LoopUnrollPass.cpp revision d9e079706e782c5f054322a0d15f063a1a683230
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 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 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 83char LoopUnroll::ID = 0; 84INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 85INITIALIZE_PASS_DEPENDENCY(LoopInfo) 86INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 87INITIALIZE_PASS_DEPENDENCY(LCSSA) 88INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 89 90Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); } 91 92/// ApproximateLoopSize - Approximate the size of the loop. 93static 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 // If we can identify the induction variable, we know that it will become 103 // constant when we unroll the loop, so factor that into our loop size 104 // estimate. 105 // FIXME: We have to divide by InlineConstants::InstrCost because the 106 // measure returned by CountCodeReductionForConstant is not an instruction 107 // count, but rather a weight as defined by InlineConstants. It would 108 // probably be a good idea to standardize on a single weighting scheme by 109 // pushing more of the logic for weighting into CodeMetrics. 110 if (PHINode *IndVar = L->getCanonicalInductionVariable()) { 111 unsigned SizeDecrease = Metrics.CountCodeReductionForConstant(IndVar); 112 // NOTE: Because SizeDecrease is a fuzzy estimate, we don't want to allow 113 // it to totally negate the cost of unrolling a loop. 114 SizeDecrease = SizeDecrease > LoopSize / 2 ? LoopSize / 2 : SizeDecrease; 115 } 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) { 126 127 LoopInfo *LI = &getAnalysis<LoopInfo>(); 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 CurrentThreshold = UnrollThreshold; 139 if (Header->getParent()->hasFnAttr(Attribute::OptimizeForSize) && 140 UnrollThreshold.getNumOccurrences() == 0) 141 CurrentThreshold = OptSizeUnrollThreshold; 142 143 // Find trip count 144 unsigned TripCount = L->getSmallConstantTripCount(); 145 unsigned Count = UnrollCount; 146 147 // Automatically select an unroll count. 148 if (Count == 0) { 149 // Conservative heuristic: if we know the trip count, see if we can 150 // completely unroll (subject to the threshold, checked below); otherwise 151 // try to find greatest modulo of the trip count which is still under 152 // threshold value. 153 if (TripCount == 0) 154 return false; 155 Count = TripCount; 156 } 157 158 // Enforce the threshold. 159 if (CurrentThreshold != NoThreshold) { 160 unsigned NumInlineCandidates; 161 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates); 162 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 163 if (NumInlineCandidates != 0) { 164 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 165 return false; 166 } 167 uint64_t Size = (uint64_t)LoopSize*Count; 168 if (TripCount != 1 && Size > CurrentThreshold) { 169 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 170 << " because size: " << Size << ">" << CurrentThreshold << "\n"); 171 if (!UnrollAllowPartial) { 172 DEBUG(dbgs() << " will not try to unroll partially because " 173 << "-unroll-allow-partial not given\n"); 174 return false; 175 } 176 // Reduce unroll count to be modulo of TripCount for partial unrolling 177 Count = CurrentThreshold / LoopSize; 178 while (Count != 0 && TripCount%Count != 0) { 179 Count--; 180 } 181 if (Count < 2) { 182 DEBUG(dbgs() << " could not unroll partially\n"); 183 return false; 184 } 185 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 186 } 187 } 188 189 // Unroll the loop. 190 Function *F = L->getHeader()->getParent(); 191 if (!UnrollLoop(L, Count, LI, &LPM)) 192 return false; 193 194 // FIXME: Reconstruct dom info, because it is not preserved properly. 195 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) 196 DT->runOnFunction(*F); 197 return true; 198} 199