IVUsers.cpp revision 266715
1//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===// 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 bookkeeping for "interesting" users of expressions 11// computed from induction variables. 12// 13//===----------------------------------------------------------------------===// 14 15#define DEBUG_TYPE "iv-users" 16#include "llvm/Analysis/IVUsers.h" 17#include "llvm/ADT/STLExtras.h" 18#include "llvm/Analysis/Dominators.h" 19#include "llvm/Analysis/LoopPass.h" 20#include "llvm/Analysis/ScalarEvolutionExpressions.h" 21#include "llvm/Analysis/ValueTracking.h" 22#include "llvm/Assembly/Writer.h" 23#include "llvm/IR/Constants.h" 24#include "llvm/IR/DataLayout.h" 25#include "llvm/IR/DerivedTypes.h" 26#include "llvm/IR/Instructions.h" 27#include "llvm/IR/Type.h" 28#include "llvm/Support/Debug.h" 29#include "llvm/Support/raw_ostream.h" 30#include <algorithm> 31using namespace llvm; 32 33char IVUsers::ID = 0; 34INITIALIZE_PASS_BEGIN(IVUsers, "iv-users", 35 "Induction Variable Users", false, true) 36INITIALIZE_PASS_DEPENDENCY(LoopInfo) 37INITIALIZE_PASS_DEPENDENCY(DominatorTree) 38INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 39INITIALIZE_PASS_END(IVUsers, "iv-users", 40 "Induction Variable Users", false, true) 41 42Pass *llvm::createIVUsersPass() { 43 return new IVUsers(); 44} 45 46/// isInteresting - Test whether the given expression is "interesting" when 47/// used by the given expression, within the context of analyzing the 48/// given loop. 49static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L, 50 ScalarEvolution *SE, LoopInfo *LI) { 51 // An addrec is interesting if it's affine or if it has an interesting start. 52 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 53 // Keep things simple. Don't touch loop-variant strides unless they're 54 // only used outside the loop and we can simplify them. 55 if (AR->getLoop() == L) 56 return AR->isAffine() || 57 (!L->contains(I) && 58 SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR); 59 // Otherwise recurse to see if the start value is interesting, and that 60 // the step value is not interesting, since we don't yet know how to 61 // do effective SCEV expansions for addrecs with interesting steps. 62 return isInteresting(AR->getStart(), I, L, SE, LI) && 63 !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI); 64 } 65 66 // An add is interesting if exactly one of its operands is interesting. 67 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) { 68 bool AnyInterestingYet = false; 69 for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end(); 70 OI != OE; ++OI) 71 if (isInteresting(*OI, I, L, SE, LI)) { 72 if (AnyInterestingYet) 73 return false; 74 AnyInterestingYet = true; 75 } 76 return AnyInterestingYet; 77 } 78 79 // Nothing else is interesting here. 80 return false; 81} 82 83/// Return true if all loop headers that dominate this block are in simplified 84/// form. 85static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT, 86 const LoopInfo *LI, 87 SmallPtrSet<Loop*,16> &SimpleLoopNests) { 88 Loop *NearestLoop = 0; 89 for (DomTreeNode *Rung = DT->getNode(BB); 90 Rung; Rung = Rung->getIDom()) { 91 BasicBlock *DomBB = Rung->getBlock(); 92 Loop *DomLoop = LI->getLoopFor(DomBB); 93 if (DomLoop && DomLoop->getHeader() == DomBB) { 94 // If the domtree walk reaches a loop with no preheader, return false. 95 if (!DomLoop->isLoopSimplifyForm()) 96 return false; 97 // If we have already checked this loop nest, stop checking. 98 if (SimpleLoopNests.count(DomLoop)) 99 break; 100 // If we have not already checked this loop nest, remember the loop 101 // header nearest to BB. The nearest loop may not contain BB. 102 if (!NearestLoop) 103 NearestLoop = DomLoop; 104 } 105 } 106 if (NearestLoop) 107 SimpleLoopNests.insert(NearestLoop); 108 return true; 109} 110 111/// AddUsersImpl - Inspect the specified instruction. If it is a 112/// reducible SCEV, recursively add its users to the IVUsesByStride set and 113/// return true. Otherwise, return false. 114bool IVUsers::AddUsersImpl(Instruction *I, 115 SmallPtrSet<Loop*,16> &SimpleLoopNests) { 116 // Add this IV user to the Processed set before returning false to ensure that 117 // all IV users are members of the set. See IVUsers::isIVUserOrOperand. 118 if (!Processed.insert(I)) 119 return true; // Instruction already handled. 120 121 if (!SE->isSCEVable(I->getType())) 122 return false; // Void and FP expressions cannot be reduced. 123 124 // IVUsers is used by LSR which assumes that all SCEV expressions are safe to 125 // pass to SCEVExpander. Expressions are not safe to expand if they represent 126 // operations that are not safe to speculate, namely integer division. 127 if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I, TD)) 128 return false; 129 130 // LSR is not APInt clean, do not touch integers bigger than 64-bits. 131 // Also avoid creating IVs of non-native types. For example, we don't want a 132 // 64-bit IV in 32-bit code just because the loop has one 64-bit cast. 133 uint64_t Width = SE->getTypeSizeInBits(I->getType()); 134 if (Width > 64 || (TD && !TD->isLegalInteger(Width))) 135 return false; 136 137 // Get the symbolic expression for this instruction. 138 const SCEV *ISE = SE->getSCEV(I); 139 140 // If we've come to an uninteresting expression, stop the traversal and 141 // call this a user. 142 if (!isInteresting(ISE, I, L, SE, LI)) 143 return false; 144 145 SmallPtrSet<Instruction *, 4> UniqueUsers; 146 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 147 UI != E; ++UI) { 148 Instruction *User = cast<Instruction>(*UI); 149 if (!UniqueUsers.insert(User)) 150 continue; 151 152 // Do not infinitely recurse on PHI nodes. 153 if (isa<PHINode>(User) && Processed.count(User)) 154 continue; 155 156 // Only consider IVUsers that are dominated by simplified loop 157 // headers. Otherwise, SCEVExpander will crash. 158 BasicBlock *UseBB = User->getParent(); 159 // A phi's use is live out of its predecessor block. 160 if (PHINode *PHI = dyn_cast<PHINode>(User)) { 161 unsigned OperandNo = UI.getOperandNo(); 162 unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo); 163 UseBB = PHI->getIncomingBlock(ValNo); 164 } 165 if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests)) 166 return false; 167 168 // Descend recursively, but not into PHI nodes outside the current loop. 169 // It's important to see the entire expression outside the loop to get 170 // choices that depend on addressing mode use right, although we won't 171 // consider references outside the loop in all cases. 172 // If User is already in Processed, we don't want to recurse into it again, 173 // but do want to record a second reference in the same instruction. 174 bool AddUserToIVUsers = false; 175 if (LI->getLoopFor(User->getParent()) != L) { 176 if (isa<PHINode>(User) || Processed.count(User) || 177 !AddUsersImpl(User, SimpleLoopNests)) { 178 DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n' 179 << " OF SCEV: " << *ISE << '\n'); 180 AddUserToIVUsers = true; 181 } 182 } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) { 183 DEBUG(dbgs() << "FOUND USER: " << *User << '\n' 184 << " OF SCEV: " << *ISE << '\n'); 185 AddUserToIVUsers = true; 186 } 187 188 if (AddUserToIVUsers) { 189 // Okay, we found a user that we cannot reduce. 190 IVStrideUse &NewUse = AddUser(User, I); 191 // Autodetect the post-inc loop set, populating NewUse.PostIncLoops. 192 // The regular return value here is discarded; instead of recording 193 // it, we just recompute it when we need it. 194 const SCEV *OriginalISE = ISE; 195 ISE = TransformForPostIncUse(NormalizeAutodetect, 196 ISE, User, I, 197 NewUse.PostIncLoops, 198 *SE, *DT); 199 200 // PostIncNormalization effectively simplifies the expression under 201 // pre-increment assumptions. Those assumptions (no wrapping) might not 202 // hold for the post-inc value. Catch such cases by making sure the 203 // transformation is invertible. 204 if (OriginalISE != ISE) { 205 const SCEV *DenormalizedISE = 206 TransformForPostIncUse(Denormalize, ISE, User, I, 207 NewUse.PostIncLoops, *SE, *DT); 208 209 // If we normalized the expression, but denormalization doesn't give the 210 // original one, discard this user. 211 if (OriginalISE != DenormalizedISE) { 212 DEBUG(dbgs() << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): " 213 << *ISE << '\n'); 214 IVUses.pop_back(); 215 return false; 216 } 217 } 218 DEBUG(if (SE->getSCEV(I) != ISE) 219 dbgs() << " NORMALIZED TO: " << *ISE << '\n'); 220 } 221 } 222 return true; 223} 224 225bool IVUsers::AddUsersIfInteresting(Instruction *I) { 226 // SCEVExpander can only handle users that are dominated by simplified loop 227 // entries. Keep track of all loops that are only dominated by other simple 228 // loops so we don't traverse the domtree for each user. 229 SmallPtrSet<Loop*,16> SimpleLoopNests; 230 231 return AddUsersImpl(I, SimpleLoopNests); 232} 233 234IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) { 235 IVUses.push_back(new IVStrideUse(this, User, Operand)); 236 return IVUses.back(); 237} 238 239IVUsers::IVUsers() 240 : LoopPass(ID) { 241 initializeIVUsersPass(*PassRegistry::getPassRegistry()); 242} 243 244void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 245 AU.addRequired<LoopInfo>(); 246 AU.addRequired<DominatorTree>(); 247 AU.addRequired<ScalarEvolution>(); 248 AU.setPreservesAll(); 249} 250 251bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 252 253 L = l; 254 LI = &getAnalysis<LoopInfo>(); 255 DT = &getAnalysis<DominatorTree>(); 256 SE = &getAnalysis<ScalarEvolution>(); 257 TD = getAnalysisIfAvailable<DataLayout>(); 258 259 // Find all uses of induction variables in this loop, and categorize 260 // them by stride. Start by finding all of the PHI nodes in the header for 261 // this loop. If they are induction variables, inspect their uses. 262 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 263 (void)AddUsersIfInteresting(I); 264 265 return false; 266} 267 268void IVUsers::print(raw_ostream &OS, const Module *M) const { 269 OS << "IV Users for loop "; 270 WriteAsOperand(OS, L->getHeader(), false); 271 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 272 OS << " with backedge-taken count " 273 << *SE->getBackedgeTakenCount(L); 274 } 275 OS << ":\n"; 276 277 for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(), 278 E = IVUses.end(); UI != E; ++UI) { 279 OS << " "; 280 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 281 OS << " = " << *getReplacementExpr(*UI); 282 for (PostIncLoopSet::const_iterator 283 I = UI->PostIncLoops.begin(), 284 E = UI->PostIncLoops.end(); I != E; ++I) { 285 OS << " (post-inc with loop "; 286 WriteAsOperand(OS, (*I)->getHeader(), false); 287 OS << ")"; 288 } 289 OS << " in "; 290 UI->getUser()->print(OS); 291 OS << '\n'; 292 } 293} 294 295#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP) 296void IVUsers::dump() const { 297 print(dbgs()); 298} 299#endif 300 301void IVUsers::releaseMemory() { 302 Processed.clear(); 303 IVUses.clear(); 304} 305 306/// getReplacementExpr - Return a SCEV expression which computes the 307/// value of the OperandValToReplace. 308const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const { 309 return SE->getSCEV(IU.getOperandValToReplace()); 310} 311 312/// getExpr - Return the expression for the use. 313const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const { 314 return 315 TransformForPostIncUse(Normalize, getReplacementExpr(IU), 316 IU.getUser(), IU.getOperandValToReplace(), 317 const_cast<PostIncLoopSet &>(IU.getPostIncLoops()), 318 *SE, *DT); 319} 320 321static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) { 322 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 323 if (AR->getLoop() == L) 324 return AR; 325 return findAddRecForLoop(AR->getStart(), L); 326 } 327 328 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) { 329 for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end(); 330 I != E; ++I) 331 if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L)) 332 return AR; 333 return 0; 334 } 335 336 return 0; 337} 338 339const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const { 340 if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L)) 341 return AR->getStepRecurrence(*SE); 342 return 0; 343} 344 345void IVStrideUse::transformToPostInc(const Loop *L) { 346 PostIncLoops.insert(L); 347} 348 349void IVStrideUse::deleted() { 350 // Remove this user from the list. 351 Parent->Processed.erase(this->getUser()); 352 Parent->IVUses.erase(this); 353 // this now dangles! 354} 355