcode(), B->getOperand(0), RepOp, TD, TLI);
331  }
332
333  // Same for CmpInsts.
334  if (CmpInst *C = dyn_cast<CmpInst>(I)) {
335    if (C->getOperand(0) == Op)
336      return SimplifyCmpInst(C->getPredicate(), RepOp, C->getOperand(1), TD,
337                             TLI);
338    if (C->getOperand(1) == Op)
339      return SimplifyCmpInst(C->getPredicate(), C->getOperand(0), RepOp, TD,
340                             TLI);
341  }
342
343  // TODO: We could hand off more cases to instsimplify here.
344
345  // If all operands are constant after substituting Op for RepOp then we can
346  // constant fold the instruction.
347  if (Constant *CRepOp = dyn_cast<Constant>(RepOp)) {
348    // Build a list of all constant operands.
349    SmallVector<Constant*, 8> ConstOps;
350    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
351      if (I->getOperand(i) == Op)
352        ConstOps.push_back(CRepOp);
353      else if (Constant *COp = dyn_cast<Constant>(I->getOperand(i)))
354        ConstOps.push_back(COp);
355      else
356        break;
357    }
358
359    // All operands were constants, fold it.
360    if (ConstOps.size() == I->getNumOperands()) {
361      if (CmpInst *C = dyn_cast<CmpInst>(I))
362        return ConstantFoldCompareInstOperands(C->getPredicate(), ConstOps[0],
363                                               ConstOps[1], TD, TLI);
364
365      if (LoadInst *LI = dyn_cast<LoadInst>(I))
366        if (!LI->isVolatile())
367          return ConstantFoldLoadFromConstPtr(ConstOps[0], TD);
368
369      return ConstantFoldInstOperands(I->getOpcode(), I->getType(),
370                                      ConstOps, TD, TLI);
371    }
372  }
373
374  return nullptr;
375}
376
377/// foldSelectICmpAndOr - We want to turn:
378///   (select (icmp eq (and X, C1), 0), Y, (or Y, C2))
379/// into:
380///   (or (shl (and X, C1), C3), y)
381/// iff:
382///   C1 and C2 are both powers of 2
383/// where:
384///   C3 = Log(C2) - Log(C1)
385///
386/// This transform handles cases where:
387/// 1. The icmp predicate is inverted
388/// 2. The select operands are reversed
389/// 3. The magnitude of C2 and C1 are flipped
390///
391/// This also tries to turn
392/// --- Single bit tests:
393/// if ((x & C) == 0) x |= C	to  x |= C
394/// if ((x & C) != 0) x ^= C	to  x &= ~C
395/// if ((x & C) == 0) x ^= C	to  x |= C
396/// if ((x & C) != 0) x &= ~C	to  x &= ~C
397/// if ((x & C) == 0) x &= ~C	to  nothing
398static Value *foldSelectICmpAndOr(SelectInst &SI, Value *TrueVal,
399                                  Value *FalseVal,
400                                  InstCombiner::BuilderTy *Builder) {
401  const ICmpInst *IC = dyn_cast<ICmpInst>(SI.getCondition());
402  if (!IC || !IC->isEquality() || !SI.getType()->isIntegerTy())
403    return nullptr;
404
405  Value *CmpLHS = IC->getOperand(0);
406  Value *CmpRHS = IC->getOperand(1);
407
408  if (!match(CmpRHS, m_Zero()))
409    return nullptr;
410
411  Value *X;
412  const APInt *C1;
413  if (!match(CmpLHS, m_And(m_Value(X), m_Power2(C1))))
414    return nullptr;
415
416  const APInt *C2;
417  if (match(TrueVal, m_Specific(X))) {
418    // if ((X & C) != 0) X ^= C becomes X &= ~C
419    if (match(FalseVal, m_Xor(m_Specific(X), m_APInt(C2))) && C1 == C2)
420      return Builder->CreateAnd(X, ~(*C1));
421    // if ((X & C) != 0) X &= ~C becomes X &= ~C
422    if (match(FalseVal, m_And(m_Specific(X), m_APInt(C2))) && *C1 == ~(*C2))
423      return FalseVal;
424  } else if (match(FalseVal, m_Specific(X))) {
425    // if ((X & C) == 0) X ^= C becomes X |= C
426    if (match(TrueVal, m_Xor(m_Specific(X), m_APInt(C2))) && C1 == C2)
427      return Builder->CreateOr(X, *C1);
428    // if ((X & C) == 0) X &= ~C becomes nothing
429    if (match(TrueVal, m_And(m_Specific(X), m_APInt(C2))) && *C1 == ~(*C2))
430      return X;
431    // if ((X & C) == 0) X |= C becomes X |= C
432    if (match(TrueVal, m_Or(m_Specific(X), m_APInt(C2))) && C1 == C2)
433      return TrueVal;
434  }
435
436  bool OrOnTrueVal = false;
437  bool OrOnFalseVal = match(FalseVal, m_Or(m_Specific(TrueVal), m_Power2(C2)));
438  if (!OrOnFalseVal)
439    OrOnTrueVal = match(TrueVal, m_Or(m_Specific(FalseVal), m_Power2(C2)));
440
441  if (!OrOnFalseVal && !OrOnTrueVal)
442    return nullptr;
443
444  Value *V = CmpLHS;
445  Value *Y = OrOnFalseVal ? TrueVal : FalseVal;
446
447  unsigned C1Log = C1->logBase2();
448  unsigned C2Log = C2->logBase2();
449  if (C2Log > C1Log) {
450    V = Builder->CreateZExtOrTrunc(V, Y->getType());
451    V = Builder->CreateShl(V, C2Log - C1Log);
452  } else if (C1Log > C2Log) {
453    V = Builder->CreateLShr(V, C1Log - C2Log);
454    V = Builder->CreateZExtOrTrunc(V, Y->getType());
455  } else
456    V = Builder->CreateZExtOrTrunc(V, Y->getType());
457
458  ICmpInst::Predicate Pred = IC->getPredicate();
459  if ((Pred == ICmpInst::ICMP_NE && OrOnFalseVal) ||
460      (Pred == ICmpInst::ICMP_EQ && OrOnTrueVal))
461    V = Builder->CreateXor(V, *C2);
462
463  return Builder->CreateOr(V, Y);
464}
465
466/// visitSelectInstWithICmp - Visit a SelectInst that has an
467/// ICmpInst as its first operand.
468///
469Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
470                                                   ICmpInst *ICI) {
471  bool Changed = false;
472  ICmpInst::Predicate Pred = ICI->getPredicate();
473  Value *CmpLHS = ICI->getOperand(0);
474  Value *CmpRHS = ICI->getOperand(1);
475  Value *TrueVal = SI.getTrueValue();
476  Value *FalseVal = SI.getFalseValue();
477
478  // Check cases where the comparison is with a constant that
479  // can be adjusted to fit the min/max idiom. We may move or edit ICI
480  // here, so make sure the select is the only user.
481  if (ICI->hasOneUse())
482    if (ConstantInt *CI = dyn_cast<ConstantInt>(CmpRHS)) {
483      // X < MIN ? T : F  -->  F
484      if ((Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT)
485          && CI->isMinValue(Pred == ICmpInst::ICMP_SLT))
486        return ReplaceInstUsesWith(SI, FalseVal);
487      // X > MAX ? T : F  -->  F
488      else if ((Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_UGT)
489               && CI->isMaxValue(Pred == ICmpInst::ICMP_SGT))
490        return ReplaceInstUsesWith(SI, FalseVal);
491      switch (Pred) {
492      default: break;
493      case ICmpInst::ICMP_ULT:
494      case ICmpInst::ICMP_SLT:
495      case ICmpInst::ICMP_UGT:
496      case ICmpInst::ICMP_SGT: {
497        // These transformations only work for selects over integers.
498        IntegerType *SelectTy = dyn_cast<IntegerType>(SI.getType());
499        if (!SelectTy)
500          break;
501
502        Constant *AdjustedRHS;
503        if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_SGT)
504          AdjustedRHS = ConstantInt::get(CI->getContext(), CI->getValue() + 1);
505        else // (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_SLT)
506          AdjustedRHS = ConstantInt::get(CI->getContext(), CI->getValue() - 1);
507
508        // X > C ? X : C+1  -->  X < C+1 ? C+1 : X
509        // X < C ? X : C-1  -->  X > C-1 ? C-1 : X
510        if ((CmpLHS == TrueVal && AdjustedRHS == FalseVal) ||
511            (CmpLHS == FalseVal && AdjustedRHS == TrueVal))
512          ; // Nothing to do here. Values match without any sign/zero extension.
513
514        // Types do not match. Instead of calculating this with mixed types
515        // promote all to the larger type. This enables scalar evolution to
516        // analyze this expression.
517        else if (CmpRHS->getType()->getScalarSizeInBits()
518                 < SelectTy->getBitWidth()) {
519          Constant *sextRHS = ConstantExpr::getSExt(AdjustedRHS, SelectTy);
520
521          // X = sext x; x >s c ? X : C+1 --> X = sext x; X <s C+1 ? C+1 : X
522          // X = sext x; x <s c ? X : C-1 --> X = sext x; X >s C-1 ? C-1 : X
523          // X = sext x; x >u c ? X : C+1 --> X = sext x; X <u C+1 ? C+1 : X
524          // X = sext x; x <u c ? X : C-1 --> X = sext x; X >u C-1 ? C-1 : X
525          if (match(TrueVal, m_SExt(m_Specific(CmpLHS))) &&
526                sextRHS == FalseVal) {
527            CmpLHS = TrueVal;
528            AdjustedRHS = sextRHS;
529          } else if (match(FalseVal, m_SExt(m_Specific(CmpLHS))) &&
530                     sextRHS == TrueVal) {
531            CmpLHS = FalseVal;
532            AdjustedRHS = sextRHS;
533          } else if (ICI->isUnsigned()) {
534            Constant *zextRHS = ConstantExpr::getZExt(AdjustedRHS, SelectTy);
535            // X = zext x; x >u c ? X : C+1 --> X = zext x; X <u C+1 ? C+1 : X
536            // X = zext x; x <u c ? X : C-1 --> X = zext x; X >u C-1 ? C-1 : X
537            // zext + signed compare cannot be changed:
538            //    0xff <s 0x00, but 0x00ff >s 0x0000
539            if (match(TrueVal, m_ZExt(m_Specific(CmpLHS))) &&
540                zextRHS == FalseVal) {
541              CmpLHS = TrueVal;
542              AdjustedRHS = zextRHS;
543            } else if (match(FalseVal, m_ZExt(m_Specific(CmpLHS))) &&
544                       zextRHS == TrueVal) {
545              CmpLHS = FalseVal;
546              AdjustedRHS = zextRHS;
547            } else
548              break;
549          } else
550            break;
551        } else
552          break;
553
554        Pred = ICmpInst::getSwappedPredicate(Pred);
555        CmpRHS = AdjustedRHS;
556        std::swap(FalseVal, TrueVal);
557        ICI->setPredicate(Pred);
558        ICI->setOperand(0, CmpLHS);
559        ICI->setOperand(1, CmpRHS);
560        SI.setOperand(1, TrueVal);
561        SI.setOperand(2, FalseVal);
562
563        // Move ICI instruction right before the select instruction. Otherwise
564        // the sext/zext value may be defined after the ICI instruction uses it.
565        ICI->moveBefore(&SI);
566
567        Changed = true;
568        break;
569      }
570      }
571    }
572
573  // Transform (X >s -1) ? C1 : C2 --> ((X >>s 31) & (C2 - C1)) + C1
574  // and       (X <s  0) ? C2 : C1 --> ((X >>s 31) & (C2 - C1)) + C1
575  // FIXME: Type and constness constraints could be lifted, but we have to
576  //        watch code size carefully. We should consider xor instead of
577  //        sub/add when we decide to do that.
578  if (IntegerType *Ty = dyn_cast<IntegerType>(CmpLHS->getType())) {
579    if (TrueVal->getType() == Ty) {
580      if (ConstantInt *Cmp = dyn_cast<ConstantInt>(CmpRHS)) {
581        ConstantInt *C1 = nullptr, *C2 = nullptr;
582        if (Pred == ICmpInst::ICMP_SGT && Cmp->isAllOnesValue()) {
583          C1 = dyn_cast<ConstantInt>(TrueVal);
584          C2 = dyn_cast<ConstantInt>(FalseVal);
585        } else if (Pred == ICmpInst::ICMP_SLT && Cmp->isNullValue()) {
586          C1 = dyn_cast<ConstantInt>(FalseVal);
587          C2 = dyn_cast<ConstantInt>(TrueVal);
588        }
589        if (C1 && C2) {
590          // This shift results in either -1 or 0.
591          Value *AShr = Builder->CreateAShr(CmpLHS, Ty->getBitWidth()-1);
592
593          // Check if we can express the operation with a single or.
594          if (C2->isAllOnesValue())
595            return ReplaceInstUsesWith(SI, Builder->CreateOr(AShr, C1));
596
597          Value *And = Builder->CreateAnd(AShr, C2->getValue()-C1->getValue());
598          return ReplaceInstUsesWith(SI, Builder->CreateAdd(And, C1));
599        }
600      }
601    }
602  }
603
604  // If we have an equality comparison then we know the value in one of the
605  // arms of the select. See if substituting this value into the arm and
606  // simplifying the result yields the same value as the other arm.
607  if (Pred == ICmpInst::ICMP_EQ) {
608    if (SimplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, DL, TLI) == TrueVal ||
609        SimplifyWithOpReplaced(FalseVal, CmpRHS, CmpLHS, DL, TLI) == TrueVal)
610      return ReplaceInstUsesWith(SI, FalseVal);
611    if (SimplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, DL, TLI) == FalseVal ||
612        SimplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, DL, TLI) == FalseVal)
613      return ReplaceInstUsesWith(SI, FalseVal);
614  } else if (Pred == ICmpInst::ICMP_NE) {
615    if (SimplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, DL, TLI) == FalseVal ||
616        SimplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, DL, TLI) == FalseVal)
617      return ReplaceInstUsesWith(SI, TrueVal);
618    if (SimplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, DL, TLI) == TrueVal ||
619        SimplifyWithOpReplaced(FalseVal, CmpRHS, CmpLHS, DL, TLI) == TrueVal)
620      return ReplaceInstUsesWith(SI, TrueVal);
621  }
622
623  // NOTE: if we wanted to, this is where to detect integer MIN/MAX
624
625  if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS)) {
626    if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
627      // Transform (X == C) ? X : Y -> (X == C) ? C : Y
628      SI.setOperand(1, CmpRHS);
629      Changed = true;
630    } else if (CmpLHS == FalseVal && Pred == ICmpInst::ICMP_NE) {
631      // Transform (X != C) ? Y : X -> (X != C) ? Y : C
632      SI.setOperand(2, CmpRHS);
633      Changed = true;
634    }
635  }
636
637  if (Value *V = foldSelectICmpAndOr(SI, TrueVal, FalseVal, Builder))
638    return ReplaceInstUsesWith(SI, V);
639
640  return Changed ? &SI : nullptr;
641}
642
643
644/// CanSelectOperandBeMappingIntoPredBlock - SI is a select whose condition is a
645/// PHI node (but the two may be in different blocks).  See if the true/false
646/// values (V) are live in all of the predecessor blocks of the PHI.  For
647/// example, cases like this cannot be mapped:
648///
649///   X = phi [ C1, BB1], [C2, BB2]
650///   Y = add
651///   Z = select X, Y, 0
652///
653/// because Y is not live in BB1/BB2.
654///
655static bool CanSelectOperandBeMappingIntoPredBlock(const Value *V,
656                                                   const SelectInst &SI) {
657  // If the value is a non-instruction value like a constant or argument, it
658  // can always be mapped.
659  const Instruction *I = dyn_cast<Instruction>(V);
660  if (!I) return true;
661
662  // If V is a PHI node defined in the same block as the condition PHI, we can
663  // map the arguments.
664  const PHINode *CondPHI = cast<PHINode>(SI.getCondition());
665
666  if (const PHINode *VP = dyn_cast<PHINode>(I))
667    if (VP->getParent() == CondPHI->getParent())
668      return true;
669
670  // Otherwise, if the PHI and select are defined in the same block and if V is
671  // defined in a different block, then we can transform it.
672  if (SI.getParent() == CondPHI->getParent() &&
673      I->getParent() != CondPHI->getParent())
674    return true;
675
676  // Otherwise we have a 'hard' case and we can't tell without doing more
677  // detailed dominator based analysis, punt.
678  return false;
679}
680
681/// FoldSPFofSPF - We have an SPF (e.g. a min or max) of an SPF of the form:
682///   SPF2(SPF1(A, B), C)
683Instruction *InstCombiner::FoldSPFofSPF(Instruction *Inner,
684                                        SelectPatternFlavor SPF1,
685                                        Value *A, Value *B,
686                                        Instruction &Outer,
687                                        SelectPatternFlavor SPF2, Value *C) {
688  if (C == A || C == B) {
689    // MAX(MAX(A, B), B) -> MAX(A, B)
690    // MIN(MIN(a, b), a) -> MIN(a, b)
691    if (SPF1 == SPF2)
692      return ReplaceInstUsesWith(Outer, Inner);
693
694    // MAX(MIN(a, b), a) -> a
695    // MIN(MAX(a, b), a) -> a
696    if ((SPF1 == SPF_SMIN && SPF2 == SPF_SMAX) ||
697        (SPF1 == SPF_SMAX && SPF2 == SPF_SMIN) ||
698        (SPF1 == SPF_UMIN && SPF2 == SPF_UMAX) ||
699        (SPF1 == SPF_UMAX && SPF2 == SPF_UMIN))
700      return ReplaceInstUsesWith(Outer, C);
701  }
702
703  if (SPF1 == SPF2) {
704    if (ConstantInt *CB = dyn_cast<ConstantInt>(B)) {
705      if (ConstantInt *CC = dyn_cast<ConstantInt>(C)) {
706        APInt ACB = CB->getValue();
707        APInt ACC = CC->getValue();
708
709        // MIN(MIN(A, 23), 97) -> MIN(A, 23)
710        // MAX(MAX(A, 97), 23) -> MAX(A, 97)
711        if ((SPF1 == SPF_UMIN && ACB.ule(ACC)) ||
712            (SPF1 == SPF_SMIN && ACB.sle(ACC)) ||
713            (SPF1 == SPF_UMAX && ACB.uge(ACC)) ||
714            (SPF1 == SPF_SMAX && ACB.sge(ACC)))
715          return ReplaceInstUsesWith(Outer, Inner);
716
717        // MIN(MIN(A, 97), 23) -> MIN(A, 23)
718        // MAX(MAX(A, 23), 97) -> MAX(A, 97)
719        if ((SPF1 == SPF_UMIN && ACB.ugt(ACC)) ||
720            (SPF1 == SPF_SMIN && ACB.sgt(ACC)) ||
721            (SPF1 == SPF_UMAX && ACB.ult(ACC)) ||
722            (SPF1 == SPF_SMAX && ACB.slt(ACC))) {
723          Outer.replaceUsesOfWith(Inner, A);
724          return &Outer;
725        }
726      }
727    }
728  }
729
730  // ABS(ABS(X)) -> ABS(X)
731  // NABS(NABS(X)) -> NABS(X)
732  if (SPF1 == SPF2 && (SPF1 == SPF_ABS || SPF1 == SPF_NABS)) {
733    return ReplaceInstUsesWith(Outer, Inner);
734  }
735
736  // ABS(NABS(X)) -> ABS(X)
737  // NABS(ABS(X)) -> NABS(X)
738  if ((SPF1 == SPF_ABS && SPF2 == SPF_NABS) ||
739      (SPF1 == SPF_NABS && SPF2 == SPF_ABS)) {
740    SelectInst *SI = cast<SelectInst>(Inner);
741    Value *NewSI = Builder->CreateSelect(
742        SI->getCondition(), SI->getFalseValue(), SI->getTrueValue());
743    return ReplaceInstUsesWith(Outer, NewSI);
744  }
745  return nullptr;
746}
747
748/// foldSelectICmpAnd - If one of the constants is zero (we know they can't
749/// both be) and we have an icmp instruction with zero, and we have an 'and'
750/// with the non-constant value and a power of two we can turn the select
751/// into a shift on the result of the 'and'.
752static Value *foldSelectICmpAnd(const SelectInst &SI, ConstantInt *TrueVal,
753                                ConstantInt *FalseVal,
754                                InstCombiner::BuilderTy *Builder) {
755  const ICmpInst *IC = dyn_cast<ICmpInst>(SI.getCondition());
756  if (!IC || !IC->isEquality() || !SI.getType()->isIntegerTy())
757    return nullptr;
758
759  if (!match(IC->getOperand(1), m_Zero()))
760    return nullptr;
761
762  ConstantInt *AndRHS;
763  Value *LHS = IC->getOperand(0);
764  if (!match(LHS, m_And(m_Value(), m_ConstantInt(AndRHS))))
765    return nullptr;
766
767  // If both select arms are non-zero see if we have a select of the form
768  // 'x ? 2^n + C : C'. Then we can offset both arms by C, use the logic
769  // for 'x ? 2^n : 0' and fix the thing up at the end.
770  ConstantInt *Offset = nullptr;
771  if (!TrueVal->isZero() && !FalseVal->isZero()) {
772    if ((TrueVal->getValue() - FalseVal->getValue()).isPowerOf2())
773      Offset = FalseVal;
774    else if ((FalseVal->getValue() - TrueVal->getValue()).isPowerOf2())
775      Offset = TrueVal;
776    else
777      return nullptr;
778
779    // Adjust TrueVal and FalseVal to the offset.
780    TrueVal = ConstantInt::get(Builder->getContext(),
781                               TrueVal->getValue() - Offset->getValue());
782    FalseVal = ConstantInt::get(Builder->getContext(),
783                                FalseVal->getValue() - Offset->getValue());
784  }
785
786  // Make sure the mask in the 'and' and one of the select arms is a power of 2.
787  if (!AndRHS->getValue().isPowerOf2() ||
788      (!TrueVal->getValue().isPowerOf2() &&
789       !FalseVal->getValue().isPowerOf2()))
790    return nullptr;
791
792  // Determine which shift is needed to transform result of the 'and' into the
793  // desired result.
794  ConstantInt *ValC = !TrueVal->isZero() ? TrueVal : FalseVal;
795  unsigned ValZeros = ValC->getValue().logBase2();
796  unsigned AndZeros = AndRHS->getValue().logBase2();
797
798  // If types don't match we can still convert the select by introducing a zext
799  // or a trunc of the 'and'. The trunc case requires that all of the truncated
800  // bits are zero, we can figure that out by looking at the 'and' mask.
801  if (AndZeros >= ValC->getBitWidth())
802    return nullptr;
803
804  Value *V = Builder->CreateZExtOrTrunc(LHS, SI.getType());
805  if (ValZeros > AndZeros)
806    V = Builder->CreateShl(V, ValZeros - AndZeros);
807  else if (ValZeros < AndZeros)
808    V = Builder->CreateLShr(V, AndZeros - ValZeros);
809
810  // Okay, now we know that everything is set up, we just don't know whether we
811  // have a icmp_ne or icmp_eq and whether the true or false val is the zero.
812  bool ShouldNotVal = !TrueVal->isZero();
813  ShouldNotVal ^= IC->getPredicate() == ICmpInst::ICMP_NE;
814  if (ShouldNotVal)
815    V = Builder->CreateXor(V, ValC);
816
817  // Apply an offset if needed.
818  if (Offset)
819    V = Builder->CreateAdd(V, Offset);
820  return V;
821}
822
823Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
824  Value *CondVal = SI.getCondition();
825  Value *TrueVal = SI.getTrueValue();
826  Value *FalseVal = SI.getFalseValue();
827
828  if (Value *V = SimplifySelectInst(CondVal, TrueVal, FalseVal, DL))
829    return ReplaceInstUsesWith(SI, V);
830
831  if (SI.getType()->isIntegerTy(1)) {
832    if (ConstantInt *C = dyn_cast<ConstantInt>(TrueVal)) {
833      if (C->getZExtValue()) {
834        // Change: A = select B, true, C --> A = or B, C
835        return BinaryOperator::CreateOr(CondVal, FalseVal);
836      }
837      // Change: A = select B, false, C --> A = and !B, C
838      Value *NotCond = Builder->CreateNot(CondVal, "not."+CondVal->getName());
839      return BinaryOperator::CreateAnd(NotCond, FalseVal);
840    }
841    if (ConstantInt *C = dyn_cast<ConstantInt>(FalseVal)) {
842      if (C->getZExtValue() == false) {
843        // Change: A = select B, C, false --> A = and B, C
844        return BinaryOperator::CreateAnd(CondVal, TrueVal);
845      }
846      // Change: A = select B, C, true --> A = or !B, C
847      Value *NotCond = Builder->CreateNot(CondVal, "not."+CondVal->getName());
848      return BinaryOperator::CreateOr(NotCond, TrueVal);
849    }
850
851    // select a, b, a  -> a&b
852    // select a, a, b  -> a|b
853    if (CondVal == TrueVal)
854      return BinaryOperator::CreateOr(CondVal, FalseVal);
855    if (CondVal == FalseVal)
856      return BinaryOperator::CreateAnd(CondVal, TrueVal);
857
858    // select a, ~a, b -> (~a)&b
859    // select a, b, ~a -> (~a)|b
860    if (match(TrueVal, m_Not(m_Specific(CondVal))))
861      return BinaryOperator::CreateAnd(TrueVal, FalseVal);
862    if (match(FalseVal, m_Not(m_Specific(CondVal))))
863      return BinaryOperator::CreateOr(TrueVal, FalseVal);
864  }
865
866  // Selecting between two integer constants?
867  if (ConstantInt *TrueValC = dyn_cast<ConstantInt>(TrueVal))
868    if (ConstantInt *FalseValC = dyn_cast<ConstantInt>(FalseVal)) {
869      // select C, 1, 0 -> zext C to int
870      if (FalseValC->isZero() && TrueValC->getValue() == 1)
871        return new ZExtInst(CondVal, SI.getType());
872
873      // select C, -1, 0 -> sext C to int
874      if (FalseValC->isZero() && TrueValC->isAllOnesValue())
875        return new SExtInst(CondVal, SI.getType());
876
877      // select C, 0, 1 -> zext !C to int
878      if (TrueValC->isZero() && FalseValC->getValue() == 1) {
879        Value *NotCond = Builder->CreateNot(CondVal, "not."+CondVal->getName());
880        return new ZExtInst(NotCond, SI.getType());
881      }
882
883      // select C, 0, -1 -> sext !C to int
884      if (TrueValC->isZero() && FalseValC->isAllOnesValue()) {
885        Value *NotCond = Builder->CreateNot(CondVal, "not."+CondVal->getName());
886        return new SExtInst(NotCond, SI.getType());
887      }
888
889      if (Value *V = foldSelectICmpAnd(SI, TrueValC, FalseValC, Builder))
890        return ReplaceInstUsesWith(SI, V);
891    }
892
893  // See if we are selecting two values based on a comparison of the two values.
894  if (FCmpInst *FCI = dyn_cast<FCmpInst>(CondVal)) {
895    if (FCI->getOperand(0) == TrueVal && FCI->getOperand(1) == FalseVal) {
896      // Transform (X == Y) ? X : Y  -> Y
897      if (FCI->getPredicate() == FCmpInst::FCMP_OEQ) {
898        // This is not safe in general for floating point:
899        // consider X== -0, Y== +0.
900        // It becomes safe if either operand is a nonzero constant.
901        ConstantFP *CFPt, *CFPf;
902        if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
903              !CFPt->getValueAPF().isZero()) ||
904            ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
905             !CFPf->getValueAPF().isZero()))
906        return ReplaceInstUsesWith(SI, FalseVal);
907      }
908      // Transform (X une Y) ? X : Y  -> X
909      if (FCI->getPredicate() == FCmpInst::FCMP_UNE) {
910        // This is not safe in general for floating point:
911        // consider X== -0, Y== +0.
912        // It becomes safe if either operand is a nonzero constant.
913        ConstantFP *CFPt, *CFPf;
914        if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
915              !CFPt->getValueAPF().isZero()) ||
916            ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
917             !CFPf->getValueAPF().isZero()))
918        return ReplaceInstUsesWith(SI, TrueVal);
919      }
920      // NOTE: if we wanted to, this is where to detect MIN/MAX
921
922    } else if (FCI->getOperand(0) == FalseVal && FCI->getOperand(1) == TrueVal){
923      // Transform (X == Y) ? Y : X  -> X
924      if (FCI->getPredicate() == FCmpInst::FCMP_OEQ) {
925        // This is not safe in general for floating point:
926        // consider X== -0, Y== +0.
927        // It becomes safe if either operand is a nonzero constant.
928        ConstantFP *CFPt, *CFPf;
929        if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
930              !CFPt->getValueAPF().isZero()) ||
931            ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
932             !CFPf->getValueAPF().isZero()))
933          return ReplaceInstUsesWith(SI, FalseVal);
934      }
935      // Transform (X une Y) ? Y : X  -> Y
936      if (FCI->getPredicate() == FCmpInst::FCMP_UNE) {
937        // This is not safe in general for floating point:
938        // consider X== -0, Y== +0.
939        // It becomes safe if either operand is a nonzero constant.
940        ConstantFP *CFPt, *CFPf;
941        if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
942              !CFPt->getValueAPF().isZero()) ||
943            ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
944             !CFPf->getValueAPF().isZero()))
945          return ReplaceInstUsesWith(SI, TrueVal);
946      }
947      // NOTE: if we wanted to, this is where to detect MIN/MAX
948    }
949    // NOTE: if we wanted to, this is where to detect ABS
950  }
951
952  // See if we are selecting two values based on a comparison of the two values.
953  if (ICmpInst *ICI = dyn_cast<ICmpInst>(CondVal))
954    if (Instruction *Result = visitSelectInstWithICmp(SI, ICI))
955      return Result;
956
957  if (Instruction *TI = dyn_cast<Instruction>(TrueVal))
958    if (Instruction *FI = dyn_cast<Instruction>(FalseVal))
959      if (TI->hasOneUse() && FI->hasOneUse()) {
960        Instruction *AddOp = nullptr, *SubOp = nullptr;
961
962        // Turn (select C, (op X, Y), (op X, Z)) -> (op X, (select C, Y, Z))
963        if (TI->getOpcode() == FI->getOpcode())
964          if (Instruction *IV = FoldSelectOpOp(SI, TI, FI))
965            return IV;
966
967        // Turn select C, (X+Y), (X-Y) --> (X+(select C, Y, (-Y))).  This is
968        // even legal for FP.
969        if ((TI->getOpcode() == Instruction::Sub &&
970             FI->getOpcode() == Instruction::Add) ||
971            (TI->getOpcode() == Instruction::FSub &&
972             FI->getOpcode() == Instruction::FAdd)) {
973          AddOp = FI; SubOp = TI;
974        } else if ((FI->getOpcode() == Instruction::Sub &&
975                    TI->getOpcode() == Instruction::Add) ||
976                   (FI->getOpcode() == Instruction::FSub &&
977                    TI->getOpcode() == Instruction::FAdd)) {
978          AddOp = TI; SubOp = FI;
979        }
980
981        if (AddOp) {
982          Value *OtherAddOp = nullptr;
983          if (SubOp->getOperand(0) == AddOp->getOperand(0)) {
984            OtherAddOp = AddOp->getOperand(1);
985          } else if (SubOp->getOperand(0) == AddOp->getOperand(1)) {
986            OtherAddOp = AddOp->getOperand(0);
987          }
988
989          if (OtherAddOp) {
990            // So at this point we know we have (Y -> OtherAddOp):
991            //        select C, (add X, Y), (sub X, Z)
992            Value *NegVal;  // Compute -Z
993            if (SI.getType()->isFPOrFPVectorTy()) {
994              NegVal = Builder->CreateFNeg(SubOp->getOperand(1));
995              if (Instruction *NegInst = dyn_cast<Instruction>(NegVal)) {
996                FastMathFlags Flags = AddOp->getFastMathFlags();
997                Flags &= SubOp->getFastMathFlags();
998                NegInst->setFastMathFlags(Flags);
999              }
1000            } else {
1001              NegVal = Builder->CreateNeg(SubOp->getOperand(1));
1002            }
1003
1004            Value *NewTrueOp = OtherAddOp;
1005            Value *NewFalseOp = NegVal;
1006            if (AddOp != TI)
1007              std::swap(NewTrueOp, NewFalseOp);
1008            Value *NewSel =
1009              Builder->CreateSelect(CondVal, NewTrueOp,
1010                                    NewFalseOp, SI.getName() + ".p");
1011
1012            if (SI.getType()->isFPOrFPVectorTy()) {
1013              Instruction *RI =
1014                BinaryOperator::CreateFAdd(SubOp->getOperand(0), NewSel);
1015
1016              FastMathFlags Flags = AddOp->getFastMathFlags();
1017              Flags &= SubOp->getFastMathFlags();
1018              RI->setFastMathFlags(Flags);
1019              return RI;
1020            } else
1021              return BinaryOperator::CreateAdd(SubOp->getOperand(0), NewSel);
1022          }
1023        }
1024      }
1025
1026  // See if we can fold the select into one of our operands.
1027  if (SI.getType()->isIntegerTy()) {
1028    if (Instruction *FoldI = FoldSelectIntoOp(SI, TrueVal, FalseVal))
1029      return FoldI;
1030
1031    // MAX(MAX(a, b), a) -> MAX(a, b)
1032    // MIN(MIN(a, b), a) -> MIN(a, b)
1033    // MAX(MIN(a, b), a) -> a
1034    // MIN(MAX(a, b), a) -> a
1035    Value *LHS, *RHS, *LHS2, *RHS2;
1036    if (SelectPatternFlavor SPF = MatchSelectPattern(&SI, LHS, RHS)) {
1037      if (SelectPatternFlavor SPF2 = MatchSelectPattern(LHS, LHS2, RHS2))
1038        if (Instruction *R = FoldSPFofSPF(cast<Instruction>(LHS),SPF2,LHS2,RHS2,
1039                                          SI, SPF, RHS))
1040          return R;
1041      if (SelectPatternFlavor SPF2 = MatchSelectPattern(RHS, LHS2, RHS2))
1042        if (Instruction *R = FoldSPFofSPF(cast<Instruction>(RHS),SPF2,LHS2,RHS2,
1043                                          SI, SPF, LHS))
1044          return R;
1045    }
1046
1047    // TODO.
1048    // ABS(-X) -> ABS(X)
1049  }
1050
1051  // See if we can fold the select into a phi node if the condition is a select.
1052  if (isa<PHINode>(SI.getCondition()))
1053    // The true/false values have to be live in the PHI predecessor's blocks.
1054    if (CanSelectOperandBeMappingIntoPredBlock(TrueVal, SI) &&
1055        CanSelectOperandBeMappingIntoPredBlock(FalseVal, SI))
1056      if (Instruction *NV = FoldOpIntoPhi(SI))
1057        return NV;
1058
1059  if (SelectInst *TrueSI = dyn_cast<SelectInst>(TrueVal)) {
1060    if (TrueSI->getCondition() == CondVal) {
1061      if (SI.getTrueValue() == TrueSI->getTrueValue())
1062        return nullptr;
1063      SI.setOperand(1, TrueSI->getTrueValue());
1064      return &SI;
1065    }
1066  }
1067  if (SelectInst *FalseSI = dyn_cast<SelectInst>(FalseVal)) {
1068    if (FalseSI->getCondition() == CondVal) {
1069      if (SI.getFalseValue() == FalseSI->getFalseValue())
1070        return nullptr;
1071      SI.setOperand(2, FalseSI->getFalseValue());
1072      return &SI;
1073    }
1074  }
1075
1076  if (BinaryOperator::isNot(CondVal)) {
1077    SI.setOperand(0, BinaryOperator::getNotArgument(CondVal));
1078    SI.setOperand(1, FalseVal);
1079    SI.setOperand(2, TrueVal);
1080    return &SI;
1081  }
1082
1083  if (VectorType* VecTy = dyn_cast<VectorType>(SI.getType())) {
1084    unsigned VWidth = VecTy->getNumElements();
1085    APInt UndefElts(VWidth, 0);
1086    APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
1087    if (Value *V = SimplifyDemandedVectorElts(&SI, AllOnesEltMask, UndefElts)) {
1088      if (V != &SI)
1089        return ReplaceInstUsesWith(SI, V);
1090      return &SI;
1091    }
1092
1093    if (isa<ConstantAggregateZero>(CondVal)) {
1094      return ReplaceInstUsesWith(SI, FalseVal);
1095    }
1096  }
1097
1098  return nullptr;
1099}
1100