IVUsers.cpp revision 18395d2c3d03115a6b6f1496274304fc95f6178e
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 bool &isSigned, 86 ScalarEvolution *SE, DominatorTree *DT) { 87 SCEVHandle TheAddRec = Start; // Initialize to zero. 88 bool isSExt = false; 89 bool isZExt = false; 90 91 // If the outer level is an AddExpr, the operands are all start values except 92 // for a nested AddRecExpr. 93 if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) { 94 for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i) 95 if (const SCEVAddRecExpr *AddRec = 96 dyn_cast<SCEVAddRecExpr>(AE->getOperand(i))) { 97 if (AddRec->getLoop() == L) 98 TheAddRec = SE->getAddExpr(AddRec, TheAddRec); 99 else 100 return false; // Nested IV of some sort? 101 } else { 102 Start = SE->getAddExpr(Start, AE->getOperand(i)); 103 } 104 105 } else if (const SCEVZeroExtendExpr *Z = dyn_cast<SCEVZeroExtendExpr>(SH)) { 106 TheAddRec = Z->getOperand(); 107 isZExt = true; 108 } else if (const SCEVSignExtendExpr *S = dyn_cast<SCEVSignExtendExpr>(SH)) { 109 TheAddRec = S->getOperand(); 110 isSExt = true; 111 } else if (isa<SCEVAddRecExpr>(SH)) { 112 TheAddRec = SH; 113 } else { 114 return false; // not analyzable. 115 } 116 117 const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(TheAddRec); 118 if (!AddRec || AddRec->getLoop() != L) return false; 119 120 // Use getSCEVAtScope to attempt to simplify other loops out of 121 // the picture. 122 SCEVHandle AddRecStart = AddRec->getStart(); 123 SCEVHandle BetterAddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop); 124 if (!isa<SCEVCouldNotCompute>(BetterAddRecStart)) 125 AddRecStart = BetterAddRecStart; 126 127 // FIXME: If Start contains an SCEVAddRecExpr from a different loop, other 128 // than an outer loop of the current loop, reject it. LSR has no concept of 129 // operating on more than one loop at a time so don't confuse it with such 130 // expressions. 131 if (containsAddRecFromDifferentLoop(AddRecStart, L)) 132 return false; 133 134 if (isSExt || isZExt) 135 Start = SE->getTruncateExpr(Start, AddRec->getType()); 136 137 Start = SE->getAddExpr(Start, AddRecStart); 138 139 if (!isa<SCEVConstant>(AddRec->getStepRecurrence(*SE))) { 140 // If stride is an instruction, make sure it dominates the loop preheader. 141 // Otherwise we could end up with a use before def situation. 142 BasicBlock *Preheader = L->getLoopPreheader(); 143 if (!AddRec->getStepRecurrence(*SE)->dominates(Preheader, DT)) 144 return false; 145 146 DOUT << "[" << L->getHeader()->getName() 147 << "] Variable stride: " << *AddRec << "\n"; 148 } 149 150 Stride = AddRec->getStepRecurrence(*SE); 151 isSigned = isSExt; 152 return true; 153} 154 155/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression 156/// and now we need to decide whether the user should use the preinc or post-inc 157/// value. If this user should use the post-inc version of the IV, return true. 158/// 159/// Choosing wrong here can break dominance properties (if we choose to use the 160/// post-inc value when we cannot) or it can end up adding extra live-ranges to 161/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we 162/// should use the post-inc value). 163static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV, 164 Loop *L, LoopInfo *LI, DominatorTree *DT, 165 Pass *P) { 166 // If the user is in the loop, use the preinc value. 167 if (L->contains(User->getParent())) return false; 168 169 BasicBlock *LatchBlock = L->getLoopLatch(); 170 171 // Ok, the user is outside of the loop. If it is dominated by the latch 172 // block, use the post-inc value. 173 if (DT->dominates(LatchBlock, User->getParent())) 174 return true; 175 176 // There is one case we have to be careful of: PHI nodes. These little guys 177 // can live in blocks that are not dominated by the latch block, but (since 178 // their uses occur in the predecessor block, not the block the PHI lives in) 179 // should still use the post-inc value. Check for this case now. 180 PHINode *PN = dyn_cast<PHINode>(User); 181 if (!PN) return false; // not a phi, not dominated by latch block. 182 183 // Look at all of the uses of IV by the PHI node. If any use corresponds to 184 // a block that is not dominated by the latch block, give up and use the 185 // preincremented value. 186 unsigned NumUses = 0; 187 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 188 if (PN->getIncomingValue(i) == IV) { 189 ++NumUses; 190 if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i))) 191 return false; 192 } 193 194 // Okay, all uses of IV by PN are in predecessor blocks that really are 195 // dominated by the latch block. Use the post-incremented value. 196 return true; 197} 198 199/// AddUsersIfInteresting - Inspect the specified instruction. If it is a 200/// reducible SCEV, recursively add its users to the IVUsesByStride set and 201/// return true. Otherwise, return false. 202bool IVUsers::AddUsersIfInteresting(Instruction *I) { 203 if (!SE->isSCEVable(I->getType())) 204 return false; // Void and FP expressions cannot be reduced. 205 206 // LSR is not APInt clean, do not touch integers bigger than 64-bits. 207 if (SE->getTypeSizeInBits(I->getType()) > 64) 208 return false; 209 210 if (!Processed.insert(I)) 211 return true; // Instruction already handled. 212 213 // Get the symbolic expression for this instruction. 214 SCEVHandle ISE = SE->getSCEV(I); 215 if (isa<SCEVCouldNotCompute>(ISE)) return false; 216 217 // Get the start and stride for this expression. 218 Loop *UseLoop = LI->getLoopFor(I->getParent()); 219 SCEVHandle Start = SE->getIntegerSCEV(0, ISE->getType()); 220 SCEVHandle Stride = Start; 221 bool isSigned = false; // Arbitrary initial value - pacifies compiler. 222 223 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, isSigned, SE, DT)) 224 return false; // Non-reducible symbolic expression, bail out. 225 226 SmallPtrSet<Instruction *, 4> UniqueUsers; 227 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 228 UI != E; ++UI) { 229 Instruction *User = cast<Instruction>(*UI); 230 if (!UniqueUsers.insert(User)) 231 continue; 232 233 // Do not infinitely recurse on PHI nodes. 234 if (isa<PHINode>(User) && Processed.count(User)) 235 continue; 236 237 // Descend recursively, but not into PHI nodes outside the current loop. 238 // It's important to see the entire expression outside the loop to get 239 // choices that depend on addressing mode use right, although we won't 240 // consider references ouside the loop in all cases. 241 // If User is already in Processed, we don't want to recurse into it again, 242 // but do want to record a second reference in the same instruction. 243 bool AddUserToIVUsers = false; 244 if (LI->getLoopFor(User->getParent()) != L) { 245 if (isa<PHINode>(User) || Processed.count(User) || 246 !AddUsersIfInteresting(User)) { 247 DOUT << "FOUND USER in other loop: " << *User 248 << " OF SCEV: " << *ISE << "\n"; 249 AddUserToIVUsers = true; 250 } 251 } else if (Processed.count(User) || 252 !AddUsersIfInteresting(User)) { 253 DOUT << "FOUND USER: " << *User 254 << " OF SCEV: " << *ISE << "\n"; 255 AddUserToIVUsers = true; 256 } 257 258 if (AddUserToIVUsers) { 259 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 260 if (!StrideUses) { // First occurrence of this stride? 261 StrideOrder.push_back(Stride); 262 StrideUses = new IVUsersOfOneStride(Stride); 263 IVUses.push_back(StrideUses); 264 IVUsesByStride[Stride] = StrideUses; 265 } 266 267 // Okay, we found a user that we cannot reduce. Analyze the instruction 268 // and decide what to do with it. If we are a use inside of the loop, use 269 // the value before incrementation, otherwise use it after incrementation. 270 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) { 271 // The value used will be incremented by the stride more than we are 272 // expecting, so subtract this off. 273 SCEVHandle NewStart = SE->getMinusSCEV(Start, Stride); 274 StrideUses->addUser(NewStart, User, I, isSigned); 275 StrideUses->Users.back().setIsUseOfPostIncrementedValue(true); 276 DOUT << " USING POSTINC SCEV, START=" << *NewStart<< "\n"; 277 } else { 278 StrideUses->addUser(Start, User, I, isSigned); 279 } 280 } 281 } 282 return true; 283} 284 285IVUsers::IVUsers() 286 : LoopPass(&ID) { 287} 288 289void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 290 AU.addRequired<LoopInfo>(); 291 AU.addRequired<DominatorTree>(); 292 AU.addRequired<ScalarEvolution>(); 293 AU.setPreservesAll(); 294} 295 296bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 297 298 L = l; 299 LI = &getAnalysis<LoopInfo>(); 300 DT = &getAnalysis<DominatorTree>(); 301 SE = &getAnalysis<ScalarEvolution>(); 302 303 // Find all uses of induction variables in this loop, and categorize 304 // them by stride. Start by finding all of the PHI nodes in the header for 305 // this loop. If they are induction variables, inspect their uses. 306 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 307 AddUsersIfInteresting(I); 308 309 return false; 310} 311 312/// getReplacementExpr - Return a SCEV expression which computes the 313/// value of the OperandValToReplace of the given IVStrideUse. 314SCEVHandle IVUsers::getReplacementExpr(const IVStrideUse &U) const { 315 const Type *UseTy = U.getOperandValToReplace()->getType(); 316 // Start with zero. 317 SCEVHandle RetVal = SE->getIntegerSCEV(0, U.getParent()->Stride->getType()); 318 // Create the basic add recurrence. 319 RetVal = SE->getAddRecExpr(RetVal, U.getParent()->Stride, L); 320 // Add the offset in a separate step, because it may be loop-variant. 321 RetVal = SE->getAddExpr(RetVal, U.getOffset()); 322 // For uses of post-incremented values, add an extra stride to compute 323 // the actual replacement value. 324 if (U.isUseOfPostIncrementedValue()) 325 RetVal = SE->getAddExpr(RetVal, U.getParent()->Stride); 326 // Evaluate the expression out of the loop, if possible. 327 if (!L->contains(U.getUser()->getParent())) { 328 SCEVHandle ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop()); 329 if (!isa<SCEVCouldNotCompute>(ExitVal) && ExitVal->isLoopInvariant(L)) 330 RetVal = ExitVal; 331 } 332 // Promote the result to the type of the use. 333 if (SE->getTypeSizeInBits(RetVal->getType()) != 334 SE->getTypeSizeInBits(UseTy)) { 335 if (U.isSigned()) 336 RetVal = SE->getSignExtendExpr(RetVal, UseTy); 337 else 338 RetVal = SE->getZeroExtendExpr(RetVal, UseTy); 339 } 340 return RetVal; 341} 342 343void IVUsers::print(raw_ostream &OS, const Module *M) const { 344 OS << "IV Users for loop "; 345 WriteAsOperand(OS, L->getHeader(), false); 346 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 347 OS << " with backedge-taken count " 348 << *SE->getBackedgeTakenCount(L); 349 } 350 OS << ":\n"; 351 352 for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) { 353 std::map<SCEVHandle, IVUsersOfOneStride*>::const_iterator SI = 354 IVUsesByStride.find(StrideOrder[Stride]); 355 assert(SI != IVUsesByStride.end() && "Stride doesn't exist!"); 356 OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n"; 357 358 for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(), 359 E = SI->second->Users.end(); UI != E; ++UI) { 360 OS << " "; 361 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 362 OS << " = "; 363 OS << *getReplacementExpr(*UI); 364 if (UI->isUseOfPostIncrementedValue()) 365 OS << " (post-inc)"; 366 OS << " in "; 367 UI->getUser()->print(OS); 368 } 369 } 370} 371 372void IVUsers::print(std::ostream &o, const Module *M) const { 373 raw_os_ostream OS(o); 374 print(OS, M); 375} 376 377void IVUsers::dump() const { 378 print(errs()); 379} 380 381void IVUsers::releaseMemory() { 382 IVUsesByStride.clear(); 383 StrideOrder.clear(); 384 Processed.clear(); 385} 386 387void IVStrideUse::deleted() { 388 // Remove this user from the list. 389 Parent->Users.erase(this); 390 // this now dangles! 391} 392