IVUsers.cpp revision 4e8a98519ebf75ed47456ea42706aaa57ecd2c27
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(SCEVHandle 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 (!LoopInfoBase<BasicBlock>::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 SCEVHandle &SH, Loop *L, Loop *UseLoop, 84 SCEVHandle &Start, SCEVHandle &Stride, 85 ScalarEvolution *SE, DominatorTree *DT) { 86 SCEVHandle 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 SCEVHandle AddRecStart = AddRec->getStart(); 113 AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop); 114 SCEVHandle AddRecStride = AddRec->getStepRecurrence(*SE); 115 AddRecStride = SE->getSCEVAtScope(AddRecStride, UseLoop); 116 117 // FIXME: If Start contains an SCEVAddRecExpr from a different loop, other 118 // than an outer loop of the current loop, reject it. LSR has no concept of 119 // operating on more than one loop at a time so don't confuse it with such 120 // expressions. 121 if (containsAddRecFromDifferentLoop(AddRecStart, L)) 122 return false; 123 124 Start = SE->getAddExpr(Start, AddRecStart); 125 126 // If stride is an instruction, make sure it dominates the loop preheader. 127 // Otherwise we could end up with a use before def situation. 128 if (!isa<SCEVConstant>(AddRecStride)) { 129 BasicBlock *Preheader = L->getLoopPreheader(); 130 if (!AddRecStride->dominates(Preheader, DT)) 131 return false; 132 133 DOUT << "[" << L->getHeader()->getName() 134 << "] Variable stride: " << *AddRec << "\n"; 135 } 136 137 Stride = AddRecStride; 138 return true; 139} 140 141/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression 142/// and now we need to decide whether the user should use the preinc or post-inc 143/// value. If this user should use the post-inc version of the IV, return true. 144/// 145/// Choosing wrong here can break dominance properties (if we choose to use the 146/// post-inc value when we cannot) or it can end up adding extra live-ranges to 147/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we 148/// should use the post-inc value). 149static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV, 150 Loop *L, LoopInfo *LI, DominatorTree *DT, 151 Pass *P) { 152 // If the user is in the loop, use the preinc value. 153 if (L->contains(User->getParent())) return false; 154 155 BasicBlock *LatchBlock = L->getLoopLatch(); 156 157 // Ok, the user is outside of the loop. If it is dominated by the latch 158 // block, use the post-inc value. 159 if (DT->dominates(LatchBlock, User->getParent())) 160 return true; 161 162 // There is one case we have to be careful of: PHI nodes. These little guys 163 // can live in blocks that are not dominated by the latch block, but (since 164 // their uses occur in the predecessor block, not the block the PHI lives in) 165 // should still use the post-inc value. Check for this case now. 166 PHINode *PN = dyn_cast<PHINode>(User); 167 if (!PN) return false; // not a phi, not dominated by latch block. 168 169 // Look at all of the uses of IV by the PHI node. If any use corresponds to 170 // a block that is not dominated by the latch block, give up and use the 171 // preincremented value. 172 unsigned NumUses = 0; 173 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 174 if (PN->getIncomingValue(i) == IV) { 175 ++NumUses; 176 if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i))) 177 return false; 178 } 179 180 // Okay, all uses of IV by PN are in predecessor blocks that really are 181 // dominated by the latch block. Use the post-incremented value. 182 return true; 183} 184 185/// AddUsersIfInteresting - Inspect the specified instruction. If it is a 186/// reducible SCEV, recursively add its users to the IVUsesByStride set and 187/// return true. Otherwise, return false. 188bool IVUsers::AddUsersIfInteresting(Instruction *I) { 189 if (!SE->isSCEVable(I->getType())) 190 return false; // Void and FP expressions cannot be reduced. 191 192 // LSR is not APInt clean, do not touch integers bigger than 64-bits. 193 if (SE->getTypeSizeInBits(I->getType()) > 64) 194 return false; 195 196 if (!Processed.insert(I)) 197 return true; // Instruction already handled. 198 199 // Get the symbolic expression for this instruction. 200 SCEVHandle ISE = SE->getSCEV(I); 201 if (isa<SCEVCouldNotCompute>(ISE)) return false; 202 203 // Get the start and stride for this expression. 204 Loop *UseLoop = LI->getLoopFor(I->getParent()); 205 SCEVHandle Start = SE->getIntegerSCEV(0, ISE->getType()); 206 SCEVHandle Stride = Start; 207 208 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT)) 209 return false; // Non-reducible symbolic expression, bail out. 210 211 SmallPtrSet<Instruction *, 4> UniqueUsers; 212 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 213 UI != E; ++UI) { 214 Instruction *User = cast<Instruction>(*UI); 215 if (!UniqueUsers.insert(User)) 216 continue; 217 218 // Do not infinitely recurse on PHI nodes. 219 if (isa<PHINode>(User) && Processed.count(User)) 220 continue; 221 222 // Descend recursively, but not into PHI nodes outside the current loop. 223 // It's important to see the entire expression outside the loop to get 224 // choices that depend on addressing mode use right, although we won't 225 // consider references ouside the loop in all cases. 226 // If User is already in Processed, we don't want to recurse into it again, 227 // but do want to record a second reference in the same instruction. 228 bool AddUserToIVUsers = false; 229 if (LI->getLoopFor(User->getParent()) != L) { 230 if (isa<PHINode>(User) || Processed.count(User) || 231 !AddUsersIfInteresting(User)) { 232 DOUT << "FOUND USER in other loop: " << *User 233 << " OF SCEV: " << *ISE << "\n"; 234 AddUserToIVUsers = true; 235 } 236 } else if (Processed.count(User) || 237 !AddUsersIfInteresting(User)) { 238 DOUT << "FOUND USER: " << *User 239 << " OF SCEV: " << *ISE << "\n"; 240 AddUserToIVUsers = true; 241 } 242 243 if (AddUserToIVUsers) { 244 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 245 if (!StrideUses) { // First occurrence of this stride? 246 StrideOrder.push_back(Stride); 247 StrideUses = new IVUsersOfOneStride(Stride); 248 IVUses.push_back(StrideUses); 249 IVUsesByStride[Stride] = StrideUses; 250 } 251 252 // Okay, we found a user that we cannot reduce. Analyze the instruction 253 // and decide what to do with it. If we are a use inside of the loop, use 254 // the value before incrementation, otherwise use it after incrementation. 255 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) { 256 // The value used will be incremented by the stride more than we are 257 // expecting, so subtract this off. 258 SCEVHandle NewStart = SE->getMinusSCEV(Start, Stride); 259 StrideUses->addUser(NewStart, User, I); 260 StrideUses->Users.back().setIsUseOfPostIncrementedValue(true); 261 DOUT << " USING POSTINC SCEV, START=" << *NewStart<< "\n"; 262 } else { 263 StrideUses->addUser(Start, User, I); 264 } 265 } 266 } 267 return true; 268} 269 270IVUsers::IVUsers() 271 : LoopPass(&ID) { 272} 273 274void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 275 AU.addRequired<LoopInfo>(); 276 AU.addRequired<DominatorTree>(); 277 AU.addRequired<ScalarEvolution>(); 278 AU.setPreservesAll(); 279} 280 281bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 282 283 L = l; 284 LI = &getAnalysis<LoopInfo>(); 285 DT = &getAnalysis<DominatorTree>(); 286 SE = &getAnalysis<ScalarEvolution>(); 287 288 // Find all uses of induction variables in this loop, and categorize 289 // them by stride. Start by finding all of the PHI nodes in the header for 290 // this loop. If they are induction variables, inspect their uses. 291 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 292 AddUsersIfInteresting(I); 293 294 return false; 295} 296 297/// getReplacementExpr - Return a SCEV expression which computes the 298/// value of the OperandValToReplace of the given IVStrideUse. 299SCEVHandle IVUsers::getReplacementExpr(const IVStrideUse &U) const { 300 // Start with zero. 301 SCEVHandle RetVal = SE->getIntegerSCEV(0, U.getParent()->Stride->getType()); 302 // Create the basic add recurrence. 303 RetVal = SE->getAddRecExpr(RetVal, U.getParent()->Stride, L); 304 // Add the offset in a separate step, because it may be loop-variant. 305 RetVal = SE->getAddExpr(RetVal, U.getOffset()); 306 // For uses of post-incremented values, add an extra stride to compute 307 // the actual replacement value. 308 if (U.isUseOfPostIncrementedValue()) 309 RetVal = SE->getAddExpr(RetVal, U.getParent()->Stride); 310 // Evaluate the expression out of the loop, if possible. 311 if (!L->contains(U.getUser()->getParent())) { 312 SCEVHandle ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop()); 313 if (ExitVal->isLoopInvariant(L)) 314 RetVal = ExitVal; 315 } 316 return RetVal; 317} 318 319void IVUsers::print(raw_ostream &OS, const Module *M) const { 320 OS << "IV Users for loop "; 321 WriteAsOperand(OS, L->getHeader(), false); 322 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 323 OS << " with backedge-taken count " 324 << *SE->getBackedgeTakenCount(L); 325 } 326 OS << ":\n"; 327 328 for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) { 329 std::map<SCEVHandle, IVUsersOfOneStride*>::const_iterator SI = 330 IVUsesByStride.find(StrideOrder[Stride]); 331 assert(SI != IVUsesByStride.end() && "Stride doesn't exist!"); 332 OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n"; 333 334 for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(), 335 E = SI->second->Users.end(); UI != E; ++UI) { 336 OS << " "; 337 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 338 OS << " = "; 339 OS << *getReplacementExpr(*UI); 340 if (UI->isUseOfPostIncrementedValue()) 341 OS << " (post-inc)"; 342 OS << " in "; 343 UI->getUser()->print(OS); 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