LoopUnrollPass.cpp revision ed38f1ca2ede643b058211ec09b205ed30d2a256
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 28using namespace llvm; 29 30static cl::opt<unsigned> 31UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, 32 cl::desc("The cut-off point for automatic loop unrolling")); 33 34static cl::opt<unsigned> 35UnrollCount("unroll-count", cl::init(0), cl::Hidden, 36 cl::desc("Use this unroll count for all loops, for testing purposes")); 37 38static cl::opt<bool> 39UnrollAllowPartial("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 43namespace { 44 class LoopUnroll : public LoopPass { 45 public: 46 static char ID; // Pass ID, replacement for typeid 47 LoopUnroll(int T = -1, int C = -1, int P = -1) : LoopPass(ID) { 48 CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T); 49 CurrentCount = (C == -1) ? UnrollCount : unsigned(C); 50 CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P; 51 52 UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0); 53 54 initializeLoopUnrollPass(*PassRegistry::getPassRegistry()); 55 } 56 57 /// A magic value for use with the Threshold parameter to indicate 58 /// that the loop unroll should be performed regardless of how much 59 /// code expansion would result. 60 static const unsigned NoThreshold = UINT_MAX; 61 62 // Threshold to use when optsize is specified (and there is no 63 // explicit -unroll-threshold). 64 static const unsigned OptSizeUnrollThreshold = 50; 65 66 unsigned CurrentCount; 67 unsigned CurrentThreshold; 68 bool CurrentAllowPartial; 69 bool UserThreshold; // CurrentThreshold is user-specified. 70 71 bool runOnLoop(Loop *L, LPPassManager &LPM); 72 73 /// This transformation requires natural loop information & requires that 74 /// loop preheaders be inserted into the CFG... 75 /// 76 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 77 AU.addRequired<LoopInfo>(); 78 AU.addPreserved<LoopInfo>(); 79 AU.addRequiredID(LoopSimplifyID); 80 AU.addPreservedID(LoopSimplifyID); 81 AU.addRequiredID(LCSSAID); 82 AU.addPreservedID(LCSSAID); 83 AU.addRequired<ScalarEvolution>(); 84 AU.addPreserved<ScalarEvolution>(); 85 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. 86 // If loop unroll does not preserve dom info then LCSSA pass on next 87 // loop will receive invalid dom info. 88 // For now, recreate dom info, if loop is unrolled. 89 AU.addPreserved<DominatorTree>(); 90 } 91 }; 92} 93 94char LoopUnroll::ID = 0; 95INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 96INITIALIZE_PASS_DEPENDENCY(LoopInfo) 97INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 98INITIALIZE_PASS_DEPENDENCY(LCSSA) 99INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 100INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 101 102Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial) { 103 return new LoopUnroll(Threshold, Count, AllowPartial); 104} 105 106/// ApproximateLoopSize - Approximate the size of the loop. 107static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls, 108 const TargetData *TD) { 109 CodeMetrics Metrics; 110 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 111 I != E; ++I) 112 Metrics.analyzeBasicBlock(*I, TD); 113 NumCalls = Metrics.NumInlineCandidates; 114 115 unsigned LoopSize = Metrics.NumInsts; 116 117 // Don't allow an estimate of size zero. This would allows unrolling of loops 118 // with huge iteration counts, which is a compile time problem even if it's 119 // not a problem for code quality. 120 if (LoopSize == 0) LoopSize = 1; 121 122 return LoopSize; 123} 124 125bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { 126 LoopInfo *LI = &getAnalysis<LoopInfo>(); 127 ScalarEvolution *SE = &getAnalysis<ScalarEvolution>(); 128 129 BasicBlock *Header = L->getHeader(); 130 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() 131 << "] Loop %" << Header->getName() << "\n"); 132 (void)Header; 133 134 // Determine the current unrolling threshold. While this is normally set 135 // from UnrollThreshold, it is overridden to a smaller value if the current 136 // function is marked as optimize-for-size, and the unroll threshold was 137 // not user specified. 138 unsigned Threshold = CurrentThreshold; 139 if (!UserThreshold && 140 Header->getParent()->hasFnAttr(Attribute::OptimizeForSize)) 141 Threshold = OptSizeUnrollThreshold; 142 143 // Find trip count and trip multiple if count is not available 144 unsigned TripCount = 0; 145 unsigned TripMultiple = 1; 146 // Find "latch trip count". UnrollLoop assumes that control cannot exit 147 // via the loop latch on any iteration prior to TripCount. The loop may exit 148 // early via an earlier branch. 149 BasicBlock *LatchBlock = L->getLoopLatch(); 150 if (LatchBlock) { 151 TripCount = SE->getSmallConstantTripCount(L, LatchBlock); 152 TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock); 153 } 154 // Automatically select an unroll count. 155 unsigned Count = CurrentCount; 156 if (Count == 0) { 157 // Conservative heuristic: if we know the trip count, see if we can 158 // completely unroll (subject to the threshold, checked below); otherwise 159 // try to find greatest modulo of the trip count which is still under 160 // threshold value. 161 if (TripCount == 0) 162 return false; 163 Count = TripCount; 164 } 165 166 // Enforce the threshold. 167 if (Threshold != NoThreshold) { 168 const TargetData *TD = getAnalysisIfAvailable<TargetData>(); 169 unsigned NumInlineCandidates; 170 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates, TD); 171 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 172 if (NumInlineCandidates != 0) { 173 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 174 return false; 175 } 176 uint64_t Size = (uint64_t)LoopSize*Count; 177 if (TripCount != 1 && Size > Threshold) { 178 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 179 << " because size: " << Size << ">" << Threshold << "\n"); 180 if (!CurrentAllowPartial) { 181 DEBUG(dbgs() << " will not try to unroll partially because " 182 << "-unroll-allow-partial not given\n"); 183 return false; 184 } 185 // Reduce unroll count to be modulo of TripCount for partial unrolling 186 Count = Threshold / LoopSize; 187 while (Count != 0 && TripCount%Count != 0) { 188 Count--; 189 } 190 if (Count < 2) { 191 DEBUG(dbgs() << " could not unroll partially\n"); 192 return false; 193 } 194 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 195 } 196 } 197 198 // Unroll the loop. 199 if (!UnrollLoop(L, Count, TripCount, TripMultiple, LI, &LPM)) 200 return false; 201 202 return true; 203} 204