IVUsers.cpp revision d9ef1a860e01321cacbd57b6f7c154f399c813d4
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/Constants.h" 18#include "llvm/Instructions.h" 19#include "llvm/Type.h" 20#include "llvm/DerivedTypes.h" 21#include "llvm/Analysis/Dominators.h" 22#include "llvm/Analysis/LoopInfo.h" 23#include "llvm/Analysis/LoopPass.h" 24#include "llvm/Analysis/ScalarEvolutionExpressions.h" 25#include "llvm/ADT/STLExtras.h" 26#include "llvm/Support/Debug.h" 27#include "llvm/Support/raw_ostream.h" 28#include <algorithm> 29using namespace llvm; 30 31char IVUsers::ID = 0; 32static RegisterPass<IVUsers> 33X("iv-users", "Induction Variable Users", false, true); 34 35Pass *llvm::createIVUsersPass() { 36 return new IVUsers(); 37} 38 39/// containsAddRecFromDifferentLoop - Determine whether expression S involves a 40/// subexpression that is an AddRec from a loop other than L. An outer loop 41/// of L is OK, but not an inner loop nor a disjoint loop. 42static bool containsAddRecFromDifferentLoop(const SCEV *S, Loop *L) { 43 // This is very common, put it first. 44 if (isa<SCEVConstant>(S)) 45 return false; 46 if (const SCEVCommutativeExpr *AE = dyn_cast<SCEVCommutativeExpr>(S)) { 47 for (unsigned int i=0; i< AE->getNumOperands(); i++) 48 if (containsAddRecFromDifferentLoop(AE->getOperand(i), L)) 49 return true; 50 return false; 51 } 52 if (const SCEVAddRecExpr *AE = dyn_cast<SCEVAddRecExpr>(S)) { 53 if (const Loop *newLoop = AE->getLoop()) { 54 if (newLoop == L) 55 return false; 56 // if newLoop is an outer loop of L, this is OK. 57 if (!LoopInfo::isNotAlreadyContainedIn(L, newLoop)) 58 return false; 59 } 60 return true; 61 } 62 if (const SCEVUDivExpr *DE = dyn_cast<SCEVUDivExpr>(S)) 63 return containsAddRecFromDifferentLoop(DE->getLHS(), L) || 64 containsAddRecFromDifferentLoop(DE->getRHS(), L); 65#if 0 66 // SCEVSDivExpr has been backed out temporarily, but will be back; we'll 67 // need this when it is. 68 if (const SCEVSDivExpr *DE = dyn_cast<SCEVSDivExpr>(S)) 69 return containsAddRecFromDifferentLoop(DE->getLHS(), L) || 70 containsAddRecFromDifferentLoop(DE->getRHS(), L); 71#endif 72 if (const SCEVCastExpr *CE = dyn_cast<SCEVCastExpr>(S)) 73 return containsAddRecFromDifferentLoop(CE->getOperand(), L); 74 return false; 75} 76 77/// getSCEVStartAndStride - Compute the start and stride of this expression, 78/// returning false if the expression is not a start/stride pair, or true if it 79/// is. The stride must be a loop invariant expression, but the start may be 80/// a mix of loop invariant and loop variant expressions. The start cannot, 81/// however, contain an AddRec from a different loop, unless that loop is an 82/// outer loop of the current loop. 83static bool getSCEVStartAndStride(const SCEV *&SH, Loop *L, Loop *UseLoop, 84 const SCEV *&Start, const SCEV *&Stride, 85 ScalarEvolution *SE, DominatorTree *DT) { 86 const SCEV *TheAddRec = Start; // Initialize to zero. 87 88 // If the outer level is an AddExpr, the operands are all start values except 89 // for a nested AddRecExpr. 90 if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) { 91 for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i) 92 if (const SCEVAddRecExpr *AddRec = 93 dyn_cast<SCEVAddRecExpr>(AE->getOperand(i))) { 94 if (AddRec->getLoop() == L) 95 TheAddRec = SE->getAddExpr(AddRec, TheAddRec); 96 else 97 return false; // Nested IV of some sort? 98 } else { 99 Start = SE->getAddExpr(Start, AE->getOperand(i)); 100 } 101 } else if (isa<SCEVAddRecExpr>(SH)) { 102 TheAddRec = SH; 103 } else { 104 return false; // not analyzable. 105 } 106 107 const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(TheAddRec); 108 if (!AddRec || AddRec->getLoop() != L) return false; 109 110 // Use getSCEVAtScope to attempt to simplify other loops out of 111 // the picture. 112 const SCEV *AddRecStart = AddRec->getStart(); 113 AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop); 114 const SCEV *AddRecStride = AddRec->getStepRecurrence(*SE); 115 116 // FIXME: If Start contains an SCEVAddRecExpr from a different loop, other 117 // than an outer loop of the current loop, reject it. LSR has no concept of 118 // operating on more than one loop at a time so don't confuse it with such 119 // expressions. 120 if (containsAddRecFromDifferentLoop(AddRecStart, L)) 121 return false; 122 123 Start = SE->getAddExpr(Start, AddRecStart); 124 125 // If stride is an instruction, make sure it dominates the loop preheader. 126 // Otherwise we could end up with a use before def situation. 127 if (!isa<SCEVConstant>(AddRecStride)) { 128 BasicBlock *Preheader = L->getLoopPreheader(); 129 if (!AddRecStride->dominates(Preheader, DT)) 130 return false; 131 132 DOUT << "[" << L->getHeader()->getName() 133 << "] Variable stride: " << *AddRec << "\n"; 134 } 135 136 Stride = AddRecStride; 137 return true; 138} 139 140/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression 141/// and now we need to decide whether the user should use the preinc or post-inc 142/// value. If this user should use the post-inc version of the IV, return true. 143/// 144/// Choosing wrong here can break dominance properties (if we choose to use the 145/// post-inc value when we cannot) or it can end up adding extra live-ranges to 146/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we 147/// should use the post-inc value). 148static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV, 149 Loop *L, LoopInfo *LI, DominatorTree *DT, 150 Pass *P) { 151 // If the user is in the loop, use the preinc value. 152 if (L->contains(User->getParent())) return false; 153 154 BasicBlock *LatchBlock = L->getLoopLatch(); 155 156 // Ok, the user is outside of the loop. If it is dominated by the latch 157 // block, use the post-inc value. 158 if (DT->dominates(LatchBlock, User->getParent())) 159 return true; 160 161 // There is one case we have to be careful of: PHI nodes. These little guys 162 // can live in blocks that are not dominated by the latch block, but (since 163 // their uses occur in the predecessor block, not the block the PHI lives in) 164 // should still use the post-inc value. Check for this case now. 165 PHINode *PN = dyn_cast<PHINode>(User); 166 if (!PN) return false; // not a phi, not dominated by latch block. 167 168 // Look at all of the uses of IV by the PHI node. If any use corresponds to 169 // a block that is not dominated by the latch block, give up and use the 170 // preincremented value. 171 unsigned NumUses = 0; 172 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 173 if (PN->getIncomingValue(i) == IV) { 174 ++NumUses; 175 if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i))) 176 return false; 177 } 178 179 // Okay, all uses of IV by PN are in predecessor blocks that really are 180 // dominated by the latch block. Use the post-incremented value. 181 return true; 182} 183 184/// AddUsersIfInteresting - Inspect the specified instruction. If it is a 185/// reducible SCEV, recursively add its users to the IVUsesByStride set and 186/// return true. Otherwise, return false. 187bool IVUsers::AddUsersIfInteresting(Instruction *I) { 188 if (!SE->isSCEVable(I->getType())) 189 return false; // Void and FP expressions cannot be reduced. 190 191 // LSR is not APInt clean, do not touch integers bigger than 64-bits. 192 if (SE->getTypeSizeInBits(I->getType()) > 64) 193 return false; 194 195 if (!Processed.insert(I)) 196 return true; // Instruction already handled. 197 198 // Get the symbolic expression for this instruction. 199 const SCEV *ISE = SE->getSCEV(I); 200 if (isa<SCEVCouldNotCompute>(ISE)) return false; 201 202 // Get the start and stride for this expression. 203 Loop *UseLoop = LI->getLoopFor(I->getParent()); 204 const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType()); 205 const SCEV *Stride = Start; 206 207 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT)) 208 return false; // Non-reducible symbolic expression, bail out. 209 210 SmallPtrSet<Instruction *, 4> UniqueUsers; 211 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 212 UI != E; ++UI) { 213 Instruction *User = cast<Instruction>(*UI); 214 if (!UniqueUsers.insert(User)) 215 continue; 216 217 // Do not infinitely recurse on PHI nodes. 218 if (isa<PHINode>(User) && Processed.count(User)) 219 continue; 220 221 // Descend recursively, but not into PHI nodes outside the current loop. 222 // It's important to see the entire expression outside the loop to get 223 // choices that depend on addressing mode use right, although we won't 224 // consider references ouside the loop in all cases. 225 // If User is already in Processed, we don't want to recurse into it again, 226 // but do want to record a second reference in the same instruction. 227 bool AddUserToIVUsers = false; 228 if (LI->getLoopFor(User->getParent()) != L) { 229 if (isa<PHINode>(User) || Processed.count(User) || 230 !AddUsersIfInteresting(User)) { 231 DOUT << "FOUND USER in other loop: " << *User 232 << " OF SCEV: " << *ISE << "\n"; 233 AddUserToIVUsers = true; 234 } 235 } else if (Processed.count(User) || 236 !AddUsersIfInteresting(User)) { 237 DOUT << "FOUND USER: " << *User 238 << " OF SCEV: " << *ISE << "\n"; 239 AddUserToIVUsers = true; 240 } 241 242 if (AddUserToIVUsers) { 243 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 244 if (!StrideUses) { // First occurrence of this stride? 245 StrideOrder.push_back(Stride); 246 StrideUses = new IVUsersOfOneStride(Stride); 247 IVUses.push_back(StrideUses); 248 IVUsesByStride[Stride] = StrideUses; 249 } 250 251 // Okay, we found a user that we cannot reduce. Analyze the instruction 252 // and decide what to do with it. If we are a use inside of the loop, use 253 // the value before incrementation, otherwise use it after incrementation. 254 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) { 255 // The value used will be incremented by the stride more than we are 256 // expecting, so subtract this off. 257 const SCEV *NewStart = SE->getMinusSCEV(Start, Stride); 258 StrideUses->addUser(NewStart, User, I); 259 StrideUses->Users.back().setIsUseOfPostIncrementedValue(true); 260 DOUT << " USING POSTINC SCEV, START=" << *NewStart<< "\n"; 261 } else { 262 StrideUses->addUser(Start, User, I); 263 } 264 } 265 } 266 return true; 267} 268 269IVUsers::IVUsers() 270 : LoopPass(&ID) { 271} 272 273void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 274 AU.addRequired<LoopInfo>(); 275 AU.addRequired<DominatorTree>(); 276 AU.addRequired<ScalarEvolution>(); 277 AU.setPreservesAll(); 278} 279 280bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 281 282 L = l; 283 LI = &getAnalysis<LoopInfo>(); 284 DT = &getAnalysis<DominatorTree>(); 285 SE = &getAnalysis<ScalarEvolution>(); 286 287 // Find all uses of induction variables in this loop, and categorize 288 // them by stride. Start by finding all of the PHI nodes in the header for 289 // this loop. If they are induction variables, inspect their uses. 290 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 291 AddUsersIfInteresting(I); 292 293 return false; 294} 295 296/// getReplacementExpr - Return a SCEV expression which computes the 297/// value of the OperandValToReplace of the given IVStrideUse. 298const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const { 299 // Start with zero. 300 const SCEV *RetVal = SE->getIntegerSCEV(0, U.getParent()->Stride->getType()); 301 // Create the basic add recurrence. 302 RetVal = SE->getAddRecExpr(RetVal, U.getParent()->Stride, L); 303 // Add the offset in a separate step, because it may be loop-variant. 304 RetVal = SE->getAddExpr(RetVal, U.getOffset()); 305 // For uses of post-incremented values, add an extra stride to compute 306 // the actual replacement value. 307 if (U.isUseOfPostIncrementedValue()) 308 RetVal = SE->getAddExpr(RetVal, U.getParent()->Stride); 309 // Evaluate the expression out of the loop, if possible. 310 if (!L->contains(U.getUser()->getParent())) { 311 const SCEV *ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop()); 312 if (ExitVal->isLoopInvariant(L)) 313 RetVal = ExitVal; 314 } 315 return RetVal; 316} 317 318void IVUsers::print(raw_ostream &OS, const Module *M) const { 319 OS << "IV Users for loop "; 320 WriteAsOperand(OS, L->getHeader(), false); 321 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 322 OS << " with backedge-taken count " 323 << *SE->getBackedgeTakenCount(L); 324 } 325 OS << ":\n"; 326 327 for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) { 328 std::map<const SCEV *, IVUsersOfOneStride*>::const_iterator SI = 329 IVUsesByStride.find(StrideOrder[Stride]); 330 assert(SI != IVUsesByStride.end() && "Stride doesn't exist!"); 331 OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n"; 332 333 for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(), 334 E = SI->second->Users.end(); UI != E; ++UI) { 335 OS << " "; 336 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 337 OS << " = "; 338 OS << *getReplacementExpr(*UI); 339 if (UI->isUseOfPostIncrementedValue()) 340 OS << " (post-inc)"; 341 OS << " in "; 342 UI->getUser()->print(OS); 343 OS << '\n'; 344 } 345 } 346} 347 348void IVUsers::print(std::ostream &o, const Module *M) const { 349 raw_os_ostream OS(o); 350 print(OS, M); 351} 352 353void IVUsers::dump() const { 354 print(errs()); 355} 356 357void IVUsers::releaseMemory() { 358 IVUsesByStride.clear(); 359 StrideOrder.clear(); 360 Processed.clear(); 361} 362 363void IVStrideUse::deleted() { 364 // Remove this user from the list. 365 Parent->Users.erase(this); 366 // this now dangles! 367} 368