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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 #include "llvm/Analysis/IVUsers.h"
00016 #include "llvm/ADT/STLExtras.h"
00017 #include "llvm/Analysis/LoopPass.h"
00018 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
00019 #include "llvm/Analysis/ValueTracking.h"
00020 #include "llvm/IR/Constants.h"
00021 #include "llvm/IR/DataLayout.h"
00022 #include "llvm/IR/DerivedTypes.h"
00023 #include "llvm/IR/Dominators.h"
00024 #include "llvm/IR/Instructions.h"
00025 #include "llvm/IR/Module.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 #define DEBUG_TYPE "iv-users"
00033 
00034 char IVUsers::ID = 0;
00035 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
00036                       "Induction Variable Users", false, true)
00037 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
00038 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
00039 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
00040 INITIALIZE_PASS_END(IVUsers, "iv-users",
00041                       "Induction Variable Users", false, true)
00042 
00043 Pass *llvm::createIVUsersPass() {
00044   return new IVUsers();
00045 }
00046 
00047 /// isInteresting - Test whether the given expression is "interesting" when
00048 /// used by the given expression, within the context of analyzing the
00049 /// given loop.
00050 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
00051                           ScalarEvolution *SE, LoopInfo *LI) {
00052   // An addrec is interesting if it's affine or if it has an interesting start.
00053   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00054     // Keep things simple. Don't touch loop-variant strides unless they're
00055     // only used outside the loop and we can simplify them.
00056     if (AR->getLoop() == L)
00057       return AR->isAffine() ||
00058              (!L->contains(I) &&
00059               SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
00060     // Otherwise recurse to see if the start value is interesting, and that
00061     // the step value is not interesting, since we don't yet know how to
00062     // do effective SCEV expansions for addrecs with interesting steps.
00063     return isInteresting(AR->getStart(), I, L, SE, LI) &&
00064           !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
00065   }
00066 
00067   // An add is interesting if exactly one of its operands is interesting.
00068   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00069     bool AnyInterestingYet = false;
00070     for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
00071          OI != OE; ++OI)
00072       if (isInteresting(*OI, I, L, SE, LI)) {
00073         if (AnyInterestingYet)
00074           return false;
00075         AnyInterestingYet = true;
00076       }
00077     return AnyInterestingYet;
00078   }
00079 
00080   // Nothing else is interesting here.
00081   return false;
00082 }
00083 
00084 /// Return true if all loop headers that dominate this block are in simplified
00085 /// form.
00086 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
00087                                  const LoopInfo *LI,
00088                                  SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
00089   Loop *NearestLoop = nullptr;
00090   for (DomTreeNode *Rung = DT->getNode(BB);
00091        Rung; Rung = Rung->getIDom()) {
00092     BasicBlock *DomBB = Rung->getBlock();
00093     Loop *DomLoop = LI->getLoopFor(DomBB);
00094     if (DomLoop && DomLoop->getHeader() == DomBB) {
00095       // If the domtree walk reaches a loop with no preheader, return false.
00096       if (!DomLoop->isLoopSimplifyForm())
00097         return false;
00098       // If we have already checked this loop nest, stop checking.
00099       if (SimpleLoopNests.count(DomLoop))
00100         break;
00101       // If we have not already checked this loop nest, remember the loop
00102       // header nearest to BB. The nearest loop may not contain BB.
00103       if (!NearestLoop)
00104         NearestLoop = DomLoop;
00105     }
00106   }
00107   if (NearestLoop)
00108     SimpleLoopNests.insert(NearestLoop);
00109   return true;
00110 }
00111 
00112 /// AddUsersImpl - Inspect the specified instruction.  If it is a
00113 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
00114 /// return true.  Otherwise, return false.
00115 bool IVUsers::AddUsersImpl(Instruction *I,
00116                            SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
00117   const DataLayout &DL = I->getModule()->getDataLayout();
00118 
00119   // Add this IV user to the Processed set before returning false to ensure that
00120   // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
00121   if (!Processed.insert(I).second)
00122     return true;    // Instruction already handled.
00123 
00124   if (!SE->isSCEVable(I->getType()))
00125     return false;   // Void and FP expressions cannot be reduced.
00126 
00127   // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
00128   // pass to SCEVExpander. Expressions are not safe to expand if they represent
00129   // operations that are not safe to speculate, namely integer division.
00130   if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
00131     return false;
00132 
00133   // LSR is not APInt clean, do not touch integers bigger than 64-bits.
00134   // Also avoid creating IVs of non-native types. For example, we don't want a
00135   // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
00136   uint64_t Width = SE->getTypeSizeInBits(I->getType());
00137   if (Width > 64 || !DL.isLegalInteger(Width))
00138     return false;
00139 
00140   // Get the symbolic expression for this instruction.
00141   const SCEV *ISE = SE->getSCEV(I);
00142 
00143   // If we've come to an uninteresting expression, stop the traversal and
00144   // call this a user.
00145   if (!isInteresting(ISE, I, L, SE, LI))
00146     return false;
00147 
00148   SmallPtrSet<Instruction *, 4> UniqueUsers;
00149   for (Use &U : I->uses()) {
00150     Instruction *User = cast<Instruction>(U.getUser());
00151     if (!UniqueUsers.insert(User).second)
00152       continue;
00153 
00154     // Do not infinitely recurse on PHI nodes.
00155     if (isa<PHINode>(User) && Processed.count(User))
00156       continue;
00157 
00158     // Only consider IVUsers that are dominated by simplified loop
00159     // headers. Otherwise, SCEVExpander will crash.
00160     BasicBlock *UseBB = User->getParent();
00161     // A phi's use is live out of its predecessor block.
00162     if (PHINode *PHI = dyn_cast<PHINode>(User)) {
00163       unsigned OperandNo = U.getOperandNo();
00164       unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
00165       UseBB = PHI->getIncomingBlock(ValNo);
00166     }
00167     if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
00168       return false;
00169 
00170     // Descend recursively, but not into PHI nodes outside the current loop.
00171     // It's important to see the entire expression outside the loop to get
00172     // choices that depend on addressing mode use right, although we won't
00173     // consider references outside the loop in all cases.
00174     // If User is already in Processed, we don't want to recurse into it again,
00175     // but do want to record a second reference in the same instruction.
00176     bool AddUserToIVUsers = false;
00177     if (LI->getLoopFor(User->getParent()) != L) {
00178       if (isa<PHINode>(User) || Processed.count(User) ||
00179           !AddUsersImpl(User, SimpleLoopNests)) {
00180         DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
00181                      << "   OF SCEV: " << *ISE << '\n');
00182         AddUserToIVUsers = true;
00183       }
00184     } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
00185       DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
00186                    << "   OF SCEV: " << *ISE << '\n');
00187       AddUserToIVUsers = true;
00188     }
00189 
00190     if (AddUserToIVUsers) {
00191       // Okay, we found a user that we cannot reduce.
00192       IVStrideUse &NewUse = AddUser(User, I);
00193       // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
00194       // The regular return value here is discarded; instead of recording
00195       // it, we just recompute it when we need it.
00196       const SCEV *OriginalISE = ISE;
00197       ISE = TransformForPostIncUse(NormalizeAutodetect,
00198                                    ISE, User, I,
00199                                    NewUse.PostIncLoops,
00200                                    *SE, *DT);
00201 
00202       // PostIncNormalization effectively simplifies the expression under
00203       // pre-increment assumptions. Those assumptions (no wrapping) might not
00204       // hold for the post-inc value. Catch such cases by making sure the
00205       // transformation is invertible.
00206       if (OriginalISE != ISE) {
00207         const SCEV *DenormalizedISE =
00208           TransformForPostIncUse(Denormalize, ISE, User, I,
00209               NewUse.PostIncLoops, *SE, *DT);
00210 
00211         // If we normalized the expression, but denormalization doesn't give the
00212         // original one, discard this user.
00213         if (OriginalISE != DenormalizedISE) {
00214           DEBUG(dbgs() << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
00215                        << *ISE << '\n');
00216           IVUses.pop_back();
00217           return false;
00218         }
00219       }
00220       DEBUG(if (SE->getSCEV(I) != ISE)
00221               dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
00222     }
00223   }
00224   return true;
00225 }
00226 
00227 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
00228   // SCEVExpander can only handle users that are dominated by simplified loop
00229   // entries. Keep track of all loops that are only dominated by other simple
00230   // loops so we don't traverse the domtree for each user.
00231   SmallPtrSet<Loop*,16> SimpleLoopNests;
00232 
00233   return AddUsersImpl(I, SimpleLoopNests);
00234 }
00235 
00236 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
00237   IVUses.push_back(new IVStrideUse(this, User, Operand));
00238   return IVUses.back();
00239 }
00240 
00241 IVUsers::IVUsers()
00242     : LoopPass(ID) {
00243   initializeIVUsersPass(*PassRegistry::getPassRegistry());
00244 }
00245 
00246 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
00247   AU.addRequired<LoopInfoWrapperPass>();
00248   AU.addRequired<DominatorTreeWrapperPass>();
00249   AU.addRequired<ScalarEvolution>();
00250   AU.setPreservesAll();
00251 }
00252 
00253 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
00254 
00255   L = l;
00256   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
00257   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
00258   SE = &getAnalysis<ScalarEvolution>();
00259 
00260   // Find all uses of induction variables in this loop, and categorize
00261   // them by stride.  Start by finding all of the PHI nodes in the header for
00262   // this loop.  If they are induction variables, inspect their uses.
00263   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
00264     (void)AddUsersIfInteresting(I);
00265 
00266   return false;
00267 }
00268 
00269 void IVUsers::print(raw_ostream &OS, const Module *M) const {
00270   OS << "IV Users for loop ";
00271   L->getHeader()->printAsOperand(OS, false);
00272   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
00273     OS << " with backedge-taken count "
00274        << *SE->getBackedgeTakenCount(L);
00275   }
00276   OS << ":\n";
00277 
00278   for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
00279        E = IVUses.end(); UI != E; ++UI) {
00280     OS << "  ";
00281     UI->getOperandValToReplace()->printAsOperand(OS, false);
00282     OS << " = " << *getReplacementExpr(*UI);
00283     for (PostIncLoopSet::const_iterator
00284          I = UI->PostIncLoops.begin(),
00285          E = UI->PostIncLoops.end(); I != E; ++I) {
00286       OS << " (post-inc with loop ";
00287       (*I)->getHeader()->printAsOperand(OS, false);
00288       OS << ")";
00289     }
00290     OS << " in  ";
00291     if (UI->getUser())
00292       UI->getUser()->print(OS);
00293     else
00294       OS << "Printing <null> User";
00295     OS << '\n';
00296   }
00297 }
00298 
00299 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
00300 void IVUsers::dump() const {
00301   print(dbgs());
00302 }
00303 #endif
00304 
00305 void IVUsers::releaseMemory() {
00306   Processed.clear();
00307   IVUses.clear();
00308 }
00309 
00310 /// getReplacementExpr - Return a SCEV expression which computes the
00311 /// value of the OperandValToReplace.
00312 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
00313   return SE->getSCEV(IU.getOperandValToReplace());
00314 }
00315 
00316 /// getExpr - Return the expression for the use.
00317 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
00318   return
00319     TransformForPostIncUse(Normalize, getReplacementExpr(IU),
00320                            IU.getUser(), IU.getOperandValToReplace(),
00321                            const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
00322                            *SE, *DT);
00323 }
00324 
00325 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
00326   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00327     if (AR->getLoop() == L)
00328       return AR;
00329     return findAddRecForLoop(AR->getStart(), L);
00330   }
00331 
00332   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00333     for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
00334          I != E; ++I)
00335       if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
00336         return AR;
00337     return nullptr;
00338   }
00339 
00340   return nullptr;
00341 }
00342 
00343 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
00344   if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
00345     return AR->getStepRecurrence(*SE);
00346   return nullptr;
00347 }
00348 
00349 void IVStrideUse::transformToPostInc(const Loop *L) {
00350   PostIncLoops.insert(L);
00351 }
00352 
00353 void IVStrideUse::deleted() {
00354   // Remove this user from the list.
00355   Parent->Processed.erase(this->getUser());
00356   Parent->IVUses.erase(this);
00357   // this now dangles!
00358 }