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 "llvm/Target/TargetData.h" 26 #include <climits> 27 28 using namespace llvm; 29 30 static cl::opt<unsigned> 31 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, 32 cl::desc("The cut-off point for automatic loop unrolling")); 33 34 static cl::opt<unsigned> 35 UnrollCount("unroll-count", cl::init(0), cl::Hidden, 36 cl::desc("Use this unroll count for all loops, for testing purposes")); 37 38 static cl::opt<bool> 39 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden, 40 cl::desc("Allows loops to be partially unrolled until " 41 "-unroll-threshold loop size is reached.")); 42 43 // Temporary flag to be removed in 3.0 44 static cl::opt<bool> 45 NoSCEVUnroll("disable-unroll-scev", cl::init(false), cl::Hidden, 46 cl::desc("Use ScalarEvolution to analyze loop trip counts for unrolling")); 47 48 namespace { 49 class LoopUnroll : public LoopPass { 50 public: 51 static char ID; // Pass ID, replacement for typeid 52 LoopUnroll(int T = -1, int C = -1, int P = -1) : LoopPass(ID) { 53 CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T); 54 CurrentCount = (C == -1) ? UnrollCount : unsigned(C); 55 CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P; 56 57 UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0); 58 59 initializeLoopUnrollPass(*PassRegistry::getPassRegistry()); 60 } 61 62 /// A magic value for use with the Threshold parameter to indicate 63 /// that the loop unroll should be performed regardless of how much 64 /// code expansion would result. 65 static const unsigned NoThreshold = UINT_MAX; 66 67 // Threshold to use when optsize is specified (and there is no 68 // explicit -unroll-threshold). 69 static const unsigned OptSizeUnrollThreshold = 50; 70 71 unsigned CurrentCount; 72 unsigned CurrentThreshold; 73 bool CurrentAllowPartial; 74 bool UserThreshold; // CurrentThreshold is user-specified. 75 76 bool runOnLoop(Loop *L, LPPassManager &LPM); 77 78 /// This transformation requires natural loop information & requires that 79 /// loop preheaders be inserted into the CFG... 80 /// 81 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 82 AU.addRequired<LoopInfo>(); 83 AU.addPreserved<LoopInfo>(); 84 AU.addRequiredID(LoopSimplifyID); 85 AU.addPreservedID(LoopSimplifyID); 86 AU.addRequiredID(LCSSAID); 87 AU.addPreservedID(LCSSAID); 88 AU.addRequired<ScalarEvolution>(); 89 AU.addPreserved<ScalarEvolution>(); 90 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. 91 // If loop unroll does not preserve dom info then LCSSA pass on next 92 // loop will receive invalid dom info. 93 // For now, recreate dom info, if loop is unrolled. 94 AU.addPreserved<DominatorTree>(); 95 } 96 }; 97 } 98 99 char LoopUnroll::ID = 0; 100 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 101 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 102 INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 103 INITIALIZE_PASS_DEPENDENCY(LCSSA) 104 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 105 106 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial) { 107 return new LoopUnroll(Threshold, Count, AllowPartial); 108 } 109 110 /// ApproximateLoopSize - Approximate the size of the loop. 111 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls, 112 const TargetData *TD) { 113 CodeMetrics Metrics; 114 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 115 I != E; ++I) 116 Metrics.analyzeBasicBlock(*I, TD); 117 NumCalls = Metrics.NumInlineCandidates; 118 119 unsigned LoopSize = Metrics.NumInsts; 120 121 // Don't allow an estimate of size zero. This would allows unrolling of loops 122 // with huge iteration counts, which is a compile time problem even if it's 123 // not a problem for code quality. 124 if (LoopSize == 0) LoopSize = 1; 125 126 return LoopSize; 127 } 128 129 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { 130 LoopInfo *LI = &getAnalysis<LoopInfo>(); 131 ScalarEvolution *SE = &getAnalysis<ScalarEvolution>(); 132 133 BasicBlock *Header = L->getHeader(); 134 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() 135 << "] Loop %" << Header->getName() << "\n"); 136 (void)Header; 137 138 // Determine the current unrolling threshold. While this is normally set 139 // from UnrollThreshold, it is overridden to a smaller value if the current 140 // function is marked as optimize-for-size, and the unroll threshold was 141 // not user specified. 142 unsigned Threshold = CurrentThreshold; 143 if (!UserThreshold && 144 Header->getParent()->hasFnAttr(Attribute::OptimizeForSize)) 145 Threshold = OptSizeUnrollThreshold; 146 147 // Find trip count and trip multiple if count is not available 148 unsigned TripCount = 0; 149 unsigned TripMultiple = 1; 150 if (!NoSCEVUnroll) { 151 // Find "latch trip count". UnrollLoop assumes that control cannot exit 152 // via the loop latch on any iteration prior to TripCount. The loop may exit 153 // early via an earlier branch. 154 BasicBlock *LatchBlock = L->getLoopLatch(); 155 if (LatchBlock) { 156 TripCount = SE->getSmallConstantTripCount(L, LatchBlock); 157 TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock); 158 } 159 } 160 else { 161 TripCount = L->getSmallConstantTripCount(); 162 if (TripCount == 0) 163 TripMultiple = L->getSmallConstantTripMultiple(); 164 } 165 // Automatically select an unroll count. 166 unsigned Count = CurrentCount; 167 if (Count == 0) { 168 // Conservative heuristic: if we know the trip count, see if we can 169 // completely unroll (subject to the threshold, checked below); otherwise 170 // try to find greatest modulo of the trip count which is still under 171 // threshold value. 172 if (TripCount == 0) 173 return false; 174 Count = TripCount; 175 } 176 177 // Enforce the threshold. 178 if (Threshold != NoThreshold) { 179 const TargetData *TD = getAnalysisIfAvailable<TargetData>(); 180 unsigned NumInlineCandidates; 181 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates, TD); 182 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 183 if (NumInlineCandidates != 0) { 184 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 185 return false; 186 } 187 uint64_t Size = (uint64_t)LoopSize*Count; 188 if (TripCount != 1 && Size > Threshold) { 189 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 190 << " because size: " << Size << ">" << Threshold << "\n"); 191 if (!CurrentAllowPartial) { 192 DEBUG(dbgs() << " will not try to unroll partially because " 193 << "-unroll-allow-partial not given\n"); 194 return false; 195 } 196 // Reduce unroll count to be modulo of TripCount for partial unrolling 197 Count = Threshold / LoopSize; 198 while (Count != 0 && TripCount%Count != 0) { 199 Count--; 200 } 201 if (Count < 2) { 202 DEBUG(dbgs() << " could not unroll partially\n"); 203 return false; 204 } 205 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 206 } 207 } 208 209 // Unroll the loop. 210 if (!UnrollLoop(L, Count, TripCount, TripMultiple, LI, &LPM)) 211 return false; 212 213 return true; 214 } 215
