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/Transforms/Utils/Local.h"
19 using namespace llvm;
20
21 #define DEBUG_TYPE "instcombine"
22
23 /// If we have something like phi [add (a,b), add(a,c)] and if a/b/c and the
24 /// adds all have a single use, turn this into a phi and a single binop.
FoldPHIArgBinOpIntoPHI(PHINode & PN)25 Instruction *InstCombiner::FoldPHIArgBinOpIntoPHI(PHINode &PN) {
26 Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
27 assert(isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst));
28 unsigned Opc = FirstInst->getOpcode();
29 Value *LHSVal = FirstInst->getOperand(0);
30 Value *RHSVal = FirstInst->getOperand(1);
31
32 Type *LHSType = LHSVal->getType();
33 Type *RHSType = RHSVal->getType();
34
35 bool isNUW = false, isNSW = false, isExact = false;
36 if (OverflowingBinaryOperator *BO =
37 dyn_cast<OverflowingBinaryOperator>(FirstInst)) {
38 isNUW = BO->hasNoUnsignedWrap();
39 isNSW = BO->hasNoSignedWrap();
40 } else if (PossiblyExactOperator *PEO =
41 dyn_cast<PossiblyExactOperator>(FirstInst))
42 isExact = PEO->isExact();
43
44 // Scan to see if all operands are the same opcode, and all have one use.
45 for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
46 Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
47 if (!I || I->getOpcode() != Opc || !I->hasOneUse() ||
48 // Verify type of the LHS matches so we don't fold cmp's of different
49 // types.
50 I->getOperand(0)->getType() != LHSType ||
51 I->getOperand(1)->getType() != RHSType)
52 return nullptr;
53
54 // If they are CmpInst instructions, check their predicates
55 if (CmpInst *CI = dyn_cast<CmpInst>(I))
56 if (CI->getPredicate() != cast<CmpInst>(FirstInst)->getPredicate())
57 return nullptr;
58
59 if (isNUW)
60 isNUW = cast<OverflowingBinaryOperator>(I)->hasNoUnsignedWrap();
61 if (isNSW)
62 isNSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
63 if (isExact)
64 isExact = cast<PossiblyExactOperator>(I)->isExact();
65
66 // Keep track of which operand needs a phi node.
67 if (I->getOperand(0) != LHSVal) LHSVal = nullptr;
68 if (I->getOperand(1) != RHSVal) RHSVal = nullptr;
69 }
70
71 // If both LHS and RHS would need a PHI, don't do this transformation,
72 // because it would increase the number of PHIs entering the block,
73 // which leads to higher register pressure. This is especially
74 // bad when the PHIs are in the header of a loop.
75 if (!LHSVal && !RHSVal)
76 return nullptr;
77
78 // Otherwise, this is safe to transform!
79
80 Value *InLHS = FirstInst->getOperand(0);
81 Value *InRHS = FirstInst->getOperand(1);
82 PHINode *NewLHS = nullptr, *NewRHS = nullptr;
83 if (!LHSVal) {
84 NewLHS = PHINode::Create(LHSType, PN.getNumIncomingValues(),
85 FirstInst->getOperand(0)->getName() + ".pn");
86 NewLHS->addIncoming(InLHS, PN.getIncomingBlock(0));
87 InsertNewInstBefore(NewLHS, PN);
88 LHSVal = NewLHS;
89 }
90
91 if (!RHSVal) {
92 NewRHS = PHINode::Create(RHSType, PN.getNumIncomingValues(),
93 FirstInst->getOperand(1)->getName() + ".pn");
94 NewRHS->addIncoming(InRHS, PN.getIncomingBlock(0));
95 InsertNewInstBefore(NewRHS, PN);
96 RHSVal = NewRHS;
97 }
98
99 // Add all operands to the new PHIs.
100 if (NewLHS || NewRHS) {
101 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
102 Instruction *InInst = cast<Instruction>(PN.getIncomingValue(i));
103 if (NewLHS) {
104 Value *NewInLHS = InInst->getOperand(0);
105 NewLHS->addIncoming(NewInLHS, PN.getIncomingBlock(i));
106 }
107 if (NewRHS) {
108 Value *NewInRHS = InInst->getOperand(1);
109 NewRHS->addIncoming(NewInRHS, PN.getIncomingBlock(i));
110 }
111 }
112 }
113
114 if (CmpInst *CIOp = dyn_cast<CmpInst>(FirstInst)) {
115 CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
116 LHSVal, RHSVal);
117 NewCI->setDebugLoc(FirstInst->getDebugLoc());
118 return NewCI;
119 }
120
121 BinaryOperator *BinOp = cast<BinaryOperator>(FirstInst);
122 BinaryOperator *NewBinOp =
123 BinaryOperator::Create(BinOp->getOpcode(), LHSVal, RHSVal);
124 if (isNUW) NewBinOp->setHasNoUnsignedWrap();
125 if (isNSW) NewBinOp->setHasNoSignedWrap();
126 if (isExact) NewBinOp->setIsExact();
127 NewBinOp->setDebugLoc(FirstInst->getDebugLoc());
128 return NewBinOp;
129 }
130
FoldPHIArgGEPIntoPHI(PHINode & PN)131 Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
132 GetElementPtrInst *FirstInst =cast<GetElementPtrInst>(PN.getIncomingValue(0));
133
134 SmallVector<Value*, 16> FixedOperands(FirstInst->op_begin(),
135 FirstInst->op_end());
136 // This is true if all GEP bases are allocas and if all indices into them are
137 // constants.
138 bool AllBasePointersAreAllocas = true;
139
140 // We don't want to replace this phi if the replacement would require
141 // more than one phi, which leads to higher register pressure. This is
142 // especially bad when the PHIs are in the header of a loop.
143 bool NeededPhi = false;
144
145 bool AllInBounds = true;
146
147 // Scan to see if all operands are the same opcode, and all have one use.
148 for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
149 GetElementPtrInst *GEP= dyn_cast<GetElementPtrInst>(PN.getIncomingValue(i));
150 if (!GEP || !GEP->hasOneUse() || GEP->getType() != FirstInst->getType() ||
151 GEP->getNumOperands() != FirstInst->getNumOperands())
152 return nullptr;
153
154 AllInBounds &= GEP->isInBounds();
155
156 // Keep track of whether or not all GEPs are of alloca pointers.
157 if (AllBasePointersAreAllocas &&
158 (!isa<AllocaInst>(GEP->getOperand(0)) ||
159 !GEP->hasAllConstantIndices()))
160 AllBasePointersAreAllocas = false;
161
162 // Compare the operand lists.
163 for (unsigned op = 0, e = FirstInst->getNumOperands(); op != e; ++op) {
164 if (FirstInst->getOperand(op) == GEP->getOperand(op))
165 continue;
166
167 // Don't merge two GEPs when two operands differ (introducing phi nodes)
168 // if one of the PHIs has a constant for the index. The index may be
169 // substantially cheaper to compute for the constants, so making it a
170 // variable index could pessimize the path. This also handles the case
171 // for struct indices, which must always be constant.
172 if (isa<ConstantInt>(FirstInst->getOperand(op)) ||
173 isa<ConstantInt>(GEP->getOperand(op)))
174 return nullptr;
175
176 if (FirstInst->getOperand(op)->getType() !=GEP->getOperand(op)->getType())
177 return nullptr;
178
179 // If we already needed a PHI for an earlier operand, and another operand
180 // also requires a PHI, we'd be introducing more PHIs than we're
181 // eliminating, which increases register pressure on entry to the PHI's
182 // block.
183 if (NeededPhi)
184 return nullptr;
185
186 FixedOperands[op] = nullptr; // Needs a PHI.
187 NeededPhi = true;
188 }
189 }
190
191 // If all of the base pointers of the PHI'd GEPs are from allocas, don't
192 // bother doing this transformation. At best, this will just save a bit of
193 // offset calculation, but all the predecessors will have to materialize the
194 // stack address into a register anyway. We'd actually rather *clone* the
195 // load up into the predecessors so that we have a load of a gep of an alloca,
196 // which can usually all be folded into the load.
197 if (AllBasePointersAreAllocas)
198 return nullptr;
199
200 // Otherwise, this is safe to transform. Insert PHI nodes for each operand
201 // that is variable.
202 SmallVector<PHINode*, 16> OperandPhis(FixedOperands.size());
203
204 bool HasAnyPHIs = false;
205 for (unsigned i = 0, e = FixedOperands.size(); i != e; ++i) {
206 if (FixedOperands[i]) continue; // operand doesn't need a phi.
207 Value *FirstOp = FirstInst->getOperand(i);
208 PHINode *NewPN = PHINode::Create(FirstOp->getType(), e,
209 FirstOp->getName()+".pn");
210 InsertNewInstBefore(NewPN, PN);
211
212 NewPN->addIncoming(FirstOp, PN.getIncomingBlock(0));
213 OperandPhis[i] = NewPN;
214 FixedOperands[i] = NewPN;
215 HasAnyPHIs = true;
216 }
217
218
219 // Add all operands to the new PHIs.
220 if (HasAnyPHIs) {
221 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
222 GetElementPtrInst *InGEP =cast<GetElementPtrInst>(PN.getIncomingValue(i));
223 BasicBlock *InBB = PN.getIncomingBlock(i);
224
225 for (unsigned op = 0, e = OperandPhis.size(); op != e; ++op)
226 if (PHINode *OpPhi = OperandPhis[op])
227 OpPhi->addIncoming(InGEP->getOperand(op), InBB);
228 }
229 }
230
231 Value *Base = FixedOperands[0];
232 GetElementPtrInst *NewGEP =
233 GetElementPtrInst::Create(FirstInst->getSourceElementType(), Base,
234 makeArrayRef(FixedOperands).slice(1));
235 if (AllInBounds) NewGEP->setIsInBounds();
236 NewGEP->setDebugLoc(FirstInst->getDebugLoc());
237 return NewGEP;
238 }
239
240
241 /// Return true if we know that it is safe to sink the load out of the block
242 /// that defines it. This means that it must be obvious the value of the load is
243 /// not changed from the point of the load to the end of the block it is in.
244 ///
245 /// Finally, it is safe, but not profitable, to sink a load targeting a
246 /// non-address-taken alloca. Doing so will cause us to not promote the alloca
247 /// to a register.
isSafeAndProfitableToSinkLoad(LoadInst * L)248 static bool isSafeAndProfitableToSinkLoad(LoadInst *L) {
249 BasicBlock::iterator BBI = L->getIterator(), E = L->getParent()->end();
250
251 for (++BBI; BBI != E; ++BBI)
252 if (BBI->mayWriteToMemory())
253 return false;
254
255 // Check for non-address taken alloca. If not address-taken already, it isn't
256 // profitable to do this xform.
257 if (AllocaInst *AI = dyn_cast<AllocaInst>(L->getOperand(0))) {
258 bool isAddressTaken = false;
259 for (User *U : AI->users()) {
260 if (isa<LoadInst>(U)) continue;
261 if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
262 // If storing TO the alloca, then the address isn't taken.
263 if (SI->getOperand(1) == AI) continue;
264 }
265 isAddressTaken = true;
266 break;
267 }
268
269 if (!isAddressTaken && AI->isStaticAlloca())
270 return false;
271 }
272
273 // If this load is a load from a GEP with a constant offset from an alloca,
274 // then we don't want to sink it. In its present form, it will be
275 // load [constant stack offset]. Sinking it will cause us to have to
276 // materialize the stack addresses in each predecessor in a register only to
277 // do a shared load from register in the successor.
278 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(L->getOperand(0)))
279 if (AllocaInst *AI = dyn_cast<AllocaInst>(GEP->getOperand(0)))
280 if (AI->isStaticAlloca() && GEP->hasAllConstantIndices())
281 return false;
282
283 return true;
284 }
285
FoldPHIArgLoadIntoPHI(PHINode & PN)286 Instruction *InstCombiner::FoldPHIArgLoadIntoPHI(PHINode &PN) {
287 LoadInst *FirstLI = cast<LoadInst>(PN.getIncomingValue(0));
288
289 // FIXME: This is overconservative; this transform is allowed in some cases
290 // for atomic operations.
291 if (FirstLI->isAtomic())
292 return nullptr;
293
294 // When processing loads, we need to propagate two bits of information to the
295 // sunk load: whether it is volatile, and what its alignment is. We currently
296 // don't sink loads when some have their alignment specified and some don't.
297 // visitLoadInst will propagate an alignment onto the load when TD is around,
298 // and if TD isn't around, we can't handle the mixed case.
299 bool isVolatile = FirstLI->isVolatile();
300 unsigned LoadAlignment = FirstLI->getAlignment();
301 unsigned LoadAddrSpace = FirstLI->getPointerAddressSpace();
302
303 // We can't sink the load if the loaded value could be modified between the
304 // load and the PHI.
305 if (FirstLI->getParent() != PN.getIncomingBlock(0) ||
306 !isSafeAndProfitableToSinkLoad(FirstLI))
307 return nullptr;
308
309 // If the PHI is of volatile loads and the load block has multiple
310 // successors, sinking it would remove a load of the volatile value from
311 // the path through the other successor.
312 if (isVolatile &&
313 FirstLI->getParent()->getTerminator()->getNumSuccessors() != 1)
314 return nullptr;
315
316 // Check to see if all arguments are the same operation.
317 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
318 LoadInst *LI = dyn_cast<LoadInst>(PN.getIncomingValue(i));
319 if (!LI || !LI->hasOneUse())
320 return nullptr;
321
322 // We can't sink the load if the loaded value could be modified between
323 // the load and the PHI.
324 if (LI->isVolatile() != isVolatile ||
325 LI->getParent() != PN.getIncomingBlock(i) ||
326 LI->getPointerAddressSpace() != LoadAddrSpace ||
327 !isSafeAndProfitableToSinkLoad(LI))
328 return nullptr;
329
330 // If some of the loads have an alignment specified but not all of them,
331 // we can't do the transformation.
332 if ((LoadAlignment != 0) != (LI->getAlignment() != 0))
333 return nullptr;
334
335 LoadAlignment = std::min(LoadAlignment, LI->getAlignment());
336
337 // If the PHI is of volatile loads and the load block has multiple
338 // successors, sinking it would remove a load of the volatile value from
339 // the path through the other successor.
340 if (isVolatile &&
341 LI->getParent()->getTerminator()->getNumSuccessors() != 1)
342 return nullptr;
343 }
344
345 // Okay, they are all the same operation. Create a new PHI node of the
346 // correct type, and PHI together all of the LHS's of the instructions.
347 PHINode *NewPN = PHINode::Create(FirstLI->getOperand(0)->getType(),
348 PN.getNumIncomingValues(),
349 PN.getName()+".in");
350
351 Value *InVal = FirstLI->getOperand(0);
352 NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
353 LoadInst *NewLI = new LoadInst(NewPN, "", isVolatile, LoadAlignment);
354
355 unsigned KnownIDs[] = {
356 LLVMContext::MD_tbaa,
357 LLVMContext::MD_range,
358 LLVMContext::MD_invariant_load,
359 LLVMContext::MD_alias_scope,
360 LLVMContext::MD_noalias,
361 LLVMContext::MD_nonnull,
362 LLVMContext::MD_align,
363 LLVMContext::MD_dereferenceable,
364 LLVMContext::MD_dereferenceable_or_null,
365 };
366
367 for (unsigned ID : KnownIDs)
368 NewLI->setMetadata(ID, FirstLI->getMetadata(ID));
369
370 // Add all operands to the new PHI and combine TBAA metadata.
371 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
372 LoadInst *LI = cast<LoadInst>(PN.getIncomingValue(i));
373 combineMetadata(NewLI, LI, KnownIDs);
374 Value *NewInVal = LI->getOperand(0);
375 if (NewInVal != InVal)
376 InVal = nullptr;
377 NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
378 }
379
380 if (InVal) {
381 // The new PHI unions all of the same values together. This is really
382 // common, so we handle it intelligently here for compile-time speed.
383 NewLI->setOperand(0, InVal);
384 delete NewPN;
385 } else {
386 InsertNewInstBefore(NewPN, PN);
387 }
388
389 // If this was a volatile load that we are merging, make sure to loop through
390 // and mark all the input loads as non-volatile. If we don't do this, we will
391 // insert a new volatile load and the old ones will not be deletable.
392 if (isVolatile)
393 for (Value *IncValue : PN.incoming_values())
394 cast<LoadInst>(IncValue)->setVolatile(false);
395
396 NewLI->setDebugLoc(FirstLI->getDebugLoc());
397 return NewLI;
398 }
399
400 /// TODO: This function could handle other cast types, but then it might
401 /// require special-casing a cast from the 'i1' type. See the comment in
402 /// FoldPHIArgOpIntoPHI() about pessimizing illegal integer types.
FoldPHIArgZextsIntoPHI(PHINode & Phi)403 Instruction *InstCombiner::FoldPHIArgZextsIntoPHI(PHINode &Phi) {
404 // We cannot create a new instruction after the PHI if the terminator is an
405 // EHPad because there is no valid insertion point.
406 if (TerminatorInst *TI = Phi.getParent()->getTerminator())
407 if (TI->isEHPad())
408 return nullptr;
409
410 // Early exit for the common case of a phi with two operands. These are
411 // handled elsewhere. See the comment below where we check the count of zexts
412 // and constants for more details.
413 unsigned NumIncomingValues = Phi.getNumIncomingValues();
414 if (NumIncomingValues < 3)
415 return nullptr;
416
417 // Find the narrower type specified by the first zext.
418 Type *NarrowType = nullptr;
419 for (Value *V : Phi.incoming_values()) {
420 if (auto *Zext = dyn_cast<ZExtInst>(V)) {
421 NarrowType = Zext->getSrcTy();
422 break;
423 }
424 }
425 if (!NarrowType)
426 return nullptr;
427
428 // Walk the phi operands checking that we only have zexts or constants that
429 // we can shrink for free. Store the new operands for the new phi.
430 SmallVector<Value *, 4> NewIncoming;
431 unsigned NumZexts = 0;
432 unsigned NumConsts = 0;
433 for (Value *V : Phi.incoming_values()) {
434 if (auto *Zext = dyn_cast<ZExtInst>(V)) {
435 // All zexts must be identical and have one use.
436 if (Zext->getSrcTy() != NarrowType || !Zext->hasOneUse())
437 return nullptr;
438 NewIncoming.push_back(Zext->getOperand(0));
439 NumZexts++;
440 } else if (auto *C = dyn_cast<Constant>(V)) {
441 // Make sure that constants can fit in the new type.
442 Constant *Trunc = ConstantExpr::getTrunc(C, NarrowType);
443 if (ConstantExpr::getZExt(Trunc, C->getType()) != C)
444 return nullptr;
445 NewIncoming.push_back(Trunc);
446 NumConsts++;
447 } else {
448 // If it's not a cast or a constant, bail out.
449 return nullptr;
450 }
451 }
452
453 // The more common cases of a phi with no constant operands or just one
454 // variable operand are handled by FoldPHIArgOpIntoPHI() and FoldOpIntoPhi()
455 // respectively. FoldOpIntoPhi() wants to do the opposite transform that is
456 // performed here. It tries to replicate a cast in the phi operand's basic
457 // block to expose other folding opportunities. Thus, InstCombine will
458 // infinite loop without this check.
459 if (NumConsts == 0 || NumZexts < 2)
460 return nullptr;
461
462 // All incoming values are zexts or constants that are safe to truncate.
463 // Create a new phi node of the narrow type, phi together all of the new
464 // operands, and zext the result back to the original type.
465 PHINode *NewPhi = PHINode::Create(NarrowType, NumIncomingValues,
466 Phi.getName() + ".shrunk");
467 for (unsigned i = 0; i != NumIncomingValues; ++i)
468 NewPhi->addIncoming(NewIncoming[i], Phi.getIncomingBlock(i));
469
470 InsertNewInstBefore(NewPhi, Phi);
471 return CastInst::CreateZExtOrBitCast(NewPhi, Phi.getType());
472 }
473
474 /// If all operands to a PHI node are the same "unary" operator and they all are
475 /// only used by the PHI, PHI together their inputs, and do the operation once,
476 /// to the result of the PHI.
FoldPHIArgOpIntoPHI(PHINode & PN)477 Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
478 // We cannot create a new instruction after the PHI if the terminator is an
479 // EHPad because there is no valid insertion point.
480 if (TerminatorInst *TI = PN.getParent()->getTerminator())
481 if (TI->isEHPad())
482 return nullptr;
483
484 Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
485
486 if (isa<GetElementPtrInst>(FirstInst))
487 return FoldPHIArgGEPIntoPHI(PN);
488 if (isa<LoadInst>(FirstInst))
489 return FoldPHIArgLoadIntoPHI(PN);
490
491 // Scan the instruction, looking for input operations that can be folded away.
492 // If all input operands to the phi are the same instruction (e.g. a cast from
493 // the same type or "+42") we can pull the operation through the PHI, reducing
494 // code size and simplifying code.
495 Constant *ConstantOp = nullptr;
496 Type *CastSrcTy = nullptr;
497 bool isNUW = false, isNSW = false, isExact = false;
498
499 if (isa<CastInst>(FirstInst)) {
500 CastSrcTy = FirstInst->getOperand(0)->getType();
501
502 // Be careful about transforming integer PHIs. We don't want to pessimize
503 // the code by turning an i32 into an i1293.
504 if (PN.getType()->isIntegerTy() && CastSrcTy->isIntegerTy()) {
505 if (!ShouldChangeType(PN.getType(), CastSrcTy))
506 return nullptr;
507 }
508 } else if (isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst)) {
509 // Can fold binop, compare or shift here if the RHS is a constant,
510 // otherwise call FoldPHIArgBinOpIntoPHI.
511 ConstantOp = dyn_cast<Constant>(FirstInst->getOperand(1));
512 if (!ConstantOp)
513 return FoldPHIArgBinOpIntoPHI(PN);
514
515 if (OverflowingBinaryOperator *BO =
516 dyn_cast<OverflowingBinaryOperator>(FirstInst)) {
517 isNUW = BO->hasNoUnsignedWrap();
518 isNSW = BO->hasNoSignedWrap();
519 } else if (PossiblyExactOperator *PEO =
520 dyn_cast<PossiblyExactOperator>(FirstInst))
521 isExact = PEO->isExact();
522 } else {
523 return nullptr; // Cannot fold this operation.
524 }
525
526 // Check to see if all arguments are the same operation.
527 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
528 Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
529 if (!I || !I->hasOneUse() || !I->isSameOperationAs(FirstInst))
530 return nullptr;
531 if (CastSrcTy) {
532 if (I->getOperand(0)->getType() != CastSrcTy)
533 return nullptr; // Cast operation must match.
534 } else if (I->getOperand(1) != ConstantOp) {
535 return nullptr;
536 }
537
538 if (isNUW)
539 isNUW = cast<OverflowingBinaryOperator>(I)->hasNoUnsignedWrap();
540 if (isNSW)
541 isNSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
542 if (isExact)
543 isExact = cast<PossiblyExactOperator>(I)->isExact();
544 }
545
546 // Okay, they are all the same operation. Create a new PHI node of the
547 // correct type, and PHI together all of the LHS's of the instructions.
548 PHINode *NewPN = PHINode::Create(FirstInst->getOperand(0)->getType(),
549 PN.getNumIncomingValues(),
550 PN.getName()+".in");
551
552 Value *InVal = FirstInst->getOperand(0);
553 NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
554
555 // Add all operands to the new PHI.
556 for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
557 Value *NewInVal = cast<Instruction>(PN.getIncomingValue(i))->getOperand(0);
558 if (NewInVal != InVal)
559 InVal = nullptr;
560 NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
561 }
562
563 Value *PhiVal;
564 if (InVal) {
565 // The new PHI unions all of the same values together. This is really
566 // common, so we handle it intelligently here for compile-time speed.
567 PhiVal = InVal;
568 delete NewPN;
569 } else {
570 InsertNewInstBefore(NewPN, PN);
571 PhiVal = NewPN;
572 }
573
574 // Insert and return the new operation.
575 if (CastInst *FirstCI = dyn_cast<CastInst>(FirstInst)) {
576 CastInst *NewCI = CastInst::Create(FirstCI->getOpcode(), PhiVal,
577 PN.getType());
578 NewCI->setDebugLoc(FirstInst->getDebugLoc());
579 return NewCI;
580 }
581
582 if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(FirstInst)) {
583 BinOp = BinaryOperator::Create(BinOp->getOpcode(), PhiVal, ConstantOp);
584 if (isNUW) BinOp->setHasNoUnsignedWrap();
585 if (isNSW) BinOp->setHasNoSignedWrap();
586 if (isExact) BinOp->setIsExact();
587 BinOp->setDebugLoc(FirstInst->getDebugLoc());
588 return BinOp;
589 }
590
591 CmpInst *CIOp = cast<CmpInst>(FirstInst);
592 CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
593 PhiVal, ConstantOp);
594 NewCI->setDebugLoc(FirstInst->getDebugLoc());
595 return NewCI;
596 }
597
598 /// Return true if this PHI node is only used by a PHI node cycle that is dead.
DeadPHICycle(PHINode * PN,SmallPtrSetImpl<PHINode * > & PotentiallyDeadPHIs)599 static bool DeadPHICycle(PHINode *PN,
600 SmallPtrSetImpl<PHINode*> &PotentiallyDeadPHIs) {
601 if (PN->use_empty()) return true;
602 if (!PN->hasOneUse()) return false;
603
604 // Remember this node, and if we find the cycle, return.
605 if (!PotentiallyDeadPHIs.insert(PN).second)
606 return true;
607
608 // Don't scan crazily complex things.
609 if (PotentiallyDeadPHIs.size() == 16)
610 return false;
611
612 if (PHINode *PU = dyn_cast<PHINode>(PN->user_back()))
613 return DeadPHICycle(PU, PotentiallyDeadPHIs);
614
615 return false;
616 }
617
618 /// Return true if this phi node is always equal to NonPhiInVal.
619 /// This happens with mutually cyclic phi nodes like:
620 /// z = some value; x = phi (y, z); y = phi (x, z)
PHIsEqualValue(PHINode * PN,Value * NonPhiInVal,SmallPtrSetImpl<PHINode * > & ValueEqualPHIs)621 static bool PHIsEqualValue(PHINode *PN, Value *NonPhiInVal,
622 SmallPtrSetImpl<PHINode*> &ValueEqualPHIs) {
623 // See if we already saw this PHI node.
624 if (!ValueEqualPHIs.insert(PN).second)
625 return true;
626
627 // Don't scan crazily complex things.
628 if (ValueEqualPHIs.size() == 16)
629 return false;
630
631 // Scan the operands to see if they are either phi nodes or are equal to
632 // the value.
633 for (Value *Op : PN->incoming_values()) {
634 if (PHINode *OpPN = dyn_cast<PHINode>(Op)) {
635 if (!PHIsEqualValue(OpPN, NonPhiInVal, ValueEqualPHIs))
636 return false;
637 } else if (Op != NonPhiInVal)
638 return false;
639 }
640
641 return true;
642 }
643
644
645 namespace {
646 struct PHIUsageRecord {
647 unsigned PHIId; // The ID # of the PHI (something determinstic to sort on)
648 unsigned Shift; // The amount shifted.
649 Instruction *Inst; // The trunc instruction.
650
PHIUsageRecord__anonfafdb65b0111::PHIUsageRecord651 PHIUsageRecord(unsigned pn, unsigned Sh, Instruction *User)
652 : PHIId(pn), Shift(Sh), Inst(User) {}
653
operator <__anonfafdb65b0111::PHIUsageRecord654 bool operator<(const PHIUsageRecord &RHS) const {
655 if (PHIId < RHS.PHIId) return true;
656 if (PHIId > RHS.PHIId) return false;
657 if (Shift < RHS.Shift) return true;
658 if (Shift > RHS.Shift) return false;
659 return Inst->getType()->getPrimitiveSizeInBits() <
660 RHS.Inst->getType()->getPrimitiveSizeInBits();
661 }
662 };
663
664 struct LoweredPHIRecord {
665 PHINode *PN; // The PHI that was lowered.
666 unsigned Shift; // The amount shifted.
667 unsigned Width; // The width extracted.
668
LoweredPHIRecord__anonfafdb65b0111::LoweredPHIRecord669 LoweredPHIRecord(PHINode *pn, unsigned Sh, Type *Ty)
670 : PN(pn), Shift(Sh), Width(Ty->getPrimitiveSizeInBits()) {}
671
672 // Ctor form used by DenseMap.
LoweredPHIRecord__anonfafdb65b0111::LoweredPHIRecord673 LoweredPHIRecord(PHINode *pn, unsigned Sh)
674 : PN(pn), Shift(Sh), Width(0) {}
675 };
676 }
677
678 namespace llvm {
679 template<>
680 struct DenseMapInfo<LoweredPHIRecord> {
getEmptyKeyllvm::DenseMapInfo681 static inline LoweredPHIRecord getEmptyKey() {
682 return LoweredPHIRecord(nullptr, 0);
683 }
getTombstoneKeyllvm::DenseMapInfo684 static inline LoweredPHIRecord getTombstoneKey() {
685 return LoweredPHIRecord(nullptr, 1);
686 }
getHashValuellvm::DenseMapInfo687 static unsigned getHashValue(const LoweredPHIRecord &Val) {
688 return DenseMapInfo<PHINode*>::getHashValue(Val.PN) ^ (Val.Shift>>3) ^
689 (Val.Width>>3);
690 }
isEqualllvm::DenseMapInfo691 static bool isEqual(const LoweredPHIRecord &LHS,
692 const LoweredPHIRecord &RHS) {
693 return LHS.PN == RHS.PN && LHS.Shift == RHS.Shift &&
694 LHS.Width == RHS.Width;
695 }
696 };
697 }
698
699
700 /// This is an integer PHI and we know that it has an illegal type: see if it is
701 /// only used by trunc or trunc(lshr) operations. If so, we split the PHI into
702 /// the various pieces being extracted. This sort of thing is introduced when
703 /// SROA promotes an aggregate to large integer values.
704 ///
705 /// TODO: The user of the trunc may be an bitcast to float/double/vector or an
706 /// inttoptr. We should produce new PHIs in the right type.
707 ///
SliceUpIllegalIntegerPHI(PHINode & FirstPhi)708 Instruction *InstCombiner::SliceUpIllegalIntegerPHI(PHINode &FirstPhi) {
709 // PHIUsers - Keep track of all of the truncated values extracted from a set
710 // of PHIs, along with their offset. These are the things we want to rewrite.
711 SmallVector<PHIUsageRecord, 16> PHIUsers;
712
713 // PHIs are often mutually cyclic, so we keep track of a whole set of PHI
714 // nodes which are extracted from. PHIsToSlice is a set we use to avoid
715 // revisiting PHIs, PHIsInspected is a ordered list of PHIs that we need to
716 // check the uses of (to ensure they are all extracts).
717 SmallVector<PHINode*, 8> PHIsToSlice;
718 SmallPtrSet<PHINode*, 8> PHIsInspected;
719
720 PHIsToSlice.push_back(&FirstPhi);
721 PHIsInspected.insert(&FirstPhi);
722
723 for (unsigned PHIId = 0; PHIId != PHIsToSlice.size(); ++PHIId) {
724 PHINode *PN = PHIsToSlice[PHIId];
725
726 // Scan the input list of the PHI. If any input is an invoke, and if the
727 // input is defined in the predecessor, then we won't be split the critical
728 // edge which is required to insert a truncate. Because of this, we have to
729 // bail out.
730 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
731 InvokeInst *II = dyn_cast<InvokeInst>(PN->getIncomingValue(i));
732 if (!II) continue;
733 if (II->getParent() != PN->getIncomingBlock(i))
734 continue;
735
736 // If we have a phi, and if it's directly in the predecessor, then we have
737 // a critical edge where we need to put the truncate. Since we can't
738 // split the edge in instcombine, we have to bail out.
739 return nullptr;
740 }
741
742 for (User *U : PN->users()) {
743 Instruction *UserI = cast<Instruction>(U);
744
745 // If the user is a PHI, inspect its uses recursively.
746 if (PHINode *UserPN = dyn_cast<PHINode>(UserI)) {
747 if (PHIsInspected.insert(UserPN).second)
748 PHIsToSlice.push_back(UserPN);
749 continue;
750 }
751
752 // Truncates are always ok.
753 if (isa<TruncInst>(UserI)) {
754 PHIUsers.push_back(PHIUsageRecord(PHIId, 0, UserI));
755 continue;
756 }
757
758 // Otherwise it must be a lshr which can only be used by one trunc.
759 if (UserI->getOpcode() != Instruction::LShr ||
760 !UserI->hasOneUse() || !isa<TruncInst>(UserI->user_back()) ||
761 !isa<ConstantInt>(UserI->getOperand(1)))
762 return nullptr;
763
764 unsigned Shift = cast<ConstantInt>(UserI->getOperand(1))->getZExtValue();
765 PHIUsers.push_back(PHIUsageRecord(PHIId, Shift, UserI->user_back()));
766 }
767 }
768
769 // If we have no users, they must be all self uses, just nuke the PHI.
770 if (PHIUsers.empty())
771 return ReplaceInstUsesWith(FirstPhi, UndefValue::get(FirstPhi.getType()));
772
773 // If this phi node is transformable, create new PHIs for all the pieces
774 // extracted out of it. First, sort the users by their offset and size.
775 array_pod_sort(PHIUsers.begin(), PHIUsers.end());
776
777 DEBUG(dbgs() << "SLICING UP PHI: " << FirstPhi << '\n';
778 for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
779 dbgs() << "AND USER PHI #" << i << ": " << *PHIsToSlice[i] << '\n';
780 );
781
782 // PredValues - This is a temporary used when rewriting PHI nodes. It is
783 // hoisted out here to avoid construction/destruction thrashing.
784 DenseMap<BasicBlock*, Value*> PredValues;
785
786 // ExtractedVals - Each new PHI we introduce is saved here so we don't
787 // introduce redundant PHIs.
788 DenseMap<LoweredPHIRecord, PHINode*> ExtractedVals;
789
790 for (unsigned UserI = 0, UserE = PHIUsers.size(); UserI != UserE; ++UserI) {
791 unsigned PHIId = PHIUsers[UserI].PHIId;
792 PHINode *PN = PHIsToSlice[PHIId];
793 unsigned Offset = PHIUsers[UserI].Shift;
794 Type *Ty = PHIUsers[UserI].Inst->getType();
795
796 PHINode *EltPHI;
797
798 // If we've already lowered a user like this, reuse the previously lowered
799 // value.
800 if ((EltPHI = ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)]) == nullptr) {
801
802 // Otherwise, Create the new PHI node for this user.
803 EltPHI = PHINode::Create(Ty, PN->getNumIncomingValues(),
804 PN->getName()+".off"+Twine(Offset), PN);
805 assert(EltPHI->getType() != PN->getType() &&
806 "Truncate didn't shrink phi?");
807
808 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
809 BasicBlock *Pred = PN->getIncomingBlock(i);
810 Value *&PredVal = PredValues[Pred];
811
812 // If we already have a value for this predecessor, reuse it.
813 if (PredVal) {
814 EltPHI->addIncoming(PredVal, Pred);
815 continue;
816 }
817
818 // Handle the PHI self-reuse case.
819 Value *InVal = PN->getIncomingValue(i);
820 if (InVal == PN) {
821 PredVal = EltPHI;
822 EltPHI->addIncoming(PredVal, Pred);
823 continue;
824 }
825
826 if (PHINode *InPHI = dyn_cast<PHINode>(PN)) {
827 // If the incoming value was a PHI, and if it was one of the PHIs we
828 // already rewrote it, just use the lowered value.
829 if (Value *Res = ExtractedVals[LoweredPHIRecord(InPHI, Offset, Ty)]) {
830 PredVal = Res;
831 EltPHI->addIncoming(PredVal, Pred);
832 continue;
833 }
834 }
835
836 // Otherwise, do an extract in the predecessor.
837 Builder->SetInsertPoint(Pred->getTerminator());
838 Value *Res = InVal;
839 if (Offset)
840 Res = Builder->CreateLShr(Res, ConstantInt::get(InVal->getType(),
841 Offset), "extract");
842 Res = Builder->CreateTrunc(Res, Ty, "extract.t");
843 PredVal = Res;
844 EltPHI->addIncoming(Res, Pred);
845
846 // If the incoming value was a PHI, and if it was one of the PHIs we are
847 // rewriting, we will ultimately delete the code we inserted. This
848 // means we need to revisit that PHI to make sure we extract out the
849 // needed piece.
850 if (PHINode *OldInVal = dyn_cast<PHINode>(PN->getIncomingValue(i)))
851 if (PHIsInspected.count(OldInVal)) {
852 unsigned RefPHIId = std::find(PHIsToSlice.begin(),PHIsToSlice.end(),
853 OldInVal)-PHIsToSlice.begin();
854 PHIUsers.push_back(PHIUsageRecord(RefPHIId, Offset,
855 cast<Instruction>(Res)));
856 ++UserE;
857 }
858 }
859 PredValues.clear();
860
861 DEBUG(dbgs() << " Made element PHI for offset " << Offset << ": "
862 << *EltPHI << '\n');
863 ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)] = EltPHI;
864 }
865
866 // Replace the use of this piece with the PHI node.
867 ReplaceInstUsesWith(*PHIUsers[UserI].Inst, EltPHI);
868 }
869
870 // Replace all the remaining uses of the PHI nodes (self uses and the lshrs)
871 // with undefs.
872 Value *Undef = UndefValue::get(FirstPhi.getType());
873 for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
874 ReplaceInstUsesWith(*PHIsToSlice[i], Undef);
875 return ReplaceInstUsesWith(FirstPhi, Undef);
876 }
877
878 // PHINode simplification
879 //
visitPHINode(PHINode & PN)880 Instruction *InstCombiner::visitPHINode(PHINode &PN) {
881 if (Value *V = SimplifyInstruction(&PN, DL, TLI, DT, AC))
882 return ReplaceInstUsesWith(PN, V);
883
884 if (Instruction *Result = FoldPHIArgZextsIntoPHI(PN))
885 return Result;
886
887 // If all PHI operands are the same operation, pull them through the PHI,
888 // reducing code size.
889 if (isa<Instruction>(PN.getIncomingValue(0)) &&
890 isa<Instruction>(PN.getIncomingValue(1)) &&
891 cast<Instruction>(PN.getIncomingValue(0))->getOpcode() ==
892 cast<Instruction>(PN.getIncomingValue(1))->getOpcode() &&
893 // FIXME: The hasOneUse check will fail for PHIs that use the value more
894 // than themselves more than once.
895 PN.getIncomingValue(0)->hasOneUse())
896 if (Instruction *Result = FoldPHIArgOpIntoPHI(PN))
897 return Result;
898
899 // If this is a trivial cycle in the PHI node graph, remove it. Basically, if
900 // this PHI only has a single use (a PHI), and if that PHI only has one use (a
901 // PHI)... break the cycle.
902 if (PN.hasOneUse()) {
903 Instruction *PHIUser = cast<Instruction>(PN.user_back());
904 if (PHINode *PU = dyn_cast<PHINode>(PHIUser)) {
905 SmallPtrSet<PHINode*, 16> PotentiallyDeadPHIs;
906 PotentiallyDeadPHIs.insert(&PN);
907 if (DeadPHICycle(PU, PotentiallyDeadPHIs))
908 return ReplaceInstUsesWith(PN, UndefValue::get(PN.getType()));
909 }
910
911 // If this phi has a single use, and if that use just computes a value for
912 // the next iteration of a loop, delete the phi. This occurs with unused
913 // induction variables, e.g. "for (int j = 0; ; ++j);". Detecting this
914 // common case here is good because the only other things that catch this
915 // are induction variable analysis (sometimes) and ADCE, which is only run
916 // late.
917 if (PHIUser->hasOneUse() &&
918 (isa<BinaryOperator>(PHIUser) || isa<GetElementPtrInst>(PHIUser)) &&
919 PHIUser->user_back() == &PN) {
920 return ReplaceInstUsesWith(PN, UndefValue::get(PN.getType()));
921 }
922 }
923
924 // We sometimes end up with phi cycles that non-obviously end up being the
925 // same value, for example:
926 // z = some value; x = phi (y, z); y = phi (x, z)
927 // where the phi nodes don't necessarily need to be in the same block. Do a
928 // quick check to see if the PHI node only contains a single non-phi value, if
929 // so, scan to see if the phi cycle is actually equal to that value.
930 {
931 unsigned InValNo = 0, NumIncomingVals = PN.getNumIncomingValues();
932 // Scan for the first non-phi operand.
933 while (InValNo != NumIncomingVals &&
934 isa<PHINode>(PN.getIncomingValue(InValNo)))
935 ++InValNo;
936
937 if (InValNo != NumIncomingVals) {
938 Value *NonPhiInVal = PN.getIncomingValue(InValNo);
939
940 // Scan the rest of the operands to see if there are any conflicts, if so
941 // there is no need to recursively scan other phis.
942 for (++InValNo; InValNo != NumIncomingVals; ++InValNo) {
943 Value *OpVal = PN.getIncomingValue(InValNo);
944 if (OpVal != NonPhiInVal && !isa<PHINode>(OpVal))
945 break;
946 }
947
948 // If we scanned over all operands, then we have one unique value plus
949 // phi values. Scan PHI nodes to see if they all merge in each other or
950 // the value.
951 if (InValNo == NumIncomingVals) {
952 SmallPtrSet<PHINode*, 16> ValueEqualPHIs;
953 if (PHIsEqualValue(&PN, NonPhiInVal, ValueEqualPHIs))
954 return ReplaceInstUsesWith(PN, NonPhiInVal);
955 }
956 }
957 }
958
959 // If there are multiple PHIs, sort their operands so that they all list
960 // the blocks in the same order. This will help identical PHIs be eliminated
961 // by other passes. Other passes shouldn't depend on this for correctness
962 // however.
963 PHINode *FirstPN = cast<PHINode>(PN.getParent()->begin());
964 if (&PN != FirstPN)
965 for (unsigned i = 0, e = FirstPN->getNumIncomingValues(); i != e; ++i) {
966 BasicBlock *BBA = PN.getIncomingBlock(i);
967 BasicBlock *BBB = FirstPN->getIncomingBlock(i);
968 if (BBA != BBB) {
969 Value *VA = PN.getIncomingValue(i);
970 unsigned j = PN.getBasicBlockIndex(BBB);
971 Value *VB = PN.getIncomingValue(j);
972 PN.setIncomingBlock(i, BBB);
973 PN.setIncomingValue(i, VB);
974 PN.setIncomingBlock(j, BBA);
975 PN.setIncomingValue(j, VA);
976 // NOTE: Instcombine normally would want us to "return &PN" if we
977 // modified any of the operands of an instruction. However, since we
978 // aren't adding or removing uses (just rearranging them) we don't do
979 // this in this case.
980 }
981 }
982
983 // If this is an integer PHI and we know that it has an illegal type, see if
984 // it is only used by trunc or trunc(lshr) operations. If so, we split the
985 // PHI into the various pieces being extracted. This sort of thing is
986 // introduced when SROA promotes an aggregate to a single large integer type.
987 if (PN.getType()->isIntegerTy() &&
988 !DL.isLegalInteger(PN.getType()->getPrimitiveSizeInBits()))
989 if (Instruction *Res = SliceUpIllegalIntegerPHI(PN))
990 return Res;
991
992 return nullptr;
993 }
994