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 "InstCombineInternal.h"
15 #include "llvm/ADT/APInt.h"
16 #include "llvm/ADT/Optional.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/Analysis/AssumptionCache.h"
20 #include "llvm/Analysis/CmpInstAnalysis.h"
21 #include "llvm/Analysis/InstructionSimplify.h"
22 #include "llvm/Analysis/ValueTracking.h"
23 #include "llvm/IR/BasicBlock.h"
24 #include "llvm/IR/Constant.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/IRBuilder.h"
28 #include "llvm/IR/InstrTypes.h"
29 #include "llvm/IR/Instruction.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/IntrinsicInst.h"
32 #include "llvm/IR/Intrinsics.h"
33 #include "llvm/IR/Operator.h"
34 #include "llvm/IR/PatternMatch.h"
35 #include "llvm/IR/Type.h"
36 #include "llvm/IR/User.h"
37 #include "llvm/IR/Value.h"
38 #include "llvm/Support/Casting.h"
39 #include "llvm/Support/ErrorHandling.h"
40 #include "llvm/Support/KnownBits.h"
41 #include "llvm/Transforms/InstCombine/InstCombineWorklist.h"
42 #include <cassert>
43 #include <utility>
44
45 using namespace llvm;
46 using namespace PatternMatch;
47
48 #define DEBUG_TYPE "instcombine"
49
createMinMax(InstCombiner::BuilderTy & Builder,SelectPatternFlavor SPF,Value * A,Value * B)50 static Value *createMinMax(InstCombiner::BuilderTy &Builder,
51 SelectPatternFlavor SPF, Value *A, Value *B) {
52 CmpInst::Predicate Pred = getMinMaxPred(SPF);
53 assert(CmpInst::isIntPredicate(Pred) && "Expected integer predicate");
54 return Builder.CreateSelect(Builder.CreateICmp(Pred, A, B), A, B);
55 }
56
57 /// Fold
58 /// %A = icmp eq/ne i8 %x, 0
59 /// %B = op i8 %x, %z
60 /// %C = select i1 %A, i8 %B, i8 %y
61 /// To
62 /// %C = select i1 %A, i8 %z, i8 %y
63 /// OP: binop with an identity constant
64 /// TODO: support for non-commutative and FP opcodes
foldSelectBinOpIdentity(SelectInst & Sel)65 static Instruction *foldSelectBinOpIdentity(SelectInst &Sel) {
66
67 Value *Cond = Sel.getCondition();
68 Value *X, *Z;
69 Constant *C;
70 CmpInst::Predicate Pred;
71 if (!match(Cond, m_ICmp(Pred, m_Value(X), m_Constant(C))) ||
72 !ICmpInst::isEquality(Pred))
73 return nullptr;
74
75 bool IsEq = Pred == ICmpInst::ICMP_EQ;
76 auto *BO =
77 dyn_cast<BinaryOperator>(IsEq ? Sel.getTrueValue() : Sel.getFalseValue());
78 // TODO: support for undefs
79 if (BO && match(BO, m_c_BinOp(m_Specific(X), m_Value(Z))) &&
80 ConstantExpr::getBinOpIdentity(BO->getOpcode(), X->getType()) == C) {
81 Sel.setOperand(IsEq ? 1 : 2, Z);
82 return &Sel;
83 }
84 return nullptr;
85 }
86
87 /// This folds:
88 /// select (icmp eq (and X, C1)), TC, FC
89 /// iff C1 is a power 2 and the difference between TC and FC is a power-of-2.
90 /// To something like:
91 /// (shr (and (X, C1)), (log2(C1) - log2(TC-FC))) + FC
92 /// Or:
93 /// (shl (and (X, C1)), (log2(TC-FC) - log2(C1))) + FC
94 /// With some variations depending if FC is larger than TC, or the shift
95 /// isn't needed, or the bit widths don't match.
foldSelectICmpAnd(SelectInst & Sel,ICmpInst * Cmp,InstCombiner::BuilderTy & Builder)96 static Value *foldSelectICmpAnd(SelectInst &Sel, ICmpInst *Cmp,
97 InstCombiner::BuilderTy &Builder) {
98 const APInt *SelTC, *SelFC;
99 if (!match(Sel.getTrueValue(), m_APInt(SelTC)) ||
100 !match(Sel.getFalseValue(), m_APInt(SelFC)))
101 return nullptr;
102
103 // If this is a vector select, we need a vector compare.
104 Type *SelType = Sel.getType();
105 if (SelType->isVectorTy() != Cmp->getType()->isVectorTy())
106 return nullptr;
107
108 Value *V;
109 APInt AndMask;
110 bool CreateAnd = false;
111 ICmpInst::Predicate Pred = Cmp->getPredicate();
112 if (ICmpInst::isEquality(Pred)) {
113 if (!match(Cmp->getOperand(1), m_Zero()))
114 return nullptr;
115
116 V = Cmp->getOperand(0);
117 const APInt *AndRHS;
118 if (!match(V, m_And(m_Value(), m_Power2(AndRHS))))
119 return nullptr;
120
121 AndMask = *AndRHS;
122 } else if (decomposeBitTestICmp(Cmp->getOperand(0), Cmp->getOperand(1),
123 Pred, V, AndMask)) {
124 assert(ICmpInst::isEquality(Pred) && "Not equality test?");
125 if (!AndMask.isPowerOf2())
126 return nullptr;
127
128 CreateAnd = true;
129 } else {
130 return nullptr;
131 }
132
133 // In general, when both constants are non-zero, we would need an offset to
134 // replace the select. This would require more instructions than we started
135 // with. But there's one special-case that we handle here because it can
136 // simplify/reduce the instructions.
137 APInt TC = *SelTC;
138 APInt FC = *SelFC;
139 if (!TC.isNullValue() && !FC.isNullValue()) {
140 // If the select constants differ by exactly one bit and that's the same
141 // bit that is masked and checked by the select condition, the select can
142 // be replaced by bitwise logic to set/clear one bit of the constant result.
143 if (TC.getBitWidth() != AndMask.getBitWidth() || (TC ^ FC) != AndMask)
144 return nullptr;
145 if (CreateAnd) {
146 // If we have to create an 'and', then we must kill the cmp to not
147 // increase the instruction count.
148 if (!Cmp->hasOneUse())
149 return nullptr;
150 V = Builder.CreateAnd(V, ConstantInt::get(SelType, AndMask));
151 }
152 bool ExtraBitInTC = TC.ugt(FC);
153 if (Pred == ICmpInst::ICMP_EQ) {
154 // If the masked bit in V is clear, clear or set the bit in the result:
155 // (V & AndMaskC) == 0 ? TC : FC --> (V & AndMaskC) ^ TC
156 // (V & AndMaskC) == 0 ? TC : FC --> (V & AndMaskC) | TC
157 Constant *C = ConstantInt::get(SelType, TC);
158 return ExtraBitInTC ? Builder.CreateXor(V, C) : Builder.CreateOr(V, C);
159 }
160 if (Pred == ICmpInst::ICMP_NE) {
161 // If the masked bit in V is set, set or clear the bit in the result:
162 // (V & AndMaskC) != 0 ? TC : FC --> (V & AndMaskC) | FC
163 // (V & AndMaskC) != 0 ? TC : FC --> (V & AndMaskC) ^ FC
164 Constant *C = ConstantInt::get(SelType, FC);
165 return ExtraBitInTC ? Builder.CreateOr(V, C) : Builder.CreateXor(V, C);
166 }
167 llvm_unreachable("Only expecting equality predicates");
168 }
169
170 // Make sure one of the select arms is a power-of-2.
171 if (!TC.isPowerOf2() && !FC.isPowerOf2())
172 return nullptr;
173
174 // Determine which shift is needed to transform result of the 'and' into the
175 // desired result.
176 const APInt &ValC = !TC.isNullValue() ? TC : FC;
177 unsigned ValZeros = ValC.logBase2();
178 unsigned AndZeros = AndMask.logBase2();
179
180 // Insert the 'and' instruction on the input to the truncate.
181 if (CreateAnd)
182 V = Builder.CreateAnd(V, ConstantInt::get(V->getType(), AndMask));
183
184 // If types don't match, we can still convert the select by introducing a zext
185 // or a trunc of the 'and'.
186 if (ValZeros > AndZeros) {
187 V = Builder.CreateZExtOrTrunc(V, SelType);
188 V = Builder.CreateShl(V, ValZeros - AndZeros);
189 } else if (ValZeros < AndZeros) {
190 V = Builder.CreateLShr(V, AndZeros - ValZeros);
191 V = Builder.CreateZExtOrTrunc(V, SelType);
192 } else {
193 V = Builder.CreateZExtOrTrunc(V, SelType);
194 }
195
196 // Okay, now we know that everything is set up, we just don't know whether we
197 // have a icmp_ne or icmp_eq and whether the true or false val is the zero.
198 bool ShouldNotVal = !TC.isNullValue();
199 ShouldNotVal ^= Pred == ICmpInst::ICMP_NE;
200 if (ShouldNotVal)
201 V = Builder.CreateXor(V, ValC);
202
203 return V;
204 }
205
206 /// We want to turn code that looks like this:
207 /// %C = or %A, %B
208 /// %D = select %cond, %C, %A
209 /// into:
210 /// %C = select %cond, %B, 0
211 /// %D = or %A, %C
212 ///
213 /// Assuming that the specified instruction is an operand to the select, return
214 /// a bitmask indicating which operands of this instruction are foldable if they
215 /// equal the other incoming value of the select.
getSelectFoldableOperands(BinaryOperator * I)216 static unsigned getSelectFoldableOperands(BinaryOperator *I) {
217 switch (I->getOpcode()) {
218 case Instruction::Add:
219 case Instruction::Mul:
220 case Instruction::And:
221 case Instruction::Or:
222 case Instruction::Xor:
223 return 3; // Can fold through either operand.
224 case Instruction::Sub: // Can only fold on the amount subtracted.
225 case Instruction::Shl: // Can only fold on the shift amount.
226 case Instruction::LShr:
227 case Instruction::AShr:
228 return 1;
229 default:
230 return 0; // Cannot fold
231 }
232 }
233
234 /// For the same transformation as the previous function, return the identity
235 /// constant that goes into the select.
getSelectFoldableConstant(BinaryOperator * I)236 static APInt getSelectFoldableConstant(BinaryOperator *I) {
237 switch (I->getOpcode()) {
238 default: llvm_unreachable("This cannot happen!");
239 case Instruction::Add:
240 case Instruction::Sub:
241 case Instruction::Or:
242 case Instruction::Xor:
243 case Instruction::Shl:
244 case Instruction::LShr:
245 case Instruction::AShr:
246 return APInt::getNullValue(I->getType()->getScalarSizeInBits());
247 case Instruction::And:
248 return APInt::getAllOnesValue(I->getType()->getScalarSizeInBits());
249 case Instruction::Mul:
250 return APInt(I->getType()->getScalarSizeInBits(), 1);
251 }
252 }
253
254 /// We have (select c, TI, FI), and we know that TI and FI have the same opcode.
foldSelectOpOp(SelectInst & SI,Instruction * TI,Instruction * FI)255 Instruction *InstCombiner::foldSelectOpOp(SelectInst &SI, Instruction *TI,
256 Instruction *FI) {
257 // Don't break up min/max patterns. The hasOneUse checks below prevent that
258 // for most cases, but vector min/max with bitcasts can be transformed. If the
259 // one-use restrictions are eased for other patterns, we still don't want to
260 // obfuscate min/max.
261 if ((match(&SI, m_SMin(m_Value(), m_Value())) ||
262 match(&SI, m_SMax(m_Value(), m_Value())) ||
263 match(&SI, m_UMin(m_Value(), m_Value())) ||
264 match(&SI, m_UMax(m_Value(), m_Value()))))
265 return nullptr;
266
267 // If this is a cast from the same type, merge.
268 if (TI->getNumOperands() == 1 && TI->isCast()) {
269 Type *FIOpndTy = FI->getOperand(0)->getType();
270 if (TI->getOperand(0)->getType() != FIOpndTy)
271 return nullptr;
272
273 // The select condition may be a vector. We may only change the operand
274 // type if the vector width remains the same (and matches the condition).
275 Type *CondTy = SI.getCondition()->getType();
276 if (CondTy->isVectorTy()) {
277 if (!FIOpndTy->isVectorTy())
278 return nullptr;
279 if (CondTy->getVectorNumElements() != FIOpndTy->getVectorNumElements())
280 return nullptr;
281
282 // TODO: If the backend knew how to deal with casts better, we could
283 // remove this limitation. For now, there's too much potential to create
284 // worse codegen by promoting the select ahead of size-altering casts
285 // (PR28160).
286 //
287 // Note that ValueTracking's matchSelectPattern() looks through casts
288 // without checking 'hasOneUse' when it matches min/max patterns, so this
289 // transform may end up happening anyway.
290 if (TI->getOpcode() != Instruction::BitCast &&
291 (!TI->hasOneUse() || !FI->hasOneUse()))
292 return nullptr;
293 } else if (!TI->hasOneUse() || !FI->hasOneUse()) {
294 // TODO: The one-use restrictions for a scalar select could be eased if
295 // the fold of a select in visitLoadInst() was enhanced to match a pattern
296 // that includes a cast.
297 return nullptr;
298 }
299
300 // Fold this by inserting a select from the input values.
301 Value *NewSI =
302 Builder.CreateSelect(SI.getCondition(), TI->getOperand(0),
303 FI->getOperand(0), SI.getName() + ".v", &SI);
304 return CastInst::Create(Instruction::CastOps(TI->getOpcode()), NewSI,
305 TI->getType());
306 }
307
308 // Only handle binary operators (including two-operand getelementptr) with
309 // one-use here. As with the cast case above, it may be possible to relax the
310 // one-use constraint, but that needs be examined carefully since it may not
311 // reduce the total number of instructions.
312 if (TI->getNumOperands() != 2 || FI->getNumOperands() != 2 ||
313 (!isa<BinaryOperator>(TI) && !isa<GetElementPtrInst>(TI)) ||
314 !TI->hasOneUse() || !FI->hasOneUse())
315 return nullptr;
316
317 // Figure out if the operations have any operands in common.
318 Value *MatchOp, *OtherOpT, *OtherOpF;
319 bool MatchIsOpZero;
320 if (TI->getOperand(0) == FI->getOperand(0)) {
321 MatchOp = TI->getOperand(0);
322 OtherOpT = TI->getOperand(1);
323 OtherOpF = FI->getOperand(1);
324 MatchIsOpZero = true;
325 } else if (TI->getOperand(1) == FI->getOperand(1)) {
326 MatchOp = TI->getOperand(1);
327 OtherOpT = TI->getOperand(0);
328 OtherOpF = FI->getOperand(0);
329 MatchIsOpZero = false;
330 } else if (!TI->isCommutative()) {
331 return nullptr;
332 } else if (TI->getOperand(0) == FI->getOperand(1)) {
333 MatchOp = TI->getOperand(0);
334 OtherOpT = TI->getOperand(1);
335 OtherOpF = FI->getOperand(0);
336 MatchIsOpZero = true;
337 } else if (TI->getOperand(1) == FI->getOperand(0)) {
338 MatchOp = TI->getOperand(1);
339 OtherOpT = TI->getOperand(0);
340 OtherOpF = FI->getOperand(1);
341 MatchIsOpZero = true;
342 } else {
343 return nullptr;
344 }
345
346 // If we reach here, they do have operations in common.
347 Value *NewSI = Builder.CreateSelect(SI.getCondition(), OtherOpT, OtherOpF,
348 SI.getName() + ".v", &SI);
349 Value *Op0 = MatchIsOpZero ? MatchOp : NewSI;
350 Value *Op1 = MatchIsOpZero ? NewSI : MatchOp;
351 if (auto *BO = dyn_cast<BinaryOperator>(TI)) {
352 return BinaryOperator::Create(BO->getOpcode(), Op0, Op1);
353 }
354 if (auto *TGEP = dyn_cast<GetElementPtrInst>(TI)) {
355 auto *FGEP = cast<GetElementPtrInst>(FI);
356 Type *ElementType = TGEP->getResultElementType();
357 return TGEP->isInBounds() && FGEP->isInBounds()
358 ? GetElementPtrInst::CreateInBounds(ElementType, Op0, {Op1})
359 : GetElementPtrInst::Create(ElementType, Op0, {Op1});
360 }
361 llvm_unreachable("Expected BinaryOperator or GEP");
362 return nullptr;
363 }
364
isSelect01(const APInt & C1I,const APInt & C2I)365 static bool isSelect01(const APInt &C1I, const APInt &C2I) {
366 if (!C1I.isNullValue() && !C2I.isNullValue()) // One side must be zero.
367 return false;
368 return C1I.isOneValue() || C1I.isAllOnesValue() ||
369 C2I.isOneValue() || C2I.isAllOnesValue();
370 }
371
372 /// Try to fold the select into one of the operands to allow further
373 /// optimization.
foldSelectIntoOp(SelectInst & SI,Value * TrueVal,Value * FalseVal)374 Instruction *InstCombiner::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
375 Value *FalseVal) {
376 // See the comment above GetSelectFoldableOperands for a description of the
377 // transformation we are doing here.
378 if (auto *TVI = dyn_cast<BinaryOperator>(TrueVal)) {
379 if (TVI->hasOneUse() && !isa<Constant>(FalseVal)) {
380 if (unsigned SFO = getSelectFoldableOperands(TVI)) {
381 unsigned OpToFold = 0;
382 if ((SFO & 1) && FalseVal == TVI->getOperand(0)) {
383 OpToFold = 1;
384 } else if ((SFO & 2) && FalseVal == TVI->getOperand(1)) {
385 OpToFold = 2;
386 }
387
388 if (OpToFold) {
389 APInt CI = getSelectFoldableConstant(TVI);
390 Value *OOp = TVI->getOperand(2-OpToFold);
391 // Avoid creating select between 2 constants unless it's selecting
392 // between 0, 1 and -1.
393 const APInt *OOpC;
394 bool OOpIsAPInt = match(OOp, m_APInt(OOpC));
395 if (!isa<Constant>(OOp) || (OOpIsAPInt && isSelect01(CI, *OOpC))) {
396 Value *C = ConstantInt::get(OOp->getType(), CI);
397 Value *NewSel = Builder.CreateSelect(SI.getCondition(), OOp, C);
398 NewSel->takeName(TVI);
399 BinaryOperator *BO = BinaryOperator::Create(TVI->getOpcode(),
400 FalseVal, NewSel);
401 BO->copyIRFlags(TVI);
402 return BO;
403 }
404 }
405 }
406 }
407 }
408
409 if (auto *FVI = dyn_cast<BinaryOperator>(FalseVal)) {
410 if (FVI->hasOneUse() && !isa<Constant>(TrueVal)) {
411 if (unsigned SFO = getSelectFoldableOperands(FVI)) {
412 unsigned OpToFold = 0;
413 if ((SFO & 1) && TrueVal == FVI->getOperand(0)) {
414 OpToFold = 1;
415 } else if ((SFO & 2) && TrueVal == FVI->getOperand(1)) {
416 OpToFold = 2;
417 }
418
419 if (OpToFold) {
420 APInt CI = getSelectFoldableConstant(FVI);
421 Value *OOp = FVI->getOperand(2-OpToFold);
422 // Avoid creating select between 2 constants unless it's selecting
423 // between 0, 1 and -1.
424 const APInt *OOpC;
425 bool OOpIsAPInt = match(OOp, m_APInt(OOpC));
426 if (!isa<Constant>(OOp) || (OOpIsAPInt && isSelect01(CI, *OOpC))) {
427 Value *C = ConstantInt::get(OOp->getType(), CI);
428 Value *NewSel = Builder.CreateSelect(SI.getCondition(), C, OOp);
429 NewSel->takeName(FVI);
430 BinaryOperator *BO = BinaryOperator::Create(FVI->getOpcode(),
431 TrueVal, NewSel);
432 BO->copyIRFlags(FVI);
433 return BO;
434 }
435 }
436 }
437 }
438 }
439
440 return nullptr;
441 }
442
443 /// We want to turn:
444 /// (select (icmp eq (and X, Y), 0), (and (lshr X, Z), 1), 1)
445 /// into:
446 /// zext (icmp ne i32 (and X, (or Y, (shl 1, Z))), 0)
447 /// Note:
448 /// Z may be 0 if lshr is missing.
449 /// Worst-case scenario is that we will replace 5 instructions with 5 different
450 /// instructions, but we got rid of select.
foldSelectICmpAndAnd(Type * SelType,const ICmpInst * Cmp,Value * TVal,Value * FVal,InstCombiner::BuilderTy & Builder)451 static Instruction *foldSelectICmpAndAnd(Type *SelType, const ICmpInst *Cmp,
452 Value *TVal, Value *FVal,
453 InstCombiner::BuilderTy &Builder) {
454 if (!(Cmp->hasOneUse() && Cmp->getOperand(0)->hasOneUse() &&
455 Cmp->getPredicate() == ICmpInst::ICMP_EQ &&
456 match(Cmp->getOperand(1), m_Zero()) && match(FVal, m_One())))
457 return nullptr;
458
459 // The TrueVal has general form of: and %B, 1
460 Value *B;
461 if (!match(TVal, m_OneUse(m_And(m_Value(B), m_One()))))
462 return nullptr;
463
464 // Where %B may be optionally shifted: lshr %X, %Z.
465 Value *X, *Z;
466 const bool HasShift = match(B, m_OneUse(m_LShr(m_Value(X), m_Value(Z))));
467 if (!HasShift)
468 X = B;
469
470 Value *Y;
471 if (!match(Cmp->getOperand(0), m_c_And(m_Specific(X), m_Value(Y))))
472 return nullptr;
473
474 // ((X & Y) == 0) ? ((X >> Z) & 1) : 1 --> (X & (Y | (1 << Z))) != 0
475 // ((X & Y) == 0) ? (X & 1) : 1 --> (X & (Y | 1)) != 0
476 Constant *One = ConstantInt::get(SelType, 1);
477 Value *MaskB = HasShift ? Builder.CreateShl(One, Z) : One;
478 Value *FullMask = Builder.CreateOr(Y, MaskB);
479 Value *MaskedX = Builder.CreateAnd(X, FullMask);
480 Value *ICmpNeZero = Builder.CreateIsNotNull(MaskedX);
481 return new ZExtInst(ICmpNeZero, SelType);
482 }
483
484 /// We want to turn:
485 /// (select (icmp eq (and X, C1), 0), Y, (or Y, C2))
486 /// into:
487 /// (or (shl (and X, C1), C3), Y)
488 /// iff:
489 /// C1 and C2 are both powers of 2
490 /// where:
491 /// C3 = Log(C2) - Log(C1)
492 ///
493 /// This transform handles cases where:
494 /// 1. The icmp predicate is inverted
495 /// 2. The select operands are reversed
496 /// 3. The magnitude of C2 and C1 are flipped
foldSelectICmpAndOr(const ICmpInst * IC,Value * TrueVal,Value * FalseVal,InstCombiner::BuilderTy & Builder)497 static Value *foldSelectICmpAndOr(const ICmpInst *IC, Value *TrueVal,
498 Value *FalseVal,
499 InstCombiner::BuilderTy &Builder) {
500 // Only handle integer compares. Also, if this is a vector select, we need a
501 // vector compare.
502 if (!TrueVal->getType()->isIntOrIntVectorTy() ||
503 TrueVal->getType()->isVectorTy() != IC->getType()->isVectorTy())
504 return nullptr;
505
506 Value *CmpLHS = IC->getOperand(0);
507 Value *CmpRHS = IC->getOperand(1);
508
509 Value *V;
510 unsigned C1Log;
511 bool IsEqualZero;
512 bool NeedAnd = false;
513 if (IC->isEquality()) {
514 if (!match(CmpRHS, m_Zero()))
515 return nullptr;
516
517 const APInt *C1;
518 if (!match(CmpLHS, m_And(m_Value(), m_Power2(C1))))
519 return nullptr;
520
521 V = CmpLHS;
522 C1Log = C1->logBase2();
523 IsEqualZero = IC->getPredicate() == ICmpInst::ICMP_EQ;
524 } else if (IC->getPredicate() == ICmpInst::ICMP_SLT ||
525 IC->getPredicate() == ICmpInst::ICMP_SGT) {
526 // We also need to recognize (icmp slt (trunc (X)), 0) and
527 // (icmp sgt (trunc (X)), -1).
528 IsEqualZero = IC->getPredicate() == ICmpInst::ICMP_SGT;
529 if ((IsEqualZero && !match(CmpRHS, m_AllOnes())) ||
530 (!IsEqualZero && !match(CmpRHS, m_Zero())))
531 return nullptr;
532
533 if (!match(CmpLHS, m_OneUse(m_Trunc(m_Value(V)))))
534 return nullptr;
535
536 C1Log = CmpLHS->getType()->getScalarSizeInBits() - 1;
537 NeedAnd = true;
538 } else {
539 return nullptr;
540 }
541
542 const APInt *C2;
543 bool OrOnTrueVal = false;
544 bool OrOnFalseVal = match(FalseVal, m_Or(m_Specific(TrueVal), m_Power2(C2)));
545 if (!OrOnFalseVal)
546 OrOnTrueVal = match(TrueVal, m_Or(m_Specific(FalseVal), m_Power2(C2)));
547
548 if (!OrOnFalseVal && !OrOnTrueVal)
549 return nullptr;
550
551 Value *Y = OrOnFalseVal ? TrueVal : FalseVal;
552
553 unsigned C2Log = C2->logBase2();
554
555 bool NeedXor = (!IsEqualZero && OrOnFalseVal) || (IsEqualZero && OrOnTrueVal);
556 bool NeedShift = C1Log != C2Log;
557 bool NeedZExtTrunc = Y->getType()->getScalarSizeInBits() !=
558 V->getType()->getScalarSizeInBits();
559
560 // Make sure we don't create more instructions than we save.
561 Value *Or = OrOnFalseVal ? FalseVal : TrueVal;
562 if ((NeedShift + NeedXor + NeedZExtTrunc) >
563 (IC->hasOneUse() + Or->hasOneUse()))
564 return nullptr;
565
566 if (NeedAnd) {
567 // Insert the AND instruction on the input to the truncate.
568 APInt C1 = APInt::getOneBitSet(V->getType()->getScalarSizeInBits(), C1Log);
569 V = Builder.CreateAnd(V, ConstantInt::get(V->getType(), C1));
570 }
571
572 if (C2Log > C1Log) {
573 V = Builder.CreateZExtOrTrunc(V, Y->getType());
574 V = Builder.CreateShl(V, C2Log - C1Log);
575 } else if (C1Log > C2Log) {
576 V = Builder.CreateLShr(V, C1Log - C2Log);
577 V = Builder.CreateZExtOrTrunc(V, Y->getType());
578 } else
579 V = Builder.CreateZExtOrTrunc(V, Y->getType());
580
581 if (NeedXor)
582 V = Builder.CreateXor(V, *C2);
583
584 return Builder.CreateOr(V, Y);
585 }
586
587 /// Transform patterns such as: (a > b) ? a - b : 0
588 /// into: ((a > b) ? a : b) - b)
589 /// This produces a canonical max pattern that is more easily recognized by the
590 /// backend and converted into saturated subtraction instructions if those
591 /// exist.
592 /// There are 8 commuted/swapped variants of this pattern.
593 /// TODO: Also support a - UMIN(a,b) patterns.
canonicalizeSaturatedSubtract(const ICmpInst * ICI,const Value * TrueVal,const Value * FalseVal,InstCombiner::BuilderTy & Builder)594 static Value *canonicalizeSaturatedSubtract(const ICmpInst *ICI,
595 const Value *TrueVal,
596 const Value *FalseVal,
597 InstCombiner::BuilderTy &Builder) {
598 ICmpInst::Predicate Pred = ICI->getPredicate();
599 if (!ICmpInst::isUnsigned(Pred))
600 return nullptr;
601
602 // (b > a) ? 0 : a - b -> (b <= a) ? a - b : 0
603 if (match(TrueVal, m_Zero())) {
604 Pred = ICmpInst::getInversePredicate(Pred);
605 std::swap(TrueVal, FalseVal);
606 }
607 if (!match(FalseVal, m_Zero()))
608 return nullptr;
609
610 Value *A = ICI->getOperand(0);
611 Value *B = ICI->getOperand(1);
612 if (Pred == ICmpInst::ICMP_ULE || Pred == ICmpInst::ICMP_ULT) {
613 // (b < a) ? a - b : 0 -> (a > b) ? a - b : 0
614 std::swap(A, B);
615 Pred = ICmpInst::getSwappedPredicate(Pred);
616 }
617
618 assert((Pred == ICmpInst::ICMP_UGE || Pred == ICmpInst::ICMP_UGT) &&
619 "Unexpected isUnsigned predicate!");
620
621 // Account for swapped form of subtraction: ((a > b) ? b - a : 0).
622 bool IsNegative = false;
623 if (match(TrueVal, m_Sub(m_Specific(B), m_Specific(A))))
624 IsNegative = true;
625 else if (!match(TrueVal, m_Sub(m_Specific(A), m_Specific(B))))
626 return nullptr;
627
628 // If sub is used anywhere else, we wouldn't be able to eliminate it
629 // afterwards.
630 if (!TrueVal->hasOneUse())
631 return nullptr;
632
633 // All checks passed, convert to canonical unsigned saturated subtraction
634 // form: sub(max()).
635 // (a > b) ? a - b : 0 -> ((a > b) ? a : b) - b)
636 Value *Max = Builder.CreateSelect(Builder.CreateICmp(Pred, A, B), A, B);
637 return IsNegative ? Builder.CreateSub(B, Max) : Builder.CreateSub(Max, B);
638 }
639
640 /// Attempt to fold a cttz/ctlz followed by a icmp plus select into a single
641 /// call to cttz/ctlz with flag 'is_zero_undef' cleared.
642 ///
643 /// For example, we can fold the following code sequence:
644 /// \code
645 /// %0 = tail call i32 @llvm.cttz.i32(i32 %x, i1 true)
646 /// %1 = icmp ne i32 %x, 0
647 /// %2 = select i1 %1, i32 %0, i32 32
648 /// \code
649 ///
650 /// into:
651 /// %0 = tail call i32 @llvm.cttz.i32(i32 %x, i1 false)
foldSelectCttzCtlz(ICmpInst * ICI,Value * TrueVal,Value * FalseVal,InstCombiner::BuilderTy & Builder)652 static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal,
653 InstCombiner::BuilderTy &Builder) {
654 ICmpInst::Predicate Pred = ICI->getPredicate();
655 Value *CmpLHS = ICI->getOperand(0);
656 Value *CmpRHS = ICI->getOperand(1);
657
658 // Check if the condition value compares a value for equality against zero.
659 if (!ICI->isEquality() || !match(CmpRHS, m_Zero()))
660 return nullptr;
661
662 Value *Count = FalseVal;
663 Value *ValueOnZero = TrueVal;
664 if (Pred == ICmpInst::ICMP_NE)
665 std::swap(Count, ValueOnZero);
666
667 // Skip zero extend/truncate.
668 Value *V = nullptr;
669 if (match(Count, m_ZExt(m_Value(V))) ||
670 match(Count, m_Trunc(m_Value(V))))
671 Count = V;
672
673 // Check if the value propagated on zero is a constant number equal to the
674 // sizeof in bits of 'Count'.
675 unsigned SizeOfInBits = Count->getType()->getScalarSizeInBits();
676 if (!match(ValueOnZero, m_SpecificInt(SizeOfInBits)))
677 return nullptr;
678
679 // Check that 'Count' is a call to intrinsic cttz/ctlz. Also check that the
680 // input to the cttz/ctlz is used as LHS for the compare instruction.
681 if (match(Count, m_Intrinsic<Intrinsic::cttz>(m_Specific(CmpLHS))) ||
682 match(Count, m_Intrinsic<Intrinsic::ctlz>(m_Specific(CmpLHS)))) {
683 IntrinsicInst *II = cast<IntrinsicInst>(Count);
684 // Explicitly clear the 'undef_on_zero' flag.
685 IntrinsicInst *NewI = cast<IntrinsicInst>(II->clone());
686 NewI->setArgOperand(1, ConstantInt::getFalse(NewI->getContext()));
687 Builder.Insert(NewI);
688 return Builder.CreateZExtOrTrunc(NewI, ValueOnZero->getType());
689 }
690
691 return nullptr;
692 }
693
694 /// Return true if we find and adjust an icmp+select pattern where the compare
695 /// is with a constant that can be incremented or decremented to match the
696 /// minimum or maximum idiom.
adjustMinMax(SelectInst & Sel,ICmpInst & Cmp)697 static bool adjustMinMax(SelectInst &Sel, ICmpInst &Cmp) {
698 ICmpInst::Predicate Pred = Cmp.getPredicate();
699 Value *CmpLHS = Cmp.getOperand(0);
700 Value *CmpRHS = Cmp.getOperand(1);
701 Value *TrueVal = Sel.getTrueValue();
702 Value *FalseVal = Sel.getFalseValue();
703
704 // We may move or edit the compare, so make sure the select is the only user.
705 const APInt *CmpC;
706 if (!Cmp.hasOneUse() || !match(CmpRHS, m_APInt(CmpC)))
707 return false;
708
709 // These transforms only work for selects of integers or vector selects of
710 // integer vectors.
711 Type *SelTy = Sel.getType();
712 auto *SelEltTy = dyn_cast<IntegerType>(SelTy->getScalarType());
713 if (!SelEltTy || SelTy->isVectorTy() != Cmp.getType()->isVectorTy())
714 return false;
715
716 Constant *AdjustedRHS;
717 if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_SGT)
718 AdjustedRHS = ConstantInt::get(CmpRHS->getType(), *CmpC + 1);
719 else if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_SLT)
720 AdjustedRHS = ConstantInt::get(CmpRHS->getType(), *CmpC - 1);
721 else
722 return false;
723
724 // X > C ? X : C+1 --> X < C+1 ? C+1 : X
725 // X < C ? X : C-1 --> X > C-1 ? C-1 : X
726 if ((CmpLHS == TrueVal && AdjustedRHS == FalseVal) ||
727 (CmpLHS == FalseVal && AdjustedRHS == TrueVal)) {
728 ; // Nothing to do here. Values match without any sign/zero extension.
729 }
730 // Types do not match. Instead of calculating this with mixed types, promote
731 // all to the larger type. This enables scalar evolution to analyze this
732 // expression.
733 else if (CmpRHS->getType()->getScalarSizeInBits() < SelEltTy->getBitWidth()) {
734 Constant *SextRHS = ConstantExpr::getSExt(AdjustedRHS, SelTy);
735
736 // X = sext x; x >s c ? X : C+1 --> X = sext x; X <s C+1 ? C+1 : X
737 // X = sext x; x <s c ? X : C-1 --> X = sext x; X >s C-1 ? C-1 : X
738 // X = sext x; x >u c ? X : C+1 --> X = sext x; X <u C+1 ? C+1 : X
739 // X = sext x; x <u c ? X : C-1 --> X = sext x; X >u C-1 ? C-1 : X
740 if (match(TrueVal, m_SExt(m_Specific(CmpLHS))) && SextRHS == FalseVal) {
741 CmpLHS = TrueVal;
742 AdjustedRHS = SextRHS;
743 } else if (match(FalseVal, m_SExt(m_Specific(CmpLHS))) &&
744 SextRHS == TrueVal) {
745 CmpLHS = FalseVal;
746 AdjustedRHS = SextRHS;
747 } else if (Cmp.isUnsigned()) {
748 Constant *ZextRHS = ConstantExpr::getZExt(AdjustedRHS, SelTy);
749 // X = zext x; x >u c ? X : C+1 --> X = zext x; X <u C+1 ? C+1 : X
750 // X = zext x; x <u c ? X : C-1 --> X = zext x; X >u C-1 ? C-1 : X
751 // zext + signed compare cannot be changed:
752 // 0xff <s 0x00, but 0x00ff >s 0x0000
753 if (match(TrueVal, m_ZExt(m_Specific(CmpLHS))) && ZextRHS == FalseVal) {
754 CmpLHS = TrueVal;
755 AdjustedRHS = ZextRHS;
756 } else if (match(FalseVal, m_ZExt(m_Specific(CmpLHS))) &&
757 ZextRHS == TrueVal) {
758 CmpLHS = FalseVal;
759 AdjustedRHS = ZextRHS;
760 } else {
761 return false;
762 }
763 } else {
764 return false;
765 }
766 } else {
767 return false;
768 }
769
770 Pred = ICmpInst::getSwappedPredicate(Pred);
771 CmpRHS = AdjustedRHS;
772 std::swap(FalseVal, TrueVal);
773 Cmp.setPredicate(Pred);
774 Cmp.setOperand(0, CmpLHS);
775 Cmp.setOperand(1, CmpRHS);
776 Sel.setOperand(1, TrueVal);
777 Sel.setOperand(2, FalseVal);
778 Sel.swapProfMetadata();
779
780 // Move the compare instruction right before the select instruction. Otherwise
781 // the sext/zext value may be defined after the compare instruction uses it.
782 Cmp.moveBefore(&Sel);
783
784 return true;
785 }
786
787 /// If this is an integer min/max (icmp + select) with a constant operand,
788 /// create the canonical icmp for the min/max operation and canonicalize the
789 /// constant to the 'false' operand of the select:
790 /// select (icmp Pred X, C1), C2, X --> select (icmp Pred' X, C2), X, C2
791 /// Note: if C1 != C2, this will change the icmp constant to the existing
792 /// constant operand of the select.
793 static Instruction *
canonicalizeMinMaxWithConstant(SelectInst & Sel,ICmpInst & Cmp,InstCombiner::BuilderTy & Builder)794 canonicalizeMinMaxWithConstant(SelectInst &Sel, ICmpInst &Cmp,
795 InstCombiner::BuilderTy &Builder) {
796 if (!Cmp.hasOneUse() || !isa<Constant>(Cmp.getOperand(1)))
797 return nullptr;
798
799 // Canonicalize the compare predicate based on whether we have min or max.
800 Value *LHS, *RHS;
801 SelectPatternResult SPR = matchSelectPattern(&Sel, LHS, RHS);
802 if (!SelectPatternResult::isMinOrMax(SPR.Flavor))
803 return nullptr;
804
805 // Is this already canonical?
806 ICmpInst::Predicate CanonicalPred = getMinMaxPred(SPR.Flavor);
807 if (Cmp.getOperand(0) == LHS && Cmp.getOperand(1) == RHS &&
808 Cmp.getPredicate() == CanonicalPred)
809 return nullptr;
810
811 // Create the canonical compare and plug it into the select.
812 Sel.setCondition(Builder.CreateICmp(CanonicalPred, LHS, RHS));
813
814 // If the select operands did not change, we're done.
815 if (Sel.getTrueValue() == LHS && Sel.getFalseValue() == RHS)
816 return &Sel;
817
818 // If we are swapping the select operands, swap the metadata too.
819 assert(Sel.getTrueValue() == RHS && Sel.getFalseValue() == LHS &&
820 "Unexpected results from matchSelectPattern");
821 Sel.setTrueValue(LHS);
822 Sel.setFalseValue(RHS);
823 Sel.swapProfMetadata();
824 return &Sel;
825 }
826
827 /// There are many select variants for each of ABS/NABS.
828 /// In matchSelectPattern(), there are different compare constants, compare
829 /// predicates/operands and select operands.
830 /// In isKnownNegation(), there are different formats of negated operands.
831 /// Canonicalize all these variants to 1 pattern.
832 /// This makes CSE more likely.
canonicalizeAbsNabs(SelectInst & Sel,ICmpInst & Cmp,InstCombiner::BuilderTy & Builder)833 static Instruction *canonicalizeAbsNabs(SelectInst &Sel, ICmpInst &Cmp,
834 InstCombiner::BuilderTy &Builder) {
835 if (!Cmp.hasOneUse() || !isa<Constant>(Cmp.getOperand(1)))
836 return nullptr;
837
838 // Choose a sign-bit check for the compare (likely simpler for codegen).
839 // ABS: (X <s 0) ? -X : X
840 // NABS: (X <s 0) ? X : -X
841 Value *LHS, *RHS;
842 SelectPatternFlavor SPF = matchSelectPattern(&Sel, LHS, RHS).Flavor;
843 if (SPF != SelectPatternFlavor::SPF_ABS &&
844 SPF != SelectPatternFlavor::SPF_NABS)
845 return nullptr;
846
847 Value *TVal = Sel.getTrueValue();
848 Value *FVal = Sel.getFalseValue();
849 assert(isKnownNegation(TVal, FVal) &&
850 "Unexpected result from matchSelectPattern");
851
852 // The compare may use the negated abs()/nabs() operand, or it may use
853 // negation in non-canonical form such as: sub A, B.
854 bool CmpUsesNegatedOp = match(Cmp.getOperand(0), m_Neg(m_Specific(TVal))) ||
855 match(Cmp.getOperand(0), m_Neg(m_Specific(FVal)));
856
857 bool CmpCanonicalized = !CmpUsesNegatedOp &&
858 match(Cmp.getOperand(1), m_ZeroInt()) &&
859 Cmp.getPredicate() == ICmpInst::ICMP_SLT;
860 bool RHSCanonicalized = match(RHS, m_Neg(m_Specific(LHS)));
861
862 // Is this already canonical?
863 if (CmpCanonicalized && RHSCanonicalized)
864 return nullptr;
865
866 // If RHS is used by other instructions except compare and select, don't
867 // canonicalize it to not increase the instruction count.
868 if (!(RHS->hasOneUse() || (RHS->hasNUses(2) && CmpUsesNegatedOp)))
869 return nullptr;
870
871 // Create the canonical compare: icmp slt LHS 0.
872 if (!CmpCanonicalized) {
873 Cmp.setPredicate(ICmpInst::ICMP_SLT);
874 Cmp.setOperand(1, ConstantInt::getNullValue(Cmp.getOperand(0)->getType()));
875 if (CmpUsesNegatedOp)
876 Cmp.setOperand(0, LHS);
877 }
878
879 // Create the canonical RHS: RHS = sub (0, LHS).
880 if (!RHSCanonicalized) {
881 assert(RHS->hasOneUse() && "RHS use number is not right");
882 RHS = Builder.CreateNeg(LHS);
883 if (TVal == LHS) {
884 Sel.setFalseValue(RHS);
885 FVal = RHS;
886 } else {
887 Sel.setTrueValue(RHS);
888 TVal = RHS;
889 }
890 }
891
892 // If the select operands do not change, we're done.
893 if (SPF == SelectPatternFlavor::SPF_NABS) {
894 if (TVal == LHS)
895 return &Sel;
896 assert(FVal == LHS && "Unexpected results from matchSelectPattern");
897 } else {
898 if (FVal == LHS)
899 return &Sel;
900 assert(TVal == LHS && "Unexpected results from matchSelectPattern");
901 }
902
903 // We are swapping the select operands, so swap the metadata too.
904 Sel.setTrueValue(FVal);
905 Sel.setFalseValue(TVal);
906 Sel.swapProfMetadata();
907 return &Sel;
908 }
909
910 /// Visit a SelectInst that has an ICmpInst as its first operand.
foldSelectInstWithICmp(SelectInst & SI,ICmpInst * ICI)911 Instruction *InstCombiner::foldSelectInstWithICmp(SelectInst &SI,
912 ICmpInst *ICI) {
913 Value *TrueVal = SI.getTrueValue();
914 Value *FalseVal = SI.getFalseValue();
915
916 if (Instruction *NewSel = canonicalizeMinMaxWithConstant(SI, *ICI, Builder))
917 return NewSel;
918
919 if (Instruction *NewAbs = canonicalizeAbsNabs(SI, *ICI, Builder))
920 return NewAbs;
921
922 bool Changed = adjustMinMax(SI, *ICI);
923
924 if (Value *V = foldSelectICmpAnd(SI, ICI, Builder))
925 return replaceInstUsesWith(SI, V);
926
927 // NOTE: if we wanted to, this is where to detect integer MIN/MAX
928 ICmpInst::Predicate Pred = ICI->getPredicate();
929 Value *CmpLHS = ICI->getOperand(0);
930 Value *CmpRHS = ICI->getOperand(1);
931 if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS)) {
932 if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
933 // Transform (X == C) ? X : Y -> (X == C) ? C : Y
934 SI.setOperand(1, CmpRHS);
935 Changed = true;
936 } else if (CmpLHS == FalseVal && Pred == ICmpInst::ICMP_NE) {
937 // Transform (X != C) ? Y : X -> (X != C) ? Y : C
938 SI.setOperand(2, CmpRHS);
939 Changed = true;
940 }
941 }
942
943 // FIXME: This code is nearly duplicated in InstSimplify. Using/refactoring
944 // decomposeBitTestICmp() might help.
945 {
946 unsigned BitWidth =
947 DL.getTypeSizeInBits(TrueVal->getType()->getScalarType());
948 APInt MinSignedValue = APInt::getSignedMinValue(BitWidth);
949 Value *X;
950 const APInt *Y, *C;
951 bool TrueWhenUnset;
952 bool IsBitTest = false;
953 if (ICmpInst::isEquality(Pred) &&
954 match(CmpLHS, m_And(m_Value(X), m_Power2(Y))) &&
955 match(CmpRHS, m_Zero())) {
956 IsBitTest = true;
957 TrueWhenUnset = Pred == ICmpInst::ICMP_EQ;
958 } else if (Pred == ICmpInst::ICMP_SLT && match(CmpRHS, m_Zero())) {
959 X = CmpLHS;
960 Y = &MinSignedValue;
961 IsBitTest = true;
962 TrueWhenUnset = false;
963 } else if (Pred == ICmpInst::ICMP_SGT && match(CmpRHS, m_AllOnes())) {
964 X = CmpLHS;
965 Y = &MinSignedValue;
966 IsBitTest = true;
967 TrueWhenUnset = true;
968 }
969 if (IsBitTest) {
970 Value *V = nullptr;
971 // (X & Y) == 0 ? X : X ^ Y --> X & ~Y
972 if (TrueWhenUnset && TrueVal == X &&
973 match(FalseVal, m_Xor(m_Specific(X), m_APInt(C))) && *Y == *C)
974 V = Builder.CreateAnd(X, ~(*Y));
975 // (X & Y) != 0 ? X ^ Y : X --> X & ~Y
976 else if (!TrueWhenUnset && FalseVal == X &&
977 match(TrueVal, m_Xor(m_Specific(X), m_APInt(C))) && *Y == *C)
978 V = Builder.CreateAnd(X, ~(*Y));
979 // (X & Y) == 0 ? X ^ Y : X --> X | Y
980 else if (TrueWhenUnset && FalseVal == X &&
981 match(TrueVal, m_Xor(m_Specific(X), m_APInt(C))) && *Y == *C)
982 V = Builder.CreateOr(X, *Y);
983 // (X & Y) != 0 ? X : X ^ Y --> X | Y
984 else if (!TrueWhenUnset && TrueVal == X &&
985 match(FalseVal, m_Xor(m_Specific(X), m_APInt(C))) && *Y == *C)
986 V = Builder.CreateOr(X, *Y);
987
988 if (V)
989 return replaceInstUsesWith(SI, V);
990 }
991 }
992
993 if (Instruction *V =
994 foldSelectICmpAndAnd(SI.getType(), ICI, TrueVal, FalseVal, Builder))
995 return V;
996
997 if (Value *V = foldSelectICmpAndOr(ICI, TrueVal, FalseVal, Builder))
998 return replaceInstUsesWith(SI, V);
999
1000 if (Value *V = foldSelectCttzCtlz(ICI, TrueVal, FalseVal, Builder))
1001 return replaceInstUsesWith(SI, V);
1002
1003 if (Value *V = canonicalizeSaturatedSubtract(ICI, TrueVal, FalseVal, Builder))
1004 return replaceInstUsesWith(SI, V);
1005
1006 return Changed ? &SI : nullptr;
1007 }
1008
1009 /// SI is a select whose condition is a PHI node (but the two may be in
1010 /// different blocks). See if the true/false values (V) are live in all of the
1011 /// predecessor blocks of the PHI. For example, cases like this can't be mapped:
1012 ///
1013 /// X = phi [ C1, BB1], [C2, BB2]
1014 /// Y = add
1015 /// Z = select X, Y, 0
1016 ///
1017 /// because Y is not live in BB1/BB2.
canSelectOperandBeMappingIntoPredBlock(const Value * V,const SelectInst & SI)1018 static bool canSelectOperandBeMappingIntoPredBlock(const Value *V,
1019 const SelectInst &SI) {
1020 // If the value is a non-instruction value like a constant or argument, it
1021 // can always be mapped.
1022 const Instruction *I = dyn_cast<Instruction>(V);
1023 if (!I) return true;
1024
1025 // If V is a PHI node defined in the same block as the condition PHI, we can
1026 // map the arguments.
1027 const PHINode *CondPHI = cast<PHINode>(SI.getCondition());
1028
1029 if (const PHINode *VP = dyn_cast<PHINode>(I))
1030 if (VP->getParent() == CondPHI->getParent())
1031 return true;
1032
1033 // Otherwise, if the PHI and select are defined in the same block and if V is
1034 // defined in a different block, then we can transform it.
1035 if (SI.getParent() == CondPHI->getParent() &&
1036 I->getParent() != CondPHI->getParent())
1037 return true;
1038
1039 // Otherwise we have a 'hard' case and we can't tell without doing more
1040 // detailed dominator based analysis, punt.
1041 return false;
1042 }
1043
1044 /// We have an SPF (e.g. a min or max) of an SPF of the form:
1045 /// SPF2(SPF1(A, B), C)
foldSPFofSPF(Instruction * Inner,SelectPatternFlavor SPF1,Value * A,Value * B,Instruction & Outer,SelectPatternFlavor SPF2,Value * C)1046 Instruction *InstCombiner::foldSPFofSPF(Instruction *Inner,
1047 SelectPatternFlavor SPF1,
1048 Value *A, Value *B,
1049 Instruction &Outer,
1050 SelectPatternFlavor SPF2, Value *C) {
1051 if (Outer.getType() != Inner->getType())
1052 return nullptr;
1053
1054 if (C == A || C == B) {
1055 // MAX(MAX(A, B), B) -> MAX(A, B)
1056 // MIN(MIN(a, b), a) -> MIN(a, b)
1057 if (SPF1 == SPF2 && SelectPatternResult::isMinOrMax(SPF1))
1058 return replaceInstUsesWith(Outer, Inner);
1059
1060 // MAX(MIN(a, b), a) -> a
1061 // MIN(MAX(a, b), a) -> a
1062 if ((SPF1 == SPF_SMIN && SPF2 == SPF_SMAX) ||
1063 (SPF1 == SPF_SMAX && SPF2 == SPF_SMIN) ||
1064 (SPF1 == SPF_UMIN && SPF2 == SPF_UMAX) ||
1065 (SPF1 == SPF_UMAX && SPF2 == SPF_UMIN))
1066 return replaceInstUsesWith(Outer, C);
1067 }
1068
1069 if (SPF1 == SPF2) {
1070 const APInt *CB, *CC;
1071 if (match(B, m_APInt(CB)) && match(C, m_APInt(CC))) {
1072 // MIN(MIN(A, 23), 97) -> MIN(A, 23)
1073 // MAX(MAX(A, 97), 23) -> MAX(A, 97)
1074 if ((SPF1 == SPF_UMIN && CB->ule(*CC)) ||
1075 (SPF1 == SPF_SMIN && CB->sle(*CC)) ||
1076 (SPF1 == SPF_UMAX && CB->uge(*CC)) ||
1077 (SPF1 == SPF_SMAX && CB->sge(*CC)))
1078 return replaceInstUsesWith(Outer, Inner);
1079
1080 // MIN(MIN(A, 97), 23) -> MIN(A, 23)
1081 // MAX(MAX(A, 23), 97) -> MAX(A, 97)
1082 if ((SPF1 == SPF_UMIN && CB->ugt(*CC)) ||
1083 (SPF1 == SPF_SMIN && CB->sgt(*CC)) ||
1084 (SPF1 == SPF_UMAX && CB->ult(*CC)) ||
1085 (SPF1 == SPF_SMAX && CB->slt(*CC))) {
1086 Outer.replaceUsesOfWith(Inner, A);
1087 return &Outer;
1088 }
1089 }
1090 }
1091
1092 // ABS(ABS(X)) -> ABS(X)
1093 // NABS(NABS(X)) -> NABS(X)
1094 if (SPF1 == SPF2 && (SPF1 == SPF_ABS || SPF1 == SPF_NABS)) {
1095 return replaceInstUsesWith(Outer, Inner);
1096 }
1097
1098 // ABS(NABS(X)) -> ABS(X)
1099 // NABS(ABS(X)) -> NABS(X)
1100 if ((SPF1 == SPF_ABS && SPF2 == SPF_NABS) ||
1101 (SPF1 == SPF_NABS && SPF2 == SPF_ABS)) {
1102 SelectInst *SI = cast<SelectInst>(Inner);
1103 Value *NewSI =
1104 Builder.CreateSelect(SI->getCondition(), SI->getFalseValue(),
1105 SI->getTrueValue(), SI->getName(), SI);
1106 return replaceInstUsesWith(Outer, NewSI);
1107 }
1108
1109 auto IsFreeOrProfitableToInvert =
1110 [&](Value *V, Value *&NotV, bool &ElidesXor) {
1111 if (match(V, m_Not(m_Value(NotV)))) {
1112 // If V has at most 2 uses then we can get rid of the xor operation
1113 // entirely.
1114 ElidesXor |= !V->hasNUsesOrMore(3);
1115 return true;
1116 }
1117
1118 if (IsFreeToInvert(V, !V->hasNUsesOrMore(3))) {
1119 NotV = nullptr;
1120 return true;
1121 }
1122
1123 return false;
1124 };
1125
1126 Value *NotA, *NotB, *NotC;
1127 bool ElidesXor = false;
1128
1129 // MIN(MIN(~A, ~B), ~C) == ~MAX(MAX(A, B), C)
1130 // MIN(MAX(~A, ~B), ~C) == ~MAX(MIN(A, B), C)
1131 // MAX(MIN(~A, ~B), ~C) == ~MIN(MAX(A, B), C)
1132 // MAX(MAX(~A, ~B), ~C) == ~MIN(MIN(A, B), C)
1133 //
1134 // This transform is performance neutral if we can elide at least one xor from
1135 // the set of three operands, since we'll be tacking on an xor at the very
1136 // end.
1137 if (SelectPatternResult::isMinOrMax(SPF1) &&
1138 SelectPatternResult::isMinOrMax(SPF2) &&
1139 IsFreeOrProfitableToInvert(A, NotA, ElidesXor) &&
1140 IsFreeOrProfitableToInvert(B, NotB, ElidesXor) &&
1141 IsFreeOrProfitableToInvert(C, NotC, ElidesXor) && ElidesXor) {
1142 if (!NotA)
1143 NotA = Builder.CreateNot(A);
1144 if (!NotB)
1145 NotB = Builder.CreateNot(B);
1146 if (!NotC)
1147 NotC = Builder.CreateNot(C);
1148
1149 Value *NewInner = createMinMax(Builder, getInverseMinMaxFlavor(SPF1), NotA,
1150 NotB);
1151 Value *NewOuter = Builder.CreateNot(
1152 createMinMax(Builder, getInverseMinMaxFlavor(SPF2), NewInner, NotC));
1153 return replaceInstUsesWith(Outer, NewOuter);
1154 }
1155
1156 return nullptr;
1157 }
1158
1159 /// Turn select C, (X + Y), (X - Y) --> (X + (select C, Y, (-Y))).
1160 /// This is even legal for FP.
foldAddSubSelect(SelectInst & SI,InstCombiner::BuilderTy & Builder)1161 static Instruction *foldAddSubSelect(SelectInst &SI,
1162 InstCombiner::BuilderTy &Builder) {
1163 Value *CondVal = SI.getCondition();
1164 Value *TrueVal = SI.getTrueValue();
1165 Value *FalseVal = SI.getFalseValue();
1166 auto *TI = dyn_cast<Instruction>(TrueVal);
1167 auto *FI = dyn_cast<Instruction>(FalseVal);
1168 if (!TI || !FI || !TI->hasOneUse() || !FI->hasOneUse())
1169 return nullptr;
1170
1171 Instruction *AddOp = nullptr, *SubOp = nullptr;
1172 if ((TI->getOpcode() == Instruction::Sub &&
1173 FI->getOpcode() == Instruction::Add) ||
1174 (TI->getOpcode() == Instruction::FSub &&
1175 FI->getOpcode() == Instruction::FAdd)) {
1176 AddOp = FI;
1177 SubOp = TI;
1178 } else if ((FI->getOpcode() == Instruction::Sub &&
1179 TI->getOpcode() == Instruction::Add) ||
1180 (FI->getOpcode() == Instruction::FSub &&
1181 TI->getOpcode() == Instruction::FAdd)) {
1182 AddOp = TI;
1183 SubOp = FI;
1184 }
1185
1186 if (AddOp) {
1187 Value *OtherAddOp = nullptr;
1188 if (SubOp->getOperand(0) == AddOp->getOperand(0)) {
1189 OtherAddOp = AddOp->getOperand(1);
1190 } else if (SubOp->getOperand(0) == AddOp->getOperand(1)) {
1191 OtherAddOp = AddOp->getOperand(0);
1192 }
1193
1194 if (OtherAddOp) {
1195 // So at this point we know we have (Y -> OtherAddOp):
1196 // select C, (add X, Y), (sub X, Z)
1197 Value *NegVal; // Compute -Z
1198 if (SI.getType()->isFPOrFPVectorTy()) {
1199 NegVal = Builder.CreateFNeg(SubOp->getOperand(1));
1200 if (Instruction *NegInst = dyn_cast<Instruction>(NegVal)) {
1201 FastMathFlags Flags = AddOp->getFastMathFlags();
1202 Flags &= SubOp->getFastMathFlags();
1203 NegInst->setFastMathFlags(Flags);
1204 }
1205 } else {
1206 NegVal = Builder.CreateNeg(SubOp->getOperand(1));
1207 }
1208
1209 Value *NewTrueOp = OtherAddOp;
1210 Value *NewFalseOp = NegVal;
1211 if (AddOp != TI)
1212 std::swap(NewTrueOp, NewFalseOp);
1213 Value *NewSel = Builder.CreateSelect(CondVal, NewTrueOp, NewFalseOp,
1214 SI.getName() + ".p", &SI);
1215
1216 if (SI.getType()->isFPOrFPVectorTy()) {
1217 Instruction *RI =
1218 BinaryOperator::CreateFAdd(SubOp->getOperand(0), NewSel);
1219
1220 FastMathFlags Flags = AddOp->getFastMathFlags();
1221 Flags &= SubOp->getFastMathFlags();
1222 RI->setFastMathFlags(Flags);
1223 return RI;
1224 } else
1225 return BinaryOperator::CreateAdd(SubOp->getOperand(0), NewSel);
1226 }
1227 }
1228 return nullptr;
1229 }
1230
foldSelectExtConst(SelectInst & Sel)1231 Instruction *InstCombiner::foldSelectExtConst(SelectInst &Sel) {
1232 Constant *C;
1233 if (!match(Sel.getTrueValue(), m_Constant(C)) &&
1234 !match(Sel.getFalseValue(), m_Constant(C)))
1235 return nullptr;
1236
1237 Instruction *ExtInst;
1238 if (!match(Sel.getTrueValue(), m_Instruction(ExtInst)) &&
1239 !match(Sel.getFalseValue(), m_Instruction(ExtInst)))
1240 return nullptr;
1241
1242 auto ExtOpcode = ExtInst->getOpcode();
1243 if (ExtOpcode != Instruction::ZExt && ExtOpcode != Instruction::SExt)
1244 return nullptr;
1245
1246 // If we are extending from a boolean type or if we can create a select that
1247 // has the same size operands as its condition, try to narrow the select.
1248 Value *X = ExtInst->getOperand(0);
1249 Type *SmallType = X->getType();
1250 Value *Cond = Sel.getCondition();
1251 auto *Cmp = dyn_cast<CmpInst>(Cond);
1252 if (!SmallType->isIntOrIntVectorTy(1) &&
1253 (!Cmp || Cmp->getOperand(0)->getType() != SmallType))
1254 return nullptr;
1255
1256 // If the constant is the same after truncation to the smaller type and
1257 // extension to the original type, we can narrow the select.
1258 Type *SelType = Sel.getType();
1259 Constant *TruncC = ConstantExpr::getTrunc(C, SmallType);
1260 Constant *ExtC = ConstantExpr::getCast(ExtOpcode, TruncC, SelType);
1261 if (ExtC == C) {
1262 Value *TruncCVal = cast<Value>(TruncC);
1263 if (ExtInst == Sel.getFalseValue())
1264 std::swap(X, TruncCVal);
1265
1266 // select Cond, (ext X), C --> ext(select Cond, X, C')
1267 // select Cond, C, (ext X) --> ext(select Cond, C', X)
1268 Value *NewSel = Builder.CreateSelect(Cond, X, TruncCVal, "narrow", &Sel);
1269 return CastInst::Create(Instruction::CastOps(ExtOpcode), NewSel, SelType);
1270 }
1271
1272 // If one arm of the select is the extend of the condition, replace that arm
1273 // with the extension of the appropriate known bool value.
1274 if (Cond == X) {
1275 if (ExtInst == Sel.getTrueValue()) {
1276 // select X, (sext X), C --> select X, -1, C
1277 // select X, (zext X), C --> select X, 1, C
1278 Constant *One = ConstantInt::getTrue(SmallType);
1279 Constant *AllOnesOrOne = ConstantExpr::getCast(ExtOpcode, One, SelType);
1280 return SelectInst::Create(Cond, AllOnesOrOne, C, "", nullptr, &Sel);
1281 } else {
1282 // select X, C, (sext X) --> select X, C, 0
1283 // select X, C, (zext X) --> select X, C, 0
1284 Constant *Zero = ConstantInt::getNullValue(SelType);
1285 return SelectInst::Create(Cond, C, Zero, "", nullptr, &Sel);
1286 }
1287 }
1288
1289 return nullptr;
1290 }
1291
1292 /// Try to transform a vector select with a constant condition vector into a
1293 /// shuffle for easier combining with other shuffles and insert/extract.
canonicalizeSelectToShuffle(SelectInst & SI)1294 static Instruction *canonicalizeSelectToShuffle(SelectInst &SI) {
1295 Value *CondVal = SI.getCondition();
1296 Constant *CondC;
1297 if (!CondVal->getType()->isVectorTy() || !match(CondVal, m_Constant(CondC)))
1298 return nullptr;
1299
1300 unsigned NumElts = CondVal->getType()->getVectorNumElements();
1301 SmallVector<Constant *, 16> Mask;
1302 Mask.reserve(NumElts);
1303 Type *Int32Ty = Type::getInt32Ty(CondVal->getContext());
1304 for (unsigned i = 0; i != NumElts; ++i) {
1305 Constant *Elt = CondC->getAggregateElement(i);
1306 if (!Elt)
1307 return nullptr;
1308
1309 if (Elt->isOneValue()) {
1310 // If the select condition element is true, choose from the 1st vector.
1311 Mask.push_back(ConstantInt::get(Int32Ty, i));
1312 } else if (Elt->isNullValue()) {
1313 // If the select condition element is false, choose from the 2nd vector.
1314 Mask.push_back(ConstantInt::get(Int32Ty, i + NumElts));
1315 } else if (isa<UndefValue>(Elt)) {
1316 // Undef in a select condition (choose one of the operands) does not mean
1317 // the same thing as undef in a shuffle mask (any value is acceptable), so
1318 // give up.
1319 return nullptr;
1320 } else {
1321 // Bail out on a constant expression.
1322 return nullptr;
1323 }
1324 }
1325
1326 return new ShuffleVectorInst(SI.getTrueValue(), SI.getFalseValue(),
1327 ConstantVector::get(Mask));
1328 }
1329
1330 /// Reuse bitcasted operands between a compare and select:
1331 /// select (cmp (bitcast C), (bitcast D)), (bitcast' C), (bitcast' D) -->
1332 /// bitcast (select (cmp (bitcast C), (bitcast D)), (bitcast C), (bitcast D))
foldSelectCmpBitcasts(SelectInst & Sel,InstCombiner::BuilderTy & Builder)1333 static Instruction *foldSelectCmpBitcasts(SelectInst &Sel,
1334 InstCombiner::BuilderTy &Builder) {
1335 Value *Cond = Sel.getCondition();
1336 Value *TVal = Sel.getTrueValue();
1337 Value *FVal = Sel.getFalseValue();
1338
1339 CmpInst::Predicate Pred;
1340 Value *A, *B;
1341 if (!match(Cond, m_Cmp(Pred, m_Value(A), m_Value(B))))
1342 return nullptr;
1343
1344 // The select condition is a compare instruction. If the select's true/false
1345 // values are already the same as the compare operands, there's nothing to do.
1346 if (TVal == A || TVal == B || FVal == A || FVal == B)
1347 return nullptr;
1348
1349 Value *C, *D;
1350 if (!match(A, m_BitCast(m_Value(C))) || !match(B, m_BitCast(m_Value(D))))
1351 return nullptr;
1352
1353 // select (cmp (bitcast C), (bitcast D)), (bitcast TSrc), (bitcast FSrc)
1354 Value *TSrc, *FSrc;
1355 if (!match(TVal, m_BitCast(m_Value(TSrc))) ||
1356 !match(FVal, m_BitCast(m_Value(FSrc))))
1357 return nullptr;
1358
1359 // If the select true/false values are *different bitcasts* of the same source
1360 // operands, make the select operands the same as the compare operands and
1361 // cast the result. This is the canonical select form for min/max.
1362 Value *NewSel;
1363 if (TSrc == C && FSrc == D) {
1364 // select (cmp (bitcast C), (bitcast D)), (bitcast' C), (bitcast' D) -->
1365 // bitcast (select (cmp A, B), A, B)
1366 NewSel = Builder.CreateSelect(Cond, A, B, "", &Sel);
1367 } else if (TSrc == D && FSrc == C) {
1368 // select (cmp (bitcast C), (bitcast D)), (bitcast' D), (bitcast' C) -->
1369 // bitcast (select (cmp A, B), B, A)
1370 NewSel = Builder.CreateSelect(Cond, B, A, "", &Sel);
1371 } else {
1372 return nullptr;
1373 }
1374 return CastInst::CreateBitOrPointerCast(NewSel, Sel.getType());
1375 }
1376
1377 /// Try to eliminate select instructions that test the returned flag of cmpxchg
1378 /// instructions.
1379 ///
1380 /// If a select instruction tests the returned flag of a cmpxchg instruction and
1381 /// selects between the returned value of the cmpxchg instruction its compare
1382 /// operand, the result of the select will always be equal to its false value.
1383 /// For example:
1384 ///
1385 /// %0 = cmpxchg i64* %ptr, i64 %compare, i64 %new_value seq_cst seq_cst
1386 /// %1 = extractvalue { i64, i1 } %0, 1
1387 /// %2 = extractvalue { i64, i1 } %0, 0
1388 /// %3 = select i1 %1, i64 %compare, i64 %2
1389 /// ret i64 %3
1390 ///
1391 /// The returned value of the cmpxchg instruction (%2) is the original value
1392 /// located at %ptr prior to any update. If the cmpxchg operation succeeds, %2
1393 /// must have been equal to %compare. Thus, the result of the select is always
1394 /// equal to %2, and the code can be simplified to:
1395 ///
1396 /// %0 = cmpxchg i64* %ptr, i64 %compare, i64 %new_value seq_cst seq_cst
1397 /// %1 = extractvalue { i64, i1 } %0, 0
1398 /// ret i64 %1
1399 ///
foldSelectCmpXchg(SelectInst & SI)1400 static Instruction *foldSelectCmpXchg(SelectInst &SI) {
1401 // A helper that determines if V is an extractvalue instruction whose
1402 // aggregate operand is a cmpxchg instruction and whose single index is equal
1403 // to I. If such conditions are true, the helper returns the cmpxchg
1404 // instruction; otherwise, a nullptr is returned.
1405 auto isExtractFromCmpXchg = [](Value *V, unsigned I) -> AtomicCmpXchgInst * {
1406 auto *Extract = dyn_cast<ExtractValueInst>(V);
1407 if (!Extract)
1408 return nullptr;
1409 if (Extract->getIndices()[0] != I)
1410 return nullptr;
1411 return dyn_cast<AtomicCmpXchgInst>(Extract->getAggregateOperand());
1412 };
1413
1414 // If the select has a single user, and this user is a select instruction that
1415 // we can simplify, skip the cmpxchg simplification for now.
1416 if (SI.hasOneUse())
1417 if (auto *Select = dyn_cast<SelectInst>(SI.user_back()))
1418 if (Select->getCondition() == SI.getCondition())
1419 if (Select->getFalseValue() == SI.getTrueValue() ||
1420 Select->getTrueValue() == SI.getFalseValue())
1421 return nullptr;
1422
1423 // Ensure the select condition is the returned flag of a cmpxchg instruction.
1424 auto *CmpXchg = isExtractFromCmpXchg(SI.getCondition(), 1);
1425 if (!CmpXchg)
1426 return nullptr;
1427
1428 // Check the true value case: The true value of the select is the returned
1429 // value of the same cmpxchg used by the condition, and the false value is the
1430 // cmpxchg instruction's compare operand.
1431 if (auto *X = isExtractFromCmpXchg(SI.getTrueValue(), 0))
1432 if (X == CmpXchg && X->getCompareOperand() == SI.getFalseValue()) {
1433 SI.setTrueValue(SI.getFalseValue());
1434 return &SI;
1435 }
1436
1437 // Check the false value case: The false value of the select is the returned
1438 // value of the same cmpxchg used by the condition, and the true value is the
1439 // cmpxchg instruction's compare operand.
1440 if (auto *X = isExtractFromCmpXchg(SI.getFalseValue(), 0))
1441 if (X == CmpXchg && X->getCompareOperand() == SI.getTrueValue()) {
1442 SI.setTrueValue(SI.getFalseValue());
1443 return &SI;
1444 }
1445
1446 return nullptr;
1447 }
1448
1449 /// Reduce a sequence of min/max with a common operand.
factorizeMinMaxTree(SelectPatternFlavor SPF,Value * LHS,Value * RHS,InstCombiner::BuilderTy & Builder)1450 static Instruction *factorizeMinMaxTree(SelectPatternFlavor SPF, Value *LHS,
1451 Value *RHS,
1452 InstCombiner::BuilderTy &Builder) {
1453 assert(SelectPatternResult::isMinOrMax(SPF) && "Expected a min/max");
1454 // TODO: Allow FP min/max with nnan/nsz.
1455 if (!LHS->getType()->isIntOrIntVectorTy())
1456 return nullptr;
1457
1458 // Match 3 of the same min/max ops. Example: umin(umin(), umin()).
1459 Value *A, *B, *C, *D;
1460 SelectPatternResult L = matchSelectPattern(LHS, A, B);
1461 SelectPatternResult R = matchSelectPattern(RHS, C, D);
1462 if (SPF != L.Flavor || L.Flavor != R.Flavor)
1463 return nullptr;
1464
1465 // Look for a common operand. The use checks are different than usual because
1466 // a min/max pattern typically has 2 uses of each op: 1 by the cmp and 1 by
1467 // the select.
1468 Value *MinMaxOp = nullptr;
1469 Value *ThirdOp = nullptr;
1470 if (!LHS->hasNUsesOrMore(3) && RHS->hasNUsesOrMore(3)) {
1471 // If the LHS is only used in this chain and the RHS is used outside of it,
1472 // reuse the RHS min/max because that will eliminate the LHS.
1473 if (D == A || C == A) {
1474 // min(min(a, b), min(c, a)) --> min(min(c, a), b)
1475 // min(min(a, b), min(a, d)) --> min(min(a, d), b)
1476 MinMaxOp = RHS;
1477 ThirdOp = B;
1478 } else if (D == B || C == B) {
1479 // min(min(a, b), min(c, b)) --> min(min(c, b), a)
1480 // min(min(a, b), min(b, d)) --> min(min(b, d), a)
1481 MinMaxOp = RHS;
1482 ThirdOp = A;
1483 }
1484 } else if (!RHS->hasNUsesOrMore(3)) {
1485 // Reuse the LHS. This will eliminate the RHS.
1486 if (D == A || D == B) {
1487 // min(min(a, b), min(c, a)) --> min(min(a, b), c)
1488 // min(min(a, b), min(c, b)) --> min(min(a, b), c)
1489 MinMaxOp = LHS;
1490 ThirdOp = C;
1491 } else if (C == A || C == B) {
1492 // min(min(a, b), min(b, d)) --> min(min(a, b), d)
1493 // min(min(a, b), min(c, b)) --> min(min(a, b), d)
1494 MinMaxOp = LHS;
1495 ThirdOp = D;
1496 }
1497 }
1498 if (!MinMaxOp || !ThirdOp)
1499 return nullptr;
1500
1501 CmpInst::Predicate P = getMinMaxPred(SPF);
1502 Value *CmpABC = Builder.CreateICmp(P, MinMaxOp, ThirdOp);
1503 return SelectInst::Create(CmpABC, MinMaxOp, ThirdOp);
1504 }
1505
visitSelectInst(SelectInst & SI)1506 Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
1507 Value *CondVal = SI.getCondition();
1508 Value *TrueVal = SI.getTrueValue();
1509 Value *FalseVal = SI.getFalseValue();
1510 Type *SelType = SI.getType();
1511
1512 // FIXME: Remove this workaround when freeze related patches are done.
1513 // For select with undef operand which feeds into an equality comparison,
1514 // don't simplify it so loop unswitch can know the equality comparison
1515 // may have an undef operand. This is a workaround for PR31652 caused by
1516 // descrepancy about branch on undef between LoopUnswitch and GVN.
1517 if (isa<UndefValue>(TrueVal) || isa<UndefValue>(FalseVal)) {
1518 if (llvm::any_of(SI.users(), [&](User *U) {
1519 ICmpInst *CI = dyn_cast<ICmpInst>(U);
1520 if (CI && CI->isEquality())
1521 return true;
1522 return false;
1523 })) {
1524 return nullptr;
1525 }
1526 }
1527
1528 if (Value *V = SimplifySelectInst(CondVal, TrueVal, FalseVal,
1529 SQ.getWithInstruction(&SI)))
1530 return replaceInstUsesWith(SI, V);
1531
1532 if (Instruction *I = canonicalizeSelectToShuffle(SI))
1533 return I;
1534
1535 // Canonicalize a one-use integer compare with a non-canonical predicate by
1536 // inverting the predicate and swapping the select operands. This matches a
1537 // compare canonicalization for conditional branches.
1538 // TODO: Should we do the same for FP compares?
1539 CmpInst::Predicate Pred;
1540 if (match(CondVal, m_OneUse(m_ICmp(Pred, m_Value(), m_Value()))) &&
1541 !isCanonicalPredicate(Pred)) {
1542 // Swap true/false values and condition.
1543 CmpInst *Cond = cast<CmpInst>(CondVal);
1544 Cond->setPredicate(CmpInst::getInversePredicate(Pred));
1545 SI.setOperand(1, FalseVal);
1546 SI.setOperand(2, TrueVal);
1547 SI.swapProfMetadata();
1548 Worklist.Add(Cond);
1549 return &SI;
1550 }
1551
1552 if (SelType->isIntOrIntVectorTy(1) &&
1553 TrueVal->getType() == CondVal->getType()) {
1554 if (match(TrueVal, m_One())) {
1555 // Change: A = select B, true, C --> A = or B, C
1556 return BinaryOperator::CreateOr(CondVal, FalseVal);
1557 }
1558 if (match(TrueVal, m_Zero())) {
1559 // Change: A = select B, false, C --> A = and !B, C
1560 Value *NotCond = Builder.CreateNot(CondVal, "not." + CondVal->getName());
1561 return BinaryOperator::CreateAnd(NotCond, FalseVal);
1562 }
1563 if (match(FalseVal, m_Zero())) {
1564 // Change: A = select B, C, false --> A = and B, C
1565 return BinaryOperator::CreateAnd(CondVal, TrueVal);
1566 }
1567 if (match(FalseVal, m_One())) {
1568 // Change: A = select B, C, true --> A = or !B, C
1569 Value *NotCond = Builder.CreateNot(CondVal, "not." + CondVal->getName());
1570 return BinaryOperator::CreateOr(NotCond, TrueVal);
1571 }
1572
1573 // select a, a, b -> a | b
1574 // select a, b, a -> a & b
1575 if (CondVal == TrueVal)
1576 return BinaryOperator::CreateOr(CondVal, FalseVal);
1577 if (CondVal == FalseVal)
1578 return BinaryOperator::CreateAnd(CondVal, TrueVal);
1579
1580 // select a, ~a, b -> (~a) & b
1581 // select a, b, ~a -> (~a) | b
1582 if (match(TrueVal, m_Not(m_Specific(CondVal))))
1583 return BinaryOperator::CreateAnd(TrueVal, FalseVal);
1584 if (match(FalseVal, m_Not(m_Specific(CondVal))))
1585 return BinaryOperator::CreateOr(TrueVal, FalseVal);
1586 }
1587
1588 // Selecting between two integer or vector splat integer constants?
1589 //
1590 // Note that we don't handle a scalar select of vectors:
1591 // select i1 %c, <2 x i8> <1, 1>, <2 x i8> <0, 0>
1592 // because that may need 3 instructions to splat the condition value:
1593 // extend, insertelement, shufflevector.
1594 if (SelType->isIntOrIntVectorTy() &&
1595 CondVal->getType()->isVectorTy() == SelType->isVectorTy()) {
1596 // select C, 1, 0 -> zext C to int
1597 if (match(TrueVal, m_One()) && match(FalseVal, m_Zero()))
1598 return new ZExtInst(CondVal, SelType);
1599
1600 // select C, -1, 0 -> sext C to int
1601 if (match(TrueVal, m_AllOnes()) && match(FalseVal, m_Zero()))
1602 return new SExtInst(CondVal, SelType);
1603
1604 // select C, 0, 1 -> zext !C to int
1605 if (match(TrueVal, m_Zero()) && match(FalseVal, m_One())) {
1606 Value *NotCond = Builder.CreateNot(CondVal, "not." + CondVal->getName());
1607 return new ZExtInst(NotCond, SelType);
1608 }
1609
1610 // select C, 0, -1 -> sext !C to int
1611 if (match(TrueVal, m_Zero()) && match(FalseVal, m_AllOnes())) {
1612 Value *NotCond = Builder.CreateNot(CondVal, "not." + CondVal->getName());
1613 return new SExtInst(NotCond, SelType);
1614 }
1615 }
1616
1617 // See if we are selecting two values based on a comparison of the two values.
1618 if (FCmpInst *FCI = dyn_cast<FCmpInst>(CondVal)) {
1619 if (FCI->getOperand(0) == TrueVal && FCI->getOperand(1) == FalseVal) {
1620 // Transform (X == Y) ? X : Y -> Y
1621 if (FCI->getPredicate() == FCmpInst::FCMP_OEQ) {
1622 // This is not safe in general for floating point:
1623 // consider X== -0, Y== +0.
1624 // It becomes safe if either operand is a nonzero constant.
1625 ConstantFP *CFPt, *CFPf;
1626 if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
1627 !CFPt->getValueAPF().isZero()) ||
1628 ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
1629 !CFPf->getValueAPF().isZero()))
1630 return replaceInstUsesWith(SI, FalseVal);
1631 }
1632 // Transform (X une Y) ? X : Y -> X
1633 if (FCI->getPredicate() == FCmpInst::FCMP_UNE) {
1634 // This is not safe in general for floating point:
1635 // consider X== -0, Y== +0.
1636 // It becomes safe if either operand is a nonzero constant.
1637 ConstantFP *CFPt, *CFPf;
1638 if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
1639 !CFPt->getValueAPF().isZero()) ||
1640 ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
1641 !CFPf->getValueAPF().isZero()))
1642 return replaceInstUsesWith(SI, TrueVal);
1643 }
1644
1645 // Canonicalize to use ordered comparisons by swapping the select
1646 // operands.
1647 //
1648 // e.g.
1649 // (X ugt Y) ? X : Y -> (X ole Y) ? Y : X
1650 if (FCI->hasOneUse() && FCmpInst::isUnordered(FCI->getPredicate())) {
1651 FCmpInst::Predicate InvPred = FCI->getInversePredicate();
1652 IRBuilder<>::FastMathFlagGuard FMFG(Builder);
1653 Builder.setFastMathFlags(FCI->getFastMathFlags());
1654 Value *NewCond = Builder.CreateFCmp(InvPred, TrueVal, FalseVal,
1655 FCI->getName() + ".inv");
1656
1657 return SelectInst::Create(NewCond, FalseVal, TrueVal,
1658 SI.getName() + ".p");
1659 }
1660
1661 // NOTE: if we wanted to, this is where to detect MIN/MAX
1662 } else if (FCI->getOperand(0) == FalseVal && FCI->getOperand(1) == TrueVal){
1663 // Transform (X == Y) ? Y : X -> X
1664 if (FCI->getPredicate() == FCmpInst::FCMP_OEQ) {
1665 // This is not safe in general for floating point:
1666 // consider X== -0, Y== +0.
1667 // It becomes safe if either operand is a nonzero constant.
1668 ConstantFP *CFPt, *CFPf;
1669 if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
1670 !CFPt->getValueAPF().isZero()) ||
1671 ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
1672 !CFPf->getValueAPF().isZero()))
1673 return replaceInstUsesWith(SI, FalseVal);
1674 }
1675 // Transform (X une Y) ? Y : X -> Y
1676 if (FCI->getPredicate() == FCmpInst::FCMP_UNE) {
1677 // This is not safe in general for floating point:
1678 // consider X== -0, Y== +0.
1679 // It becomes safe if either operand is a nonzero constant.
1680 ConstantFP *CFPt, *CFPf;
1681 if (((CFPt = dyn_cast<ConstantFP>(TrueVal)) &&
1682 !CFPt->getValueAPF().isZero()) ||
1683 ((CFPf = dyn_cast<ConstantFP>(FalseVal)) &&
1684 !CFPf->getValueAPF().isZero()))
1685 return replaceInstUsesWith(SI, TrueVal);
1686 }
1687
1688 // Canonicalize to use ordered comparisons by swapping the select
1689 // operands.
1690 //
1691 // e.g.
1692 // (X ugt Y) ? X : Y -> (X ole Y) ? X : Y
1693 if (FCI->hasOneUse() && FCmpInst::isUnordered(FCI->getPredicate())) {
1694 FCmpInst::Predicate InvPred = FCI->getInversePredicate();
1695 IRBuilder<>::FastMathFlagGuard FMFG(Builder);
1696 Builder.setFastMathFlags(FCI->getFastMathFlags());
1697 Value *NewCond = Builder.CreateFCmp(InvPred, FalseVal, TrueVal,
1698 FCI->getName() + ".inv");
1699
1700 return SelectInst::Create(NewCond, FalseVal, TrueVal,
1701 SI.getName() + ".p");
1702 }
1703
1704 // NOTE: if we wanted to, this is where to detect MIN/MAX
1705 }
1706
1707 // Canonicalize select with fcmp to fabs(). -0.0 makes this tricky. We need
1708 // fast-math-flags (nsz) or fsub with +0.0 (not fneg) for this to work. We
1709 // also require nnan because we do not want to unintentionally change the
1710 // sign of a NaN value.
1711 Value *X = FCI->getOperand(0);
1712 FCmpInst::Predicate Pred = FCI->getPredicate();
1713 if (match(FCI->getOperand(1), m_AnyZeroFP()) && FCI->hasNoNaNs()) {
1714 // (X <= +/-0.0) ? (0.0 - X) : X --> fabs(X)
1715 // (X > +/-0.0) ? X : (0.0 - X) --> fabs(X)
1716 if ((X == FalseVal && Pred == FCmpInst::FCMP_OLE &&
1717 match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X)))) ||
1718 (X == TrueVal && Pred == FCmpInst::FCMP_OGT &&
1719 match(FalseVal, m_FSub(m_PosZeroFP(), m_Specific(X))))) {
1720 Value *Fabs = Builder.CreateIntrinsic(Intrinsic::fabs, { X }, FCI);
1721 return replaceInstUsesWith(SI, Fabs);
1722 }
1723 // With nsz:
1724 // (X < +/-0.0) ? -X : X --> fabs(X)
1725 // (X <= +/-0.0) ? -X : X --> fabs(X)
1726 // (X > +/-0.0) ? X : -X --> fabs(X)
1727 // (X >= +/-0.0) ? X : -X --> fabs(X)
1728 if (FCI->hasNoSignedZeros() &&
1729 ((X == FalseVal && match(TrueVal, m_FNeg(m_Specific(X))) &&
1730 (Pred == FCmpInst::FCMP_OLT || Pred == FCmpInst::FCMP_OLE)) ||
1731 (X == TrueVal && match(FalseVal, m_FNeg(m_Specific(X))) &&
1732 (Pred == FCmpInst::FCMP_OGT || Pred == FCmpInst::FCMP_OGE)))) {
1733 Value *Fabs = Builder.CreateIntrinsic(Intrinsic::fabs, { X }, FCI);
1734 return replaceInstUsesWith(SI, Fabs);
1735 }
1736 }
1737 }
1738
1739 // See if we are selecting two values based on a comparison of the two values.
1740 if (ICmpInst *ICI = dyn_cast<ICmpInst>(CondVal))
1741 if (Instruction *Result = foldSelectInstWithICmp(SI, ICI))
1742 return Result;
1743
1744 if (Instruction *Add = foldAddSubSelect(SI, Builder))
1745 return Add;
1746
1747 // Turn (select C, (op X, Y), (op X, Z)) -> (op X, (select C, Y, Z))
1748 auto *TI = dyn_cast<Instruction>(TrueVal);
1749 auto *FI = dyn_cast<Instruction>(FalseVal);
1750 if (TI && FI && TI->getOpcode() == FI->getOpcode())
1751 if (Instruction *IV = foldSelectOpOp(SI, TI, FI))
1752 return IV;
1753
1754 if (Instruction *I = foldSelectExtConst(SI))
1755 return I;
1756
1757 // See if we can fold the select into one of our operands.
1758 if (SelType->isIntOrIntVectorTy() || SelType->isFPOrFPVectorTy()) {
1759 if (Instruction *FoldI = foldSelectIntoOp(SI, TrueVal, FalseVal))
1760 return FoldI;
1761
1762 Value *LHS, *RHS, *LHS2, *RHS2;
1763 Instruction::CastOps CastOp;
1764 SelectPatternResult SPR = matchSelectPattern(&SI, LHS, RHS, &CastOp);
1765 auto SPF = SPR.Flavor;
1766
1767 if (SelectPatternResult::isMinOrMax(SPF)) {
1768 // Canonicalize so that
1769 // - type casts are outside select patterns.
1770 // - float clamp is transformed to min/max pattern
1771
1772 bool IsCastNeeded = LHS->getType() != SelType;
1773 Value *CmpLHS = cast<CmpInst>(CondVal)->getOperand(0);
1774 Value *CmpRHS = cast<CmpInst>(CondVal)->getOperand(1);
1775 if (IsCastNeeded ||
1776 (LHS->getType()->isFPOrFPVectorTy() &&
1777 ((CmpLHS != LHS && CmpLHS != RHS) ||
1778 (CmpRHS != LHS && CmpRHS != RHS)))) {
1779 CmpInst::Predicate Pred = getMinMaxPred(SPF, SPR.Ordered);
1780
1781 Value *Cmp;
1782 if (CmpInst::isIntPredicate(Pred)) {
1783 Cmp = Builder.CreateICmp(Pred, LHS, RHS);
1784 } else {
1785 IRBuilder<>::FastMathFlagGuard FMFG(Builder);
1786 auto FMF = cast<FPMathOperator>(SI.getCondition())->getFastMathFlags();
1787 Builder.setFastMathFlags(FMF);
1788 Cmp = Builder.CreateFCmp(Pred, LHS, RHS);
1789 }
1790
1791 Value *NewSI = Builder.CreateSelect(Cmp, LHS, RHS, SI.getName(), &SI);
1792 if (!IsCastNeeded)
1793 return replaceInstUsesWith(SI, NewSI);
1794
1795 Value *NewCast = Builder.CreateCast(CastOp, NewSI, SelType);
1796 return replaceInstUsesWith(SI, NewCast);
1797 }
1798
1799 // MAX(~a, ~b) -> ~MIN(a, b)
1800 // MIN(~a, ~b) -> ~MAX(a, b)
1801 Value *A, *B;
1802 if (match(LHS, m_Not(m_Value(A))) && match(RHS, m_Not(m_Value(B))) &&
1803 (LHS->getNumUses() <= 2 || RHS->getNumUses() <= 2)) {
1804 CmpInst::Predicate InvertedPred = getInverseMinMaxPred(SPF);
1805 Value *InvertedCmp = Builder.CreateICmp(InvertedPred, A, B);
1806 Value *NewSel = Builder.CreateSelect(InvertedCmp, A, B);
1807 return BinaryOperator::CreateNot(NewSel);
1808 }
1809
1810 if (Instruction *I = factorizeMinMaxTree(SPF, LHS, RHS, Builder))
1811 return I;
1812 }
1813
1814 if (SPF) {
1815 // MAX(MAX(a, b), a) -> MAX(a, b)
1816 // MIN(MIN(a, b), a) -> MIN(a, b)
1817 // MAX(MIN(a, b), a) -> a
1818 // MIN(MAX(a, b), a) -> a
1819 // ABS(ABS(a)) -> ABS(a)
1820 // NABS(NABS(a)) -> NABS(a)
1821 if (SelectPatternFlavor SPF2 = matchSelectPattern(LHS, LHS2, RHS2).Flavor)
1822 if (Instruction *R = foldSPFofSPF(cast<Instruction>(LHS),SPF2,LHS2,RHS2,
1823 SI, SPF, RHS))
1824 return R;
1825 if (SelectPatternFlavor SPF2 = matchSelectPattern(RHS, LHS2, RHS2).Flavor)
1826 if (Instruction *R = foldSPFofSPF(cast<Instruction>(RHS),SPF2,LHS2,RHS2,
1827 SI, SPF, LHS))
1828 return R;
1829 }
1830
1831 // TODO.
1832 // ABS(-X) -> ABS(X)
1833 }
1834
1835 // See if we can fold the select into a phi node if the condition is a select.
1836 if (auto *PN = dyn_cast<PHINode>(SI.getCondition()))
1837 // The true/false values have to be live in the PHI predecessor's blocks.
1838 if (canSelectOperandBeMappingIntoPredBlock(TrueVal, SI) &&
1839 canSelectOperandBeMappingIntoPredBlock(FalseVal, SI))
1840 if (Instruction *NV = foldOpIntoPhi(SI, PN))
1841 return NV;
1842
1843 if (SelectInst *TrueSI = dyn_cast<SelectInst>(TrueVal)) {
1844 if (TrueSI->getCondition()->getType() == CondVal->getType()) {
1845 // select(C, select(C, a, b), c) -> select(C, a, c)
1846 if (TrueSI->getCondition() == CondVal) {
1847 if (SI.getTrueValue() == TrueSI->getTrueValue())
1848 return nullptr;
1849 SI.setOperand(1, TrueSI->getTrueValue());
1850 return &SI;
1851 }
1852 // select(C0, select(C1, a, b), b) -> select(C0&C1, a, b)
1853 // We choose this as normal form to enable folding on the And and shortening
1854 // paths for the values (this helps GetUnderlyingObjects() for example).
1855 if (TrueSI->getFalseValue() == FalseVal && TrueSI->hasOneUse()) {
1856 Value *And = Builder.CreateAnd(CondVal, TrueSI->getCondition());
1857 SI.setOperand(0, And);
1858 SI.setOperand(1, TrueSI->getTrueValue());
1859 return &SI;
1860 }
1861 }
1862 }
1863 if (SelectInst *FalseSI = dyn_cast<SelectInst>(FalseVal)) {
1864 if (FalseSI->getCondition()->getType() == CondVal->getType()) {
1865 // select(C, a, select(C, b, c)) -> select(C, a, c)
1866 if (FalseSI->getCondition() == CondVal) {
1867 if (SI.getFalseValue() == FalseSI->getFalseValue())
1868 return nullptr;
1869 SI.setOperand(2, FalseSI->getFalseValue());
1870 return &SI;
1871 }
1872 // select(C0, a, select(C1, a, b)) -> select(C0|C1, a, b)
1873 if (FalseSI->getTrueValue() == TrueVal && FalseSI->hasOneUse()) {
1874 Value *Or = Builder.CreateOr(CondVal, FalseSI->getCondition());
1875 SI.setOperand(0, Or);
1876 SI.setOperand(2, FalseSI->getFalseValue());
1877 return &SI;
1878 }
1879 }
1880 }
1881
1882 auto canMergeSelectThroughBinop = [](BinaryOperator *BO) {
1883 // The select might be preventing a division by 0.
1884 switch (BO->getOpcode()) {
1885 default:
1886 return true;
1887 case Instruction::SRem:
1888 case Instruction::URem:
1889 case Instruction::SDiv:
1890 case Instruction::UDiv:
1891 return false;
1892 }
1893 };
1894
1895 // Try to simplify a binop sandwiched between 2 selects with the same
1896 // condition.
1897 // select(C, binop(select(C, X, Y), W), Z) -> select(C, binop(X, W), Z)
1898 BinaryOperator *TrueBO;
1899 if (match(TrueVal, m_OneUse(m_BinOp(TrueBO))) &&
1900 canMergeSelectThroughBinop(TrueBO)) {
1901 if (auto *TrueBOSI = dyn_cast<SelectInst>(TrueBO->getOperand(0))) {
1902 if (TrueBOSI->getCondition() == CondVal) {
1903 TrueBO->setOperand(0, TrueBOSI->getTrueValue());
1904 Worklist.Add(TrueBO);
1905 return &SI;
1906 }
1907 }
1908 if (auto *TrueBOSI = dyn_cast<SelectInst>(TrueBO->getOperand(1))) {
1909 if (TrueBOSI->getCondition() == CondVal) {
1910 TrueBO->setOperand(1, TrueBOSI->getTrueValue());
1911 Worklist.Add(TrueBO);
1912 return &SI;
1913 }
1914 }
1915 }
1916
1917 // select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W))
1918 BinaryOperator *FalseBO;
1919 if (match(FalseVal, m_OneUse(m_BinOp(FalseBO))) &&
1920 canMergeSelectThroughBinop(FalseBO)) {
1921 if (auto *FalseBOSI = dyn_cast<SelectInst>(FalseBO->getOperand(0))) {
1922 if (FalseBOSI->getCondition() == CondVal) {
1923 FalseBO->setOperand(0, FalseBOSI->getFalseValue());
1924 Worklist.Add(FalseBO);
1925 return &SI;
1926 }
1927 }
1928 if (auto *FalseBOSI = dyn_cast<SelectInst>(FalseBO->getOperand(1))) {
1929 if (FalseBOSI->getCondition() == CondVal) {
1930 FalseBO->setOperand(1, FalseBOSI->getFalseValue());
1931 Worklist.Add(FalseBO);
1932 return &SI;
1933 }
1934 }
1935 }
1936
1937 if (BinaryOperator::isNot(CondVal)) {
1938 SI.setOperand(0, BinaryOperator::getNotArgument(CondVal));
1939 SI.setOperand(1, FalseVal);
1940 SI.setOperand(2, TrueVal);
1941 return &SI;
1942 }
1943
1944 if (VectorType *VecTy = dyn_cast<VectorType>(SelType)) {
1945 unsigned VWidth = VecTy->getNumElements();
1946 APInt UndefElts(VWidth, 0);
1947 APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
1948 if (Value *V = SimplifyDemandedVectorElts(&SI, AllOnesEltMask, UndefElts)) {
1949 if (V != &SI)
1950 return replaceInstUsesWith(SI, V);
1951 return &SI;
1952 }
1953 }
1954
1955 // See if we can determine the result of this select based on a dominating
1956 // condition.
1957 BasicBlock *Parent = SI.getParent();
1958 if (BasicBlock *Dom = Parent->getSinglePredecessor()) {
1959 auto *PBI = dyn_cast_or_null<BranchInst>(Dom->getTerminator());
1960 if (PBI && PBI->isConditional() &&
1961 PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
1962 (PBI->getSuccessor(0) == Parent || PBI->getSuccessor(1) == Parent)) {
1963 bool CondIsTrue = PBI->getSuccessor(0) == Parent;
1964 Optional<bool> Implication = isImpliedCondition(
1965 PBI->getCondition(), SI.getCondition(), DL, CondIsTrue);
1966 if (Implication) {
1967 Value *V = *Implication ? TrueVal : FalseVal;
1968 return replaceInstUsesWith(SI, V);
1969 }
1970 }
1971 }
1972
1973 // If we can compute the condition, there's no need for a select.
1974 // Like the above fold, we are attempting to reduce compile-time cost by
1975 // putting this fold here with limitations rather than in InstSimplify.
1976 // The motivation for this call into value tracking is to take advantage of
1977 // the assumption cache, so make sure that is populated.
1978 if (!CondVal->getType()->isVectorTy() && !AC.assumptions().empty()) {
1979 KnownBits Known(1);
1980 computeKnownBits(CondVal, Known, 0, &SI);
1981 if (Known.One.isOneValue())
1982 return replaceInstUsesWith(SI, TrueVal);
1983 if (Known.Zero.isOneValue())
1984 return replaceInstUsesWith(SI, FalseVal);
1985 }
1986
1987 if (Instruction *BitCastSel = foldSelectCmpBitcasts(SI, Builder))
1988 return BitCastSel;
1989
1990 // Simplify selects that test the returned flag of cmpxchg instructions.
1991 if (Instruction *Select = foldSelectCmpXchg(SI))
1992 return Select;
1993
1994 if (Instruction *Select = foldSelectBinOpIdentity(SI))
1995 return Select;
1996
1997 return nullptr;
1998 }
1999