LLVM API Documentation
00001 //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===// 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 pass implements a simple loop unroller. It works best when loops have 00011 // been canonicalized by the -indvars pass, allowing it to determine the trip 00012 // counts of loops easily. 00013 // 00014 // This pass is currently extremely limited. It only currently only unrolls 00015 // single basic block loops that execute a constant number of times. 00016 // 00017 //===----------------------------------------------------------------------===// 00018 00019 #define DEBUG_TYPE "loop-unroll" 00020 #include "llvm/Transforms/Scalar.h" 00021 #include "llvm/Constants.h" 00022 #include "llvm/Function.h" 00023 #include "llvm/Instructions.h" 00024 #include "llvm/Analysis/LoopInfo.h" 00025 #include "llvm/Transforms/Utils/Cloning.h" 00026 #include "llvm/Transforms/Utils/Local.h" 00027 #include "llvm/Support/CommandLine.h" 00028 #include "llvm/Support/Debug.h" 00029 #include "llvm/ADT/Statistic.h" 00030 #include "llvm/ADT/STLExtras.h" 00031 #include "llvm/IntrinsicInst.h" 00032 #include <cstdio> 00033 #include <set> 00034 #include <algorithm> 00035 00036 using namespace llvm; 00037 00038 namespace { 00039 Statistic<> NumUnrolled("loop-unroll", "Number of loops completely unrolled"); 00040 00041 cl::opt<unsigned> 00042 UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden, 00043 cl::desc("The cut-off point for loop unrolling")); 00044 00045 class LoopUnroll : public FunctionPass { 00046 LoopInfo *LI; // The current loop information 00047 public: 00048 virtual bool runOnFunction(Function &F); 00049 bool visitLoop(Loop *L); 00050 00051 /// This transformation requires natural loop information & requires that 00052 /// loop preheaders be inserted into the CFG... 00053 /// 00054 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00055 AU.addRequiredID(LoopSimplifyID); 00056 AU.addRequired<LoopInfo>(); 00057 AU.addPreserved<LoopInfo>(); 00058 } 00059 }; 00060 RegisterOpt<LoopUnroll> X("loop-unroll", "Unroll loops"); 00061 } 00062 00063 FunctionPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); } 00064 00065 bool LoopUnroll::runOnFunction(Function &F) { 00066 bool Changed = false; 00067 LI = &getAnalysis<LoopInfo>(); 00068 00069 // Transform all the top-level loops. Copy the loop list so that the child 00070 // can update the loop tree if it needs to delete the loop. 00071 std::vector<Loop*> SubLoops(LI->begin(), LI->end()); 00072 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 00073 Changed |= visitLoop(SubLoops[i]); 00074 00075 return Changed; 00076 } 00077 00078 /// ApproximateLoopSize - Approximate the size of the loop after it has been 00079 /// unrolled. 00080 static unsigned ApproximateLoopSize(const Loop *L) { 00081 unsigned Size = 0; 00082 for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) { 00083 BasicBlock *BB = L->getBlocks()[i]; 00084 Instruction *Term = BB->getTerminator(); 00085 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 00086 if (isa<PHINode>(I) && BB == L->getHeader()) { 00087 // Ignore PHI nodes in the header. 00088 } else if (I->hasOneUse() && I->use_back() == Term) { 00089 // Ignore instructions only used by the loop terminator. 00090 } else if (DbgInfoIntrinsic *DbgI = dyn_cast<DbgInfoIntrinsic>(I)) { 00091 // Ignore debug instructions 00092 } else { 00093 ++Size; 00094 } 00095 00096 // TODO: Ignore expressions derived from PHI and constants if inval of phi 00097 // is a constant, or if operation is associative. This will get induction 00098 // variables. 00099 } 00100 } 00101 00102 return Size; 00103 } 00104 00105 // RemapInstruction - Convert the instruction operands from referencing the 00106 // current values into those specified by ValueMap. 00107 // 00108 static inline void RemapInstruction(Instruction *I, 00109 std::map<const Value *, Value*> &ValueMap) { 00110 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 00111 Value *Op = I->getOperand(op); 00112 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op); 00113 if (It != ValueMap.end()) Op = It->second; 00114 I->setOperand(op, Op); 00115 } 00116 } 00117 00118 bool LoopUnroll::visitLoop(Loop *L) { 00119 bool Changed = false; 00120 00121 // Recurse through all subloops before we process this loop. Copy the loop 00122 // list so that the child can update the loop tree if it needs to delete the 00123 // loop. 00124 std::vector<Loop*> SubLoops(L->begin(), L->end()); 00125 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 00126 Changed |= visitLoop(SubLoops[i]); 00127 00128 // We only handle single basic block loops right now. 00129 if (L->getBlocks().size() != 1) 00130 return Changed; 00131 00132 BasicBlock *BB = L->getHeader(); 00133 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); 00134 if (BI == 0) return Changed; // Must end in a conditional branch 00135 00136 ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount()); 00137 if (!TripCountC) return Changed; // Must have constant trip count! 00138 00139 unsigned TripCount = TripCountC->getRawValue(); 00140 if (TripCount != TripCountC->getRawValue() || TripCount == 0) 00141 return Changed; // More than 2^32 iterations??? 00142 00143 unsigned LoopSize = ApproximateLoopSize(L); 00144 DEBUG(std::cerr << "Loop Unroll: F[" << BB->getParent()->getName() 00145 << "] Loop %" << BB->getName() << " Loop Size = " << LoopSize 00146 << " Trip Count = " << TripCount << " - "); 00147 uint64_t Size = (uint64_t)LoopSize*(uint64_t)TripCount; 00148 if (Size > UnrollThreshold) { 00149 DEBUG(std::cerr << "TOO LARGE: " << Size << ">" << UnrollThreshold << "\n"); 00150 return Changed; 00151 } 00152 DEBUG(std::cerr << "UNROLLING!\n"); 00153 00154 BasicBlock *LoopExit = BI->getSuccessor(L->contains(BI->getSuccessor(0))); 00155 00156 // Create a new basic block to temporarily hold all of the cloned code. 00157 BasicBlock *NewBlock = new BasicBlock(); 00158 00159 // For the first iteration of the loop, we should use the precloned values for 00160 // PHI nodes. Insert associations now. 00161 std::map<const Value*, Value*> LastValueMap; 00162 std::vector<PHINode*> OrigPHINode; 00163 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) { 00164 PHINode *PN = cast<PHINode>(I); 00165 OrigPHINode.push_back(PN); 00166 if (Instruction *I =dyn_cast<Instruction>(PN->getIncomingValueForBlock(BB))) 00167 if (I->getParent() == BB) 00168 LastValueMap[I] = I; 00169 } 00170 00171 // Remove the exit branch from the loop 00172 BB->getInstList().erase(BI); 00173 00174 assert(TripCount != 0 && "Trip count of 0 is impossible!"); 00175 for (unsigned It = 1; It != TripCount; ++It) { 00176 char SuffixBuffer[100]; 00177 sprintf(SuffixBuffer, ".%d", It); 00178 std::map<const Value*, Value*> ValueMap; 00179 BasicBlock *New = CloneBasicBlock(BB, ValueMap, SuffixBuffer); 00180 00181 // Loop over all of the PHI nodes in the block, changing them to use the 00182 // incoming values from the previous block. 00183 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 00184 PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]); 00185 Value *InVal = NewPHI->getIncomingValueForBlock(BB); 00186 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 00187 if (InValI->getParent() == BB) 00188 InVal = LastValueMap[InValI]; 00189 ValueMap[OrigPHINode[i]] = InVal; 00190 New->getInstList().erase(NewPHI); 00191 } 00192 00193 for (BasicBlock::iterator I = New->begin(), E = New->end(); I != E; ++I) 00194 RemapInstruction(I, ValueMap); 00195 00196 // Now that all of the instructions are remapped, splice them into the end 00197 // of the NewBlock. 00198 NewBlock->getInstList().splice(NewBlock->end(), New->getInstList()); 00199 delete New; 00200 00201 // LastValue map now contains values from this iteration. 00202 std::swap(LastValueMap, ValueMap); 00203 } 00204 00205 // If there was more than one iteration, replace any uses of values computed 00206 // in the loop with values computed during the last iteration of the loop. 00207 if (TripCount != 1) { 00208 std::set<User*> Users; 00209 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 00210 Users.insert(I->use_begin(), I->use_end()); 00211 00212 // We don't want to reprocess entries with PHI nodes in them. For this 00213 // reason, we look at each operand of each user exactly once, performing the 00214 // stubstitution exactly once. 00215 for (std::set<User*>::iterator UI = Users.begin(), E = Users.end(); UI != E; 00216 ++UI) { 00217 Instruction *I = cast<Instruction>(*UI); 00218 if (I->getParent() != BB && I->getParent() != NewBlock) 00219 RemapInstruction(I, LastValueMap); 00220 } 00221 } 00222 00223 // Now that we cloned the block as many times as we needed, stitch the new 00224 // code into the original block and delete the temporary block. 00225 BB->getInstList().splice(BB->end(), NewBlock->getInstList()); 00226 delete NewBlock; 00227 00228 // Now loop over the PHI nodes in the original block, setting them to their 00229 // incoming values. 00230 BasicBlock *Preheader = L->getLoopPreheader(); 00231 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 00232 PHINode *PN = OrigPHINode[i]; 00233 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 00234 BB->getInstList().erase(PN); 00235 } 00236 00237 // Finally, add an unconditional branch to the block to continue into the exit 00238 // block. 00239 new BranchInst(LoopExit, BB); 00240 00241 // At this point, the code is well formed. We now do a quick sweep over the 00242 // inserted code, doing constant propagation and dead code elimination as we 00243 // go. 00244 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 00245 Instruction *Inst = I++; 00246 00247 if (isInstructionTriviallyDead(Inst)) 00248 BB->getInstList().erase(Inst); 00249 else if (Constant *C = ConstantFoldInstruction(Inst)) { 00250 Inst->replaceAllUsesWith(C); 00251 BB->getInstList().erase(Inst); 00252 } 00253 } 00254 00255 // Update the loop information for this loop. 00256 Loop *Parent = L->getParentLoop(); 00257 00258 // Move all of the basic blocks in the loop into the parent loop. 00259 LI->changeLoopFor(BB, Parent); 00260 00261 // Remove the loop from the parent. 00262 if (Parent) 00263 delete Parent->removeChildLoop(std::find(Parent->begin(), Parent->end(),L)); 00264 else 00265 delete LI->removeLoop(std::find(LI->begin(), LI->end(), L)); 00266 00267 00268 // FIXME: Should update dominator analyses 00269 00270 00271 // Now that everything is up-to-date that will be, we fold the loop block into 00272 // the preheader and exit block, updating our analyses as we go. 00273 LoopExit->getInstList().splice(LoopExit->begin(), BB->getInstList(), 00274 BB->getInstList().begin(), 00275 prior(BB->getInstList().end())); 00276 LoopExit->getInstList().splice(LoopExit->begin(), Preheader->getInstList(), 00277 Preheader->getInstList().begin(), 00278 prior(Preheader->getInstList().end())); 00279 00280 // Make all other blocks in the program branch to LoopExit now instead of 00281 // Preheader. 00282 Preheader->replaceAllUsesWith(LoopExit); 00283 00284 // Remove BB and LoopExit from our analyses. 00285 LI->removeBlock(Preheader); 00286 LI->removeBlock(BB); 00287 00288 // If the preheader was the entry block of this function, move the exit block 00289 // to be the new entry of the loop. 00290 Function *F = LoopExit->getParent(); 00291 if (Preheader == &F->front()) 00292 F->getBasicBlockList().splice(F->begin(), F->getBasicBlockList(), LoopExit); 00293 00294 // Actually delete the blocks now. 00295 F->getBasicBlockList().erase(Preheader); 00296 F->getBasicBlockList().erase(BB); 00297 00298 ++NumUnrolled; 00299 return true; 00300 }