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      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/Transforms/Scalar.h"
     17 #include "llvm/Analysis/CodeMetrics.h"
     18 #include "llvm/Analysis/LoopPass.h"
     19 #include "llvm/Analysis/ScalarEvolution.h"
     20 #include "llvm/Analysis/TargetTransformInfo.h"
     21 #include "llvm/IR/DataLayout.h"
     22 #include "llvm/IR/IntrinsicInst.h"
     23 #include "llvm/Support/CommandLine.h"
     24 #include "llvm/Support/Debug.h"
     25 #include "llvm/Support/raw_ostream.h"
     26 #include "llvm/Transforms/Utils/UnrollLoop.h"
     27 #include <climits>
     28 
     29 using namespace llvm;
     30 
     31 static cl::opt<unsigned>
     32 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
     33   cl::desc("The cut-off point for automatic loop unrolling"));
     34 
     35 static cl::opt<unsigned>
     36 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
     37   cl::desc("Use this unroll count for all loops, for testing purposes"));
     38 
     39 static cl::opt<bool>
     40 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
     41   cl::desc("Allows loops to be partially unrolled until "
     42            "-unroll-threshold loop size is reached."));
     43 
     44 static cl::opt<bool>
     45 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
     46   cl::desc("Unroll loops with run-time trip counts"));
     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     // Default unroll count for loops with run-time trip count if
     72     // -unroll-count is not set
     73     static const unsigned UnrollRuntimeCount = 8;
     74 
     75     unsigned CurrentCount;
     76     unsigned CurrentThreshold;
     77     bool     CurrentAllowPartial;
     78     bool     UserThreshold;        // CurrentThreshold is user-specified.
     79 
     80     bool runOnLoop(Loop *L, LPPassManager &LPM);
     81 
     82     /// This transformation requires natural loop information & requires that
     83     /// loop preheaders be inserted into the CFG...
     84     ///
     85     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     86       AU.addRequired<LoopInfo>();
     87       AU.addPreserved<LoopInfo>();
     88       AU.addRequiredID(LoopSimplifyID);
     89       AU.addPreservedID(LoopSimplifyID);
     90       AU.addRequiredID(LCSSAID);
     91       AU.addPreservedID(LCSSAID);
     92       AU.addRequired<ScalarEvolution>();
     93       AU.addPreserved<ScalarEvolution>();
     94       AU.addRequired<TargetTransformInfo>();
     95       // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
     96       // If loop unroll does not preserve dom info then LCSSA pass on next
     97       // loop will receive invalid dom info.
     98       // For now, recreate dom info, if loop is unrolled.
     99       AU.addPreserved<DominatorTree>();
    100     }
    101   };
    102 }
    103 
    104 char LoopUnroll::ID = 0;
    105 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
    106 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
    107 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
    108 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
    109 INITIALIZE_PASS_DEPENDENCY(LCSSA)
    110 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
    111 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
    112 
    113 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial) {
    114   return new LoopUnroll(Threshold, Count, AllowPartial);
    115 }
    116 
    117 /// ApproximateLoopSize - Approximate the size of the loop.
    118 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
    119                                     bool &NotDuplicatable,
    120                                     const TargetTransformInfo &TTI) {
    121   CodeMetrics Metrics;
    122   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
    123        I != E; ++I)
    124     Metrics.analyzeBasicBlock(*I, TTI);
    125   NumCalls = Metrics.NumInlineCandidates;
    126   NotDuplicatable = Metrics.notDuplicatable;
    127 
    128   unsigned LoopSize = Metrics.NumInsts;
    129 
    130   // Don't allow an estimate of size zero.  This would allows unrolling of loops
    131   // with huge iteration counts, which is a compile time problem even if it's
    132   // not a problem for code quality.
    133   if (LoopSize == 0) LoopSize = 1;
    134 
    135   return LoopSize;
    136 }
    137 
    138 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
    139   LoopInfo *LI = &getAnalysis<LoopInfo>();
    140   ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
    141   const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
    142 
    143   BasicBlock *Header = L->getHeader();
    144   DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
    145         << "] Loop %" << Header->getName() << "\n");
    146   (void)Header;
    147 
    148   // Determine the current unrolling threshold.  While this is normally set
    149   // from UnrollThreshold, it is overridden to a smaller value if the current
    150   // function is marked as optimize-for-size, and the unroll threshold was
    151   // not user specified.
    152   unsigned Threshold = CurrentThreshold;
    153   if (!UserThreshold &&
    154       Header->getParent()->getAttributes().
    155         hasAttribute(AttributeSet::FunctionIndex,
    156                      Attribute::OptimizeForSize))
    157     Threshold = OptSizeUnrollThreshold;
    158 
    159   // Find trip count and trip multiple if count is not available
    160   unsigned TripCount = 0;
    161   unsigned TripMultiple = 1;
    162   // Find "latch trip count". UnrollLoop assumes that control cannot exit
    163   // via the loop latch on any iteration prior to TripCount. The loop may exit
    164   // early via an earlier branch.
    165   BasicBlock *LatchBlock = L->getLoopLatch();
    166   if (LatchBlock) {
    167     TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
    168     TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
    169   }
    170   // Use a default unroll-count if the user doesn't specify a value
    171   // and the trip count is a run-time value.  The default is different
    172   // for run-time or compile-time trip count loops.
    173   unsigned Count = CurrentCount;
    174   if (UnrollRuntime && CurrentCount == 0 && TripCount == 0)
    175     Count = UnrollRuntimeCount;
    176 
    177   if (Count == 0) {
    178     // Conservative heuristic: if we know the trip count, see if we can
    179     // completely unroll (subject to the threshold, checked below); otherwise
    180     // try to find greatest modulo of the trip count which is still under
    181     // threshold value.
    182     if (TripCount == 0)
    183       return false;
    184     Count = TripCount;
    185   }
    186 
    187   // Enforce the threshold.
    188   if (Threshold != NoThreshold) {
    189     unsigned NumInlineCandidates;
    190     bool notDuplicatable;
    191     unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates,
    192                                             notDuplicatable, TTI);
    193     DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
    194     if (notDuplicatable) {
    195       DEBUG(dbgs() << "  Not unrolling loop which contains non duplicatable"
    196             << " instructions.\n");
    197       return false;
    198     }
    199     if (NumInlineCandidates != 0) {
    200       DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
    201       return false;
    202     }
    203     uint64_t Size = (uint64_t)LoopSize*Count;
    204     if (TripCount != 1 && Size > Threshold) {
    205       DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
    206             << " because size: " << Size << ">" << Threshold << "\n");
    207       if (!CurrentAllowPartial && !(UnrollRuntime && TripCount == 0)) {
    208         DEBUG(dbgs() << "  will not try to unroll partially because "
    209               << "-unroll-allow-partial not given\n");
    210         return false;
    211       }
    212       if (TripCount) {
    213         // Reduce unroll count to be modulo of TripCount for partial unrolling
    214         Count = Threshold / LoopSize;
    215         while (Count != 0 && TripCount%Count != 0)
    216           Count--;
    217       }
    218       else if (UnrollRuntime) {
    219         // Reduce unroll count to be a lower power-of-two value
    220         while (Count != 0 && Size > Threshold) {
    221           Count >>= 1;
    222           Size = LoopSize*Count;
    223         }
    224       }
    225       if (Count < 2) {
    226         DEBUG(dbgs() << "  could not unroll partially\n");
    227         return false;
    228       }
    229       DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
    230     }
    231   }
    232 
    233   // Unroll the loop.
    234   if (!UnrollLoop(L, Count, TripCount, UnrollRuntime, TripMultiple, LI, &LPM))
    235     return false;
    236 
    237   return true;
    238 }
    239