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1 //===- InstCombinePHI.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 visitPHINode function.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "InstCombineInternal.h"
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/ADT/SmallPtrSet.h"
17 #include "llvm/Analysis/InstructionSimplify.h"
18 #include "llvm/Analysis/ValueTracking.h"
19 #include "llvm/IR/PatternMatch.h"
20 #include "llvm/Transforms/Utils/Local.h"
21 using namespace llvm;
22 using namespace llvm::PatternMatch;
23 
24 #define DEBUG_TYPE "instcombine"
25 
26 /// If we have something like phi [add (a,b), add(a,c)] and if a/b/c and the
27 /// adds all have a single use, turn this into a phi and a single binop.
FoldPHIArgBinOpIntoPHI(PHINode & PN)28 Instruction *InstCombiner::FoldPHIArgBinOpIntoPHI(PHINode &PN) {
29   Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
30   assert(isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst));
31   unsigned Opc = FirstInst->getOpcode();
32   Value *LHSVal = FirstInst->getOperand(0);
33   Value *RHSVal = FirstInst->getOperand(1);
34 
35   Type *LHSType = LHSVal->getType();
36   Type *RHSType = RHSVal->getType();
37 
38   // Scan to see if all operands are the same opcode, and all have one use.
39   for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
40     Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
41     if (!I || I->getOpcode() != Opc || !I->hasOneUse() ||
42         // Verify type of the LHS matches so we don't fold cmp's of different
43         // types.
44         I->getOperand(0)->getType() != LHSType ||
45         I->getOperand(1)->getType() != RHSType)
46       return nullptr;
47 
48     // If they are CmpInst instructions, check their predicates
49     if (CmpInst *CI = dyn_cast<CmpInst>(I))
50       if (CI->getPredicate() != cast<CmpInst>(FirstInst)->getPredicate())
51         return nullptr;
52 
53     // Keep track of which operand needs a phi node.
54     if (I->getOperand(0) != LHSVal) LHSVal = nullptr;
55     if (I->getOperand(1) != RHSVal) RHSVal = nullptr;
56   }
57 
58   // If both LHS and RHS would need a PHI, don't do this transformation,
59   // because it would increase the number of PHIs entering the block,
60   // which leads to higher register pressure. This is especially
61   // bad when the PHIs are in the header of a loop.
62   if (!LHSVal && !RHSVal)
63     return nullptr;
64 
65   // Otherwise, this is safe to transform!
66 
67   Value *InLHS = FirstInst->getOperand(0);
68   Value *InRHS = FirstInst->getOperand(1);
69   PHINode *NewLHS = nullptr, *NewRHS = nullptr;
70   if (!LHSVal) {
71     NewLHS = PHINode::Create(LHSType, PN.getNumIncomingValues(),
72                              FirstInst->getOperand(0)->getName() + ".pn");
73     NewLHS->addIncoming(InLHS, PN.getIncomingBlock(0));
74     InsertNewInstBefore(NewLHS, PN);
75     LHSVal = NewLHS;
76   }
77 
78   if (!RHSVal) {
79     NewRHS = PHINode::Create(RHSType, PN.getNumIncomingValues(),
80                              FirstInst->getOperand(1)->getName() + ".pn");
81     NewRHS->addIncoming(InRHS, PN.getIncomingBlock(0));
82     InsertNewInstBefore(NewRHS, PN);
83     RHSVal = NewRHS;
84   }
85 
86   // Add all operands to the new PHIs.
87   if (NewLHS || NewRHS) {
88     for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
89       Instruction *InInst = cast<Instruction>(PN.getIncomingValue(i));
90       if (NewLHS) {
91         Value *NewInLHS = InInst->getOperand(0);
92         NewLHS->addIncoming(NewInLHS, PN.getIncomingBlock(i));
93       }
94       if (NewRHS) {
95         Value *NewInRHS = InInst->getOperand(1);
96         NewRHS->addIncoming(NewInRHS, PN.getIncomingBlock(i));
97       }
98     }
99   }
100 
101   if (CmpInst *CIOp = dyn_cast<CmpInst>(FirstInst)) {
102     CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
103                                      LHSVal, RHSVal);
104     NewCI->setDebugLoc(FirstInst->getDebugLoc());
105     return NewCI;
106   }
107 
108   BinaryOperator *BinOp = cast<BinaryOperator>(FirstInst);
109   BinaryOperator *NewBinOp =
110     BinaryOperator::Create(BinOp->getOpcode(), LHSVal, RHSVal);
111 
112   NewBinOp->copyIRFlags(PN.getIncomingValue(0));
113 
114   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i)
115     NewBinOp->andIRFlags(PN.getIncomingValue(i));
116 
117   NewBinOp->setDebugLoc(FirstInst->getDebugLoc());
118   return NewBinOp;
119 }
120 
FoldPHIArgGEPIntoPHI(PHINode & PN)121 Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
122   GetElementPtrInst *FirstInst =cast<GetElementPtrInst>(PN.getIncomingValue(0));
123 
124   SmallVector<Value*, 16> FixedOperands(FirstInst->op_begin(),
125                                         FirstInst->op_end());
126   // This is true if all GEP bases are allocas and if all indices into them are
127   // constants.
128   bool AllBasePointersAreAllocas = true;
129 
130   // We don't want to replace this phi if the replacement would require
131   // more than one phi, which leads to higher register pressure. This is
132   // especially bad when the PHIs are in the header of a loop.
133   bool NeededPhi = false;
134 
135   bool AllInBounds = true;
136 
137   // Scan to see if all operands are the same opcode, and all have one use.
138   for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
139     GetElementPtrInst *GEP= dyn_cast<GetElementPtrInst>(PN.getIncomingValue(i));
140     if (!GEP || !GEP->hasOneUse() || GEP->getType() != FirstInst->getType() ||
141       GEP->getNumOperands() != FirstInst->getNumOperands())
142       return nullptr;
143 
144     AllInBounds &= GEP->isInBounds();
145 
146     // Keep track of whether or not all GEPs are of alloca pointers.
147     if (AllBasePointersAreAllocas &&
148         (!isa<AllocaInst>(GEP->getOperand(0)) ||
149          !GEP->hasAllConstantIndices()))
150       AllBasePointersAreAllocas = false;
151 
152     // Compare the operand lists.
153     for (unsigned op = 0, e = FirstInst->getNumOperands(); op != e; ++op) {
154       if (FirstInst->getOperand(op) == GEP->getOperand(op))
155         continue;
156 
157       // Don't merge two GEPs when two operands differ (introducing phi nodes)
158       // if one of the PHIs has a constant for the index.  The index may be
159       // substantially cheaper to compute for the constants, so making it a
160       // variable index could pessimize the path.  This also handles the case
161       // for struct indices, which must always be constant.
162       if (isa<ConstantInt>(FirstInst->getOperand(op)) ||
163           isa<ConstantInt>(GEP->getOperand(op)))
164         return nullptr;
165 
166       if (FirstInst->getOperand(op)->getType() !=GEP->getOperand(op)->getType())
167         return nullptr;
168 
169       // If we already needed a PHI for an earlier operand, and another operand
170       // also requires a PHI, we'd be introducing more PHIs than we're
171       // eliminating, which increases register pressure on entry to the PHI's
172       // block.
173       if (NeededPhi)
174         return nullptr;
175 
176       FixedOperands[op] = nullptr;  // Needs a PHI.
177       NeededPhi = true;
178     }
179   }
180 
181   // If all of the base pointers of the PHI'd GEPs are from allocas, don't
182   // bother doing this transformation.  At best, this will just save a bit of
183   // offset calculation, but all the predecessors will have to materialize the
184   // stack address into a register anyway.  We'd actually rather *clone* the
185   // load up into the predecessors so that we have a load of a gep of an alloca,
186   // which can usually all be folded into the load.
187   if (AllBasePointersAreAllocas)
188     return nullptr;
189 
190   // Otherwise, this is safe to transform.  Insert PHI nodes for each operand
191   // that is variable.
192   SmallVector<PHINode*, 16> OperandPhis(FixedOperands.size());
193 
194   bool HasAnyPHIs = false;
195   for (unsigned i = 0, e = FixedOperands.size(); i != e; ++i) {
196     if (FixedOperands[i]) continue;  // operand doesn't need a phi.
197     Value *FirstOp = FirstInst->getOperand(i);
198     PHINode *NewPN = PHINode::Create(FirstOp->getType(), e,
199                                      FirstOp->getName()+".pn");
200     InsertNewInstBefore(NewPN, PN);
201 
202     NewPN->addIncoming(FirstOp, PN.getIncomingBlock(0));
203     OperandPhis[i] = NewPN;
204     FixedOperands[i] = NewPN;
205     HasAnyPHIs = true;
206   }
207 
208 
209   // Add all operands to the new PHIs.
210   if (HasAnyPHIs) {
211     for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
212       GetElementPtrInst *InGEP =cast<GetElementPtrInst>(PN.getIncomingValue(i));
213       BasicBlock *InBB = PN.getIncomingBlock(i);
214 
215       for (unsigned op = 0, e = OperandPhis.size(); op != e; ++op)
216         if (PHINode *OpPhi = OperandPhis[op])
217           OpPhi->addIncoming(InGEP->getOperand(op), InBB);
218     }
219   }
220 
221   Value *Base = FixedOperands[0];
222   GetElementPtrInst *NewGEP =
223       GetElementPtrInst::Create(FirstInst->getSourceElementType(), Base,
224                                 makeArrayRef(FixedOperands).slice(1));
225   if (AllInBounds) NewGEP->setIsInBounds();
226   NewGEP->setDebugLoc(FirstInst->getDebugLoc());
227   return NewGEP;
228 }
229 
230 
231 /// Return true if we know that it is safe to sink the load out of the block
232 /// that defines it. This means that it must be obvious the value of the load is
233 /// not changed from the point of the load to the end of the block it is in.
234 ///
235 /// Finally, it is safe, but not profitable, to sink a load targeting a
236 /// non-address-taken alloca.  Doing so will cause us to not promote the alloca
237 /// to a register.
isSafeAndProfitableToSinkLoad(LoadInst * L)238 static bool isSafeAndProfitableToSinkLoad(LoadInst *L) {
239   BasicBlock::iterator BBI = L->getIterator(), E = L->getParent()->end();
240 
241   for (++BBI; BBI != E; ++BBI)
242     if (BBI->mayWriteToMemory())
243       return false;
244 
245   // Check for non-address taken alloca.  If not address-taken already, it isn't
246   // profitable to do this xform.
247   if (AllocaInst *AI = dyn_cast<AllocaInst>(L->getOperand(0))) {
248     bool isAddressTaken = false;
249     for (User *U : AI->users()) {
250       if (isa<LoadInst>(U)) continue;
251       if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
252         // If storing TO the alloca, then the address isn't taken.
253         if (SI->getOperand(1) == AI) continue;
254       }
255       isAddressTaken = true;
256       break;
257     }
258 
259     if (!isAddressTaken && AI->isStaticAlloca())
260       return false;
261   }
262 
263   // If this load is a load from a GEP with a constant offset from an alloca,
264   // then we don't want to sink it.  In its present form, it will be
265   // load [constant stack offset].  Sinking it will cause us to have to
266   // materialize the stack addresses in each predecessor in a register only to
267   // do a shared load from register in the successor.
268   if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(L->getOperand(0)))
269     if (AllocaInst *AI = dyn_cast<AllocaInst>(GEP->getOperand(0)))
270       if (AI->isStaticAlloca() && GEP->hasAllConstantIndices())
271         return false;
272 
273   return true;
274 }
275 
FoldPHIArgLoadIntoPHI(PHINode & PN)276 Instruction *InstCombiner::FoldPHIArgLoadIntoPHI(PHINode &PN) {
277   LoadInst *FirstLI = cast<LoadInst>(PN.getIncomingValue(0));
278 
279   // FIXME: This is overconservative; this transform is allowed in some cases
280   // for atomic operations.
281   if (FirstLI->isAtomic())
282     return nullptr;
283 
284   // When processing loads, we need to propagate two bits of information to the
285   // sunk load: whether it is volatile, and what its alignment is.  We currently
286   // don't sink loads when some have their alignment specified and some don't.
287   // visitLoadInst will propagate an alignment onto the load when TD is around,
288   // and if TD isn't around, we can't handle the mixed case.
289   bool isVolatile = FirstLI->isVolatile();
290   unsigned LoadAlignment = FirstLI->getAlignment();
291   unsigned LoadAddrSpace = FirstLI->getPointerAddressSpace();
292 
293   // We can't sink the load if the loaded value could be modified between the
294   // load and the PHI.
295   if (FirstLI->getParent() != PN.getIncomingBlock(0) ||
296       !isSafeAndProfitableToSinkLoad(FirstLI))
297     return nullptr;
298 
299   // If the PHI is of volatile loads and the load block has multiple
300   // successors, sinking it would remove a load of the volatile value from
301   // the path through the other successor.
302   if (isVolatile &&
303       FirstLI->getParent()->getTerminator()->getNumSuccessors() != 1)
304     return nullptr;
305 
306   // Check to see if all arguments are the same operation.
307   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
308     LoadInst *LI = dyn_cast<LoadInst>(PN.getIncomingValue(i));
309     if (!LI || !LI->hasOneUse())
310       return nullptr;
311 
312     // We can't sink the load if the loaded value could be modified between
313     // the load and the PHI.
314     if (LI->isVolatile() != isVolatile ||
315         LI->getParent() != PN.getIncomingBlock(i) ||
316         LI->getPointerAddressSpace() != LoadAddrSpace ||
317         !isSafeAndProfitableToSinkLoad(LI))
318       return nullptr;
319 
320     // If some of the loads have an alignment specified but not all of them,
321     // we can't do the transformation.
322     if ((LoadAlignment != 0) != (LI->getAlignment() != 0))
323       return nullptr;
324 
325     LoadAlignment = std::min(LoadAlignment, LI->getAlignment());
326 
327     // If the PHI is of volatile loads and the load block has multiple
328     // successors, sinking it would remove a load of the volatile value from
329     // the path through the other successor.
330     if (isVolatile &&
331         LI->getParent()->getTerminator()->getNumSuccessors() != 1)
332       return nullptr;
333   }
334 
335   // Okay, they are all the same operation.  Create a new PHI node of the
336   // correct type, and PHI together all of the LHS's of the instructions.
337   PHINode *NewPN = PHINode::Create(FirstLI->getOperand(0)->getType(),
338                                    PN.getNumIncomingValues(),
339                                    PN.getName()+".in");
340 
341   Value *InVal = FirstLI->getOperand(0);
342   NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
343   LoadInst *NewLI = new LoadInst(NewPN, "", isVolatile, LoadAlignment);
344 
345   unsigned KnownIDs[] = {
346     LLVMContext::MD_tbaa,
347     LLVMContext::MD_range,
348     LLVMContext::MD_invariant_load,
349     LLVMContext::MD_alias_scope,
350     LLVMContext::MD_noalias,
351     LLVMContext::MD_nonnull,
352     LLVMContext::MD_align,
353     LLVMContext::MD_dereferenceable,
354     LLVMContext::MD_dereferenceable_or_null,
355   };
356 
357   for (unsigned ID : KnownIDs)
358     NewLI->setMetadata(ID, FirstLI->getMetadata(ID));
359 
360   // Add all operands to the new PHI and combine TBAA metadata.
361   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
362     LoadInst *LI = cast<LoadInst>(PN.getIncomingValue(i));
363     combineMetadata(NewLI, LI, KnownIDs);
364     Value *NewInVal = LI->getOperand(0);
365     if (NewInVal != InVal)
366       InVal = nullptr;
367     NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
368   }
369 
370   if (InVal) {
371     // The new PHI unions all of the same values together.  This is really
372     // common, so we handle it intelligently here for compile-time speed.
373     NewLI->setOperand(0, InVal);
374     delete NewPN;
375   } else {
376     InsertNewInstBefore(NewPN, PN);
377   }
378 
379   // If this was a volatile load that we are merging, make sure to loop through
380   // and mark all the input loads as non-volatile.  If we don't do this, we will
381   // insert a new volatile load and the old ones will not be deletable.
382   if (isVolatile)
383     for (Value *IncValue : PN.incoming_values())
384       cast<LoadInst>(IncValue)->setVolatile(false);
385 
386   NewLI->setDebugLoc(FirstLI->getDebugLoc());
387   return NewLI;
388 }
389 
390 /// TODO: This function could handle other cast types, but then it might
391 /// require special-casing a cast from the 'i1' type. See the comment in
392 /// FoldPHIArgOpIntoPHI() about pessimizing illegal integer types.
FoldPHIArgZextsIntoPHI(PHINode & Phi)393 Instruction *InstCombiner::FoldPHIArgZextsIntoPHI(PHINode &Phi) {
394   // We cannot create a new instruction after the PHI if the terminator is an
395   // EHPad because there is no valid insertion point.
396   if (TerminatorInst *TI = Phi.getParent()->getTerminator())
397     if (TI->isEHPad())
398       return nullptr;
399 
400   // Early exit for the common case of a phi with two operands. These are
401   // handled elsewhere. See the comment below where we check the count of zexts
402   // and constants for more details.
403   unsigned NumIncomingValues = Phi.getNumIncomingValues();
404   if (NumIncomingValues < 3)
405     return nullptr;
406 
407   // Find the narrower type specified by the first zext.
408   Type *NarrowType = nullptr;
409   for (Value *V : Phi.incoming_values()) {
410     if (auto *Zext = dyn_cast<ZExtInst>(V)) {
411       NarrowType = Zext->getSrcTy();
412       break;
413     }
414   }
415   if (!NarrowType)
416     return nullptr;
417 
418   // Walk the phi operands checking that we only have zexts or constants that
419   // we can shrink for free. Store the new operands for the new phi.
420   SmallVector<Value *, 4> NewIncoming;
421   unsigned NumZexts = 0;
422   unsigned NumConsts = 0;
423   for (Value *V : Phi.incoming_values()) {
424     if (auto *Zext = dyn_cast<ZExtInst>(V)) {
425       // All zexts must be identical and have one use.
426       if (Zext->getSrcTy() != NarrowType || !Zext->hasOneUse())
427         return nullptr;
428       NewIncoming.push_back(Zext->getOperand(0));
429       NumZexts++;
430     } else if (auto *C = dyn_cast<Constant>(V)) {
431       // Make sure that constants can fit in the new type.
432       Constant *Trunc = ConstantExpr::getTrunc(C, NarrowType);
433       if (ConstantExpr::getZExt(Trunc, C->getType()) != C)
434         return nullptr;
435       NewIncoming.push_back(Trunc);
436       NumConsts++;
437     } else {
438       // If it's not a cast or a constant, bail out.
439       return nullptr;
440     }
441   }
442 
443   // The more common cases of a phi with no constant operands or just one
444   // variable operand are handled by FoldPHIArgOpIntoPHI() and FoldOpIntoPhi()
445   // respectively. FoldOpIntoPhi() wants to do the opposite transform that is
446   // performed here. It tries to replicate a cast in the phi operand's basic
447   // block to expose other folding opportunities. Thus, InstCombine will
448   // infinite loop without this check.
449   if (NumConsts == 0 || NumZexts < 2)
450     return nullptr;
451 
452   // All incoming values are zexts or constants that are safe to truncate.
453   // Create a new phi node of the narrow type, phi together all of the new
454   // operands, and zext the result back to the original type.
455   PHINode *NewPhi = PHINode::Create(NarrowType, NumIncomingValues,
456                                     Phi.getName() + ".shrunk");
457   for (unsigned i = 0; i != NumIncomingValues; ++i)
458     NewPhi->addIncoming(NewIncoming[i], Phi.getIncomingBlock(i));
459 
460   InsertNewInstBefore(NewPhi, Phi);
461   return CastInst::CreateZExtOrBitCast(NewPhi, Phi.getType());
462 }
463 
464 /// If all operands to a PHI node are the same "unary" operator and they all are
465 /// only used by the PHI, PHI together their inputs, and do the operation once,
466 /// to the result of the PHI.
FoldPHIArgOpIntoPHI(PHINode & PN)467 Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
468   // We cannot create a new instruction after the PHI if the terminator is an
469   // EHPad because there is no valid insertion point.
470   if (TerminatorInst *TI = PN.getParent()->getTerminator())
471     if (TI->isEHPad())
472       return nullptr;
473 
474   Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
475 
476   if (isa<GetElementPtrInst>(FirstInst))
477     return FoldPHIArgGEPIntoPHI(PN);
478   if (isa<LoadInst>(FirstInst))
479     return FoldPHIArgLoadIntoPHI(PN);
480 
481   // Scan the instruction, looking for input operations that can be folded away.
482   // If all input operands to the phi are the same instruction (e.g. a cast from
483   // the same type or "+42") we can pull the operation through the PHI, reducing
484   // code size and simplifying code.
485   Constant *ConstantOp = nullptr;
486   Type *CastSrcTy = nullptr;
487 
488   if (isa<CastInst>(FirstInst)) {
489     CastSrcTy = FirstInst->getOperand(0)->getType();
490 
491     // Be careful about transforming integer PHIs.  We don't want to pessimize
492     // the code by turning an i32 into an i1293.
493     if (PN.getType()->isIntegerTy() && CastSrcTy->isIntegerTy()) {
494       if (!ShouldChangeType(PN.getType(), CastSrcTy))
495         return nullptr;
496     }
497   } else if (isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst)) {
498     // Can fold binop, compare or shift here if the RHS is a constant,
499     // otherwise call FoldPHIArgBinOpIntoPHI.
500     ConstantOp = dyn_cast<Constant>(FirstInst->getOperand(1));
501     if (!ConstantOp)
502       return FoldPHIArgBinOpIntoPHI(PN);
503   } else {
504     return nullptr;  // Cannot fold this operation.
505   }
506 
507   // Check to see if all arguments are the same operation.
508   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
509     Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
510     if (!I || !I->hasOneUse() || !I->isSameOperationAs(FirstInst))
511       return nullptr;
512     if (CastSrcTy) {
513       if (I->getOperand(0)->getType() != CastSrcTy)
514         return nullptr;  // Cast operation must match.
515     } else if (I->getOperand(1) != ConstantOp) {
516       return nullptr;
517     }
518   }
519 
520   // Okay, they are all the same operation.  Create a new PHI node of the
521   // correct type, and PHI together all of the LHS's of the instructions.
522   PHINode *NewPN = PHINode::Create(FirstInst->getOperand(0)->getType(),
523                                    PN.getNumIncomingValues(),
524                                    PN.getName()+".in");
525 
526   Value *InVal = FirstInst->getOperand(0);
527   NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
528 
529   // Add all operands to the new PHI.
530   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
531     Value *NewInVal = cast<Instruction>(PN.getIncomingValue(i))->getOperand(0);
532     if (NewInVal != InVal)
533       InVal = nullptr;
534     NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
535   }
536 
537   Value *PhiVal;
538   if (InVal) {
539     // The new PHI unions all of the same values together.  This is really
540     // common, so we handle it intelligently here for compile-time speed.
541     PhiVal = InVal;
542     delete NewPN;
543   } else {
544     InsertNewInstBefore(NewPN, PN);
545     PhiVal = NewPN;
546   }
547 
548   // Insert and return the new operation.
549   if (CastInst *FirstCI = dyn_cast<CastInst>(FirstInst)) {
550     CastInst *NewCI = CastInst::Create(FirstCI->getOpcode(), PhiVal,
551                                        PN.getType());
552     NewCI->setDebugLoc(FirstInst->getDebugLoc());
553     return NewCI;
554   }
555 
556   if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(FirstInst)) {
557     BinOp = BinaryOperator::Create(BinOp->getOpcode(), PhiVal, ConstantOp);
558     BinOp->copyIRFlags(PN.getIncomingValue(0));
559 
560     for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i)
561       BinOp->andIRFlags(PN.getIncomingValue(i));
562 
563     BinOp->setDebugLoc(FirstInst->getDebugLoc());
564     return BinOp;
565   }
566 
567   CmpInst *CIOp = cast<CmpInst>(FirstInst);
568   CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
569                                    PhiVal, ConstantOp);
570   NewCI->setDebugLoc(FirstInst->getDebugLoc());
571   return NewCI;
572 }
573 
574 /// Return true if this PHI node is only used by a PHI node cycle that is dead.
DeadPHICycle(PHINode * PN,SmallPtrSetImpl<PHINode * > & PotentiallyDeadPHIs)575 static bool DeadPHICycle(PHINode *PN,
576                          SmallPtrSetImpl<PHINode*> &PotentiallyDeadPHIs) {
577   if (PN->use_empty()) return true;
578   if (!PN->hasOneUse()) return false;
579 
580   // Remember this node, and if we find the cycle, return.
581   if (!PotentiallyDeadPHIs.insert(PN).second)
582     return true;
583 
584   // Don't scan crazily complex things.
585   if (PotentiallyDeadPHIs.size() == 16)
586     return false;
587 
588   if (PHINode *PU = dyn_cast<PHINode>(PN->user_back()))
589     return DeadPHICycle(PU, PotentiallyDeadPHIs);
590 
591   return false;
592 }
593 
594 /// Return true if this phi node is always equal to NonPhiInVal.
595 /// This happens with mutually cyclic phi nodes like:
596 ///   z = some value; x = phi (y, z); y = phi (x, z)
PHIsEqualValue(PHINode * PN,Value * NonPhiInVal,SmallPtrSetImpl<PHINode * > & ValueEqualPHIs)597 static bool PHIsEqualValue(PHINode *PN, Value *NonPhiInVal,
598                            SmallPtrSetImpl<PHINode*> &ValueEqualPHIs) {
599   // See if we already saw this PHI node.
600   if (!ValueEqualPHIs.insert(PN).second)
601     return true;
602 
603   // Don't scan crazily complex things.
604   if (ValueEqualPHIs.size() == 16)
605     return false;
606 
607   // Scan the operands to see if they are either phi nodes or are equal to
608   // the value.
609   for (Value *Op : PN->incoming_values()) {
610     if (PHINode *OpPN = dyn_cast<PHINode>(Op)) {
611       if (!PHIsEqualValue(OpPN, NonPhiInVal, ValueEqualPHIs))
612         return false;
613     } else if (Op != NonPhiInVal)
614       return false;
615   }
616 
617   return true;
618 }
619 
620 /// Return an existing non-zero constant if this phi node has one, otherwise
621 /// return constant 1.
GetAnyNonZeroConstInt(PHINode & PN)622 static ConstantInt *GetAnyNonZeroConstInt(PHINode &PN) {
623   assert(isa<IntegerType>(PN.getType()) && "Expect only intger type phi");
624   for (Value *V : PN.operands())
625     if (auto *ConstVA = dyn_cast<ConstantInt>(V))
626       if (!ConstVA->isZeroValue())
627         return ConstVA;
628   return ConstantInt::get(cast<IntegerType>(PN.getType()), 1);
629 }
630 
631 namespace {
632 struct PHIUsageRecord {
633   unsigned PHIId;     // The ID # of the PHI (something determinstic to sort on)
634   unsigned Shift;     // The amount shifted.
635   Instruction *Inst;  // The trunc instruction.
636 
PHIUsageRecord__anona873ded50111::PHIUsageRecord637   PHIUsageRecord(unsigned pn, unsigned Sh, Instruction *User)
638     : PHIId(pn), Shift(Sh), Inst(User) {}
639 
operator <__anona873ded50111::PHIUsageRecord640   bool operator<(const PHIUsageRecord &RHS) const {
641     if (PHIId < RHS.PHIId) return true;
642     if (PHIId > RHS.PHIId) return false;
643     if (Shift < RHS.Shift) return true;
644     if (Shift > RHS.Shift) return false;
645     return Inst->getType()->getPrimitiveSizeInBits() <
646            RHS.Inst->getType()->getPrimitiveSizeInBits();
647   }
648 };
649 
650 struct LoweredPHIRecord {
651   PHINode *PN;        // The PHI that was lowered.
652   unsigned Shift;     // The amount shifted.
653   unsigned Width;     // The width extracted.
654 
LoweredPHIRecord__anona873ded50111::LoweredPHIRecord655   LoweredPHIRecord(PHINode *pn, unsigned Sh, Type *Ty)
656     : PN(pn), Shift(Sh), Width(Ty->getPrimitiveSizeInBits()) {}
657 
658   // Ctor form used by DenseMap.
LoweredPHIRecord__anona873ded50111::LoweredPHIRecord659   LoweredPHIRecord(PHINode *pn, unsigned Sh)
660     : PN(pn), Shift(Sh), Width(0) {}
661 };
662 }
663 
664 namespace llvm {
665   template<>
666   struct DenseMapInfo<LoweredPHIRecord> {
getEmptyKeyllvm::DenseMapInfo667     static inline LoweredPHIRecord getEmptyKey() {
668       return LoweredPHIRecord(nullptr, 0);
669     }
getTombstoneKeyllvm::DenseMapInfo670     static inline LoweredPHIRecord getTombstoneKey() {
671       return LoweredPHIRecord(nullptr, 1);
672     }
getHashValuellvm::DenseMapInfo673     static unsigned getHashValue(const LoweredPHIRecord &Val) {
674       return DenseMapInfo<PHINode*>::getHashValue(Val.PN) ^ (Val.Shift>>3) ^
675              (Val.Width>>3);
676     }
isEqualllvm::DenseMapInfo677     static bool isEqual(const LoweredPHIRecord &LHS,
678                         const LoweredPHIRecord &RHS) {
679       return LHS.PN == RHS.PN && LHS.Shift == RHS.Shift &&
680              LHS.Width == RHS.Width;
681     }
682   };
683 }
684 
685 
686 /// This is an integer PHI and we know that it has an illegal type: see if it is
687 /// only used by trunc or trunc(lshr) operations. If so, we split the PHI into
688 /// the various pieces being extracted. This sort of thing is introduced when
689 /// SROA promotes an aggregate to large integer values.
690 ///
691 /// TODO: The user of the trunc may be an bitcast to float/double/vector or an
692 /// inttoptr.  We should produce new PHIs in the right type.
693 ///
SliceUpIllegalIntegerPHI(PHINode & FirstPhi)694 Instruction *InstCombiner::SliceUpIllegalIntegerPHI(PHINode &FirstPhi) {
695   // PHIUsers - Keep track of all of the truncated values extracted from a set
696   // of PHIs, along with their offset.  These are the things we want to rewrite.
697   SmallVector<PHIUsageRecord, 16> PHIUsers;
698 
699   // PHIs are often mutually cyclic, so we keep track of a whole set of PHI
700   // nodes which are extracted from. PHIsToSlice is a set we use to avoid
701   // revisiting PHIs, PHIsInspected is a ordered list of PHIs that we need to
702   // check the uses of (to ensure they are all extracts).
703   SmallVector<PHINode*, 8> PHIsToSlice;
704   SmallPtrSet<PHINode*, 8> PHIsInspected;
705 
706   PHIsToSlice.push_back(&FirstPhi);
707   PHIsInspected.insert(&FirstPhi);
708 
709   for (unsigned PHIId = 0; PHIId != PHIsToSlice.size(); ++PHIId) {
710     PHINode *PN = PHIsToSlice[PHIId];
711 
712     // Scan the input list of the PHI.  If any input is an invoke, and if the
713     // input is defined in the predecessor, then we won't be split the critical
714     // edge which is required to insert a truncate.  Because of this, we have to
715     // bail out.
716     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
717       InvokeInst *II = dyn_cast<InvokeInst>(PN->getIncomingValue(i));
718       if (!II) continue;
719       if (II->getParent() != PN->getIncomingBlock(i))
720         continue;
721 
722       // If we have a phi, and if it's directly in the predecessor, then we have
723       // a critical edge where we need to put the truncate.  Since we can't
724       // split the edge in instcombine, we have to bail out.
725       return nullptr;
726     }
727 
728     for (User *U : PN->users()) {
729       Instruction *UserI = cast<Instruction>(U);
730 
731       // If the user is a PHI, inspect its uses recursively.
732       if (PHINode *UserPN = dyn_cast<PHINode>(UserI)) {
733         if (PHIsInspected.insert(UserPN).second)
734           PHIsToSlice.push_back(UserPN);
735         continue;
736       }
737 
738       // Truncates are always ok.
739       if (isa<TruncInst>(UserI)) {
740         PHIUsers.push_back(PHIUsageRecord(PHIId, 0, UserI));
741         continue;
742       }
743 
744       // Otherwise it must be a lshr which can only be used by one trunc.
745       if (UserI->getOpcode() != Instruction::LShr ||
746           !UserI->hasOneUse() || !isa<TruncInst>(UserI->user_back()) ||
747           !isa<ConstantInt>(UserI->getOperand(1)))
748         return nullptr;
749 
750       unsigned Shift = cast<ConstantInt>(UserI->getOperand(1))->getZExtValue();
751       PHIUsers.push_back(PHIUsageRecord(PHIId, Shift, UserI->user_back()));
752     }
753   }
754 
755   // If we have no users, they must be all self uses, just nuke the PHI.
756   if (PHIUsers.empty())
757     return replaceInstUsesWith(FirstPhi, UndefValue::get(FirstPhi.getType()));
758 
759   // If this phi node is transformable, create new PHIs for all the pieces
760   // extracted out of it.  First, sort the users by their offset and size.
761   array_pod_sort(PHIUsers.begin(), PHIUsers.end());
762 
763   DEBUG(dbgs() << "SLICING UP PHI: " << FirstPhi << '\n';
764         for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
765           dbgs() << "AND USER PHI #" << i << ": " << *PHIsToSlice[i] << '\n';
766     );
767 
768   // PredValues - This is a temporary used when rewriting PHI nodes.  It is
769   // hoisted out here to avoid construction/destruction thrashing.
770   DenseMap<BasicBlock*, Value*> PredValues;
771 
772   // ExtractedVals - Each new PHI we introduce is saved here so we don't
773   // introduce redundant PHIs.
774   DenseMap<LoweredPHIRecord, PHINode*> ExtractedVals;
775 
776   for (unsigned UserI = 0, UserE = PHIUsers.size(); UserI != UserE; ++UserI) {
777     unsigned PHIId = PHIUsers[UserI].PHIId;
778     PHINode *PN = PHIsToSlice[PHIId];
779     unsigned Offset = PHIUsers[UserI].Shift;
780     Type *Ty = PHIUsers[UserI].Inst->getType();
781 
782     PHINode *EltPHI;
783 
784     // If we've already lowered a user like this, reuse the previously lowered
785     // value.
786     if ((EltPHI = ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)]) == nullptr) {
787 
788       // Otherwise, Create the new PHI node for this user.
789       EltPHI = PHINode::Create(Ty, PN->getNumIncomingValues(),
790                                PN->getName()+".off"+Twine(Offset), PN);
791       assert(EltPHI->getType() != PN->getType() &&
792              "Truncate didn't shrink phi?");
793 
794       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
795         BasicBlock *Pred = PN->getIncomingBlock(i);
796         Value *&PredVal = PredValues[Pred];
797 
798         // If we already have a value for this predecessor, reuse it.
799         if (PredVal) {
800           EltPHI->addIncoming(PredVal, Pred);
801           continue;
802         }
803 
804         // Handle the PHI self-reuse case.
805         Value *InVal = PN->getIncomingValue(i);
806         if (InVal == PN) {
807           PredVal = EltPHI;
808           EltPHI->addIncoming(PredVal, Pred);
809           continue;
810         }
811 
812         if (PHINode *InPHI = dyn_cast<PHINode>(PN)) {
813           // If the incoming value was a PHI, and if it was one of the PHIs we
814           // already rewrote it, just use the lowered value.
815           if (Value *Res = ExtractedVals[LoweredPHIRecord(InPHI, Offset, Ty)]) {
816             PredVal = Res;
817             EltPHI->addIncoming(PredVal, Pred);
818             continue;
819           }
820         }
821 
822         // Otherwise, do an extract in the predecessor.
823         Builder->SetInsertPoint(Pred->getTerminator());
824         Value *Res = InVal;
825         if (Offset)
826           Res = Builder->CreateLShr(Res, ConstantInt::get(InVal->getType(),
827                                                           Offset), "extract");
828         Res = Builder->CreateTrunc(Res, Ty, "extract.t");
829         PredVal = Res;
830         EltPHI->addIncoming(Res, Pred);
831 
832         // If the incoming value was a PHI, and if it was one of the PHIs we are
833         // rewriting, we will ultimately delete the code we inserted.  This
834         // means we need to revisit that PHI to make sure we extract out the
835         // needed piece.
836         if (PHINode *OldInVal = dyn_cast<PHINode>(PN->getIncomingValue(i)))
837           if (PHIsInspected.count(OldInVal)) {
838             unsigned RefPHIId = std::find(PHIsToSlice.begin(),PHIsToSlice.end(),
839                                           OldInVal)-PHIsToSlice.begin();
840             PHIUsers.push_back(PHIUsageRecord(RefPHIId, Offset,
841                                               cast<Instruction>(Res)));
842             ++UserE;
843           }
844       }
845       PredValues.clear();
846 
847       DEBUG(dbgs() << "  Made element PHI for offset " << Offset << ": "
848                    << *EltPHI << '\n');
849       ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)] = EltPHI;
850     }
851 
852     // Replace the use of this piece with the PHI node.
853     replaceInstUsesWith(*PHIUsers[UserI].Inst, EltPHI);
854   }
855 
856   // Replace all the remaining uses of the PHI nodes (self uses and the lshrs)
857   // with undefs.
858   Value *Undef = UndefValue::get(FirstPhi.getType());
859   for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
860     replaceInstUsesWith(*PHIsToSlice[i], Undef);
861   return replaceInstUsesWith(FirstPhi, Undef);
862 }
863 
864 // PHINode simplification
865 //
visitPHINode(PHINode & PN)866 Instruction *InstCombiner::visitPHINode(PHINode &PN) {
867   if (Value *V = SimplifyInstruction(&PN, DL, TLI, DT, AC))
868     return replaceInstUsesWith(PN, V);
869 
870   if (Instruction *Result = FoldPHIArgZextsIntoPHI(PN))
871     return Result;
872 
873   // If all PHI operands are the same operation, pull them through the PHI,
874   // reducing code size.
875   if (isa<Instruction>(PN.getIncomingValue(0)) &&
876       isa<Instruction>(PN.getIncomingValue(1)) &&
877       cast<Instruction>(PN.getIncomingValue(0))->getOpcode() ==
878       cast<Instruction>(PN.getIncomingValue(1))->getOpcode() &&
879       // FIXME: The hasOneUse check will fail for PHIs that use the value more
880       // than themselves more than once.
881       PN.getIncomingValue(0)->hasOneUse())
882     if (Instruction *Result = FoldPHIArgOpIntoPHI(PN))
883       return Result;
884 
885   // If this is a trivial cycle in the PHI node graph, remove it.  Basically, if
886   // this PHI only has a single use (a PHI), and if that PHI only has one use (a
887   // PHI)... break the cycle.
888   if (PN.hasOneUse()) {
889     Instruction *PHIUser = cast<Instruction>(PN.user_back());
890     if (PHINode *PU = dyn_cast<PHINode>(PHIUser)) {
891       SmallPtrSet<PHINode*, 16> PotentiallyDeadPHIs;
892       PotentiallyDeadPHIs.insert(&PN);
893       if (DeadPHICycle(PU, PotentiallyDeadPHIs))
894         return replaceInstUsesWith(PN, UndefValue::get(PN.getType()));
895     }
896 
897     // If this phi has a single use, and if that use just computes a value for
898     // the next iteration of a loop, delete the phi.  This occurs with unused
899     // induction variables, e.g. "for (int j = 0; ; ++j);".  Detecting this
900     // common case here is good because the only other things that catch this
901     // are induction variable analysis (sometimes) and ADCE, which is only run
902     // late.
903     if (PHIUser->hasOneUse() &&
904         (isa<BinaryOperator>(PHIUser) || isa<GetElementPtrInst>(PHIUser)) &&
905         PHIUser->user_back() == &PN) {
906       return replaceInstUsesWith(PN, UndefValue::get(PN.getType()));
907     }
908     // When a PHI is used only to be compared with zero, it is safe to replace
909     // an incoming value proved as known nonzero with any non-zero constant.
910     // For example, in the code below, the incoming value %v can be replaced
911     // with any non-zero constant based on the fact that the PHI is only used to
912     // be compared with zero and %v is a known non-zero value:
913     // %v = select %cond, 1, 2
914     // %p = phi [%v, BB] ...
915     //      icmp eq, %p, 0
916     auto *CmpInst = dyn_cast<ICmpInst>(PHIUser);
917     // FIXME: To be simple, handle only integer type for now.
918     if (CmpInst && isa<IntegerType>(PN.getType()) && CmpInst->isEquality() &&
919         match(CmpInst->getOperand(1), m_Zero())) {
920       ConstantInt *NonZeroConst = nullptr;
921       for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
922         Instruction *CtxI = PN.getIncomingBlock(i)->getTerminator();
923         Value *VA = PN.getIncomingValue(i);
924         if (isKnownNonZero(VA, DL, 0, AC, CtxI, DT)) {
925           if (!NonZeroConst)
926             NonZeroConst = GetAnyNonZeroConstInt(PN);
927           PN.setIncomingValue(i, NonZeroConst);
928         }
929       }
930     }
931   }
932 
933   // We sometimes end up with phi cycles that non-obviously end up being the
934   // same value, for example:
935   //   z = some value; x = phi (y, z); y = phi (x, z)
936   // where the phi nodes don't necessarily need to be in the same block.  Do a
937   // quick check to see if the PHI node only contains a single non-phi value, if
938   // so, scan to see if the phi cycle is actually equal to that value.
939   {
940     unsigned InValNo = 0, NumIncomingVals = PN.getNumIncomingValues();
941     // Scan for the first non-phi operand.
942     while (InValNo != NumIncomingVals &&
943            isa<PHINode>(PN.getIncomingValue(InValNo)))
944       ++InValNo;
945 
946     if (InValNo != NumIncomingVals) {
947       Value *NonPhiInVal = PN.getIncomingValue(InValNo);
948 
949       // Scan the rest of the operands to see if there are any conflicts, if so
950       // there is no need to recursively scan other phis.
951       for (++InValNo; InValNo != NumIncomingVals; ++InValNo) {
952         Value *OpVal = PN.getIncomingValue(InValNo);
953         if (OpVal != NonPhiInVal && !isa<PHINode>(OpVal))
954           break;
955       }
956 
957       // If we scanned over all operands, then we have one unique value plus
958       // phi values.  Scan PHI nodes to see if they all merge in each other or
959       // the value.
960       if (InValNo == NumIncomingVals) {
961         SmallPtrSet<PHINode*, 16> ValueEqualPHIs;
962         if (PHIsEqualValue(&PN, NonPhiInVal, ValueEqualPHIs))
963           return replaceInstUsesWith(PN, NonPhiInVal);
964       }
965     }
966   }
967 
968   // If there are multiple PHIs, sort their operands so that they all list
969   // the blocks in the same order. This will help identical PHIs be eliminated
970   // by other passes. Other passes shouldn't depend on this for correctness
971   // however.
972   PHINode *FirstPN = cast<PHINode>(PN.getParent()->begin());
973   if (&PN != FirstPN)
974     for (unsigned i = 0, e = FirstPN->getNumIncomingValues(); i != e; ++i) {
975       BasicBlock *BBA = PN.getIncomingBlock(i);
976       BasicBlock *BBB = FirstPN->getIncomingBlock(i);
977       if (BBA != BBB) {
978         Value *VA = PN.getIncomingValue(i);
979         unsigned j = PN.getBasicBlockIndex(BBB);
980         Value *VB = PN.getIncomingValue(j);
981         PN.setIncomingBlock(i, BBB);
982         PN.setIncomingValue(i, VB);
983         PN.setIncomingBlock(j, BBA);
984         PN.setIncomingValue(j, VA);
985         // NOTE: Instcombine normally would want us to "return &PN" if we
986         // modified any of the operands of an instruction.  However, since we
987         // aren't adding or removing uses (just rearranging them) we don't do
988         // this in this case.
989       }
990     }
991 
992   // If this is an integer PHI and we know that it has an illegal type, see if
993   // it is only used by trunc or trunc(lshr) operations.  If so, we split the
994   // PHI into the various pieces being extracted.  This sort of thing is
995   // introduced when SROA promotes an aggregate to a single large integer type.
996   if (PN.getType()->isIntegerTy() &&
997       !DL.isLegalInteger(PN.getType()->getPrimitiveSizeInBits()))
998     if (Instruction *Res = SliceUpIllegalIntegerPHI(PN))
999       return Res;
1000 
1001   return nullptr;
1002 }
1003