llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp

//===- UnrollLoopPeel.cpp - Loop peeling utilities ------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements some loop unrolling utilities for peeling loops
// with dynamically inferred (from PGO) trip counts. See LoopUnroll.cpp for
// unrolling loops with compile-time constant trip counts.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <limits>

using namespace llvm;

#define DEBUG_TYPE "loop-unroll"

STATISTIC(NumPeeled, "Number of loops peeled");

static cl::opt<unsigned> UnrollPeelMaxCount(
    "unroll-peel-max-count", cl::init(7), cl::Hidden,
    cl::desc("Max average trip count which will cause loop peeling."));

static cl::opt<unsigned> UnrollForcePeelCount(
    "unroll-force-peel-count", cl::init(0), cl::Hidden,
    cl::desc("Force a peel count regardless of profiling information."));

// Designates that a Phi is estimated to become invariant after an "infinite"
// number of loop iterations (i.e. only may become an invariant if the loop is
// fully unrolled).
static const unsigned InfiniteIterationsToInvariance =
    std::numeric_limits<unsigned>::max();

// Check whether we are capable of peeling this loop.
static bool canPeel(Loop *L) {
  // Make sure the loop is in simplified form
  if (!L->isLoopSimplifyForm())
    return false;

  // Only peel loops that contain a single exit
  if (!L->getExitingBlock() || !L->getUniqueExitBlock())
    return false;

  // Don't try to peel loops where the latch is not the exiting block.
  // This can be an indication of two different things:
  // 1) The loop is not rotated.
  // 2) The loop contains irreducible control flow that involves the latch.
  if (L->getLoopLatch() != L->getExitingBlock())
    return false;

  return true;
}

// This function calculates the number of iterations after which the given Phi
// becomes an invariant. The pre-calculated values are memorized in the map. The
// function (shortcut is I) is calculated according to the following definition:
// Given %x = phi <Inputs from above the loop>, ..., [%y, %back.edge].
//   If %y is a loop invariant, then I(%x) = 1.
//   If %y is a Phi from the loop header, I(%x) = I(%y) + 1.
//   Otherwise, I(%x) is infinite.
// TODO: Actually if %y is an expression that depends only on Phi %z and some
//       loop invariants, we can estimate I(%x) = I(%z) + 1. The example
//       looks like:
//         %x = phi(0, %a),  <-- becomes invariant starting from 3rd iteration.
//         %y = phi(0, 5),
//         %a = %y + 1.
static unsigned calculateIterationsToInvariance(
    PHINode *Phi, Loop *L, BasicBlock *BackEdge,
    SmallDenseMap<PHINode *, unsigned> &IterationsToInvariance) {
  assert(Phi->getParent() == L->getHeader() &&
         "Non-loop Phi should not be checked for turning into invariant.");
  assert(BackEdge == L->getLoopLatch() && "Wrong latch?");
  // If we already know the answer, take it from the map.
  auto I = IterationsToInvariance.find(Phi);
  if (I != IterationsToInvariance.end())
    return I->second;

  // Otherwise we need to analyze the input from the back edge.
  Value *Input = Phi->getIncomingValueForBlock(BackEdge);
  // Place infinity to map to avoid infinite recursion for cycled Phis. Such
  // cycles can never stop on an invariant.
  IterationsToInvariance[Phi] = InfiniteIterationsToInvariance;
  unsigned ToInvariance = InfiniteIterationsToInvariance;

  if (L->isLoopInvariant(Input))
    ToInvariance = 1u;
  else if (PHINode *IncPhi = dyn_cast<PHINode>(Input)) {
    // Only consider Phis in header block.
    if (IncPhi->getParent() != L->getHeader())
      return InfiniteIterationsToInvariance;
    // If the input becomes an invariant after X iterations, then our Phi
    // becomes an invariant after X + 1 iterations.
    unsigned InputToInvariance = calculateIterationsToInvariance(
        IncPhi, L, BackEdge, IterationsToInvariance);
    if (InputToInvariance != InfiniteIterationsToInvariance)
      ToInvariance = InputToInvariance + 1u;
  }

  // If we found that this Phi lies in an invariant chain, update the map.
  if (ToInvariance != InfiniteIterationsToInvariance)
    IterationsToInvariance[Phi] = ToInvariance;
  return ToInvariance;
}

// Return the number of iterations we want to peel off.
void llvm::computePeelCount(Loop *L, unsigned LoopSize,
                            TargetTransformInfo::UnrollingPreferences &UP,
                            unsigned &TripCount) {
  assert(LoopSize > 0 && "Zero loop size is not allowed!");
  UP.PeelCount = 0;
  if (!canPeel(L))
    return;

  // Only try to peel innermost loops.
  if (!L->empty())
    return;

  // Here we try to get rid of Phis which become invariants after 1, 2, ..., N
  // iterations of the loop. For this we compute the number for iterations after
  // which every Phi is guaranteed to become an invariant, and try to peel the
  // maximum number of iterations among these values, thus turning all those
  // Phis into invariants.
  // First, check that we can peel at least one iteration.
  if (2 * LoopSize <= UP.Threshold && UnrollPeelMaxCount > 0) {
    // Store the pre-calculated values here.
    SmallDenseMap<PHINode *, unsigned> IterationsToInvariance;
    // Now go through all Phis to calculate their the number of iterations they
    // need to become invariants.
    unsigned DesiredPeelCount = 0;
    BasicBlock *BackEdge = L->getLoopLatch();
    assert(BackEdge && "Loop is not in simplified form?");
    for (auto BI = L->getHeader()->begin(); isa<PHINode>(&*BI); ++BI) {
      PHINode *Phi = cast<PHINode>(&*BI);
      unsigned ToInvariance = calculateIterationsToInvariance(
          Phi, L, BackEdge, IterationsToInvariance);
      if (ToInvariance != InfiniteIterationsToInvariance)
        DesiredPeelCount = std::max(DesiredPeelCount, ToInvariance);
    }
    if (DesiredPeelCount > 0) {
      // Pay respect to limitations implied by loop size and the max peel count.
      unsigned MaxPeelCount = UnrollPeelMaxCount;
      MaxPeelCount = std::min(MaxPeelCount, UP.Threshold / LoopSize - 1);
      DesiredPeelCount = std::min(DesiredPeelCount, MaxPeelCount);
      // Consider max peel count limitation.
      assert(DesiredPeelCount > 0 && "Wrong loop size estimation?");
      DEBUG(dbgs() << "Peel " << DesiredPeelCount << " iteration(s) to turn"
                   << " some Phis into invariants.\n");
      UP.PeelCount = DesiredPeelCount;
      return;
    }
  }

  // Bail if we know the statically calculated trip count.
  // In this case we rather prefer partial unrolling.
  if (TripCount)
    return;

  // If the user provided a peel count, use that.
  bool UserPeelCount = UnrollForcePeelCount.getNumOccurrences() > 0;
  if (UserPeelCount) {
    DEBUG(dbgs() << "Force-peeling first " << UnrollForcePeelCount
                 << " iterations.\n");
    UP.PeelCount = UnrollForcePeelCount;
    return;
  }

  // If we don't know the trip count, but have reason to believe the average
  // trip count is low, peeling should be beneficial, since we will usually
  // hit the peeled section.
  // We only do this in the presence of profile information, since otherwise
  // our estimates of the trip count are not reliable enough.
  if (UP.AllowPeeling && L->getHeader()->getParent()->hasProfileData()) {
    Optional<unsigned> PeelCount = getLoopEstimatedTripCount(L);
    if (!PeelCount)
      return;

    DEBUG(dbgs() << "Profile-based estimated trip count is " << *PeelCount
                 << "\n");

    if (*PeelCount) {
      if ((*PeelCount <= UnrollPeelMaxCount) &&
          (LoopSize * (*PeelCount + 1) <= UP.Threshold)) {
        DEBUG(dbgs() << "Peeling first " << *PeelCount << " iterations.\n");
        UP.PeelCount = *PeelCount;
        return;
      }
      DEBUG(dbgs() << "Requested peel count: " << *PeelCount << "\n");
      DEBUG(dbgs() << "Max peel count: " << UnrollPeelMaxCount << "\n");
      DEBUG(dbgs() << "Peel cost: " << LoopSize * (*PeelCount + 1) << "\n");
      DEBUG(dbgs() << "Max peel cost: " << UP.Threshold << "\n");
    }
  }
}

/// \brief Update the branch weights of the latch of a peeled-off loop
/// iteration.
/// This sets the branch weights for the latch of the recently peeled off loop
/// iteration correctly. 
/// Our goal is to make sure that:
/// a) The total weight of all the copies of the loop body is preserved.
/// b) The total weight of the loop exit is preserved.
/// c) The body weight is reasonably distributed between the peeled iterations.
///
/// \param Header The copy of the header block that belongs to next iteration.
/// \param LatchBR The copy of the latch branch that belongs to this iteration.
/// \param IterNumber The serial number of the iteration that was just
/// peeled off.
/// \param AvgIters The average number of iterations we expect the loop to have.
/// \param[in,out] PeeledHeaderWeight The total number of dynamic loop
/// iterations that are unaccounted for. As an input, it represents the number
/// of times we expect to enter the header of the iteration currently being
/// peeled off. The output is the number of times we expect to enter the
/// header of the next iteration.
static void updateBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
                                unsigned IterNumber, unsigned AvgIters,
                                uint64_t &PeeledHeaderWeight) {
  // FIXME: Pick a more realistic distribution.
  // Currently the proportion of weight we assign to the fall-through
  // side of the branch drops linearly with the iteration number, and we use
  // a 0.9 fudge factor to make the drop-off less sharp...
  if (PeeledHeaderWeight) {
    uint64_t FallThruWeight =
        PeeledHeaderWeight * ((float)(AvgIters - IterNumber) / AvgIters * 0.9);
    uint64_t ExitWeight = PeeledHeaderWeight - FallThruWeight;
    PeeledHeaderWeight -= ExitWeight;

    unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
    MDBuilder MDB(LatchBR->getContext());
    MDNode *WeightNode =
        HeaderIdx ? MDB.createBranchWeights(ExitWeight, FallThruWeight)
                  : MDB.createBranchWeights(FallThruWeight, ExitWeight);
    LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
  }
}

/// \brief Clones the body of the loop L, putting it between \p InsertTop and \p
/// InsertBot.
/// \param IterNumber The serial number of the iteration currently being
/// peeled off.
/// \param Exit The exit block of the original loop.
/// \param[out] NewBlocks A list of the the blocks in the newly created clone
/// \param[out] VMap The value map between the loop and the new clone.
/// \param LoopBlocks A helper for DFS-traversal of the loop.
/// \param LVMap A value-map that maps instructions from the original loop to
/// instructions in the last peeled-off iteration.
static void cloneLoopBlocks(Loop *L, unsigned IterNumber, BasicBlock *InsertTop,
                            BasicBlock *InsertBot, BasicBlock *Exit,
                            SmallVectorImpl<BasicBlock *> &NewBlocks,
                            LoopBlocksDFS &LoopBlocks, ValueToValueMapTy &VMap,
                            ValueToValueMapTy &LVMap, DominatorTree *DT,
                            LoopInfo *LI) {
  BasicBlock *Header = L->getHeader();
  BasicBlock *Latch = L->getLoopLatch();
  BasicBlock *PreHeader = L->getLoopPreheader();

  Function *F = Header->getParent();
  LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
  LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
  Loop *ParentLoop = L->getParentLoop();

  // For each block in the original loop, create a new copy,
  // and update the value map with the newly created values.
  for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
    BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".peel", F);
    NewBlocks.push_back(NewBB);

    if (ParentLoop)
      ParentLoop->addBasicBlockToLoop(NewBB, *LI);

    VMap[*BB] = NewBB;

    // If dominator tree is available, insert nodes to represent cloned blocks.
    if (DT) {
      if (Header == *BB)
        DT->addNewBlock(NewBB, InsertTop);
      else {
        DomTreeNode *IDom = DT->getNode(*BB)->getIDom();
        // VMap must contain entry for IDom, as the iteration order is RPO.
        DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDom->getBlock()]));
      }
    }
  }

  // Hook-up the control flow for the newly inserted blocks.
  // The new header is hooked up directly to the "top", which is either
  // the original loop preheader (for the first iteration) or the previous
  // iteration's exiting block (for every other iteration)
  InsertTop->getTerminator()->setSuccessor(0, cast<BasicBlock>(VMap[Header]));

  // Similarly, for the latch:
  // The original exiting edge is still hooked up to the loop exit.
  // The backedge now goes to the "bottom", which is either the loop's real
  // header (for the last peeled iteration) or the copied header of the next
  // iteration (for every other iteration)
  BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
  BranchInst *LatchBR = cast<BranchInst>(NewLatch->getTerminator());
  unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
  LatchBR->setSuccessor(HeaderIdx, InsertBot);
  LatchBR->setSuccessor(1 - HeaderIdx, Exit);
  if (DT)
    DT->changeImmediateDominator(InsertBot, NewLatch);

  // The new copy of the loop body starts with a bunch of PHI nodes
  // that pick an incoming value from either the preheader, or the previous
  // loop iteration. Since this copy is no longer part of the loop, we
  // resolve this statically:
  // For the first iteration, we use the value from the preheader directly.
  // For any other iteration, we replace the phi with the value generated by
  // the immediately preceding clone of the loop body (which represents
  // the previous iteration).
  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
    PHINode *NewPHI = cast<PHINode>(VMap[&*I]);
    if (IterNumber == 0) {
      VMap[&*I] = NewPHI->getIncomingValueForBlock(PreHeader);
    } else {
      Value *LatchVal = NewPHI->getIncomingValueForBlock(Latch);
      Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
      if (LatchInst && L->contains(LatchInst))
        VMap[&*I] = LVMap[LatchInst];
      else
        VMap[&*I] = LatchVal;
    }
    cast<BasicBlock>(VMap[Header])->getInstList().erase(NewPHI);
  }

  // Fix up the outgoing values - we need to add a value for the iteration
  // we've just created. Note that this must happen *after* the incoming
  // values are adjusted, since the value going out of the latch may also be
  // a value coming into the header.
  for (BasicBlock::iterator I = Exit->begin(); isa<PHINode>(I); ++I) {
    PHINode *PHI = cast<PHINode>(I);
    Value *LatchVal = PHI->getIncomingValueForBlock(Latch);
    Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
    if (LatchInst && L->contains(LatchInst))
      LatchVal = VMap[LatchVal];
    PHI->addIncoming(LatchVal, cast<BasicBlock>(VMap[Latch]));
  }

  // LastValueMap is updated with the values for the current loop
  // which are used the next time this function is called.
  for (const auto &KV : VMap)
    LVMap[KV.first] = KV.second;
}

/// \brief Peel off the first \p PeelCount iterations of loop \p L.
///
/// Note that this does not peel them off as a single straight-line block.
/// Rather, each iteration is peeled off separately, and needs to check the
/// exit condition.
/// For loops that dynamically execute \p PeelCount iterations or less
/// this provides a benefit, since the peeled off iterations, which account
/// for the bulk of dynamic execution, can be further simplified by scalar
/// optimizations.
bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
                    ScalarEvolution *SE, DominatorTree *DT,
                    AssumptionCache *AC, bool PreserveLCSSA) {
  if (!canPeel(L))
    return false;

  LoopBlocksDFS LoopBlocks(L);
  LoopBlocks.perform(LI);

  BasicBlock *Header = L->getHeader();
  BasicBlock *PreHeader = L->getLoopPreheader();
  BasicBlock *Latch = L->getLoopLatch();
  BasicBlock *Exit = L->getUniqueExitBlock();

  Function *F = Header->getParent();

  // Set up all the necessary basic blocks. It is convenient to split the
  // preheader into 3 parts - two blocks to anchor the peeled copy of the loop
  // body, and a new preheader for the "real" loop.

  // Peeling the first iteration transforms.
  //
  // PreHeader:
  // ...
  // Header:
  //   LoopBody
  //   If (cond) goto Header
  // Exit:
  //
  // into
  //
  // InsertTop:
  //   LoopBody
  //   If (!cond) goto Exit
  // InsertBot:
  // NewPreHeader:
  // ...
  // Header:
  //  LoopBody
  //  If (cond) goto Header
  // Exit:
  //
  // Each following iteration will split the current bottom anchor in two,
  // and put the new copy of the loop body between these two blocks. That is,
  // after peeling another iteration from the example above, we'll split 
  // InsertBot, and get:
  //
  // InsertTop:
  //   LoopBody
  //   If (!cond) goto Exit
  // InsertBot:
  //   LoopBody
  //   If (!cond) goto Exit
  // InsertBot.next:
  // NewPreHeader:
  // ...
  // Header:
  //  LoopBody
  //  If (cond) goto Header
  // Exit:

  BasicBlock *InsertTop = SplitEdge(PreHeader, Header, DT, LI);
  BasicBlock *InsertBot =
      SplitBlock(InsertTop, InsertTop->getTerminator(), DT, LI);
  BasicBlock *NewPreHeader =
      SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);

  InsertTop->setName(Header->getName() + ".peel.begin");
  InsertBot->setName(Header->getName() + ".peel.next");
  NewPreHeader->setName(PreHeader->getName() + ".peel.newph");

  ValueToValueMapTy LVMap;

  // If we have branch weight information, we'll want to update it for the
  // newly created branches.
  BranchInst *LatchBR =
      cast<BranchInst>(cast<BasicBlock>(Latch)->getTerminator());
  unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);

  uint64_t TrueWeight, FalseWeight;
  uint64_t ExitWeight = 0, CurHeaderWeight = 0;
  if (LatchBR->extractProfMetadata(TrueWeight, FalseWeight)) {
    ExitWeight = HeaderIdx ? TrueWeight : FalseWeight;
    // The # of times the loop body executes is the sum of the exit block
    // weight and the # of times the backedges are taken.
    CurHeaderWeight = TrueWeight + FalseWeight;
  }

  // For each peeled-off iteration, make a copy of the loop.
  for (unsigned Iter = 0; Iter < PeelCount; ++Iter) {
    SmallVector<BasicBlock *, 8> NewBlocks;
    ValueToValueMapTy VMap;

    // Subtract the exit weight from the current header weight -- the exit
    // weight is exactly the weight of the previous iteration's header.
    // FIXME: due to the way the distribution is constructed, we need a
    // guard here to make sure we don't end up with non-positive weights.
    if (ExitWeight < CurHeaderWeight)
      CurHeaderWeight -= ExitWeight;
    else
      CurHeaderWeight = 1;

    cloneLoopBlocks(L, Iter, InsertTop, InsertBot, Exit,
                    NewBlocks, LoopBlocks, VMap, LVMap, DT, LI);

    // Remap to use values from the current iteration instead of the
    // previous one.
    remapInstructionsInBlocks(NewBlocks, VMap);

    if (DT) {
      // Latches of the cloned loops dominate over the loop exit, so idom of the
      // latter is the first cloned loop body, as original PreHeader dominates
      // the original loop body.
      if (Iter == 0)
        DT->changeImmediateDominator(Exit, cast<BasicBlock>(LVMap[Latch]));
#ifndef NDEBUG
      if (VerifyDomInfo)
        DT->verifyDomTree();
#endif
    }

    updateBranchWeights(InsertBot, cast<BranchInst>(VMap[LatchBR]), Iter,
                        PeelCount, ExitWeight);

    InsertTop = InsertBot;
    InsertBot = SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
    InsertBot->setName(Header->getName() + ".peel.next");

    F->getBasicBlockList().splice(InsertTop->getIterator(),
                                  F->getBasicBlockList(),
                                  NewBlocks[0]->getIterator(), F->end());
  }

  // Now adjust the phi nodes in the loop header to get their initial values
  // from the last peeled-off iteration instead of the preheader.
  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
    PHINode *PHI = cast<PHINode>(I);
    Value *NewVal = PHI->getIncomingValueForBlock(Latch);
    Instruction *LatchInst = dyn_cast<Instruction>(NewVal);
    if (LatchInst && L->contains(LatchInst))
      NewVal = LVMap[LatchInst];

    PHI->setIncomingValue(PHI->getBasicBlockIndex(NewPreHeader), NewVal);
  }

  // Adjust the branch weights on the loop exit.
  if (ExitWeight) {
    // The backedge count is the difference of current header weight and
    // current loop exit weight. If the current header weight is smaller than
    // the current loop exit weight, we mark the loop backedge weight as 1.
    uint64_t BackEdgeWeight = 0;
    if (ExitWeight < CurHeaderWeight)
      BackEdgeWeight = CurHeaderWeight - ExitWeight;
    else
      BackEdgeWeight = 1;
    MDBuilder MDB(LatchBR->getContext());
    MDNode *WeightNode =
        HeaderIdx ? MDB.createBranchWeights(ExitWeight, BackEdgeWeight)
                  : MDB.createBranchWeights(BackEdgeWeight, ExitWeight);
    LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
  }

  // If the loop is nested, we changed the parent loop, update SE.
  if (Loop *ParentLoop = L->getParentLoop()) {
    SE->forgetLoop(ParentLoop);

    // FIXME: Incrementally update loop-simplify
    simplifyLoop(ParentLoop, DT, LI, SE, AC, PreserveLCSSA);
  } else {
    // FIXME: Incrementally update loop-simplify
    simplifyLoop(L, DT, LI, SE, AC, PreserveLCSSA);
  }

  NumPeeled++;

  return true;
}