LLVM API Documentation

IVUsers.cpp
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00001 //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
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 bookkeeping for "interesting" users of expressions
00011 // computed from induction variables.
00012 //
00013 //===----------------------------------------------------------------------===//
00014 
00015 #define DEBUG_TYPE "iv-users"
00016 #include "llvm/Analysis/IVUsers.h"
00017 #include "llvm/ADT/STLExtras.h"
00018 #include "llvm/Analysis/LoopPass.h"
00019 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
00020 #include "llvm/Analysis/ValueTracking.h"
00021 #include "llvm/IR/Constants.h"
00022 #include "llvm/IR/DataLayout.h"
00023 #include "llvm/IR/DerivedTypes.h"
00024 #include "llvm/IR/Dominators.h"
00025 #include "llvm/IR/Instructions.h"
00026 #include "llvm/IR/Type.h"
00027 #include "llvm/Support/Debug.h"
00028 #include "llvm/Support/raw_ostream.h"
00029 #include <algorithm>
00030 using namespace llvm;
00031 
00032 char IVUsers::ID = 0;
00033 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
00034                       "Induction Variable Users", false, true)
00035 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
00036 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
00037 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
00038 INITIALIZE_PASS_END(IVUsers, "iv-users",
00039                       "Induction Variable Users", false, true)
00040 
00041 Pass *llvm::createIVUsersPass() {
00042   return new IVUsers();
00043 }
00044 
00045 /// isInteresting - Test whether the given expression is "interesting" when
00046 /// used by the given expression, within the context of analyzing the
00047 /// given loop.
00048 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
00049                           ScalarEvolution *SE, LoopInfo *LI) {
00050   // An addrec is interesting if it's affine or if it has an interesting start.
00051   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00052     // Keep things simple. Don't touch loop-variant strides unless they're
00053     // only used outside the loop and we can simplify them.
00054     if (AR->getLoop() == L)
00055       return AR->isAffine() ||
00056              (!L->contains(I) &&
00057               SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
00058     // Otherwise recurse to see if the start value is interesting, and that
00059     // the step value is not interesting, since we don't yet know how to
00060     // do effective SCEV expansions for addrecs with interesting steps.
00061     return isInteresting(AR->getStart(), I, L, SE, LI) &&
00062           !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
00063   }
00064 
00065   // An add is interesting if exactly one of its operands is interesting.
00066   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00067     bool AnyInterestingYet = false;
00068     for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
00069          OI != OE; ++OI)
00070       if (isInteresting(*OI, I, L, SE, LI)) {
00071         if (AnyInterestingYet)
00072           return false;
00073         AnyInterestingYet = true;
00074       }
00075     return AnyInterestingYet;
00076   }
00077 
00078   // Nothing else is interesting here.
00079   return false;
00080 }
00081 
00082 /// Return true if all loop headers that dominate this block are in simplified
00083 /// form.
00084 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
00085                                  const LoopInfo *LI,
00086                                  SmallPtrSet<Loop*,16> &SimpleLoopNests) {
00087   Loop *NearestLoop = nullptr;
00088   for (DomTreeNode *Rung = DT->getNode(BB);
00089        Rung; Rung = Rung->getIDom()) {
00090     BasicBlock *DomBB = Rung->getBlock();
00091     Loop *DomLoop = LI->getLoopFor(DomBB);
00092     if (DomLoop && DomLoop->getHeader() == DomBB) {
00093       // If the domtree walk reaches a loop with no preheader, return false.
00094       if (!DomLoop->isLoopSimplifyForm())
00095         return false;
00096       // If we have already checked this loop nest, stop checking.
00097       if (SimpleLoopNests.count(DomLoop))
00098         break;
00099       // If we have not already checked this loop nest, remember the loop
00100       // header nearest to BB. The nearest loop may not contain BB.
00101       if (!NearestLoop)
00102         NearestLoop = DomLoop;
00103     }
00104   }
00105   if (NearestLoop)
00106     SimpleLoopNests.insert(NearestLoop);
00107   return true;
00108 }
00109 
00110 /// AddUsersImpl - Inspect the specified instruction.  If it is a
00111 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
00112 /// return true.  Otherwise, return false.
00113 bool IVUsers::AddUsersImpl(Instruction *I,
00114                            SmallPtrSet<Loop*,16> &SimpleLoopNests) {
00115   // Add this IV user to the Processed set before returning false to ensure that
00116   // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
00117   if (!Processed.insert(I))
00118     return true;    // Instruction already handled.
00119 
00120   if (!SE->isSCEVable(I->getType()))
00121     return false;   // Void and FP expressions cannot be reduced.
00122 
00123   // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
00124   // pass to SCEVExpander. Expressions are not safe to expand if they represent
00125   // operations that are not safe to speculate, namely integer division.
00126   if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I, DL))
00127     return false;
00128 
00129   // LSR is not APInt clean, do not touch integers bigger than 64-bits.
00130   // Also avoid creating IVs of non-native types. For example, we don't want a
00131   // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
00132   uint64_t Width = SE->getTypeSizeInBits(I->getType());
00133   if (Width > 64 || (DL && !DL->isLegalInteger(Width)))
00134     return false;
00135 
00136   // Get the symbolic expression for this instruction.
00137   const SCEV *ISE = SE->getSCEV(I);
00138 
00139   // If we've come to an uninteresting expression, stop the traversal and
00140   // call this a user.
00141   if (!isInteresting(ISE, I, L, SE, LI))
00142     return false;
00143 
00144   SmallPtrSet<Instruction *, 4> UniqueUsers;
00145   for (Use &U : I->uses()) {
00146     Instruction *User = cast<Instruction>(U.getUser());
00147     if (!UniqueUsers.insert(User))
00148       continue;
00149 
00150     // Do not infinitely recurse on PHI nodes.
00151     if (isa<PHINode>(User) && Processed.count(User))
00152       continue;
00153 
00154     // Only consider IVUsers that are dominated by simplified loop
00155     // headers. Otherwise, SCEVExpander will crash.
00156     BasicBlock *UseBB = User->getParent();
00157     // A phi's use is live out of its predecessor block.
00158     if (PHINode *PHI = dyn_cast<PHINode>(User)) {
00159       unsigned OperandNo = U.getOperandNo();
00160       unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
00161       UseBB = PHI->getIncomingBlock(ValNo);
00162     }
00163     if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
00164       return false;
00165 
00166     // Descend recursively, but not into PHI nodes outside the current loop.
00167     // It's important to see the entire expression outside the loop to get
00168     // choices that depend on addressing mode use right, although we won't
00169     // consider references outside the loop in all cases.
00170     // If User is already in Processed, we don't want to recurse into it again,
00171     // but do want to record a second reference in the same instruction.
00172     bool AddUserToIVUsers = false;
00173     if (LI->getLoopFor(User->getParent()) != L) {
00174       if (isa<PHINode>(User) || Processed.count(User) ||
00175           !AddUsersImpl(User, SimpleLoopNests)) {
00176         DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
00177                      << "   OF SCEV: " << *ISE << '\n');
00178         AddUserToIVUsers = true;
00179       }
00180     } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
00181       DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
00182                    << "   OF SCEV: " << *ISE << '\n');
00183       AddUserToIVUsers = true;
00184     }
00185 
00186     if (AddUserToIVUsers) {
00187       // Okay, we found a user that we cannot reduce.
00188       IVStrideUse &NewUse = AddUser(User, I);
00189       // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
00190       // The regular return value here is discarded; instead of recording
00191       // it, we just recompute it when we need it.
00192       const SCEV *OriginalISE = ISE;
00193       ISE = TransformForPostIncUse(NormalizeAutodetect,
00194                                    ISE, User, I,
00195                                    NewUse.PostIncLoops,
00196                                    *SE, *DT);
00197 
00198       // PostIncNormalization effectively simplifies the expression under
00199       // pre-increment assumptions. Those assumptions (no wrapping) might not
00200       // hold for the post-inc value. Catch such cases by making sure the
00201       // transformation is invertible.
00202       if (OriginalISE != ISE) {
00203         const SCEV *DenormalizedISE =
00204           TransformForPostIncUse(Denormalize, ISE, User, I,
00205               NewUse.PostIncLoops, *SE, *DT);
00206 
00207         // If we normalized the expression, but denormalization doesn't give the
00208         // original one, discard this user.
00209         if (OriginalISE != DenormalizedISE) {
00210           DEBUG(dbgs() << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
00211                        << *ISE << '\n');
00212           IVUses.pop_back();
00213           return false;
00214         }
00215       }
00216       DEBUG(if (SE->getSCEV(I) != ISE)
00217               dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
00218     }
00219   }
00220   return true;
00221 }
00222 
00223 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
00224   // SCEVExpander can only handle users that are dominated by simplified loop
00225   // entries. Keep track of all loops that are only dominated by other simple
00226   // loops so we don't traverse the domtree for each user.
00227   SmallPtrSet<Loop*,16> SimpleLoopNests;
00228 
00229   return AddUsersImpl(I, SimpleLoopNests);
00230 }
00231 
00232 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
00233   IVUses.push_back(new IVStrideUse(this, User, Operand));
00234   return IVUses.back();
00235 }
00236 
00237 IVUsers::IVUsers()
00238     : LoopPass(ID) {
00239   initializeIVUsersPass(*PassRegistry::getPassRegistry());
00240 }
00241 
00242 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
00243   AU.addRequired<LoopInfo>();
00244   AU.addRequired<DominatorTreeWrapperPass>();
00245   AU.addRequired<ScalarEvolution>();
00246   AU.setPreservesAll();
00247 }
00248 
00249 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
00250 
00251   L = l;
00252   LI = &getAnalysis<LoopInfo>();
00253   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
00254   SE = &getAnalysis<ScalarEvolution>();
00255   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
00256   DL = DLP ? &DLP->getDataLayout() : nullptr;
00257 
00258   // Find all uses of induction variables in this loop, and categorize
00259   // them by stride.  Start by finding all of the PHI nodes in the header for
00260   // this loop.  If they are induction variables, inspect their uses.
00261   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
00262     (void)AddUsersIfInteresting(I);
00263 
00264   return false;
00265 }
00266 
00267 void IVUsers::print(raw_ostream &OS, const Module *M) const {
00268   OS << "IV Users for loop ";
00269   L->getHeader()->printAsOperand(OS, false);
00270   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
00271     OS << " with backedge-taken count "
00272        << *SE->getBackedgeTakenCount(L);
00273   }
00274   OS << ":\n";
00275 
00276   for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
00277        E = IVUses.end(); UI != E; ++UI) {
00278     OS << "  ";
00279     UI->getOperandValToReplace()->printAsOperand(OS, false);
00280     OS << " = " << *getReplacementExpr(*UI);
00281     for (PostIncLoopSet::const_iterator
00282          I = UI->PostIncLoops.begin(),
00283          E = UI->PostIncLoops.end(); I != E; ++I) {
00284       OS << " (post-inc with loop ";
00285       (*I)->getHeader()->printAsOperand(OS, false);
00286       OS << ")";
00287     }
00288     OS << " in  ";
00289     UI->getUser()->print(OS);
00290     OS << '\n';
00291   }
00292 }
00293 
00294 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
00295 void IVUsers::dump() const {
00296   print(dbgs());
00297 }
00298 #endif
00299 
00300 void IVUsers::releaseMemory() {
00301   Processed.clear();
00302   IVUses.clear();
00303 }
00304 
00305 /// getReplacementExpr - Return a SCEV expression which computes the
00306 /// value of the OperandValToReplace.
00307 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
00308   return SE->getSCEV(IU.getOperandValToReplace());
00309 }
00310 
00311 /// getExpr - Return the expression for the use.
00312 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
00313   return
00314     TransformForPostIncUse(Normalize, getReplacementExpr(IU),
00315                            IU.getUser(), IU.getOperandValToReplace(),
00316                            const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
00317                            *SE, *DT);
00318 }
00319 
00320 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
00321   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00322     if (AR->getLoop() == L)
00323       return AR;
00324     return findAddRecForLoop(AR->getStart(), L);
00325   }
00326 
00327   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00328     for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
00329          I != E; ++I)
00330       if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
00331         return AR;
00332     return nullptr;
00333   }
00334 
00335   return nullptr;
00336 }
00337 
00338 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
00339   if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
00340     return AR->getStepRecurrence(*SE);
00341   return nullptr;
00342 }
00343 
00344 void IVStrideUse::transformToPostInc(const Loop *L) {
00345   PostIncLoops.insert(L);
00346 }
00347 
00348 void IVStrideUse::deleted() {
00349   // Remove this user from the list.
00350   Parent->Processed.erase(this->getUser());
00351   Parent->IVUses.erase(this);
00352   // this now dangles!
00353 }