LoopUnrollPass.cpp revision 8ea298a4e341479a4bba608007bec295fd164613
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/InlineCost.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 27using namespace llvm; 28 29static cl::opt<unsigned> 30UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, 31 cl::desc("The cut-off point for automatic loop unrolling")); 32 33static cl::opt<unsigned> 34UnrollCount("unroll-count", cl::init(0), cl::Hidden, 35 cl::desc("Use this unroll count for all loops, for testing purposes")); 36 37static cl::opt<bool> 38UnrollAllowPartial("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 42namespace { 43 class LoopUnroll : public LoopPass { 44 public: 45 static char ID; // Pass ID, replacement for typeid 46 LoopUnroll() : LoopPass(ID) {} 47 48 /// A magic value for use with the Threshold parameter to indicate 49 /// that the loop unroll should be performed regardless of how much 50 /// code expansion would result. 51 static const unsigned NoThreshold = UINT_MAX; 52 53 // Threshold to use when optsize is specified (and there is no 54 // explicit -unroll-threshold). 55 static const unsigned OptSizeUnrollThreshold = 50; 56 57 unsigned CurrentThreshold; 58 59 bool runOnLoop(Loop *L, LPPassManager &LPM); 60 61 /// This transformation requires natural loop information & requires that 62 /// loop preheaders be inserted into the CFG... 63 /// 64 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 65 AU.addRequired<LoopInfo>(); 66 AU.addPreserved<LoopInfo>(); 67 AU.addRequiredID(LoopSimplifyID); 68 AU.addPreservedID(LoopSimplifyID); 69 AU.addRequiredID(LCSSAID); 70 AU.addPreservedID(LCSSAID); 71 AU.addPreserved<ScalarEvolution>(); 72 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. 73 // If loop unroll does not preserve dom info then LCSSA pass on next 74 // loop will receive invalid dom info. 75 // For now, recreate dom info, if loop is unrolled. 76 AU.addPreserved<DominatorTree>(); 77 } 78 }; 79} 80 81char LoopUnroll::ID = 0; 82INITIALIZE_PASS(LoopUnroll, "loop-unroll", "Unroll loops", false, false); 83 84Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); } 85 86/// ApproximateLoopSize - Approximate the size of the loop. 87static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) { 88 CodeMetrics Metrics; 89 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 90 I != E; ++I) 91 Metrics.analyzeBasicBlock(*I); 92 NumCalls = Metrics.NumInlineCandidates; 93 94 unsigned LoopSize = Metrics.NumInsts; 95 96 // If we can identify the induction variable, we know that it will become 97 // constant when we unroll the loop, so factor that into our loop size 98 // estimate. 99 // FIXME: We have to divide by InlineConstants::InstrCost because the 100 // measure returned by CountCodeReductionForConstant is not an instruction 101 // count, but rather a weight as defined by InlineConstants. It would 102 // probably be a good idea to standardize on a single weighting scheme by 103 // pushing more of the logic for weighting into CodeMetrics. 104 if (PHINode *IndVar = L->getCanonicalInductionVariable()) { 105 unsigned SizeDecrease = Metrics.CountCodeReductionForConstant(IndVar); 106 // NOTE: Because SizeDecrease is a fuzzy estimate, we don't want to allow 107 // it to totally negate the cost of unrolling a loop. 108 SizeDecrease = SizeDecrease > LoopSize / 2 ? LoopSize / 2 : SizeDecrease; 109 } 110 111 // Don't allow an estimate of size zero. This would allows unrolling of loops 112 // with huge iteration counts, which is a compile time problem even if it's 113 // not a problem for code quality. 114 if (LoopSize == 0) LoopSize = 1; 115 116 return LoopSize; 117} 118 119bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { 120 121 LoopInfo *LI = &getAnalysis<LoopInfo>(); 122 123 BasicBlock *Header = L->getHeader(); 124 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() 125 << "] Loop %" << Header->getName() << "\n"); 126 (void)Header; 127 128 // Determine the current unrolling threshold. While this is normally set 129 // from UnrollThreshold, it is overridden to a smaller value if the current 130 // function is marked as optimize-for-size, and the unroll threshold was 131 // not user specified. 132 CurrentThreshold = UnrollThreshold; 133 if (Header->getParent()->hasFnAttr(Attribute::OptimizeForSize) && 134 UnrollThreshold.getNumOccurrences() == 0) 135 CurrentThreshold = OptSizeUnrollThreshold; 136 137 // Find trip count 138 unsigned TripCount = L->getSmallConstantTripCount(); 139 unsigned Count = UnrollCount; 140 141 // Automatically select an unroll count. 142 if (Count == 0) { 143 // Conservative heuristic: if we know the trip count, see if we can 144 // completely unroll (subject to the threshold, checked below); otherwise 145 // try to find greatest modulo of the trip count which is still under 146 // threshold value. 147 if (TripCount == 0) 148 return false; 149 Count = TripCount; 150 } 151 152 // Enforce the threshold. 153 if (CurrentThreshold != NoThreshold) { 154 unsigned NumInlineCandidates; 155 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates); 156 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 157 if (NumInlineCandidates != 0) { 158 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 159 return false; 160 } 161 uint64_t Size = (uint64_t)LoopSize*Count; 162 if (TripCount != 1 && Size > CurrentThreshold) { 163 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 164 << " because size: " << Size << ">" << CurrentThreshold << "\n"); 165 if (!UnrollAllowPartial) { 166 DEBUG(dbgs() << " will not try to unroll partially because " 167 << "-unroll-allow-partial not given\n"); 168 return false; 169 } 170 // Reduce unroll count to be modulo of TripCount for partial unrolling 171 Count = CurrentThreshold / LoopSize; 172 while (Count != 0 && TripCount%Count != 0) { 173 Count--; 174 } 175 if (Count < 2) { 176 DEBUG(dbgs() << " could not unroll partially\n"); 177 return false; 178 } 179 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 180 } 181 } 182 183 // Unroll the loop. 184 Function *F = L->getHeader()->getParent(); 185 if (!UnrollLoop(L, Count, LI, &LPM)) 186 return false; 187 188 // FIXME: Reconstruct dom info, because it is not preserved properly. 189 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) 190 DT->runOnFunction(*F); 191 return true; 192} 193