IVUsers.cpp revision 92329c7fbe572892c17aa2d2542a10e3ea16132f
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 (newLoop->contains(L)) 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)) return false; 152 153 BasicBlock *LatchBlock = L->getLoopLatch(); 154 if (!LatchBlock) 155 return false; 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 const SCEV *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 const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType()); 206 const SCEV *Stride = Start; 207 208 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT)) 209 return false; // Non-reducible symbolic expression, bail out. 210 211 // Keep things simple. Don't touch loop-variant strides. 212 if (!Stride->isLoopInvariant(L) && L->contains(I)) 213 return false; 214 215 SmallPtrSet<Instruction *, 4> UniqueUsers; 216 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 217 UI != E; ++UI) { 218 Instruction *User = cast<Instruction>(*UI); 219 if (!UniqueUsers.insert(User)) 220 continue; 221 222 // Do not infinitely recurse on PHI nodes. 223 if (isa<PHINode>(User) && Processed.count(User)) 224 continue; 225 226 // Descend recursively, but not into PHI nodes outside the current loop. 227 // It's important to see the entire expression outside the loop to get 228 // choices that depend on addressing mode use right, although we won't 229 // consider references ouside the loop in all cases. 230 // If User is already in Processed, we don't want to recurse into it again, 231 // but do want to record a second reference in the same instruction. 232 bool AddUserToIVUsers = false; 233 if (LI->getLoopFor(User->getParent()) != L) { 234 if (isa<PHINode>(User) || Processed.count(User) || 235 !AddUsersIfInteresting(User)) { 236 DEBUG(errs() << "FOUND USER in other loop: " << *User << '\n' 237 << " OF SCEV: " << *ISE << '\n'); 238 AddUserToIVUsers = true; 239 } 240 } else if (Processed.count(User) || 241 !AddUsersIfInteresting(User)) { 242 DEBUG(errs() << "FOUND USER: " << *User << '\n' 243 << " OF SCEV: " << *ISE << '\n'); 244 AddUserToIVUsers = true; 245 } 246 247 if (AddUserToIVUsers) { 248 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 249 if (!StrideUses) { // First occurrence of this stride? 250 StrideOrder.push_back(Stride); 251 StrideUses = new IVUsersOfOneStride(Stride); 252 IVUses.push_back(StrideUses); 253 IVUsesByStride[Stride] = StrideUses; 254 } 255 256 // Okay, we found a user that we cannot reduce. Analyze the instruction 257 // and decide what to do with it. If we are a use inside of the loop, use 258 // the value before incrementation, otherwise use it after incrementation. 259 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) { 260 // The value used will be incremented by the stride more than we are 261 // expecting, so subtract this off. 262 const SCEV *NewStart = SE->getMinusSCEV(Start, Stride); 263 StrideUses->addUser(NewStart, User, I); 264 StrideUses->Users.back().setIsUseOfPostIncrementedValue(true); 265 DEBUG(errs() << " USING POSTINC SCEV, START=" << *NewStart<< "\n"); 266 } else { 267 StrideUses->addUser(Start, User, I); 268 } 269 } 270 } 271 return true; 272} 273 274void IVUsers::AddUser(const SCEV *Stride, const SCEV *Offset, 275 Instruction *User, Value *Operand) { 276 IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride]; 277 if (!StrideUses) { // First occurrence of this stride? 278 StrideOrder.push_back(Stride); 279 StrideUses = new IVUsersOfOneStride(Stride); 280 IVUses.push_back(StrideUses); 281 IVUsesByStride[Stride] = StrideUses; 282 } 283 IVUsesByStride[Stride]->addUser(Offset, User, Operand); 284} 285 286IVUsers::IVUsers() 287 : LoopPass(&ID) { 288} 289 290void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const { 291 AU.addRequired<LoopInfo>(); 292 AU.addRequired<DominatorTree>(); 293 AU.addRequired<ScalarEvolution>(); 294 AU.setPreservesAll(); 295} 296 297bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) { 298 299 L = l; 300 LI = &getAnalysis<LoopInfo>(); 301 DT = &getAnalysis<DominatorTree>(); 302 SE = &getAnalysis<ScalarEvolution>(); 303 304 // Find all uses of induction variables in this loop, and categorize 305 // them by stride. Start by finding all of the PHI nodes in the header for 306 // this loop. If they are induction variables, inspect their uses. 307 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) 308 AddUsersIfInteresting(I); 309 310 return false; 311} 312 313/// getReplacementExpr - Return a SCEV expression which computes the 314/// value of the OperandValToReplace of the given IVStrideUse. 315const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const { 316 // Start with zero. 317 const SCEV *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())) { 328 const SCEV *ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop()); 329 if (ExitVal->isLoopInvariant(L)) 330 RetVal = ExitVal; 331 } 332 return RetVal; 333} 334 335void IVUsers::print(raw_ostream &OS, const Module *M) const { 336 OS << "IV Users for loop "; 337 WriteAsOperand(OS, L->getHeader(), false); 338 if (SE->hasLoopInvariantBackedgeTakenCount(L)) { 339 OS << " with backedge-taken count " 340 << *SE->getBackedgeTakenCount(L); 341 } 342 OS << ":\n"; 343 344 for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) { 345 std::map<const SCEV *, IVUsersOfOneStride*>::const_iterator SI = 346 IVUsesByStride.find(StrideOrder[Stride]); 347 assert(SI != IVUsesByStride.end() && "Stride doesn't exist!"); 348 OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n"; 349 350 for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(), 351 E = SI->second->Users.end(); UI != E; ++UI) { 352 OS << " "; 353 WriteAsOperand(OS, UI->getOperandValToReplace(), false); 354 OS << " = "; 355 OS << *getReplacementExpr(*UI); 356 if (UI->isUseOfPostIncrementedValue()) 357 OS << " (post-inc)"; 358 OS << " in "; 359 UI->getUser()->print(OS); 360 OS << '\n'; 361 } 362 } 363} 364 365void IVUsers::dump() const { 366 print(errs()); 367} 368 369void IVUsers::releaseMemory() { 370 IVUsesByStride.clear(); 371 StrideOrder.clear(); 372 Processed.clear(); 373 IVUses.clear(); 374} 375 376void IVStrideUse::deleted() { 377 // Remove this user from the list. 378 Parent->Users.erase(this); 379 // this now dangles! 380} 381