lib/Transforms/Scalar/LoopUnrollPass.cpp

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