LLVM API Documentation
00001 //===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file defines the LoopInfo class that is used to identify natural loops 00011 // and determine the loop depth of various nodes of the CFG. Note that the 00012 // loops identified may actually be several natural loops that share the same 00013 // header node... not just a single natural loop. 00014 // 00015 //===----------------------------------------------------------------------===// 00016 00017 #include "llvm/Analysis/LoopInfo.h" 00018 #include "llvm/Constants.h" 00019 #include "llvm/Instructions.h" 00020 #include "llvm/Analysis/Dominators.h" 00021 #include "llvm/Assembly/Writer.h" 00022 #include "llvm/Support/CFG.h" 00023 #include "llvm/ADT/DepthFirstIterator.h" 00024 #include <algorithm> 00025 #include <iostream> 00026 00027 using namespace llvm; 00028 00029 static RegisterAnalysis<LoopInfo> 00030 X("loops", "Natural Loop Construction", true); 00031 00032 //===----------------------------------------------------------------------===// 00033 // Loop implementation 00034 // 00035 bool Loop::contains(const BasicBlock *BB) const { 00036 return std::find(Blocks.begin(), Blocks.end(), BB) != Blocks.end(); 00037 } 00038 00039 bool Loop::isLoopExit(const BasicBlock *BB) const { 00040 for (succ_const_iterator SI = succ_begin(BB), SE = succ_end(BB); 00041 SI != SE; ++SI) { 00042 if (!contains(*SI)) 00043 return true; 00044 } 00045 return false; 00046 } 00047 00048 /// getNumBackEdges - Calculate the number of back edges to the loop header. 00049 /// 00050 unsigned Loop::getNumBackEdges() const { 00051 unsigned NumBackEdges = 0; 00052 BasicBlock *H = getHeader(); 00053 00054 for (pred_iterator I = pred_begin(H), E = pred_end(H); I != E; ++I) 00055 if (contains(*I)) 00056 ++NumBackEdges; 00057 00058 return NumBackEdges; 00059 } 00060 00061 /// isLoopInvariant - Return true if the specified value is loop invariant 00062 /// 00063 bool Loop::isLoopInvariant(Value *V) const { 00064 if (Instruction *I = dyn_cast<Instruction>(V)) 00065 return !contains(I->getParent()); 00066 return true; // All non-instructions are loop invariant 00067 } 00068 00069 void Loop::print(std::ostream &OS, unsigned Depth) const { 00070 OS << std::string(Depth*2, ' ') << "Loop Containing: "; 00071 00072 for (unsigned i = 0; i < getBlocks().size(); ++i) { 00073 if (i) OS << ","; 00074 WriteAsOperand(OS, getBlocks()[i], false); 00075 } 00076 OS << "\n"; 00077 00078 for (iterator I = begin(), E = end(); I != E; ++I) 00079 (*I)->print(OS, Depth+2); 00080 } 00081 00082 void Loop::dump() const { 00083 print(std::cerr); 00084 } 00085 00086 00087 //===----------------------------------------------------------------------===// 00088 // LoopInfo implementation 00089 // 00090 void LoopInfo::stub() {} 00091 00092 bool LoopInfo::runOnFunction(Function &) { 00093 releaseMemory(); 00094 Calculate(getAnalysis<DominatorSet>()); // Update 00095 return false; 00096 } 00097 00098 void LoopInfo::releaseMemory() { 00099 for (std::vector<Loop*>::iterator I = TopLevelLoops.begin(), 00100 E = TopLevelLoops.end(); I != E; ++I) 00101 delete *I; // Delete all of the loops... 00102 00103 BBMap.clear(); // Reset internal state of analysis 00104 TopLevelLoops.clear(); 00105 } 00106 00107 00108 void LoopInfo::Calculate(const DominatorSet &DS) { 00109 BasicBlock *RootNode = DS.getRoot(); 00110 00111 for (df_iterator<BasicBlock*> NI = df_begin(RootNode), 00112 NE = df_end(RootNode); NI != NE; ++NI) 00113 if (Loop *L = ConsiderForLoop(*NI, DS)) 00114 TopLevelLoops.push_back(L); 00115 } 00116 00117 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const { 00118 AU.setPreservesAll(); 00119 AU.addRequired<DominatorSet>(); 00120 } 00121 00122 void LoopInfo::print(std::ostream &OS) const { 00123 for (unsigned i = 0; i < TopLevelLoops.size(); ++i) 00124 TopLevelLoops[i]->print(OS); 00125 #if 0 00126 for (std::map<BasicBlock*, Loop*>::const_iterator I = BBMap.begin(), 00127 E = BBMap.end(); I != E; ++I) 00128 OS << "BB '" << I->first->getName() << "' level = " 00129 << I->second->getLoopDepth() << "\n"; 00130 #endif 00131 } 00132 00133 static bool isNotAlreadyContainedIn(Loop *SubLoop, Loop *ParentLoop) { 00134 if (SubLoop == 0) return true; 00135 if (SubLoop == ParentLoop) return false; 00136 return isNotAlreadyContainedIn(SubLoop->getParentLoop(), ParentLoop); 00137 } 00138 00139 Loop *LoopInfo::ConsiderForLoop(BasicBlock *BB, const DominatorSet &DS) { 00140 if (BBMap.find(BB) != BBMap.end()) return 0; // Haven't processed this node? 00141 00142 std::vector<BasicBlock *> TodoStack; 00143 00144 // Scan the predecessors of BB, checking to see if BB dominates any of 00145 // them. This identifies backedges which target this node... 00146 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) 00147 if (DS.dominates(BB, *I)) // If BB dominates it's predecessor... 00148 TodoStack.push_back(*I); 00149 00150 if (TodoStack.empty()) return 0; // No backedges to this block... 00151 00152 // Create a new loop to represent this basic block... 00153 Loop *L = new Loop(BB); 00154 BBMap[BB] = L; 00155 00156 BasicBlock *EntryBlock = &BB->getParent()->getEntryBlock(); 00157 00158 while (!TodoStack.empty()) { // Process all the nodes in the loop 00159 BasicBlock *X = TodoStack.back(); 00160 TodoStack.pop_back(); 00161 00162 if (!L->contains(X) && // As of yet unprocessed?? 00163 DS.dominates(EntryBlock, X)) { // X is reachable from entry block? 00164 // Check to see if this block already belongs to a loop. If this occurs 00165 // then we have a case where a loop that is supposed to be a child of the 00166 // current loop was processed before the current loop. When this occurs, 00167 // this child loop gets added to a part of the current loop, making it a 00168 // sibling to the current loop. We have to reparent this loop. 00169 if (Loop *SubLoop = const_cast<Loop*>(getLoopFor(X))) 00170 if (SubLoop->getHeader() == X && isNotAlreadyContainedIn(SubLoop, L)) { 00171 // Remove the subloop from it's current parent... 00172 assert(SubLoop->ParentLoop && SubLoop->ParentLoop != L); 00173 Loop *SLP = SubLoop->ParentLoop; // SubLoopParent 00174 std::vector<Loop*>::iterator I = 00175 std::find(SLP->SubLoops.begin(), SLP->SubLoops.end(), SubLoop); 00176 assert(I != SLP->SubLoops.end() && "SubLoop not a child of parent?"); 00177 SLP->SubLoops.erase(I); // Remove from parent... 00178 00179 // Add the subloop to THIS loop... 00180 SubLoop->ParentLoop = L; 00181 L->SubLoops.push_back(SubLoop); 00182 } 00183 00184 // Normal case, add the block to our loop... 00185 L->Blocks.push_back(X); 00186 00187 // Add all of the predecessors of X to the end of the work stack... 00188 TodoStack.insert(TodoStack.end(), pred_begin(X), pred_end(X)); 00189 } 00190 } 00191 00192 // If there are any loops nested within this loop, create them now! 00193 for (std::vector<BasicBlock*>::iterator I = L->Blocks.begin(), 00194 E = L->Blocks.end(); I != E; ++I) 00195 if (Loop *NewLoop = ConsiderForLoop(*I, DS)) { 00196 L->SubLoops.push_back(NewLoop); 00197 NewLoop->ParentLoop = L; 00198 } 00199 00200 // Add the basic blocks that comprise this loop to the BBMap so that this 00201 // loop can be found for them. 00202 // 00203 for (std::vector<BasicBlock*>::iterator I = L->Blocks.begin(), 00204 E = L->Blocks.end(); I != E; ++I) { 00205 std::map<BasicBlock*, Loop*>::iterator BBMI = BBMap.lower_bound(*I); 00206 if (BBMI == BBMap.end() || BBMI->first != *I) // Not in map yet... 00207 BBMap.insert(BBMI, std::make_pair(*I, L)); // Must be at this level 00208 } 00209 00210 // Now that we have a list of all of the child loops of this loop, check to 00211 // see if any of them should actually be nested inside of each other. We can 00212 // accidentally pull loops our of their parents, so we must make sure to 00213 // organize the loop nests correctly now. 00214 { 00215 std::map<BasicBlock*, Loop*> ContainingLoops; 00216 for (unsigned i = 0; i != L->SubLoops.size(); ++i) { 00217 Loop *Child = L->SubLoops[i]; 00218 assert(Child->getParentLoop() == L && "Not proper child loop?"); 00219 00220 if (Loop *ContainingLoop = ContainingLoops[Child->getHeader()]) { 00221 // If there is already a loop which contains this loop, move this loop 00222 // into the containing loop. 00223 MoveSiblingLoopInto(Child, ContainingLoop); 00224 --i; // The loop got removed from the SubLoops list. 00225 } else { 00226 // This is currently considered to be a top-level loop. Check to see if 00227 // any of the contained blocks are loop headers for subloops we have 00228 // already processed. 00229 for (unsigned b = 0, e = Child->Blocks.size(); b != e; ++b) { 00230 Loop *&BlockLoop = ContainingLoops[Child->Blocks[b]]; 00231 if (BlockLoop == 0) { // Child block not processed yet... 00232 BlockLoop = Child; 00233 } else if (BlockLoop != Child) { 00234 Loop *SubLoop = BlockLoop; 00235 // Reparent all of the blocks which used to belong to BlockLoops 00236 for (unsigned j = 0, e = SubLoop->Blocks.size(); j != e; ++j) 00237 ContainingLoops[SubLoop->Blocks[j]] = Child; 00238 00239 // There is already a loop which contains this block, that means 00240 // that we should reparent the loop which the block is currently 00241 // considered to belong to to be a child of this loop. 00242 MoveSiblingLoopInto(SubLoop, Child); 00243 --i; // We just shrunk the SubLoops list. 00244 } 00245 } 00246 } 00247 } 00248 } 00249 00250 return L; 00251 } 00252 00253 /// MoveSiblingLoopInto - This method moves the NewChild loop to live inside of 00254 /// the NewParent Loop, instead of being a sibling of it. 00255 void LoopInfo::MoveSiblingLoopInto(Loop *NewChild, Loop *NewParent) { 00256 Loop *OldParent = NewChild->getParentLoop(); 00257 assert(OldParent && OldParent == NewParent->getParentLoop() && 00258 NewChild != NewParent && "Not sibling loops!"); 00259 00260 // Remove NewChild from being a child of OldParent 00261 std::vector<Loop*>::iterator I = 00262 std::find(OldParent->SubLoops.begin(), OldParent->SubLoops.end(), NewChild); 00263 assert(I != OldParent->SubLoops.end() && "Parent fields incorrect??"); 00264 OldParent->SubLoops.erase(I); // Remove from parent's subloops list 00265 NewChild->ParentLoop = 0; 00266 00267 InsertLoopInto(NewChild, NewParent); 00268 } 00269 00270 /// InsertLoopInto - This inserts loop L into the specified parent loop. If the 00271 /// parent loop contains a loop which should contain L, the loop gets inserted 00272 /// into L instead. 00273 void LoopInfo::InsertLoopInto(Loop *L, Loop *Parent) { 00274 BasicBlock *LHeader = L->getHeader(); 00275 assert(Parent->contains(LHeader) && "This loop should not be inserted here!"); 00276 00277 // Check to see if it belongs in a child loop... 00278 for (unsigned i = 0, e = Parent->SubLoops.size(); i != e; ++i) 00279 if (Parent->SubLoops[i]->contains(LHeader)) { 00280 InsertLoopInto(L, Parent->SubLoops[i]); 00281 return; 00282 } 00283 00284 // If not, insert it here! 00285 Parent->SubLoops.push_back(L); 00286 L->ParentLoop = Parent; 00287 } 00288 00289 /// changeLoopFor - Change the top-level loop that contains BB to the 00290 /// specified loop. This should be used by transformations that restructure 00291 /// the loop hierarchy tree. 00292 void LoopInfo::changeLoopFor(BasicBlock *BB, Loop *L) { 00293 Loop *&OldLoop = BBMap[BB]; 00294 assert(OldLoop && "Block not in a loop yet!"); 00295 OldLoop = L; 00296 } 00297 00298 /// changeTopLevelLoop - Replace the specified loop in the top-level loops 00299 /// list with the indicated loop. 00300 void LoopInfo::changeTopLevelLoop(Loop *OldLoop, Loop *NewLoop) { 00301 std::vector<Loop*>::iterator I = std::find(TopLevelLoops.begin(), 00302 TopLevelLoops.end(), OldLoop); 00303 assert(I != TopLevelLoops.end() && "Old loop not at top level!"); 00304 *I = NewLoop; 00305 assert(NewLoop->ParentLoop == 0 && OldLoop->ParentLoop == 0 && 00306 "Loops already embedded into a subloop!"); 00307 } 00308 00309 /// removeLoop - This removes the specified top-level loop from this loop info 00310 /// object. The loop is not deleted, as it will presumably be inserted into 00311 /// another loop. 00312 Loop *LoopInfo::removeLoop(iterator I) { 00313 assert(I != end() && "Cannot remove end iterator!"); 00314 Loop *L = *I; 00315 assert(L->getParentLoop() == 0 && "Not a top-level loop!"); 00316 TopLevelLoops.erase(TopLevelLoops.begin() + (I-begin())); 00317 return L; 00318 } 00319 00320 /// removeBlock - This method completely removes BB from all data structures, 00321 /// including all of the Loop objects it is nested in and our mapping from 00322 /// BasicBlocks to loops. 00323 void LoopInfo::removeBlock(BasicBlock *BB) { 00324 std::map<BasicBlock *, Loop*>::iterator I = BBMap.find(BB); 00325 if (I != BBMap.end()) { 00326 for (Loop *L = I->second; L; L = L->getParentLoop()) 00327 L->removeBlockFromLoop(BB); 00328 00329 BBMap.erase(I); 00330 } 00331 } 00332 00333 00334 //===----------------------------------------------------------------------===// 00335 // APIs for simple analysis of the loop. 00336 // 00337 00338 /// getExitBlocks - Return all of the successor blocks of this loop. These 00339 /// are the blocks _outside of the current loop_ which are branched to. 00340 /// 00341 void Loop::getExitBlocks(std::vector<BasicBlock*> &ExitBlocks) const { 00342 for (std::vector<BasicBlock*>::const_iterator BI = Blocks.begin(), 00343 BE = Blocks.end(); BI != BE; ++BI) 00344 for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) 00345 if (!contains(*I)) // Not in current loop? 00346 ExitBlocks.push_back(*I); // It must be an exit block... 00347 } 00348 00349 00350 /// getLoopPreheader - If there is a preheader for this loop, return it. A 00351 /// loop has a preheader if there is only one edge to the header of the loop 00352 /// from outside of the loop. If this is the case, the block branching to the 00353 /// header of the loop is the preheader node. 00354 /// 00355 /// This method returns null if there is no preheader for the loop. 00356 /// 00357 BasicBlock *Loop::getLoopPreheader() const { 00358 // Keep track of nodes outside the loop branching to the header... 00359 BasicBlock *Out = 0; 00360 00361 // Loop over the predecessors of the header node... 00362 BasicBlock *Header = getHeader(); 00363 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); 00364 PI != PE; ++PI) 00365 if (!contains(*PI)) { // If the block is not in the loop... 00366 if (Out && Out != *PI) 00367 return 0; // Multiple predecessors outside the loop 00368 Out = *PI; 00369 } 00370 00371 // Make sure there is only one exit out of the preheader... 00372 succ_iterator SI = succ_begin(Out); 00373 ++SI; 00374 if (SI != succ_end(Out)) 00375 return 0; // Multiple exits from the block, must not be a preheader. 00376 00377 00378 // If there is exactly one preheader, return it. If there was zero, then Out 00379 // is still null. 00380 return Out; 00381 } 00382 00383 /// getCanonicalInductionVariable - Check to see if the loop has a canonical 00384 /// induction variable: an integer recurrence that starts at 0 and increments by 00385 /// one each time through the loop. If so, return the phi node that corresponds 00386 /// to it. 00387 /// 00388 PHINode *Loop::getCanonicalInductionVariable() const { 00389 BasicBlock *H = getHeader(); 00390 00391 BasicBlock *Incoming = 0, *Backedge = 0; 00392 pred_iterator PI = pred_begin(H); 00393 assert(PI != pred_end(H) && "Loop must have at least one backedge!"); 00394 Backedge = *PI++; 00395 if (PI == pred_end(H)) return 0; // dead loop 00396 Incoming = *PI++; 00397 if (PI != pred_end(H)) return 0; // multiple backedges? 00398 00399 if (contains(Incoming)) { 00400 if (contains(Backedge)) 00401 return 0; 00402 std::swap(Incoming, Backedge); 00403 } else if (!contains(Backedge)) 00404 return 0; 00405 00406 // Loop over all of the PHI nodes, looking for a canonical indvar. 00407 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) { 00408 PHINode *PN = cast<PHINode>(I); 00409 if (Instruction *Inc = 00410 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge))) 00411 if (Inc->getOpcode() == Instruction::Add && Inc->getOperand(0) == PN) 00412 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1))) 00413 if (CI->equalsInt(1)) 00414 return PN; 00415 } 00416 return 0; 00417 } 00418 00419 /// getCanonicalInductionVariableIncrement - Return the LLVM value that holds 00420 /// the canonical induction variable value for the "next" iteration of the loop. 00421 /// This always succeeds if getCanonicalInductionVariable succeeds. 00422 /// 00423 Instruction *Loop::getCanonicalInductionVariableIncrement() const { 00424 if (PHINode *PN = getCanonicalInductionVariable()) { 00425 bool P1InLoop = contains(PN->getIncomingBlock(1)); 00426 return cast<Instruction>(PN->getIncomingValue(P1InLoop)); 00427 } 00428 return 0; 00429 } 00430 00431 /// getTripCount - Return a loop-invariant LLVM value indicating the number of 00432 /// times the loop will be executed. Note that this means that the backedge of 00433 /// the loop executes N-1 times. If the trip-count cannot be determined, this 00434 /// returns null. 00435 /// 00436 Value *Loop::getTripCount() const { 00437 // Canonical loops will end with a 'setne I, V', where I is the incremented 00438 // canonical induction variable and V is the trip count of the loop. 00439 Instruction *Inc = getCanonicalInductionVariableIncrement(); 00440 if (Inc == 0) return 0; 00441 PHINode *IV = cast<PHINode>(Inc->getOperand(0)); 00442 00443 BasicBlock *BackedgeBlock = 00444 IV->getIncomingBlock(contains(IV->getIncomingBlock(1))); 00445 00446 if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator())) 00447 if (BI->isConditional()) 00448 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition())) 00449 if (SCI->getOperand(0) == Inc) 00450 if (BI->getSuccessor(0) == getHeader()) { 00451 if (SCI->getOpcode() == Instruction::SetNE) 00452 return SCI->getOperand(1); 00453 } else if (SCI->getOpcode() == Instruction::SetEQ) { 00454 return SCI->getOperand(1); 00455 } 00456 00457 return 0; 00458 } 00459 00460 00461 //===-------------------------------------------------------------------===// 00462 // APIs for updating loop information after changing the CFG 00463 // 00464 00465 /// addBasicBlockToLoop - This function is used by other analyses to update loop 00466 /// information. NewBB is set to be a new member of the current loop. Because 00467 /// of this, it is added as a member of all parent loops, and is added to the 00468 /// specified LoopInfo object as being in the current basic block. It is not 00469 /// valid to replace the loop header with this method. 00470 /// 00471 void Loop::addBasicBlockToLoop(BasicBlock *NewBB, LoopInfo &LI) { 00472 assert((Blocks.empty() || LI[getHeader()] == this) && 00473 "Incorrect LI specified for this loop!"); 00474 assert(NewBB && "Cannot add a null basic block to the loop!"); 00475 assert(LI[NewBB] == 0 && "BasicBlock already in the loop!"); 00476 00477 // Add the loop mapping to the LoopInfo object... 00478 LI.BBMap[NewBB] = this; 00479 00480 // Add the basic block to this loop and all parent loops... 00481 Loop *L = this; 00482 while (L) { 00483 L->Blocks.push_back(NewBB); 00484 L = L->getParentLoop(); 00485 } 00486 } 00487 00488 /// replaceChildLoopWith - This is used when splitting loops up. It replaces 00489 /// the OldChild entry in our children list with NewChild, and updates the 00490 /// parent pointers of the two loops as appropriate. 00491 void Loop::replaceChildLoopWith(Loop *OldChild, Loop *NewChild) { 00492 assert(OldChild->ParentLoop == this && "This loop is already broken!"); 00493 assert(NewChild->ParentLoop == 0 && "NewChild already has a parent!"); 00494 std::vector<Loop*>::iterator I = std::find(SubLoops.begin(), SubLoops.end(), 00495 OldChild); 00496 assert(I != SubLoops.end() && "OldChild not in loop!"); 00497 *I = NewChild; 00498 OldChild->ParentLoop = 0; 00499 NewChild->ParentLoop = this; 00500 } 00501 00502 /// addChildLoop - Add the specified loop to be a child of this loop. 00503 /// 00504 void Loop::addChildLoop(Loop *NewChild) { 00505 assert(NewChild->ParentLoop == 0 && "NewChild already has a parent!"); 00506 NewChild->ParentLoop = this; 00507 SubLoops.push_back(NewChild); 00508 } 00509 00510 template<typename T> 00511 static void RemoveFromVector(std::vector<T*> &V, T *N) { 00512 typename std::vector<T*>::iterator I = std::find(V.begin(), V.end(), N); 00513 assert(I != V.end() && "N is not in this list!"); 00514 V.erase(I); 00515 } 00516 00517 /// removeChildLoop - This removes the specified child from being a subloop of 00518 /// this loop. The loop is not deleted, as it will presumably be inserted 00519 /// into another loop. 00520 Loop *Loop::removeChildLoop(iterator I) { 00521 assert(I != SubLoops.end() && "Cannot remove end iterator!"); 00522 Loop *Child = *I; 00523 assert(Child->ParentLoop == this && "Child is not a child of this loop!"); 00524 SubLoops.erase(SubLoops.begin()+(I-begin())); 00525 Child->ParentLoop = 0; 00526 return Child; 00527 } 00528 00529 00530 /// removeBlockFromLoop - This removes the specified basic block from the 00531 /// current loop, updating the Blocks and ExitBlocks lists as appropriate. This 00532 /// does not update the mapping in the LoopInfo class. 00533 void Loop::removeBlockFromLoop(BasicBlock *BB) { 00534 RemoveFromVector(Blocks, BB); 00535 }