IVUsers.cpp revision 00cb67359fc2515eb972cd41fa35f8a0d2b22b87
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/LoopPass.h" 23#include "llvm/Analysis/ScalarEvolutionExpressions.h" 24#include "llvm/ADT/STLExtras.h" 25#include "llvm/Support/Debug.h" 26#include "llvm/Support/raw_ostream.h" 27#include <algorithm> 28using namespace llvm; 29 30char IVUsers::ID = 0; 31static RegisterPass<IVUsers> 32X("iv-users", "Induction Variable Users", false, true); 33 34Pass *llvm::createIVUsersPass() { 35 return new IVUsers(); 36} 37 38/// containsAddRecFromDifferentLoop - Determine whether expression S involves a 39/// subexpression that is an AddRec from a loop other than L. An outer loop 40/// of L is OK, but not an inner loop nor a disjoint loop. 41static bool containsAddRecFromDifferentLoop(const SCEV *S, Loop *L) { 42 // This is very common, put it first. 43 if (isa<SCEVConstant>(S)) 44 return false; 45 if (const SCEVCommutativeExpr *AE = dyn_cast<SCEVCommutativeExpr>(S)) { 46 for (unsigned int i=0; i< AE->getNumOperands(); i++) 47 if (containsAddRecFromDifferentLoop(AE->getOperand(i), L)) 48 return true; 49 return false; 50 } 51 if (const SCEVAddRecExpr *AE = dyn_cast<SCEVAddRecExpr>(S)) { 52 if (const Loop *newLoop = AE->getLoop()) { 53 if (newLoop == L) 54 return false; 55 // if newLoop is an outer loop of L, this is OK. 56 if (!LoopInfo::isNotAlreadyContainedIn(L, newLoop)) 57 return false; 58 } 59 return true; 60 } 61 if (const SCEVUDivExpr *DE = dyn_cast<SCEVUDivExpr>(S)) 62 return containsAddRecFromDifferentLoop(DE->getLHS(), L) || 63 containsAddRecFromDifferentLoop(DE->getRHS(), L); 64#if 0 65 // SCEVSDivExpr has been backed out temporarily, but will be back; we'll 66 // need this when it is. 67 if (const SCEVSDivExpr *DE = dyn_cast<SCEVSDivExpr>(S)) 68 return containsAddRecFromDifferentLoop(DE->getLHS(), L) || 69 containsAddRecFromDifferentLoop(DE->getRHS(), L); 70#endif 71 if (const SCEVCastExpr *CE = dyn_cast<SCEVCastExpr>(S)) 72 return containsAddRecFromDifferentLoop(CE->getOperand(), L); 73 return false; 74} 75 76/// getSCEVStartAndStride - Compute the start and stride of this expression, 77/// returning false if the expression is not a start/stride pair, or true if it 78/// is. The stride must be a loop invariant expression, but the start may be 79/// a mix of loop invariant and loop variant expressions. The start cannot, 80/// however, contain an AddRec from a different loop, unless that loop is an 81/// outer loop of the current loop. 82static bool getSCEVStartAndStride(const SCEV *&SH, Loop *L, Loop *UseLoop, 83 const SCEV *&Start, const SCEV *&Stride, 84 ScalarEvolution *SE, DominatorTree *DT) { 85 const SCEV *TheAddRec = Start; // Initialize to zero. 86 87 // If the outer level is an AddExpr, the operands are all start values except 88 // for a nested AddRecExpr. 89 if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) { 90 for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i) 91 if (const SCEVAddRecExpr *AddRec = 92 dyn_cast<SCEVAddRecExpr>(AE->getOperand(i))) { 93 if (AddRec->getLoop() == L) 94 TheAddRec = SE->getAddExpr(AddRec, TheAddRec); 95 else 96 return false; // Nested IV of some sort? 97 } else { 98 Start = SE->getAddExpr(Start, AE->getOperand(i)); 99 } 100 } else if (isa<SCEVAddRecExpr>(SH)) { 101 TheAddRec = SH; 102 } else { 103 return false; // not analyzable. 104 } 105 106 const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(TheAddRec); 107 if (!AddRec || AddRec->getLoop() != L) return false; 108 109 // Use getSCEVAtScope to attempt to simplify other loops out of 110 // the picture. 111 const SCEV *AddRecStart = AddRec->getStart(); 112 AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop); 113 const SCEV *AddRecStride = AddRec->getStepRecurrence(*SE); 114 115 // FIXME: If Start contains an SCEVAddRecExpr from a different loop, other 116 // than an outer loop of the current loop, reject it. LSR has no concept of 117 // operating on more than one loop at a time so don't confuse it with such 118 // expressions. 119 if (containsAddRecFromDifferentLoop(AddRecStart, L)) 120 return false; 121 122 Start = SE->getAddExpr(Start, AddRecStart); 123 124 // If stride is an instruction, make sure it properly dominates the header. 125 // Otherwise we could end up with a use before def situation. 126 if (!isa<SCEVConstant>(AddRecStride)) { 127 BasicBlock *Header = L->getHeader(); 128 if (!AddRecStride->properlyDominates(Header, DT)) 129 return false; 130 131 DEBUG(errs() << "[" << L->getHeader()->getName() 132 << "] Variable stride: " << *AddRec << "\n"); 133 } 134 135 Stride = AddRecStride; 136 return true; 137} 138 139/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression 140/// and now we need to decide whether the user should use the preinc or post-inc 141/// value. If this user should use the post-inc version of the IV, return true. 142/// 143/// Choosing wrong here can break dominance properties (if we choose to use the 144/// post-inc value when we cannot) or it can end up adding extra live-ranges to 145/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we 146/// should use the post-inc value). 147static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV, 148 Loop *L, LoopInfo *LI, DominatorTree *DT, 149 Pass *P) { 150 // If the user is in the loop, use the preinc value. 151 if (L->contains(User->getParent())) return false; 152 153 BasicBlock *LatchBlock = L->getLoopLatch(); 154 155 // Ok, the user is outside of the loop. If it is dominated by the latch 156 // block, use the post-inc value. 157 if (DT->dominates(LatchBlock, User->getParent())) 158 return true; 159 160 // There is one case we have to be careful of: PHI nodes. These little guys 161 // can live in blocks that are not dominated by the latch block, but (since 162 // their uses occur in the predecessor block, not the block the PHI lives in) 163 // should still use the post-inc value. Check for this case now. 164 PHINode *PN = dyn_cast<PHINode>(User); 165 if (!PN) return false; // not a phi, not dominated by latch block. 166 167 // Look at all of the uses of IV by the PHI node. If any use corresponds to 168 // a block that is not dominated by the latch block, give up and use the 169 // preincremented value. 170 unsigned NumUses = 0; 171 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 172 if (PN->getIncomingValue(i) == IV) { 173 ++NumUses; 174 if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i))) 175 return false; 176 } 177 178 // Okay, all uses of IV by PN are in predecessor blocks that really are 179 // dominated by the latch block. Use the post-incremented value. 180 return true; 181} 182 183/// AddUsersIfInteresting - Inspect the specified instruction. If it is a 184/// reducible SCEV, recursively add its users to the IVUsesByStride set and 185/// return true. Otherwise, return false. 186bool IVUsers::AddUsersIfInteresting(Instruction *I) { 187 if (!SE->isSCEVable(I->getType())) 188 return false; // Void and FP expressions cannot be reduced. 189 190 // LSR is not APInt clean, do not touch integers bigger than 64-bits. 191 if (SE->getTypeSizeInBits(I->getType()) > 64) 192 return false; 193 194 if (!Processed.insert(I)) 195 return true; // Instruction already handled. 196 197 // Get the symbolic expression for this instruction. 198 const SCEV *ISE = SE->getSCEV(I); 199 if (isa<SCEVCouldNotCompute>(ISE)) return false; 200 201 // Get the start and stride for this expression. 202 Loop *UseLoop = LI->getLoopFor(I->getParent()); 203 const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType()); 204 const SCEV *Stride = Start; 205 206 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT)) 207 return false; // Non-reducible symbolic expression, bail out. 208 209 SmallPtrSet<Instruction *, 4> UniqueUsers; 210 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 211 UI != E; ++UI) { 212 Instruction *User = cast<Instruction>(*UI); 213 if (!UniqueUsers.insert(User)) 214 continue; 215 216 // Do not infinitely recurse on PHI nodes. 217 if (isa<PHINode>(User) && Processed.count(User)) 218 continue; 219 220 // Descend recursively, but not into PHI nodes outside the current loop. 221 // It's important to see the entire expression outside the loop to get 222 // choices that depend on addressing mode use right, although we won't 223 // consider references ouside the loop in all cases. 224 // If User is already in Processed, we don't want to recurse into it again, 225 // but do want to record a second reference in the same instruction. 226 bool AddUserToIVUsers = false; 227 if (LI->getLoopFor(User->getParent()) != L) { 228 if (isa<PHINode>(User) || Processed.count(User) || 229 !AddUsersIfInteresting(User)) { 230 DEBUG(errs() << "FOUND USER in other loop: " << *User << '\n' 231 << " OF SCEV: " << *ISE << '\n'); 232 AddUserToIVUsers = true; 233 } 234 } else if (Processed.count(User) || 235 !AddUsersIfInteresting(User)) { 236 DEBUG(errs() << "FOUND USER: " << *User << '\n' 237 << " OF SCEV: " << *ISE << '\n'); 238 AddUserToIVUsers = true; 239 } 240 241 if (AddUserToIVUsers) { 242 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 243 if (!StrideUses) { // First occurrence of this stride? 244 StrideOrder.push_back(Stride); 245 StrideUses = new IVUsersOfOneStride(Stride); 246 IVUses.push_back(StrideUses); 247 IVUsesByStride[Stride] = StrideUses; 248 } 249 250 // Okay, we found a user that we cannot reduce. Analyze the instruction 251 // and decide what to do with it. If we are a use inside of the loop, use 252 // the value before incrementation, otherwise use it after incrementation. 253 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) { 254 // The value used will be incremented by the stride more than we are 255 // expecting, so subtract this off. 256 const SCEV *NewStart = SE->getMinusSCEV(Start, Stride); 257 StrideUses->addUser(NewStart, User, I); 258 StrideUses->Users.back().setIsUseOfPostIncrementedValue(true); 259 DEBUG(errs() << " USING POSTINC SCEV, START=" << *NewStart<< "\n"); 260 } else { 261 StrideUses->addUser(Start, User, I); 262 } 263 } 264 } 265 return true; 266} 267 268IVUsers::IVUsers() 269 : LoopPass(&ID) { 270} 271 272void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 273 AU.addRequired<LoopInfo>(); 274 AU.addRequired<DominatorTree>(); 275 AU.addRequired<ScalarEvolution>(); 276 AU.setPreservesAll(); 277} 278 279bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 280 281 L = l; 282 LI = &getAnalysis<LoopInfo>(); 283 DT = &getAnalysis<DominatorTree>(); 284 SE = &getAnalysis<ScalarEvolution>(); 285 286 // Find all uses of induction variables in this loop, and categorize 287 // them by stride. Start by finding all of the PHI nodes in the header for 288 // this loop. If they are induction variables, inspect their uses. 289 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 290 AddUsersIfInteresting(I); 291 292 return false; 293} 294 295/// getReplacementExpr - Return a SCEV expression which computes the 296/// value of the OperandValToReplace of the given IVStrideUse. 297const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const { 298 // Start with zero. 299 const SCEV *RetVal = SE->getIntegerSCEV(0, U.getParent()->Stride->getType()); 300 // Create the basic add recurrence. 301 RetVal = SE->getAddRecExpr(RetVal, U.getParent()->Stride, L); 302 // Add the offset in a separate step, because it may be loop-variant. 303 RetVal = SE->getAddExpr(RetVal, U.getOffset()); 304 // For uses of post-incremented values, add an extra stride to compute 305 // the actual replacement value. 306 if (U.isUseOfPostIncrementedValue()) 307 RetVal = SE->getAddExpr(RetVal, U.getParent()->Stride); 308 // Evaluate the expression out of the loop, if possible. 309 if (!L->contains(U.getUser()->getParent())) { 310 const SCEV *ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop()); 311 if (ExitVal->isLoopInvariant(L)) 312 RetVal = ExitVal; 313 } 314 return RetVal; 315} 316 317void IVUsers::print(raw_ostream &OS, const Module *M) const { 318 OS << "IV Users for loop "; 319 WriteAsOperand(OS, L->getHeader(), false); 320 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 321 OS << " with backedge-taken count " 322 << *SE->getBackedgeTakenCount(L); 323 } 324 OS << ":\n"; 325 326 for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) { 327 std::map<const SCEV *, IVUsersOfOneStride*>::const_iterator SI = 328 IVUsesByStride.find(StrideOrder[Stride]); 329 assert(SI != IVUsesByStride.end() && "Stride doesn't exist!"); 330 OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n"; 331 332 for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(), 333 E = SI->second->Users.end(); UI != E; ++UI) { 334 OS << " "; 335 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 336 OS << " = "; 337 OS << *getReplacementExpr(*UI); 338 if (UI->isUseOfPostIncrementedValue()) 339 OS << " (post-inc)"; 340 OS << " in "; 341 UI->getUser()->print(OS); 342 OS << '\n'; 343 } 344 } 345} 346 347void IVUsers::dump() const { 348 print(errs()); 349} 350 351void IVUsers::releaseMemory() { 352 IVUsesByStride.clear(); 353 StrideOrder.clear(); 354 Processed.clear(); 355} 356 357void IVStrideUse::deleted() { 358 // Remove this user from the list. 359 Parent->Users.erase(this); 360 // this now dangles! 361} 362