text: //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass implements a simple loop unroller.  It works best when loops have
  // been canonicalized by the -indvars pass, allowing it to determine the trip
  // counts of loops easily.
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/Analysis/CodeMetrics.h"
  #include "llvm/Analysis/LoopPass.h"
  #include "llvm/Analysis/ScalarEvolution.h"
  #include "llvm/Analysis/TargetTransformInfo.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/DiagnosticInfo.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/Metadata.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Transforms/Utils/UnrollLoop.h"
  #include <climits>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "loop-unroll"
  
  static cl::opt<unsigned>
  UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
    cl::desc("The cut-off point for automatic loop unrolling"));
  
  static cl::opt<unsigned>
  UnrollCount("unroll-count", cl::init(0), cl::Hidden,
    cl::desc("Use this unroll count for all loops including those with "
             "unroll_count pragma values, for testing purposes"));
  
  static cl::opt<bool>
  UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
    cl::desc("Allows loops to be partially unrolled until "
             "-unroll-threshold loop size is reached."));
  
  static cl::opt<bool>
  UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
    cl::desc("Unroll loops with run-time trip counts"));
  
  static cl::opt<unsigned>
  PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
    cl::desc("Unrolled size limit for loops with an unroll(enable) or "
             "unroll_count pragma."));
  
  namespace {
    class LoopUnroll : public LoopPass {
    public:
      static char ID; // Pass ID, replacement for typeid
      LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
        CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
        CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
        CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
        CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;
  
        UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
        UserAllowPartial = (P != -1) ||
                           (UnrollAllowPartial.getNumOccurrences() > 0);
        UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
        UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);
  
        initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
      }
  
      /// A magic value for use with the Threshold parameter to indicate
      /// that the loop unroll should be performed regardless of how much
      /// code expansion would result.
      static const unsigned NoThreshold = UINT_MAX;
  
      // Threshold to use when optsize is specified (and there is no
      // explicit -unroll-threshold).
      static const unsigned OptSizeUnrollThreshold = 50;
  
      // Default unroll count for loops with run-time trip count if
      // -unroll-count is not set
      static const unsigned UnrollRuntimeCount = 8;
  
      unsigned CurrentCount;
      unsigned CurrentThreshold;
      bool     CurrentAllowPartial;
      bool     CurrentRuntime;
      bool     UserCount;            // CurrentCount is user-specified.
      bool     UserThreshold;        // CurrentThreshold is user-specified.
      bool     UserAllowPartial;     // CurrentAllowPartial is user-specified.
      bool     UserRuntime;          // CurrentRuntime is user-specified.
  
      bool runOnLoop(Loop *L, LPPassManager &LPM) override;
  
      /// This transformation requires natural loop information & requires that
      /// loop preheaders be inserted into the CFG...
      ///
      void getAnalysisUsage(AnalysisUsage &AU) const override {
        AU.addRequired<LoopInfo>();
        AU.addPreserved<LoopInfo>();
        AU.addRequiredID(LoopSimplifyID);
        AU.addPreservedID(LoopSimplifyID);
        AU.addRequiredID(LCSSAID);
        AU.addPreservedID(LCSSAID);
        AU.addRequired<ScalarEvolution>();
        AU.addPreserved<ScalarEvolution>();
        AU.addRequired<TargetTransformInfo>();
        // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
        // If loop unroll does not preserve dom info then LCSSA pass on next
        // loop will receive invalid dom info.
        // For now, recreate dom info, if loop is unrolled.
        AU.addPreserved<DominatorTreeWrapperPass>();
      }
  
      // Fill in the UnrollingPreferences parameter with values from the
      // TargetTransformationInfo.
      void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI,
                                   TargetTransformInfo::UnrollingPreferences &UP) {
        UP.Threshold = CurrentThreshold;
        UP.OptSizeThreshold = OptSizeUnrollThreshold;
        UP.PartialThreshold = CurrentThreshold;
        UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
        UP.Count = CurrentCount;
        UP.MaxCount = UINT_MAX;
        UP.Partial = CurrentAllowPartial;
        UP.Runtime = CurrentRuntime;
        TTI.getUnrollingPreferences(L, UP);
      }
  
      // Select and return an unroll count based on parameters from
      // user, unroll preferences, unroll pragmas, or a heuristic.
      // SetExplicitly is set to true if the unroll count is is set by
      // the user or a pragma rather than selected heuristically.
      unsigned
      selectUnrollCount(const Loop *L, unsigned TripCount, bool HasEnablePragma,
                        unsigned PragmaCount,
                        const TargetTransformInfo::UnrollingPreferences &UP,
                        bool &SetExplicitly);
  
  
      // Select threshold values used to limit unrolling based on a
      // total unrolled size.  Parameters Threshold and PartialThreshold
      // are set to the maximum unrolled size for fully and partially
      // unrolled loops respectively.
      void selectThresholds(const Loop *L, bool HasPragma,
                            const TargetTransformInfo::UnrollingPreferences &UP,
                            unsigned &Threshold, unsigned &PartialThreshold) {
        // Determine the current unrolling threshold.  While this is
        // normally set from UnrollThreshold, it is overridden to a
        // smaller value if the current function is marked as
        // optimize-for-size, and the unroll threshold was not user
        // specified.
        Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
        PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
        if (!UserThreshold &&
            L->getHeader()->getParent()->getAttributes().
                hasAttribute(AttributeSet::FunctionIndex,
                             Attribute::OptimizeForSize)) {
          Threshold = UP.OptSizeThreshold;
          PartialThreshold = UP.PartialOptSizeThreshold;
        }
        if (HasPragma) {
          // If the loop has an unrolling pragma, we want to be more
          // aggressive with unrolling limits.  Set thresholds to at
          // least the PragmaTheshold value which is larger than the
          // default limits.
          if (Threshold != NoThreshold)
            Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold);
          if (PartialThreshold != NoThreshold)
            PartialThreshold =
                std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
        }
      }
    };
  }
  
  char LoopUnroll::ID = 0;
  INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
  INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
  INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
  INITIALIZE_PASS_DEPENDENCY(LCSSA)
  INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
  INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
  
  Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
                                   int Runtime) {
    return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
  }
  
  Pass *llvm::createSimpleLoopUnrollPass() {
    return llvm::createLoopUnrollPass(-1, -1, 0, 0);
  }
  
  /// ApproximateLoopSize - Approximate the size of the loop.
  static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
                                      bool &NotDuplicatable,
                                      const TargetTransformInfo &TTI) {
    CodeMetrics Metrics;
    for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
         I != E; ++I)
      Metrics.analyzeBasicBlock(*I, TTI);
    NumCalls = Metrics.NumInlineCandidates;
    NotDuplicatable = Metrics.notDuplicatable;
  
    unsigned LoopSize = Metrics.NumInsts;
  
    // Don't allow an estimate of size zero.  This would allows unrolling of loops
    // with huge iteration counts, which is a compile time problem even if it's
    // not a problem for code quality.
    if (LoopSize == 0) LoopSize = 1;
  
    return LoopSize;
  }
  
  // Returns the value associated with the given metadata node name (for
  // example, "llvm.loop.unroll.count").  If no such named metadata node
  // exists, then nullptr is returned.
  static const ConstantInt *GetUnrollMetadataValue(const Loop *L,
                                                   StringRef Name) {
    MDNode *LoopID = L->getLoopID();
    if (!LoopID) return nullptr;
  
    // First operand should refer to the loop id itself.
    assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
    assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
  
    for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
      const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
      if (!MD) continue;
  
      const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
      if (!S) continue;
  
      if (Name.equals(S->getString())) {
        assert(MD->getNumOperands() == 2 &&
               "Unroll hint metadata should have two operands.");
        return cast<ConstantInt>(MD->getOperand(1));
      }
    }
    return nullptr;
  }
  
  // Returns true if the loop has an unroll(enable) pragma.
  static bool HasUnrollEnablePragma(const Loop *L) {
    const ConstantInt *EnableValue =
        GetUnrollMetadataValue(L, "llvm.loop.unroll.enable");
    return (EnableValue && EnableValue->getZExtValue());
  }
  
  // Returns true if the loop has an unroll(disable) pragma.
  static bool HasUnrollDisablePragma(const Loop *L) {
    const ConstantInt *EnableValue =
        GetUnrollMetadataValue(L, "llvm.loop.unroll.enable");
    return (EnableValue && !EnableValue->getZExtValue());
  }
  
  // If loop has an unroll_count pragma return the (necessarily
  // positive) value from the pragma.  Otherwise return 0.
  static unsigned UnrollCountPragmaValue(const Loop *L) {
    const ConstantInt *CountValue =
        GetUnrollMetadataValue(L, "llvm.loop.unroll.count");
    if (CountValue) {
      unsigned Count = CountValue->getZExtValue();
      assert(Count >= 1 && "Unroll count must be positive.");
      return Count;
    }
    return 0;
  }
  
  // Remove existing unroll metadata and add unroll disable metadata to
  // indicate the loop has already been unrolled.  This prevents a loop
  // from being unrolled more than is directed by a pragma if the loop
  // unrolling pass is run more than once (which it generally is).
  static void SetLoopAlreadyUnrolled(Loop *L) {
    MDNode *LoopID = L->getLoopID();
    if (!LoopID) return;
  
    // First remove any existing loop unrolling metadata.
    SmallVector<Value *, 4> Vals;
    // Reserve first location for self reference to the LoopID metadata node.
    Vals.push_back(nullptr);
    for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
      bool IsUnrollMetadata = false;
      MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
      if (MD) {
        const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
        IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
      }
      if (!IsUnrollMetadata) Vals.push_back(LoopID->getOperand(i));
    }
  
    // Add unroll(disable) metadata to disable future unrolling.
    LLVMContext &Context = L->getHeader()->getContext();
    SmallVector<Value *, 2> DisableOperands;
    DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.enable"));
    DisableOperands.push_back(ConstantInt::get(Type::getInt1Ty(Context), 0));
    MDNode *DisableNode = MDNode::get(Context, DisableOperands);
    Vals.push_back(DisableNode);
  
    MDNode *NewLoopID = MDNode::get(Context, Vals);
    // Set operand 0 to refer to the loop id itself.
    NewLoopID->replaceOperandWith(0, NewLoopID);
    L->setLoopID(NewLoopID);
    LoopID->replaceAllUsesWith(NewLoopID);
  }
  
  unsigned LoopUnroll::selectUnrollCount(
      const Loop *L, unsigned TripCount, bool HasEnablePragma,
      unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
      bool &SetExplicitly) {
    SetExplicitly = true;
  
    // User-specified count (either as a command-line option or
    // constructor parameter) has highest precedence.
    unsigned Count = UserCount ? CurrentCount : 0;
  
    // If there is no user-specified count, unroll pragmas have the next
    // highest precendence.
    if (Count == 0) {
      if (PragmaCount) {
        Count = PragmaCount;
      } else if (HasEnablePragma) {
        // unroll(enable) pragma without an unroll_count pragma
        // indicates to unroll loop fully.
        Count = TripCount;
      }
    }
  
    if (Count == 0)
      Count = UP.Count;
  
    if (Count == 0) {
      SetExplicitly = false;
      if (TripCount == 0)
        // Runtime trip count.
        Count = UnrollRuntimeCount;
      else
        // Conservative heuristic: if we know the trip count, see if we can
        // completely unroll (subject to the threshold, checked below); otherwise
        // try to find greatest modulo of the trip count which is still under
        // threshold value.
        Count = TripCount;
    }
    if (TripCount && Count > TripCount)
      return TripCount;
    return Count;
  }
  
  bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
    if (skipOptnoneFunction(L))
      return false;
  
    LoopInfo *LI = &getAnalysis<LoopInfo>();
    ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
    const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
  
    BasicBlock *Header = L->getHeader();
    DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
          << "] Loop %" << Header->getName() << "\n");
  
    if (HasUnrollDisablePragma(L)) {
      return false;
    }
    bool HasEnablePragma = HasUnrollEnablePragma(L);
    unsigned PragmaCount = UnrollCountPragmaValue(L);
    bool HasPragma = HasEnablePragma || PragmaCount > 0;
  
    TargetTransformInfo::UnrollingPreferences UP;
    getUnrollingPreferences(L, TTI, UP);
  
    // Find trip count and trip multiple if count is not available
    unsigned TripCount = 0;
    unsigned TripMultiple = 1;
    // Find "latch trip count". UnrollLoop assumes that control cannot exit
    // via the loop latch on any iteration prior to TripCount. The loop may exit
    // early via an earlier branch.
    BasicBlock *LatchBlock = L->getLoopLatch();
    if (LatchBlock) {
      TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
      TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
    }
  
    // Select an initial unroll count.  This may be reduced later based
    // on size thresholds.
    bool CountSetExplicitly;
    unsigned Count = selectUnrollCount(L, TripCount, HasEnablePragma, PragmaCount,
                                       UP, CountSetExplicitly);
  
    unsigned NumInlineCandidates;
    bool notDuplicatable;
    unsigned LoopSize =
        ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI);
    DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
    uint64_t UnrolledSize = (uint64_t)LoopSize * Count;
    if (notDuplicatable) {
      DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
                   << " instructions.\n");
      return false;
    }
    if (NumInlineCandidates != 0) {
      DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
      return false;
    }
  
    unsigned Threshold, PartialThreshold;
    selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold);
  
    // Given Count, TripCount and thresholds determine the type of
    // unrolling which is to be performed.
    enum { Full = 0, Partial = 1, Runtime = 2 };
    int Unrolling;
    if (TripCount && Count == TripCount) {
      if (Threshold != NoThreshold && UnrolledSize > Threshold) {
        DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
                     << " because size: " << UnrolledSize << ">" << Threshold
                     << "\n");
        Unrolling = Partial;
      } else {
        Unrolling = Full;
      }
    } else if (TripCount && Count < TripCount) {
      Unrolling = Partial;
    } else {
      Unrolling = Runtime;
    }
  
    // Reduce count based on the type of unrolling and the threshold values.
    unsigned OriginalCount = Count;
    bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime;
    if (Unrolling == Partial) {
      bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
      if (!AllowPartial && !CountSetExplicitly) {
        DEBUG(dbgs() << "  will not try to unroll partially because "
                     << "-unroll-allow-partial not given\n");
        return false;
      }
      if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) {
        // Reduce unroll count to be modulo of TripCount for partial unrolling.
        Count = PartialThreshold / LoopSize;
        while (Count != 0 && TripCount % Count != 0)
          Count--;
      }
    } else if (Unrolling == Runtime) {
      if (!AllowRuntime && !CountSetExplicitly) {
        DEBUG(dbgs() << "  will not try to unroll loop with runtime trip count "
                     << "-unroll-runtime not given\n");
        return false;
      }
      // Reduce unroll count to be the largest power-of-two factor of
      // the original count which satisfies the threshold limit.
      while (Count != 0 && UnrolledSize > PartialThreshold) {
        Count >>= 1;
        UnrolledSize = LoopSize * Count;
      }
      if (Count > UP.MaxCount)
        Count = UP.MaxCount;
      DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
    }
  
    if (HasPragma) {
      // Mark loop as unrolled to prevent unrolling beyond that
      // requested by the pragma.
      SetLoopAlreadyUnrolled(L);
  
      // Emit optimization remarks if we are unable to unroll the loop
      // as directed by a pragma.
      DebugLoc LoopLoc = L->getStartLoc();
      Function *F = Header->getParent();
      LLVMContext &Ctx = F->getContext();
      if (HasEnablePragma && PragmaCount == 0) {
        if (TripCount && Count != TripCount) {
          emitOptimizationRemarkMissed(
              Ctx, DEBUG_TYPE, *F, LoopLoc,
              "Unable to fully unroll loop as directed by unroll(enable) pragma "
              "because unrolled size is too large.");
        } else if (!TripCount) {
          emitOptimizationRemarkMissed(
              Ctx, DEBUG_TYPE, *F, LoopLoc,
              "Unable to fully unroll loop as directed by unroll(enable) pragma "
              "because loop has a runtime trip count.");
        }
      } else if (PragmaCount > 0 && Count != OriginalCount) {
        emitOptimizationRemarkMissed(
            Ctx, DEBUG_TYPE, *F, LoopLoc,
            "Unable to unroll loop the number of times directed by "
            "unroll_count pragma because unrolled size is too large.");
      }
    }
  
    if (Unrolling != Full && Count < 2) {
      // Partial unrolling by 1 is a nop.  For full unrolling, a factor
      // of 1 makes sense because loop control can be eliminated.
      return false;
    }
  
    // Unroll the loop.
    if (!UnrollLoop(L, Count, TripCount, AllowRuntime, TripMultiple, LI, this, &LPM))
      return false;
  
    return true;
  }
  