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 #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/Type.h"
00026 #include "llvm/Support/Debug.h"
00027 #include "llvm/Support/raw_ostream.h"
00028 #include <algorithm>
00029 using namespace llvm;
00030 
00031 #define DEBUG_TYPE "iv-users"
00032 
00033 char IVUsers::ID = 0;
00034 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
00035                       "Induction Variable Users", false, true)
00036 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
00037 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
00038 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
00039 INITIALIZE_PASS_END(IVUsers, "iv-users",
00040                       "Induction Variable Users", false, true)
00041 
00042 Pass *llvm::createIVUsersPass() {
00043   return new IVUsers();
00044 }
00045 
00046 /// isInteresting - Test whether the given expression is "interesting" when
00047 /// used by the given expression, within the context of analyzing the
00048 /// given loop.
00049 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
00050                           ScalarEvolution *SE, LoopInfo *LI) {
00051   // An addrec is interesting if it's affine or if it has an interesting start.
00052   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00053     // Keep things simple. Don't touch loop-variant strides unless they're
00054     // only used outside the loop and we can simplify them.
00055     if (AR->getLoop() == L)
00056       return AR->isAffine() ||
00057              (!L->contains(I) &&
00058               SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
00059     // Otherwise recurse to see if the start value is interesting, and that
00060     // the step value is not interesting, since we don't yet know how to
00061     // do effective SCEV expansions for addrecs with interesting steps.
00062     return isInteresting(AR->getStart(), I, L, SE, LI) &&
00063           !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
00064   }
00065 
00066   // An add is interesting if exactly one of its operands is interesting.
00067   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00068     bool AnyInterestingYet = false;
00069     for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
00070          OI != OE; ++OI)
00071       if (isInteresting(*OI, I, L, SE, LI)) {
00072         if (AnyInterestingYet)
00073           return false;
00074         AnyInterestingYet = true;
00075       }
00076     return AnyInterestingYet;
00077   }
00078 
00079   // Nothing else is interesting here.
00080   return false;
00081 }
00082 
00083 /// Return true if all loop headers that dominate this block are in simplified
00084 /// form.
00085 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
00086                                  const LoopInfo *LI,
00087                                  SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
00088   Loop *NearestLoop = nullptr;
00089   for (DomTreeNode *Rung = DT->getNode(BB);
00090        Rung; Rung = Rung->getIDom()) {
00091     BasicBlock *DomBB = Rung->getBlock();
00092     Loop *DomLoop = LI->getLoopFor(DomBB);
00093     if (DomLoop && DomLoop->getHeader() == DomBB) {
00094       // If the domtree walk reaches a loop with no preheader, return false.
00095       if (!DomLoop->isLoopSimplifyForm())
00096         return false;
00097       // If we have already checked this loop nest, stop checking.
00098       if (SimpleLoopNests.count(DomLoop))
00099         break;
00100       // If we have not already checked this loop nest, remember the loop
00101       // header nearest to BB. The nearest loop may not contain BB.
00102       if (!NearestLoop)
00103         NearestLoop = DomLoop;
00104     }
00105   }
00106   if (NearestLoop)
00107     SimpleLoopNests.insert(NearestLoop);
00108   return true;
00109 }
00110 
00111 /// AddUsersImpl - Inspect the specified instruction.  If it is a
00112 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
00113 /// return true.  Otherwise, return false.
00114 bool IVUsers::AddUsersImpl(Instruction *I,
00115                            SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
00116   // Add this IV user to the Processed set before returning false to ensure that
00117   // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
00118   if (!Processed.insert(I).second)
00119     return true;    // Instruction already handled.
00120 
00121   if (!SE->isSCEVable(I->getType()))
00122     return false;   // Void and FP expressions cannot be reduced.
00123 
00124   // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
00125   // pass to SCEVExpander. Expressions are not safe to expand if they represent
00126   // operations that are not safe to speculate, namely integer division.
00127   if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I, DL))
00128     return false;
00129 
00130   // LSR is not APInt clean, do not touch integers bigger than 64-bits.
00131   // Also avoid creating IVs of non-native types. For example, we don't want a
00132   // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
00133   uint64_t Width = SE->getTypeSizeInBits(I->getType());
00134   if (Width > 64 || (DL && !DL->isLegalInteger(Width)))
00135     return false;
00136 
00137   // Get the symbolic expression for this instruction.
00138   const SCEV *ISE = SE->getSCEV(I);
00139 
00140   // If we've come to an uninteresting expression, stop the traversal and
00141   // call this a user.
00142   if (!isInteresting(ISE, I, L, SE, LI))
00143     return false;
00144 
00145   SmallPtrSet<Instruction *, 4> UniqueUsers;
00146   for (Use &U : I->uses()) {
00147     Instruction *User = cast<Instruction>(U.getUser());
00148     if (!UniqueUsers.insert(User).second)
00149       continue;
00150 
00151     // Do not infinitely recurse on PHI nodes.
00152     if (isa<PHINode>(User) && Processed.count(User))
00153       continue;
00154 
00155     // Only consider IVUsers that are dominated by simplified loop
00156     // headers. Otherwise, SCEVExpander will crash.
00157     BasicBlock *UseBB = User->getParent();
00158     // A phi's use is live out of its predecessor block.
00159     if (PHINode *PHI = dyn_cast<PHINode>(User)) {
00160       unsigned OperandNo = U.getOperandNo();
00161       unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
00162       UseBB = PHI->getIncomingBlock(ValNo);
00163     }
00164     if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
00165       return false;
00166 
00167     // Descend recursively, but not into PHI nodes outside the current loop.
00168     // It's important to see the entire expression outside the loop to get
00169     // choices that depend on addressing mode use right, although we won't
00170     // consider references outside the loop in all cases.
00171     // If User is already in Processed, we don't want to recurse into it again,
00172     // but do want to record a second reference in the same instruction.
00173     bool AddUserToIVUsers = false;
00174     if (LI->getLoopFor(User->getParent()) != L) {
00175       if (isa<PHINode>(User) || Processed.count(User) ||
00176           !AddUsersImpl(User, SimpleLoopNests)) {
00177         DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
00178                      << "   OF SCEV: " << *ISE << '\n');
00179         AddUserToIVUsers = true;
00180       }
00181     } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
00182       DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
00183                    << "   OF SCEV: " << *ISE << '\n');
00184       AddUserToIVUsers = true;
00185     }
00186 
00187     if (AddUserToIVUsers) {
00188       // Okay, we found a user that we cannot reduce.
00189       IVStrideUse &NewUse = AddUser(User, I);
00190       // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
00191       // The regular return value here is discarded; instead of recording
00192       // it, we just recompute it when we need it.
00193       const SCEV *OriginalISE = ISE;
00194       ISE = TransformForPostIncUse(NormalizeAutodetect,
00195                                    ISE, User, I,
00196                                    NewUse.PostIncLoops,
00197                                    *SE, *DT);
00198 
00199       // PostIncNormalization effectively simplifies the expression under
00200       // pre-increment assumptions. Those assumptions (no wrapping) might not
00201       // hold for the post-inc value. Catch such cases by making sure the
00202       // transformation is invertible.
00203       if (OriginalISE != ISE) {
00204         const SCEV *DenormalizedISE =
00205           TransformForPostIncUse(Denormalize, ISE, User, I,
00206               NewUse.PostIncLoops, *SE, *DT);
00207 
00208         // If we normalized the expression, but denormalization doesn't give the
00209         // original one, discard this user.
00210         if (OriginalISE != DenormalizedISE) {
00211           DEBUG(dbgs() << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
00212                        << *ISE << '\n');
00213           IVUses.pop_back();
00214           return false;
00215         }
00216       }
00217       DEBUG(if (SE->getSCEV(I) != ISE)
00218               dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
00219     }
00220   }
00221   return true;
00222 }
00223 
00224 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
00225   // SCEVExpander can only handle users that are dominated by simplified loop
00226   // entries. Keep track of all loops that are only dominated by other simple
00227   // loops so we don't traverse the domtree for each user.
00228   SmallPtrSet<Loop*,16> SimpleLoopNests;
00229 
00230   return AddUsersImpl(I, SimpleLoopNests);
00231 }
00232 
00233 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
00234   IVUses.push_back(new IVStrideUse(this, User, Operand));
00235   return IVUses.back();
00236 }
00237 
00238 IVUsers::IVUsers()
00239     : LoopPass(ID) {
00240   initializeIVUsersPass(*PassRegistry::getPassRegistry());
00241 }
00242 
00243 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
00244   AU.addRequired<LoopInfo>();
00245   AU.addRequired<DominatorTreeWrapperPass>();
00246   AU.addRequired<ScalarEvolution>();
00247   AU.setPreservesAll();
00248 }
00249 
00250 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
00251 
00252   L = l;
00253   LI = &getAnalysis<LoopInfo>();
00254   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
00255   SE = &getAnalysis<ScalarEvolution>();
00256   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
00257   DL = DLP ? &DLP->getDataLayout() : nullptr;
00258 
00259   // Find all uses of induction variables in this loop, and categorize
00260   // them by stride.  Start by finding all of the PHI nodes in the header for
00261   // this loop.  If they are induction variables, inspect their uses.
00262   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
00263     (void)AddUsersIfInteresting(I);
00264 
00265   return false;
00266 }
00267 
00268 void IVUsers::print(raw_ostream &OS, const Module *M) const {
00269   OS << "IV Users for loop ";
00270   L->getHeader()->printAsOperand(OS, false);
00271   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
00272     OS << " with backedge-taken count "
00273        << *SE->getBackedgeTakenCount(L);
00274   }
00275   OS << ":\n";
00276 
00277   for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
00278        E = IVUses.end(); UI != E; ++UI) {
00279     OS << "  ";
00280     UI->getOperandValToReplace()->printAsOperand(OS, false);
00281     OS << " = " << *getReplacementExpr(*UI);
00282     for (PostIncLoopSet::const_iterator
00283          I = UI->PostIncLoops.begin(),
00284          E = UI->PostIncLoops.end(); I != E; ++I) {
00285       OS << " (post-inc with loop ";
00286       (*I)->getHeader()->printAsOperand(OS, false);
00287       OS << ")";
00288     }
00289     OS << " in  ";
00290     if (UI->getUser())
00291       UI->getUser()->print(OS);
00292     else
00293       OS << "Printing <null> User";
00294     OS << '\n';
00295   }
00296 }
00297 
00298 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
00299 void IVUsers::dump() const {
00300   print(dbgs());
00301 }
00302 #endif
00303 
00304 void IVUsers::releaseMemory() {
00305   Processed.clear();
00306   IVUses.clear();
00307 }
00308 
00309 /// getReplacementExpr - Return a SCEV expression which computes the
00310 /// value of the OperandValToReplace.
00311 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
00312   return SE->getSCEV(IU.getOperandValToReplace());
00313 }
00314 
00315 /// getExpr - Return the expression for the use.
00316 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
00317   return
00318     TransformForPostIncUse(Normalize, getReplacementExpr(IU),
00319                            IU.getUser(), IU.getOperandValToReplace(),
00320                            const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
00321                            *SE, *DT);
00322 }
00323 
00324 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
00325   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
00326     if (AR->getLoop() == L)
00327       return AR;
00328     return findAddRecForLoop(AR->getStart(), L);
00329   }
00330 
00331   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
00332     for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
00333          I != E; ++I)
00334       if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
00335         return AR;
00336     return nullptr;
00337   }
00338 
00339   return nullptr;
00340 }
00341 
00342 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
00343   if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
00344     return AR->getStepRecurrence(*SE);
00345   return nullptr;
00346 }
00347 
00348 void IVStrideUse::transformToPostInc(const Loop *L) {
00349   PostIncLoops.insert(L);
00350 }
00351 
00352 void IVStrideUse::deleted() {
00353   // Remove this user from the list.
00354   Parent->Processed.erase(this->getUser());
00355   Parent->IVUses.erase(this);
00356   // this now dangles!
00357 }