LLVM API Documentation

InstCombinePHI.cpp
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00001 //===- InstCombinePHI.cpp -------------------------------------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements the visitPHINode function.
00011 //
00012 //===----------------------------------------------------------------------===//
00013 
00014 #include "InstCombine.h"
00015 #include "llvm/ADT/STLExtras.h"
00016 #include "llvm/ADT/SmallPtrSet.h"
00017 #include "llvm/Analysis/InstructionSimplify.h"
00018 #include "llvm/IR/DataLayout.h"
00019 using namespace llvm;
00020 
00021 #define DEBUG_TYPE "instcombine"
00022 
00023 /// FoldPHIArgBinOpIntoPHI - If we have something like phi [add (a,b), add(a,c)]
00024 /// and if a/b/c and the add's all have a single use, turn this into a phi
00025 /// and a single binop.
00026 Instruction *InstCombiner::FoldPHIArgBinOpIntoPHI(PHINode &PN) {
00027   Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
00028   assert(isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst));
00029   unsigned Opc = FirstInst->getOpcode();
00030   Value *LHSVal = FirstInst->getOperand(0);
00031   Value *RHSVal = FirstInst->getOperand(1);
00032 
00033   Type *LHSType = LHSVal->getType();
00034   Type *RHSType = RHSVal->getType();
00035 
00036   bool isNUW = false, isNSW = false, isExact = false;
00037   if (OverflowingBinaryOperator *BO =
00038         dyn_cast<OverflowingBinaryOperator>(FirstInst)) {
00039     isNUW = BO->hasNoUnsignedWrap();
00040     isNSW = BO->hasNoSignedWrap();
00041   } else if (PossiblyExactOperator *PEO =
00042                dyn_cast<PossiblyExactOperator>(FirstInst))
00043     isExact = PEO->isExact();
00044 
00045   // Scan to see if all operands are the same opcode, and all have one use.
00046   for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
00047     Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
00048     if (!I || I->getOpcode() != Opc || !I->hasOneUse() ||
00049         // Verify type of the LHS matches so we don't fold cmp's of different
00050         // types.
00051         I->getOperand(0)->getType() != LHSType ||
00052         I->getOperand(1)->getType() != RHSType)
00053       return nullptr;
00054 
00055     // If they are CmpInst instructions, check their predicates
00056     if (CmpInst *CI = dyn_cast<CmpInst>(I))
00057       if (CI->getPredicate() != cast<CmpInst>(FirstInst)->getPredicate())
00058         return nullptr;
00059 
00060     if (isNUW)
00061       isNUW = cast<OverflowingBinaryOperator>(I)->hasNoUnsignedWrap();
00062     if (isNSW)
00063       isNSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
00064     if (isExact)
00065       isExact = cast<PossiblyExactOperator>(I)->isExact();
00066 
00067     // Keep track of which operand needs a phi node.
00068     if (I->getOperand(0) != LHSVal) LHSVal = nullptr;
00069     if (I->getOperand(1) != RHSVal) RHSVal = nullptr;
00070   }
00071 
00072   // If both LHS and RHS would need a PHI, don't do this transformation,
00073   // because it would increase the number of PHIs entering the block,
00074   // which leads to higher register pressure. This is especially
00075   // bad when the PHIs are in the header of a loop.
00076   if (!LHSVal && !RHSVal)
00077     return nullptr;
00078 
00079   // Otherwise, this is safe to transform!
00080 
00081   Value *InLHS = FirstInst->getOperand(0);
00082   Value *InRHS = FirstInst->getOperand(1);
00083   PHINode *NewLHS = nullptr, *NewRHS = nullptr;
00084   if (!LHSVal) {
00085     NewLHS = PHINode::Create(LHSType, PN.getNumIncomingValues(),
00086                              FirstInst->getOperand(0)->getName() + ".pn");
00087     NewLHS->addIncoming(InLHS, PN.getIncomingBlock(0));
00088     InsertNewInstBefore(NewLHS, PN);
00089     LHSVal = NewLHS;
00090   }
00091 
00092   if (!RHSVal) {
00093     NewRHS = PHINode::Create(RHSType, PN.getNumIncomingValues(),
00094                              FirstInst->getOperand(1)->getName() + ".pn");
00095     NewRHS->addIncoming(InRHS, PN.getIncomingBlock(0));
00096     InsertNewInstBefore(NewRHS, PN);
00097     RHSVal = NewRHS;
00098   }
00099 
00100   // Add all operands to the new PHIs.
00101   if (NewLHS || NewRHS) {
00102     for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00103       Instruction *InInst = cast<Instruction>(PN.getIncomingValue(i));
00104       if (NewLHS) {
00105         Value *NewInLHS = InInst->getOperand(0);
00106         NewLHS->addIncoming(NewInLHS, PN.getIncomingBlock(i));
00107       }
00108       if (NewRHS) {
00109         Value *NewInRHS = InInst->getOperand(1);
00110         NewRHS->addIncoming(NewInRHS, PN.getIncomingBlock(i));
00111       }
00112     }
00113   }
00114 
00115   if (CmpInst *CIOp = dyn_cast<CmpInst>(FirstInst)) {
00116     CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
00117                                      LHSVal, RHSVal);
00118     NewCI->setDebugLoc(FirstInst->getDebugLoc());
00119     return NewCI;
00120   }
00121 
00122   BinaryOperator *BinOp = cast<BinaryOperator>(FirstInst);
00123   BinaryOperator *NewBinOp =
00124     BinaryOperator::Create(BinOp->getOpcode(), LHSVal, RHSVal);
00125   if (isNUW) NewBinOp->setHasNoUnsignedWrap();
00126   if (isNSW) NewBinOp->setHasNoSignedWrap();
00127   if (isExact) NewBinOp->setIsExact();
00128   NewBinOp->setDebugLoc(FirstInst->getDebugLoc());
00129   return NewBinOp;
00130 }
00131 
00132 Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
00133   GetElementPtrInst *FirstInst =cast<GetElementPtrInst>(PN.getIncomingValue(0));
00134 
00135   SmallVector<Value*, 16> FixedOperands(FirstInst->op_begin(),
00136                                         FirstInst->op_end());
00137   // This is true if all GEP bases are allocas and if all indices into them are
00138   // constants.
00139   bool AllBasePointersAreAllocas = true;
00140 
00141   // We don't want to replace this phi if the replacement would require
00142   // more than one phi, which leads to higher register pressure. This is
00143   // especially bad when the PHIs are in the header of a loop.
00144   bool NeededPhi = false;
00145 
00146   bool AllInBounds = true;
00147 
00148   // Scan to see if all operands are the same opcode, and all have one use.
00149   for (unsigned i = 1; i != PN.getNumIncomingValues(); ++i) {
00150     GetElementPtrInst *GEP= dyn_cast<GetElementPtrInst>(PN.getIncomingValue(i));
00151     if (!GEP || !GEP->hasOneUse() || GEP->getType() != FirstInst->getType() ||
00152       GEP->getNumOperands() != FirstInst->getNumOperands())
00153       return nullptr;
00154 
00155     AllInBounds &= GEP->isInBounds();
00156 
00157     // Keep track of whether or not all GEPs are of alloca pointers.
00158     if (AllBasePointersAreAllocas &&
00159         (!isa<AllocaInst>(GEP->getOperand(0)) ||
00160          !GEP->hasAllConstantIndices()))
00161       AllBasePointersAreAllocas = false;
00162 
00163     // Compare the operand lists.
00164     for (unsigned op = 0, e = FirstInst->getNumOperands(); op != e; ++op) {
00165       if (FirstInst->getOperand(op) == GEP->getOperand(op))
00166         continue;
00167 
00168       // Don't merge two GEPs when two operands differ (introducing phi nodes)
00169       // if one of the PHIs has a constant for the index.  The index may be
00170       // substantially cheaper to compute for the constants, so making it a
00171       // variable index could pessimize the path.  This also handles the case
00172       // for struct indices, which must always be constant.
00173       if (isa<ConstantInt>(FirstInst->getOperand(op)) ||
00174           isa<ConstantInt>(GEP->getOperand(op)))
00175         return nullptr;
00176 
00177       if (FirstInst->getOperand(op)->getType() !=GEP->getOperand(op)->getType())
00178         return nullptr;
00179 
00180       // If we already needed a PHI for an earlier operand, and another operand
00181       // also requires a PHI, we'd be introducing more PHIs than we're
00182       // eliminating, which increases register pressure on entry to the PHI's
00183       // block.
00184       if (NeededPhi)
00185         return nullptr;
00186 
00187       FixedOperands[op] = nullptr;  // Needs a PHI.
00188       NeededPhi = true;
00189     }
00190   }
00191 
00192   // If all of the base pointers of the PHI'd GEPs are from allocas, don't
00193   // bother doing this transformation.  At best, this will just save a bit of
00194   // offset calculation, but all the predecessors will have to materialize the
00195   // stack address into a register anyway.  We'd actually rather *clone* the
00196   // load up into the predecessors so that we have a load of a gep of an alloca,
00197   // which can usually all be folded into the load.
00198   if (AllBasePointersAreAllocas)
00199     return nullptr;
00200 
00201   // Otherwise, this is safe to transform.  Insert PHI nodes for each operand
00202   // that is variable.
00203   SmallVector<PHINode*, 16> OperandPhis(FixedOperands.size());
00204 
00205   bool HasAnyPHIs = false;
00206   for (unsigned i = 0, e = FixedOperands.size(); i != e; ++i) {
00207     if (FixedOperands[i]) continue;  // operand doesn't need a phi.
00208     Value *FirstOp = FirstInst->getOperand(i);
00209     PHINode *NewPN = PHINode::Create(FirstOp->getType(), e,
00210                                      FirstOp->getName()+".pn");
00211     InsertNewInstBefore(NewPN, PN);
00212 
00213     NewPN->addIncoming(FirstOp, PN.getIncomingBlock(0));
00214     OperandPhis[i] = NewPN;
00215     FixedOperands[i] = NewPN;
00216     HasAnyPHIs = true;
00217   }
00218 
00219 
00220   // Add all operands to the new PHIs.
00221   if (HasAnyPHIs) {
00222     for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00223       GetElementPtrInst *InGEP =cast<GetElementPtrInst>(PN.getIncomingValue(i));
00224       BasicBlock *InBB = PN.getIncomingBlock(i);
00225 
00226       for (unsigned op = 0, e = OperandPhis.size(); op != e; ++op)
00227         if (PHINode *OpPhi = OperandPhis[op])
00228           OpPhi->addIncoming(InGEP->getOperand(op), InBB);
00229     }
00230   }
00231 
00232   Value *Base = FixedOperands[0];
00233   GetElementPtrInst *NewGEP =
00234     GetElementPtrInst::Create(Base, makeArrayRef(FixedOperands).slice(1));
00235   if (AllInBounds) NewGEP->setIsInBounds();
00236   NewGEP->setDebugLoc(FirstInst->getDebugLoc());
00237   return NewGEP;
00238 }
00239 
00240 
00241 /// isSafeAndProfitableToSinkLoad - Return true if we know that it is safe to
00242 /// sink the load out of the block that defines it.  This means that it must be
00243 /// obvious the value of the load is not changed from the point of the load to
00244 /// the end of the block it is in.
00245 ///
00246 /// Finally, it is safe, but not profitable, to sink a load targeting a
00247 /// non-address-taken alloca.  Doing so will cause us to not promote the alloca
00248 /// to a register.
00249 static bool isSafeAndProfitableToSinkLoad(LoadInst *L) {
00250   BasicBlock::iterator BBI = L, E = L->getParent()->end();
00251 
00252   for (++BBI; BBI != E; ++BBI)
00253     if (BBI->mayWriteToMemory())
00254       return false;
00255 
00256   // Check for non-address taken alloca.  If not address-taken already, it isn't
00257   // profitable to do this xform.
00258   if (AllocaInst *AI = dyn_cast<AllocaInst>(L->getOperand(0))) {
00259     bool isAddressTaken = false;
00260     for (User *U : AI->users()) {
00261       if (isa<LoadInst>(U)) continue;
00262       if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
00263         // If storing TO the alloca, then the address isn't taken.
00264         if (SI->getOperand(1) == AI) continue;
00265       }
00266       isAddressTaken = true;
00267       break;
00268     }
00269 
00270     if (!isAddressTaken && AI->isStaticAlloca())
00271       return false;
00272   }
00273 
00274   // If this load is a load from a GEP with a constant offset from an alloca,
00275   // then we don't want to sink it.  In its present form, it will be
00276   // load [constant stack offset].  Sinking it will cause us to have to
00277   // materialize the stack addresses in each predecessor in a register only to
00278   // do a shared load from register in the successor.
00279   if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(L->getOperand(0)))
00280     if (AllocaInst *AI = dyn_cast<AllocaInst>(GEP->getOperand(0)))
00281       if (AI->isStaticAlloca() && GEP->hasAllConstantIndices())
00282         return false;
00283 
00284   return true;
00285 }
00286 
00287 Instruction *InstCombiner::FoldPHIArgLoadIntoPHI(PHINode &PN) {
00288   LoadInst *FirstLI = cast<LoadInst>(PN.getIncomingValue(0));
00289 
00290   // FIXME: This is overconservative; this transform is allowed in some cases
00291   // for atomic operations.
00292   if (FirstLI->isAtomic())
00293     return nullptr;
00294 
00295   // When processing loads, we need to propagate two bits of information to the
00296   // sunk load: whether it is volatile, and what its alignment is.  We currently
00297   // don't sink loads when some have their alignment specified and some don't.
00298   // visitLoadInst will propagate an alignment onto the load when TD is around,
00299   // and if TD isn't around, we can't handle the mixed case.
00300   bool isVolatile = FirstLI->isVolatile();
00301   unsigned LoadAlignment = FirstLI->getAlignment();
00302   unsigned LoadAddrSpace = FirstLI->getPointerAddressSpace();
00303 
00304   // We can't sink the load if the loaded value could be modified between the
00305   // load and the PHI.
00306   if (FirstLI->getParent() != PN.getIncomingBlock(0) ||
00307       !isSafeAndProfitableToSinkLoad(FirstLI))
00308     return nullptr;
00309 
00310   // If the PHI is of volatile loads and the load block has multiple
00311   // successors, sinking it would remove a load of the volatile value from
00312   // the path through the other successor.
00313   if (isVolatile &&
00314       FirstLI->getParent()->getTerminator()->getNumSuccessors() != 1)
00315     return nullptr;
00316 
00317   // Check to see if all arguments are the same operation.
00318   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00319     LoadInst *LI = dyn_cast<LoadInst>(PN.getIncomingValue(i));
00320     if (!LI || !LI->hasOneUse())
00321       return nullptr;
00322 
00323     // We can't sink the load if the loaded value could be modified between
00324     // the load and the PHI.
00325     if (LI->isVolatile() != isVolatile ||
00326         LI->getParent() != PN.getIncomingBlock(i) ||
00327         LI->getPointerAddressSpace() != LoadAddrSpace ||
00328         !isSafeAndProfitableToSinkLoad(LI))
00329       return nullptr;
00330 
00331     // If some of the loads have an alignment specified but not all of them,
00332     // we can't do the transformation.
00333     if ((LoadAlignment != 0) != (LI->getAlignment() != 0))
00334       return nullptr;
00335 
00336     LoadAlignment = std::min(LoadAlignment, LI->getAlignment());
00337 
00338     // If the PHI is of volatile loads and the load block has multiple
00339     // successors, sinking it would remove a load of the volatile value from
00340     // the path through the other successor.
00341     if (isVolatile &&
00342         LI->getParent()->getTerminator()->getNumSuccessors() != 1)
00343       return nullptr;
00344   }
00345 
00346   // Okay, they are all the same operation.  Create a new PHI node of the
00347   // correct type, and PHI together all of the LHS's of the instructions.
00348   PHINode *NewPN = PHINode::Create(FirstLI->getOperand(0)->getType(),
00349                                    PN.getNumIncomingValues(),
00350                                    PN.getName()+".in");
00351 
00352   Value *InVal = FirstLI->getOperand(0);
00353   NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
00354 
00355   // Add all operands to the new PHI.
00356   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00357     Value *NewInVal = cast<LoadInst>(PN.getIncomingValue(i))->getOperand(0);
00358     if (NewInVal != InVal)
00359       InVal = nullptr;
00360     NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
00361   }
00362 
00363   Value *PhiVal;
00364   if (InVal) {
00365     // The new PHI unions all of the same values together.  This is really
00366     // common, so we handle it intelligently here for compile-time speed.
00367     PhiVal = InVal;
00368     delete NewPN;
00369   } else {
00370     InsertNewInstBefore(NewPN, PN);
00371     PhiVal = NewPN;
00372   }
00373 
00374   // If this was a volatile load that we are merging, make sure to loop through
00375   // and mark all the input loads as non-volatile.  If we don't do this, we will
00376   // insert a new volatile load and the old ones will not be deletable.
00377   if (isVolatile)
00378     for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
00379       cast<LoadInst>(PN.getIncomingValue(i))->setVolatile(false);
00380 
00381   LoadInst *NewLI = new LoadInst(PhiVal, "", isVolatile, LoadAlignment);
00382   NewLI->setDebugLoc(FirstLI->getDebugLoc());
00383   return NewLI;
00384 }
00385 
00386 
00387 
00388 /// FoldPHIArgOpIntoPHI - If all operands to a PHI node are the same "unary"
00389 /// operator and they all are only used by the PHI, PHI together their
00390 /// inputs, and do the operation once, to the result of the PHI.
00391 Instruction *InstCombiner::FoldPHIArgOpIntoPHI(PHINode &PN) {
00392   Instruction *FirstInst = cast<Instruction>(PN.getIncomingValue(0));
00393 
00394   if (isa<GetElementPtrInst>(FirstInst))
00395     return FoldPHIArgGEPIntoPHI(PN);
00396   if (isa<LoadInst>(FirstInst))
00397     return FoldPHIArgLoadIntoPHI(PN);
00398 
00399   // Scan the instruction, looking for input operations that can be folded away.
00400   // If all input operands to the phi are the same instruction (e.g. a cast from
00401   // the same type or "+42") we can pull the operation through the PHI, reducing
00402   // code size and simplifying code.
00403   Constant *ConstantOp = nullptr;
00404   Type *CastSrcTy = nullptr;
00405   bool isNUW = false, isNSW = false, isExact = false;
00406 
00407   if (isa<CastInst>(FirstInst)) {
00408     CastSrcTy = FirstInst->getOperand(0)->getType();
00409 
00410     // Be careful about transforming integer PHIs.  We don't want to pessimize
00411     // the code by turning an i32 into an i1293.
00412     if (PN.getType()->isIntegerTy() && CastSrcTy->isIntegerTy()) {
00413       if (!ShouldChangeType(PN.getType(), CastSrcTy))
00414         return nullptr;
00415     }
00416   } else if (isa<BinaryOperator>(FirstInst) || isa<CmpInst>(FirstInst)) {
00417     // Can fold binop, compare or shift here if the RHS is a constant,
00418     // otherwise call FoldPHIArgBinOpIntoPHI.
00419     ConstantOp = dyn_cast<Constant>(FirstInst->getOperand(1));
00420     if (!ConstantOp)
00421       return FoldPHIArgBinOpIntoPHI(PN);
00422 
00423     if (OverflowingBinaryOperator *BO =
00424         dyn_cast<OverflowingBinaryOperator>(FirstInst)) {
00425       isNUW = BO->hasNoUnsignedWrap();
00426       isNSW = BO->hasNoSignedWrap();
00427     } else if (PossiblyExactOperator *PEO =
00428                dyn_cast<PossiblyExactOperator>(FirstInst))
00429       isExact = PEO->isExact();
00430   } else {
00431     return nullptr;  // Cannot fold this operation.
00432   }
00433 
00434   // Check to see if all arguments are the same operation.
00435   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00436     Instruction *I = dyn_cast<Instruction>(PN.getIncomingValue(i));
00437     if (!I || !I->hasOneUse() || !I->isSameOperationAs(FirstInst))
00438       return nullptr;
00439     if (CastSrcTy) {
00440       if (I->getOperand(0)->getType() != CastSrcTy)
00441         return nullptr;  // Cast operation must match.
00442     } else if (I->getOperand(1) != ConstantOp) {
00443       return nullptr;
00444     }
00445 
00446     if (isNUW)
00447       isNUW = cast<OverflowingBinaryOperator>(I)->hasNoUnsignedWrap();
00448     if (isNSW)
00449       isNSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
00450     if (isExact)
00451       isExact = cast<PossiblyExactOperator>(I)->isExact();
00452   }
00453 
00454   // Okay, they are all the same operation.  Create a new PHI node of the
00455   // correct type, and PHI together all of the LHS's of the instructions.
00456   PHINode *NewPN = PHINode::Create(FirstInst->getOperand(0)->getType(),
00457                                    PN.getNumIncomingValues(),
00458                                    PN.getName()+".in");
00459 
00460   Value *InVal = FirstInst->getOperand(0);
00461   NewPN->addIncoming(InVal, PN.getIncomingBlock(0));
00462 
00463   // Add all operands to the new PHI.
00464   for (unsigned i = 1, e = PN.getNumIncomingValues(); i != e; ++i) {
00465     Value *NewInVal = cast<Instruction>(PN.getIncomingValue(i))->getOperand(0);
00466     if (NewInVal != InVal)
00467       InVal = nullptr;
00468     NewPN->addIncoming(NewInVal, PN.getIncomingBlock(i));
00469   }
00470 
00471   Value *PhiVal;
00472   if (InVal) {
00473     // The new PHI unions all of the same values together.  This is really
00474     // common, so we handle it intelligently here for compile-time speed.
00475     PhiVal = InVal;
00476     delete NewPN;
00477   } else {
00478     InsertNewInstBefore(NewPN, PN);
00479     PhiVal = NewPN;
00480   }
00481 
00482   // Insert and return the new operation.
00483   if (CastInst *FirstCI = dyn_cast<CastInst>(FirstInst)) {
00484     CastInst *NewCI = CastInst::Create(FirstCI->getOpcode(), PhiVal,
00485                                        PN.getType());
00486     NewCI->setDebugLoc(FirstInst->getDebugLoc());
00487     return NewCI;
00488   }
00489 
00490   if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(FirstInst)) {
00491     BinOp = BinaryOperator::Create(BinOp->getOpcode(), PhiVal, ConstantOp);
00492     if (isNUW) BinOp->setHasNoUnsignedWrap();
00493     if (isNSW) BinOp->setHasNoSignedWrap();
00494     if (isExact) BinOp->setIsExact();
00495     BinOp->setDebugLoc(FirstInst->getDebugLoc());
00496     return BinOp;
00497   }
00498 
00499   CmpInst *CIOp = cast<CmpInst>(FirstInst);
00500   CmpInst *NewCI = CmpInst::Create(CIOp->getOpcode(), CIOp->getPredicate(),
00501                                    PhiVal, ConstantOp);
00502   NewCI->setDebugLoc(FirstInst->getDebugLoc());
00503   return NewCI;
00504 }
00505 
00506 /// DeadPHICycle - Return true if this PHI node is only used by a PHI node cycle
00507 /// that is dead.
00508 static bool DeadPHICycle(PHINode *PN,
00509                          SmallPtrSetImpl<PHINode*> &PotentiallyDeadPHIs) {
00510   if (PN->use_empty()) return true;
00511   if (!PN->hasOneUse()) return false;
00512 
00513   // Remember this node, and if we find the cycle, return.
00514   if (!PotentiallyDeadPHIs.insert(PN))
00515     return true;
00516 
00517   // Don't scan crazily complex things.
00518   if (PotentiallyDeadPHIs.size() == 16)
00519     return false;
00520 
00521   if (PHINode *PU = dyn_cast<PHINode>(PN->user_back()))
00522     return DeadPHICycle(PU, PotentiallyDeadPHIs);
00523 
00524   return false;
00525 }
00526 
00527 /// PHIsEqualValue - Return true if this phi node is always equal to
00528 /// NonPhiInVal.  This happens with mutually cyclic phi nodes like:
00529 ///   z = some value; x = phi (y, z); y = phi (x, z)
00530 static bool PHIsEqualValue(PHINode *PN, Value *NonPhiInVal,
00531                            SmallPtrSetImpl<PHINode*> &ValueEqualPHIs) {
00532   // See if we already saw this PHI node.
00533   if (!ValueEqualPHIs.insert(PN))
00534     return true;
00535 
00536   // Don't scan crazily complex things.
00537   if (ValueEqualPHIs.size() == 16)
00538     return false;
00539 
00540   // Scan the operands to see if they are either phi nodes or are equal to
00541   // the value.
00542   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
00543     Value *Op = PN->getIncomingValue(i);
00544     if (PHINode *OpPN = dyn_cast<PHINode>(Op)) {
00545       if (!PHIsEqualValue(OpPN, NonPhiInVal, ValueEqualPHIs))
00546         return false;
00547     } else if (Op != NonPhiInVal)
00548       return false;
00549   }
00550 
00551   return true;
00552 }
00553 
00554 
00555 namespace {
00556 struct PHIUsageRecord {
00557   unsigned PHIId;     // The ID # of the PHI (something determinstic to sort on)
00558   unsigned Shift;     // The amount shifted.
00559   Instruction *Inst;  // The trunc instruction.
00560 
00561   PHIUsageRecord(unsigned pn, unsigned Sh, Instruction *User)
00562     : PHIId(pn), Shift(Sh), Inst(User) {}
00563 
00564   bool operator<(const PHIUsageRecord &RHS) const {
00565     if (PHIId < RHS.PHIId) return true;
00566     if (PHIId > RHS.PHIId) return false;
00567     if (Shift < RHS.Shift) return true;
00568     if (Shift > RHS.Shift) return false;
00569     return Inst->getType()->getPrimitiveSizeInBits() <
00570            RHS.Inst->getType()->getPrimitiveSizeInBits();
00571   }
00572 };
00573 
00574 struct LoweredPHIRecord {
00575   PHINode *PN;        // The PHI that was lowered.
00576   unsigned Shift;     // The amount shifted.
00577   unsigned Width;     // The width extracted.
00578 
00579   LoweredPHIRecord(PHINode *pn, unsigned Sh, Type *Ty)
00580     : PN(pn), Shift(Sh), Width(Ty->getPrimitiveSizeInBits()) {}
00581 
00582   // Ctor form used by DenseMap.
00583   LoweredPHIRecord(PHINode *pn, unsigned Sh)
00584     : PN(pn), Shift(Sh), Width(0) {}
00585 };
00586 }
00587 
00588 namespace llvm {
00589   template<>
00590   struct DenseMapInfo<LoweredPHIRecord> {
00591     static inline LoweredPHIRecord getEmptyKey() {
00592       return LoweredPHIRecord(nullptr, 0);
00593     }
00594     static inline LoweredPHIRecord getTombstoneKey() {
00595       return LoweredPHIRecord(nullptr, 1);
00596     }
00597     static unsigned getHashValue(const LoweredPHIRecord &Val) {
00598       return DenseMapInfo<PHINode*>::getHashValue(Val.PN) ^ (Val.Shift>>3) ^
00599              (Val.Width>>3);
00600     }
00601     static bool isEqual(const LoweredPHIRecord &LHS,
00602                         const LoweredPHIRecord &RHS) {
00603       return LHS.PN == RHS.PN && LHS.Shift == RHS.Shift &&
00604              LHS.Width == RHS.Width;
00605     }
00606   };
00607 }
00608 
00609 
00610 /// SliceUpIllegalIntegerPHI - This is an integer PHI and we know that it has an
00611 /// illegal type: see if it is only used by trunc or trunc(lshr) operations.  If
00612 /// so, we split the PHI into the various pieces being extracted.  This sort of
00613 /// thing is introduced when SROA promotes an aggregate to large integer values.
00614 ///
00615 /// TODO: The user of the trunc may be an bitcast to float/double/vector or an
00616 /// inttoptr.  We should produce new PHIs in the right type.
00617 ///
00618 Instruction *InstCombiner::SliceUpIllegalIntegerPHI(PHINode &FirstPhi) {
00619   // PHIUsers - Keep track of all of the truncated values extracted from a set
00620   // of PHIs, along with their offset.  These are the things we want to rewrite.
00621   SmallVector<PHIUsageRecord, 16> PHIUsers;
00622 
00623   // PHIs are often mutually cyclic, so we keep track of a whole set of PHI
00624   // nodes which are extracted from. PHIsToSlice is a set we use to avoid
00625   // revisiting PHIs, PHIsInspected is a ordered list of PHIs that we need to
00626   // check the uses of (to ensure they are all extracts).
00627   SmallVector<PHINode*, 8> PHIsToSlice;
00628   SmallPtrSet<PHINode*, 8> PHIsInspected;
00629 
00630   PHIsToSlice.push_back(&FirstPhi);
00631   PHIsInspected.insert(&FirstPhi);
00632 
00633   for (unsigned PHIId = 0; PHIId != PHIsToSlice.size(); ++PHIId) {
00634     PHINode *PN = PHIsToSlice[PHIId];
00635 
00636     // Scan the input list of the PHI.  If any input is an invoke, and if the
00637     // input is defined in the predecessor, then we won't be split the critical
00638     // edge which is required to insert a truncate.  Because of this, we have to
00639     // bail out.
00640     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
00641       InvokeInst *II = dyn_cast<InvokeInst>(PN->getIncomingValue(i));
00642       if (!II) continue;
00643       if (II->getParent() != PN->getIncomingBlock(i))
00644         continue;
00645 
00646       // If we have a phi, and if it's directly in the predecessor, then we have
00647       // a critical edge where we need to put the truncate.  Since we can't
00648       // split the edge in instcombine, we have to bail out.
00649       return nullptr;
00650     }
00651 
00652     for (User *U : PN->users()) {
00653       Instruction *UserI = cast<Instruction>(U);
00654 
00655       // If the user is a PHI, inspect its uses recursively.
00656       if (PHINode *UserPN = dyn_cast<PHINode>(UserI)) {
00657         if (PHIsInspected.insert(UserPN))
00658           PHIsToSlice.push_back(UserPN);
00659         continue;
00660       }
00661 
00662       // Truncates are always ok.
00663       if (isa<TruncInst>(UserI)) {
00664         PHIUsers.push_back(PHIUsageRecord(PHIId, 0, UserI));
00665         continue;
00666       }
00667 
00668       // Otherwise it must be a lshr which can only be used by one trunc.
00669       if (UserI->getOpcode() != Instruction::LShr ||
00670           !UserI->hasOneUse() || !isa<TruncInst>(UserI->user_back()) ||
00671           !isa<ConstantInt>(UserI->getOperand(1)))
00672         return nullptr;
00673 
00674       unsigned Shift = cast<ConstantInt>(UserI->getOperand(1))->getZExtValue();
00675       PHIUsers.push_back(PHIUsageRecord(PHIId, Shift, UserI->user_back()));
00676     }
00677   }
00678 
00679   // If we have no users, they must be all self uses, just nuke the PHI.
00680   if (PHIUsers.empty())
00681     return ReplaceInstUsesWith(FirstPhi, UndefValue::get(FirstPhi.getType()));
00682 
00683   // If this phi node is transformable, create new PHIs for all the pieces
00684   // extracted out of it.  First, sort the users by their offset and size.
00685   array_pod_sort(PHIUsers.begin(), PHIUsers.end());
00686 
00687   DEBUG(dbgs() << "SLICING UP PHI: " << FirstPhi << '\n';
00688         for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
00689           dbgs() << "AND USER PHI #" << i << ": " << *PHIsToSlice[i] << '\n';
00690     );
00691 
00692   // PredValues - This is a temporary used when rewriting PHI nodes.  It is
00693   // hoisted out here to avoid construction/destruction thrashing.
00694   DenseMap<BasicBlock*, Value*> PredValues;
00695 
00696   // ExtractedVals - Each new PHI we introduce is saved here so we don't
00697   // introduce redundant PHIs.
00698   DenseMap<LoweredPHIRecord, PHINode*> ExtractedVals;
00699 
00700   for (unsigned UserI = 0, UserE = PHIUsers.size(); UserI != UserE; ++UserI) {
00701     unsigned PHIId = PHIUsers[UserI].PHIId;
00702     PHINode *PN = PHIsToSlice[PHIId];
00703     unsigned Offset = PHIUsers[UserI].Shift;
00704     Type *Ty = PHIUsers[UserI].Inst->getType();
00705 
00706     PHINode *EltPHI;
00707 
00708     // If we've already lowered a user like this, reuse the previously lowered
00709     // value.
00710     if ((EltPHI = ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)]) == nullptr) {
00711 
00712       // Otherwise, Create the new PHI node for this user.
00713       EltPHI = PHINode::Create(Ty, PN->getNumIncomingValues(),
00714                                PN->getName()+".off"+Twine(Offset), PN);
00715       assert(EltPHI->getType() != PN->getType() &&
00716              "Truncate didn't shrink phi?");
00717 
00718       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
00719         BasicBlock *Pred = PN->getIncomingBlock(i);
00720         Value *&PredVal = PredValues[Pred];
00721 
00722         // If we already have a value for this predecessor, reuse it.
00723         if (PredVal) {
00724           EltPHI->addIncoming(PredVal, Pred);
00725           continue;
00726         }
00727 
00728         // Handle the PHI self-reuse case.
00729         Value *InVal = PN->getIncomingValue(i);
00730         if (InVal == PN) {
00731           PredVal = EltPHI;
00732           EltPHI->addIncoming(PredVal, Pred);
00733           continue;
00734         }
00735 
00736         if (PHINode *InPHI = dyn_cast<PHINode>(PN)) {
00737           // If the incoming value was a PHI, and if it was one of the PHIs we
00738           // already rewrote it, just use the lowered value.
00739           if (Value *Res = ExtractedVals[LoweredPHIRecord(InPHI, Offset, Ty)]) {
00740             PredVal = Res;
00741             EltPHI->addIncoming(PredVal, Pred);
00742             continue;
00743           }
00744         }
00745 
00746         // Otherwise, do an extract in the predecessor.
00747         Builder->SetInsertPoint(Pred, Pred->getTerminator());
00748         Value *Res = InVal;
00749         if (Offset)
00750           Res = Builder->CreateLShr(Res, ConstantInt::get(InVal->getType(),
00751                                                           Offset), "extract");
00752         Res = Builder->CreateTrunc(Res, Ty, "extract.t");
00753         PredVal = Res;
00754         EltPHI->addIncoming(Res, Pred);
00755 
00756         // If the incoming value was a PHI, and if it was one of the PHIs we are
00757         // rewriting, we will ultimately delete the code we inserted.  This
00758         // means we need to revisit that PHI to make sure we extract out the
00759         // needed piece.
00760         if (PHINode *OldInVal = dyn_cast<PHINode>(PN->getIncomingValue(i)))
00761           if (PHIsInspected.count(OldInVal)) {
00762             unsigned RefPHIId = std::find(PHIsToSlice.begin(),PHIsToSlice.end(),
00763                                           OldInVal)-PHIsToSlice.begin();
00764             PHIUsers.push_back(PHIUsageRecord(RefPHIId, Offset,
00765                                               cast<Instruction>(Res)));
00766             ++UserE;
00767           }
00768       }
00769       PredValues.clear();
00770 
00771       DEBUG(dbgs() << "  Made element PHI for offset " << Offset << ": "
00772                    << *EltPHI << '\n');
00773       ExtractedVals[LoweredPHIRecord(PN, Offset, Ty)] = EltPHI;
00774     }
00775 
00776     // Replace the use of this piece with the PHI node.
00777     ReplaceInstUsesWith(*PHIUsers[UserI].Inst, EltPHI);
00778   }
00779 
00780   // Replace all the remaining uses of the PHI nodes (self uses and the lshrs)
00781   // with undefs.
00782   Value *Undef = UndefValue::get(FirstPhi.getType());
00783   for (unsigned i = 1, e = PHIsToSlice.size(); i != e; ++i)
00784     ReplaceInstUsesWith(*PHIsToSlice[i], Undef);
00785   return ReplaceInstUsesWith(FirstPhi, Undef);
00786 }
00787 
00788 // PHINode simplification
00789 //
00790 Instruction *InstCombiner::visitPHINode(PHINode &PN) {
00791   if (Value *V = SimplifyInstruction(&PN, DL, TLI))
00792     return ReplaceInstUsesWith(PN, V);
00793 
00794   // If all PHI operands are the same operation, pull them through the PHI,
00795   // reducing code size.
00796   if (isa<Instruction>(PN.getIncomingValue(0)) &&
00797       isa<Instruction>(PN.getIncomingValue(1)) &&
00798       cast<Instruction>(PN.getIncomingValue(0))->getOpcode() ==
00799       cast<Instruction>(PN.getIncomingValue(1))->getOpcode() &&
00800       // FIXME: The hasOneUse check will fail for PHIs that use the value more
00801       // than themselves more than once.
00802       PN.getIncomingValue(0)->hasOneUse())
00803     if (Instruction *Result = FoldPHIArgOpIntoPHI(PN))
00804       return Result;
00805 
00806   // If this is a trivial cycle in the PHI node graph, remove it.  Basically, if
00807   // this PHI only has a single use (a PHI), and if that PHI only has one use (a
00808   // PHI)... break the cycle.
00809   if (PN.hasOneUse()) {
00810     Instruction *PHIUser = cast<Instruction>(PN.user_back());
00811     if (PHINode *PU = dyn_cast<PHINode>(PHIUser)) {
00812       SmallPtrSet<PHINode*, 16> PotentiallyDeadPHIs;
00813       PotentiallyDeadPHIs.insert(&PN);
00814       if (DeadPHICycle(PU, PotentiallyDeadPHIs))
00815         return ReplaceInstUsesWith(PN, UndefValue::get(PN.getType()));
00816     }
00817 
00818     // If this phi has a single use, and if that use just computes a value for
00819     // the next iteration of a loop, delete the phi.  This occurs with unused
00820     // induction variables, e.g. "for (int j = 0; ; ++j);".  Detecting this
00821     // common case here is good because the only other things that catch this
00822     // are induction variable analysis (sometimes) and ADCE, which is only run
00823     // late.
00824     if (PHIUser->hasOneUse() &&
00825         (isa<BinaryOperator>(PHIUser) || isa<GetElementPtrInst>(PHIUser)) &&
00826         PHIUser->user_back() == &PN) {
00827       return ReplaceInstUsesWith(PN, UndefValue::get(PN.getType()));
00828     }
00829   }
00830 
00831   // We sometimes end up with phi cycles that non-obviously end up being the
00832   // same value, for example:
00833   //   z = some value; x = phi (y, z); y = phi (x, z)
00834   // where the phi nodes don't necessarily need to be in the same block.  Do a
00835   // quick check to see if the PHI node only contains a single non-phi value, if
00836   // so, scan to see if the phi cycle is actually equal to that value.
00837   {
00838     unsigned InValNo = 0, NumIncomingVals = PN.getNumIncomingValues();
00839     // Scan for the first non-phi operand.
00840     while (InValNo != NumIncomingVals &&
00841            isa<PHINode>(PN.getIncomingValue(InValNo)))
00842       ++InValNo;
00843 
00844     if (InValNo != NumIncomingVals) {
00845       Value *NonPhiInVal = PN.getIncomingValue(InValNo);
00846 
00847       // Scan the rest of the operands to see if there are any conflicts, if so
00848       // there is no need to recursively scan other phis.
00849       for (++InValNo; InValNo != NumIncomingVals; ++InValNo) {
00850         Value *OpVal = PN.getIncomingValue(InValNo);
00851         if (OpVal != NonPhiInVal && !isa<PHINode>(OpVal))
00852           break;
00853       }
00854 
00855       // If we scanned over all operands, then we have one unique value plus
00856       // phi values.  Scan PHI nodes to see if they all merge in each other or
00857       // the value.
00858       if (InValNo == NumIncomingVals) {
00859         SmallPtrSet<PHINode*, 16> ValueEqualPHIs;
00860         if (PHIsEqualValue(&PN, NonPhiInVal, ValueEqualPHIs))
00861           return ReplaceInstUsesWith(PN, NonPhiInVal);
00862       }
00863     }
00864   }
00865 
00866   // If there are multiple PHIs, sort their operands so that they all list
00867   // the blocks in the same order. This will help identical PHIs be eliminated
00868   // by other passes. Other passes shouldn't depend on this for correctness
00869   // however.
00870   PHINode *FirstPN = cast<PHINode>(PN.getParent()->begin());
00871   if (&PN != FirstPN)
00872     for (unsigned i = 0, e = FirstPN->getNumIncomingValues(); i != e; ++i) {
00873       BasicBlock *BBA = PN.getIncomingBlock(i);
00874       BasicBlock *BBB = FirstPN->getIncomingBlock(i);
00875       if (BBA != BBB) {
00876         Value *VA = PN.getIncomingValue(i);
00877         unsigned j = PN.getBasicBlockIndex(BBB);
00878         Value *VB = PN.getIncomingValue(j);
00879         PN.setIncomingBlock(i, BBB);
00880         PN.setIncomingValue(i, VB);
00881         PN.setIncomingBlock(j, BBA);
00882         PN.setIncomingValue(j, VA);
00883         // NOTE: Instcombine normally would want us to "return &PN" if we
00884         // modified any of the operands of an instruction.  However, since we
00885         // aren't adding or removing uses (just rearranging them) we don't do
00886         // this in this case.
00887       }
00888     }
00889 
00890   // If this is an integer PHI and we know that it has an illegal type, see if
00891   // it is only used by trunc or trunc(lshr) operations.  If so, we split the
00892   // PHI into the various pieces being extracted.  This sort of thing is
00893   // introduced when SROA promotes an aggregate to a single large integer type.
00894   if (PN.getType()->isIntegerTy() && DL &&
00895       !DL->isLegalInteger(PN.getType()->getPrimitiveSizeInBits()))
00896     if (Instruction *Res = SliceUpIllegalIntegerPHI(PN))
00897       return Res;
00898 
00899   return nullptr;
00900 }