1 //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===// 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 pass implements a simple loop unroller. It works best when loops have 11 // been canonicalized by the -indvars pass, allowing it to determine the trip 12 // counts of loops easily. 13 //===----------------------------------------------------------------------===// 14 15 #define DEBUG_TYPE "loop-unroll" 16 #include "llvm/IntrinsicInst.h" 17 #include "llvm/Transforms/Scalar.h" 18 #include "llvm/Analysis/LoopPass.h" 19 #include "llvm/Analysis/CodeMetrics.h" 20 #include "llvm/Analysis/ScalarEvolution.h" 21 #include "llvm/Support/CommandLine.h" 22 #include "llvm/Support/Debug.h" 23 #include "llvm/Support/raw_ostream.h" 24 #include "llvm/Transforms/Utils/UnrollLoop.h" 25 #include <climits> 26 27 using namespace llvm; 28 29 static cl::opt<unsigned> 30 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, 31 cl::desc("The cut-off point for automatic loop unrolling")); 32 33 static cl::opt<unsigned> 34 UnrollCount("unroll-count", cl::init(0), cl::Hidden, 35 cl::desc("Use this unroll count for all loops, for testing purposes")); 36 37 static cl::opt<bool> 38 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden, 39 cl::desc("Allows loops to be partially unrolled until " 40 "-unroll-threshold loop size is reached.")); 41 42 namespace { 43 class LoopUnroll : public LoopPass { 44 public: 45 static char ID; // Pass ID, replacement for typeid 46 LoopUnroll() : LoopPass(ID) { 47 initializeLoopUnrollPass(*PassRegistry::getPassRegistry()); 48 } 49 50 /// A magic value for use with the Threshold parameter to indicate 51 /// that the loop unroll should be performed regardless of how much 52 /// code expansion would result. 53 static const unsigned NoThreshold = UINT_MAX; 54 55 // Threshold to use when optsize is specified (and there is no 56 // explicit -unroll-threshold). 57 static const unsigned OptSizeUnrollThreshold = 50; 58 59 unsigned CurrentThreshold; 60 61 bool runOnLoop(Loop *L, LPPassManager &LPM); 62 63 /// This transformation requires natural loop information & requires that 64 /// loop preheaders be inserted into the CFG... 65 /// 66 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 67 AU.addRequired<LoopInfo>(); 68 AU.addPreserved<LoopInfo>(); 69 AU.addRequiredID(LoopSimplifyID); 70 AU.addPreservedID(LoopSimplifyID); 71 AU.addRequiredID(LCSSAID); 72 AU.addPreservedID(LCSSAID); 73 AU.addPreserved<ScalarEvolution>(); 74 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. 75 // If loop unroll does not preserve dom info then LCSSA pass on next 76 // loop will receive invalid dom info. 77 // For now, recreate dom info, if loop is unrolled. 78 AU.addPreserved<DominatorTree>(); 79 } 80 }; 81 } 82 83 char LoopUnroll::ID = 0; 84 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 85 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 86 INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 87 INITIALIZE_PASS_DEPENDENCY(LCSSA) 88 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 89 90 Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); } 91 92 /// ApproximateLoopSize - Approximate the size of the loop. 93 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) { 94 CodeMetrics Metrics; 95 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 96 I != E; ++I) 97 Metrics.analyzeBasicBlock(*I); 98 NumCalls = Metrics.NumInlineCandidates; 99 100 unsigned LoopSize = Metrics.NumInsts; 101 102 // Don't allow an estimate of size zero. This would allows unrolling of loops 103 // with huge iteration counts, which is a compile time problem even if it's 104 // not a problem for code quality. 105 if (LoopSize == 0) LoopSize = 1; 106 107 return LoopSize; 108 } 109 110 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { 111 LoopInfo *LI = &getAnalysis<LoopInfo>(); 112 113 BasicBlock *Header = L->getHeader(); 114 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() 115 << "] Loop %" << Header->getName() << "\n"); 116 (void)Header; 117 118 // Determine the current unrolling threshold. While this is normally set 119 // from UnrollThreshold, it is overridden to a smaller value if the current 120 // function is marked as optimize-for-size, and the unroll threshold was 121 // not user specified. 122 CurrentThreshold = UnrollThreshold; 123 if (Header->getParent()->hasFnAttr(Attribute::OptimizeForSize) && 124 UnrollThreshold.getNumOccurrences() == 0) 125 CurrentThreshold = OptSizeUnrollThreshold; 126 127 // Find trip count 128 unsigned TripCount = L->getSmallConstantTripCount(); 129 unsigned Count = UnrollCount; 130 131 // Automatically select an unroll count. 132 if (Count == 0) { 133 // Conservative heuristic: if we know the trip count, see if we can 134 // completely unroll (subject to the threshold, checked below); otherwise 135 // try to find greatest modulo of the trip count which is still under 136 // threshold value. 137 if (TripCount == 0) 138 return false; 139 Count = TripCount; 140 } 141 142 // Enforce the threshold. 143 if (CurrentThreshold != NoThreshold) { 144 unsigned NumInlineCandidates; 145 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates); 146 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 147 if (NumInlineCandidates != 0) { 148 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 149 return false; 150 } 151 uint64_t Size = (uint64_t)LoopSize*Count; 152 if (TripCount != 1 && Size > CurrentThreshold) { 153 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 154 << " because size: " << Size << ">" << CurrentThreshold << "\n"); 155 if (!UnrollAllowPartial) { 156 DEBUG(dbgs() << " will not try to unroll partially because " 157 << "-unroll-allow-partial not given\n"); 158 return false; 159 } 160 // Reduce unroll count to be modulo of TripCount for partial unrolling 161 Count = CurrentThreshold / LoopSize; 162 while (Count != 0 && TripCount%Count != 0) { 163 Count--; 164 } 165 if (Count < 2) { 166 DEBUG(dbgs() << " could not unroll partially\n"); 167 return false; 168 } 169 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 170 } 171 } 172 173 // Unroll the loop. 174 Function *F = L->getHeader()->getParent(); 175 if (!UnrollLoop(L, Count, LI, &LPM)) 176 return false; 177 178 // FIXME: Reconstruct dom info, because it is not preserved properly. 179 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) 180 DT->runOnFunction(*F); 181 return true; 182 } 183