Beispiel #1
0
static void shuffleValueUseLists(Value *V, std::minstd_rand0 &Gen,
                                 DenseSet<Value *> &Seen) {
  if (!Seen.insert(V).second)
    return;

  if (auto *C = dyn_cast<Constant>(V))
    if (!isa<GlobalValue>(C))
      for (Value *Op : C->operands())
        shuffleValueUseLists(Op, Gen, Seen);

  if (V->use_empty() || std::next(V->use_begin()) == V->use_end())
    // Nothing to shuffle for 0 or 1 users.
    return;

  // Generate random numbers between 10 and 99, which will line up nicely in
  // debug output.  We're not worried about collisons here.
  DEBUG(dbgs() << "V = "; V->dump());
  std::uniform_int_distribution<short> Dist(10, 99);
  SmallDenseMap<const Use *, short, 16> Order;
  for (const Use &U : V->uses()) {
    auto I = Dist(Gen);
    Order[&U] = I;
    DEBUG(dbgs() << " - order: " << I << ", U = "; U.getUser()->dump());
  }

  DEBUG(dbgs() << " => shuffle\n");
  V->sortUseList(
      [&Order](const Use &L, const Use &R) { return Order[&L] < Order[&R]; });

  DEBUG({
    for (const Use &U : V->uses())
      DEBUG(dbgs() << " - order: " << Order.lookup(&U) << ", U = ";
            U.getUser()->dump());
  });
Beispiel #2
0
/// Sort local variables so that variables appearing inside of helper
/// expressions come first.
static SmallVector<DbgVariable *, 8>
sortLocalVars(SmallVectorImpl<DbgVariable *> &Input) {
  SmallVector<DbgVariable *, 8> Result;
  SmallVector<PointerIntPair<DbgVariable *, 1>, 8> WorkList;
  // Map back from a DIVariable to its containing DbgVariable.
  SmallDenseMap<const DILocalVariable *, DbgVariable *> DbgVar;
  // Set of DbgVariables in Result.
  SmallDenseSet<DbgVariable *, 8> Visited;
  // For cycle detection.
  SmallDenseSet<DbgVariable *, 8> Visiting;

  // Initialize the worklist and the DIVariable lookup table.
  for (auto Var : reverse(Input)) {
    DbgVar.insert({Var->getVariable(), Var});
    WorkList.push_back({Var, 0});
  }

  // Perform a stable topological sort by doing a DFS.
  while (!WorkList.empty()) {
    auto Item = WorkList.back();
    DbgVariable *Var = Item.getPointer();
    bool visitedAllDependencies = Item.getInt();
    WorkList.pop_back();

    // Dependency is in a different lexical scope or a global.
    if (!Var)
      continue;

    // Already handled.
    if (Visited.count(Var))
      continue;

    // Add to Result if all dependencies are visited.
    if (visitedAllDependencies) {
      Visited.insert(Var);
      Result.push_back(Var);
      continue;
    }

    // Detect cycles.
    auto Res = Visiting.insert(Var);
    if (!Res.second) {
      assert(false && "dependency cycle in local variables");
      return Result;
    }

    // Push dependencies and this node onto the worklist, so that this node is
    // visited again after all of its dependencies are handled.
    WorkList.push_back({Var, 1});
    for (auto *Dependency : dependencies(Var)) {
      auto Dep = dyn_cast_or_null<const DILocalVariable>(Dependency);
      WorkList.push_back({DbgVar[Dep], 0});
    }
  }
  return Result;
}
Beispiel #3
0
// 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;
}
Beispiel #4
0
static bool IsEquivalentPHI(PHINode *PHI,
                          SmallDenseMap<BasicBlock*, Value*, 8> &ValueMapping) {
  unsigned PHINumValues = PHI->getNumIncomingValues();
  if (PHINumValues != ValueMapping.size())
    return false;

  // Scan the phi to see if it matches.
  for (unsigned i = 0, e = PHINumValues; i != e; ++i)
    if (ValueMapping[PHI->getIncomingBlock(i)] !=
        PHI->getIncomingValue(i)) {
      return false;
    }

  return true;
}
Beispiel #5
0
/// Remove dead functions that are not included in DNR (Do Not Remove) list.
bool Inliner::removeDeadFunctions(CallGraph &CG, bool AlwaysInlineOnly) {
  SmallVector<CallGraphNode*, 16> FunctionsToRemove;
  SmallVector<CallGraphNode *, 16> DeadFunctionsInComdats;
  SmallDenseMap<const Comdat *, int, 16> ComdatEntriesAlive;

  auto RemoveCGN = [&](CallGraphNode *CGN) {
    // Remove any call graph edges from the function to its callees.
    CGN->removeAllCalledFunctions();

    // Remove any edges from the external node to the function's call graph
    // node.  These edges might have been made irrelegant due to
    // optimization of the program.
    CG.getExternalCallingNode()->removeAnyCallEdgeTo(CGN);

    // Removing the node for callee from the call graph and delete it.
    FunctionsToRemove.push_back(CGN);
  };

  // Scan for all of the functions, looking for ones that should now be removed
  // from the program.  Insert the dead ones in the FunctionsToRemove set.
  for (CallGraph::iterator I = CG.begin(), E = CG.end(); I != E; ++I) {
    CallGraphNode *CGN = I->second;
    Function *F = CGN->getFunction();
    if (!F || F->isDeclaration())
      continue;

    // Handle the case when this function is called and we only want to care
    // about always-inline functions. This is a bit of a hack to share code
    // between here and the InlineAlways pass.
    if (AlwaysInlineOnly && !F->hasFnAttribute(Attribute::AlwaysInline))
      continue;

    // If the only remaining users of the function are dead constants, remove
    // them.
    F->removeDeadConstantUsers();

    if (!F->isDefTriviallyDead())
      continue;

    // It is unsafe to drop a function with discardable linkage from a COMDAT
    // without also dropping the other members of the COMDAT.
    // The inliner doesn't visit non-function entities which are in COMDAT
    // groups so it is unsafe to do so *unless* the linkage is local.
    if (!F->hasLocalLinkage()) {
      if (const Comdat *C = F->getComdat()) {
        --ComdatEntriesAlive[C];
        DeadFunctionsInComdats.push_back(CGN);
        continue;
      }
    }

    RemoveCGN(CGN);
  }
  if (!DeadFunctionsInComdats.empty()) {
    // Count up all the entities in COMDAT groups
    auto ComdatGroupReferenced = [&](const Comdat *C) {
      auto I = ComdatEntriesAlive.find(C);
      if (I != ComdatEntriesAlive.end())
        ++(I->getSecond());
    };
    for (const Function &F : CG.getModule())
      if (const Comdat *C = F.getComdat())
        ComdatGroupReferenced(C);
    for (const GlobalVariable &GV : CG.getModule().globals())
      if (const Comdat *C = GV.getComdat())
        ComdatGroupReferenced(C);
    for (const GlobalAlias &GA : CG.getModule().aliases())
      if (const Comdat *C = GA.getComdat())
        ComdatGroupReferenced(C);
    for (CallGraphNode *CGN : DeadFunctionsInComdats) {
      Function *F = CGN->getFunction();
      const Comdat *C = F->getComdat();
      int NumAlive = ComdatEntriesAlive[C];
      // We can remove functions in a COMDAT group if the entire group is dead.
      assert(NumAlive >= 0);
      if (NumAlive > 0)
        continue;

      RemoveCGN(CGN);
    }
  }

  if (FunctionsToRemove.empty())
    return false;

  // Now that we know which functions to delete, do so.  We didn't want to do
  // this inline, because that would invalidate our CallGraph::iterator
  // objects. :(
  //
  // Note that it doesn't matter that we are iterating over a non-stable order
  // here to do this, it doesn't matter which order the functions are deleted
  // in.
  array_pod_sort(FunctionsToRemove.begin(), FunctionsToRemove.end());
  FunctionsToRemove.erase(std::unique(FunctionsToRemove.begin(),
                                      FunctionsToRemove.end()),
                          FunctionsToRemove.end());
  for (SmallVectorImpl<CallGraphNode *>::iterator I = FunctionsToRemove.begin(),
                                                  E = FunctionsToRemove.end();
       I != E; ++I) {
    delete CG.removeFunctionFromModule(*I);
    ++NumDeleted;
  }
  return true;
}
Beispiel #6
0
/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
/// if unrolling was successful, or false if the loop was unmodified. Unrolling
/// can only fail when the loop's latch block is not terminated by a conditional
/// branch instruction. However, if the trip count (and multiple) are not known,
/// loop unrolling will mostly produce more code that is no faster.
///
/// TripCount is generally defined as the number of times the loop header
/// executes. UnrollLoop relaxes the definition to permit early exits: here
/// TripCount is the iteration on which control exits LatchBlock if no early
/// exits were taken. Note that UnrollLoop assumes that the loop counter test
/// terminates LatchBlock in order to remove unnecesssary instances of the
/// test. In other words, control may exit the loop prior to TripCount
/// iterations via an early branch, but control may not exit the loop from the
/// LatchBlock's terminator prior to TripCount iterations.
///
/// Similarly, TripMultiple divides the number of times that the LatchBlock may
/// execute without exiting the loop.
///
/// The LoopInfo Analysis that is passed will be kept consistent.
///
/// If a LoopPassManager is passed in, and the loop is fully removed, it will be
/// removed from the LoopPassManager as well. LPM can also be NULL.
///
/// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
/// available from the Pass it must also preserve those analyses.
bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
                      bool AllowRuntime, unsigned TripMultiple, LoopInfo *LI,
                      Pass *PP, LPPassManager *LPM, AssumptionCache *AC) {
  BasicBlock *Preheader = L->getLoopPreheader();
  if (!Preheader) {
    DEBUG(dbgs() << "  Can't unroll; loop preheader-insertion failed.\n");
    return false;
  }

  BasicBlock *LatchBlock = L->getLoopLatch();
  if (!LatchBlock) {
    DEBUG(dbgs() << "  Can't unroll; loop exit-block-insertion failed.\n");
    return false;
  }

  // Loops with indirectbr cannot be cloned.
  if (!L->isSafeToClone()) {
    DEBUG(dbgs() << "  Can't unroll; Loop body cannot be cloned.\n");
    return false;
  }

  BasicBlock *Header = L->getHeader();
  BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());

  if (!BI || BI->isUnconditional()) {
    // The loop-rotate pass can be helpful to avoid this in many cases.
    DEBUG(dbgs() <<
             "  Can't unroll; loop not terminated by a conditional branch.\n");
    return false;
  }

  if (Header->hasAddressTaken()) {
    // The loop-rotate pass can be helpful to avoid this in many cases.
    DEBUG(dbgs() <<
          "  Won't unroll loop: address of header block is taken.\n");
    return false;
  }

  if (TripCount != 0)
    DEBUG(dbgs() << "  Trip Count = " << TripCount << "\n");
  if (TripMultiple != 1)
    DEBUG(dbgs() << "  Trip Multiple = " << TripMultiple << "\n");

  // Effectively "DCE" unrolled iterations that are beyond the tripcount
  // and will never be executed.
  if (TripCount != 0 && Count > TripCount)
    Count = TripCount;

  // Don't enter the unroll code if there is nothing to do. This way we don't
  // need to support "partial unrolling by 1".
  if (TripCount == 0 && Count < 2)
    return false;

  assert(Count > 0);
  assert(TripMultiple > 0);
  assert(TripCount == 0 || TripCount % TripMultiple == 0);

  // Are we eliminating the loop control altogether?
  bool CompletelyUnroll = Count == TripCount;

  // We assume a run-time trip count if the compiler cannot
  // figure out the loop trip count and the unroll-runtime
  // flag is specified.
  bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);

  if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM))
    return false;

  // Notify ScalarEvolution that the loop will be substantially changed,
  // if not outright eliminated.
  ScalarEvolution *SE =
      PP ? PP->getAnalysisIfAvailable<ScalarEvolution>() : nullptr;
  if (SE)
    SE->forgetLoop(L);

  // If we know the trip count, we know the multiple...
  unsigned BreakoutTrip = 0;
  if (TripCount != 0) {
    BreakoutTrip = TripCount % Count;
    TripMultiple = 0;
  } else {
    // Figure out what multiple to use.
    BreakoutTrip = TripMultiple =
      (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
  }

  // Report the unrolling decision.
  DebugLoc LoopLoc = L->getStartLoc();
  Function *F = Header->getParent();
  LLVMContext &Ctx = F->getContext();

  if (CompletelyUnroll) {
    DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
          << " with trip count " << TripCount << "!\n");
    emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
                           Twine("completely unrolled loop with ") +
                               Twine(TripCount) + " iterations");
  } else {
    auto EmitDiag = [&](const Twine &T) {
      emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
                             "unrolled loop by a factor of " + Twine(Count) +
                                 T);
    };

    DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
          << " by " << Count);
    if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
      DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
      EmitDiag(" with a breakout at trip " + Twine(BreakoutTrip));
    } else if (TripMultiple != 1) {
      DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
      EmitDiag(" with " + Twine(TripMultiple) + " trips per branch");
    } else if (RuntimeTripCount) {
      DEBUG(dbgs() << " with run-time trip count");
      EmitDiag(" with run-time trip count");
    }
    DEBUG(dbgs() << "!\n");
  }

  bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
  BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);

  // For the first iteration of the loop, we should use the precloned values for
  // PHI nodes.  Insert associations now.
  ValueToValueMapTy LastValueMap;
  std::vector<PHINode*> OrigPHINode;
  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
    OrigPHINode.push_back(cast<PHINode>(I));
  }

  std::vector<BasicBlock*> Headers;
  std::vector<BasicBlock*> Latches;
  Headers.push_back(Header);
  Latches.push_back(LatchBlock);

  // The current on-the-fly SSA update requires blocks to be processed in
  // reverse postorder so that LastValueMap contains the correct value at each
  // exit.
  LoopBlocksDFS DFS(L);
  DFS.perform(LI);

  // Stash the DFS iterators before adding blocks to the loop.
  LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
  LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();

  for (unsigned It = 1; It != Count; ++It) {
    std::vector<BasicBlock*> NewBlocks;
    SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
    NewLoops[L] = L;

    for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
      ValueToValueMapTy VMap;
      BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
      Header->getParent()->getBasicBlockList().push_back(New);

      // Tell LI about New.
      if (*BB == Header) {
        assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
        L->addBasicBlockToLoop(New, *LI);
      } else {
        // Figure out which loop New is in.
        const Loop *OldLoop = LI->getLoopFor(*BB);
        assert(OldLoop && "Should (at least) be in the loop being unrolled!");

        Loop *&NewLoop = NewLoops[OldLoop];
        if (!NewLoop) {
          // Found a new sub-loop.
          assert(*BB == OldLoop->getHeader() &&
                 "Header should be first in RPO");

          Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
          assert(NewLoopParent &&
                 "Expected parent loop before sub-loop in RPO");
          NewLoop = new Loop;
          NewLoopParent->addChildLoop(NewLoop);

          // Forget the old loop, since its inputs may have changed.
          if (SE)
            SE->forgetLoop(OldLoop);
        }
        NewLoop->addBasicBlockToLoop(New, *LI);
      }

      if (*BB == Header)
        // Loop over all of the PHI nodes in the block, changing them to use
        // the incoming values from the previous block.
        for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
          PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]);
          Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
          if (Instruction *InValI = dyn_cast<Instruction>(InVal))
            if (It > 1 && L->contains(InValI))
              InVal = LastValueMap[InValI];
          VMap[OrigPHINode[i]] = InVal;
          New->getInstList().erase(NewPHI);
        }

      // Update our running map of newest clones
      LastValueMap[*BB] = New;
      for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
           VI != VE; ++VI)
        LastValueMap[VI->first] = VI->second;

      // Add phi entries for newly created values to all exit blocks.
      for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB);
           SI != SE; ++SI) {
        if (L->contains(*SI))
          continue;
        for (BasicBlock::iterator BBI = (*SI)->begin();
             PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
          Value *Incoming = phi->getIncomingValueForBlock(*BB);
          ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
          if (It != LastValueMap.end())
            Incoming = It->second;
          phi->addIncoming(Incoming, New);
        }
      }
      // Keep track of new headers and latches as we create them, so that
      // we can insert the proper branches later.
      if (*BB == Header)
        Headers.push_back(New);
      if (*BB == LatchBlock)
        Latches.push_back(New);

      NewBlocks.push_back(New);
    }

    // Remap all instructions in the most recent iteration
    for (unsigned i = 0; i < NewBlocks.size(); ++i)
      for (BasicBlock::iterator I = NewBlocks[i]->begin(),
           E = NewBlocks[i]->end(); I != E; ++I)
        ::RemapInstruction(I, LastValueMap);
  }

  // Loop over the PHI nodes in the original block, setting incoming values.
  for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
    PHINode *PN = OrigPHINode[i];
    if (CompletelyUnroll) {
      PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
      Header->getInstList().erase(PN);
    }
    else if (Count > 1) {
      Value *InVal = PN->removeIncomingValue(LatchBlock, false);
      // If this value was defined in the loop, take the value defined by the
      // last iteration of the loop.
      if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
        if (L->contains(InValI))
          InVal = LastValueMap[InVal];
      }
      assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
      PN->addIncoming(InVal, Latches.back());
    }
  }

  // Now that all the basic blocks for the unrolled iterations are in place,
  // set up the branches to connect them.
  for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
    // The original branch was replicated in each unrolled iteration.
    BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());

    // The branch destination.
    unsigned j = (i + 1) % e;
    BasicBlock *Dest = Headers[j];
    bool NeedConditional = true;

    if (RuntimeTripCount && j != 0) {
      NeedConditional = false;
    }

    // For a complete unroll, make the last iteration end with a branch
    // to the exit block.
    if (CompletelyUnroll && j == 0) {
      Dest = LoopExit;
      NeedConditional = false;
    }

    // If we know the trip count or a multiple of it, we can safely use an
    // unconditional branch for some iterations.
    if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
      NeedConditional = false;
    }

    if (NeedConditional) {
      // Update the conditional branch's successor for the following
      // iteration.
      Term->setSuccessor(!ContinueOnTrue, Dest);
    } else {
      // Remove phi operands at this loop exit
      if (Dest != LoopExit) {
        BasicBlock *BB = Latches[i];
        for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
             SI != SE; ++SI) {
          if (*SI == Headers[i])
            continue;
          for (BasicBlock::iterator BBI = (*SI)->begin();
               PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
            Phi->removeIncomingValue(BB, false);
          }
        }
      }
      // Replace the conditional branch with an unconditional one.
      BranchInst::Create(Dest, Term);
      Term->eraseFromParent();
    }
  }

  // Merge adjacent basic blocks, if possible.
  SmallPtrSet<Loop *, 4> ForgottenLoops;
  for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
    BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
    if (Term->isUnconditional()) {
      BasicBlock *Dest = Term->getSuccessor(0);
      if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM,
                                                      ForgottenLoops))
        std::replace(Latches.begin(), Latches.end(), Dest, Fold);
    }
  }

  // FIXME: We could register any cloned assumptions instead of clearing the
  // whole function's cache.
  AC->clear();

  DominatorTree *DT = nullptr;
  if (PP) {
    // FIXME: Reconstruct dom info, because it is not preserved properly.
    // Incrementally updating domtree after loop unrolling would be easy.
    if (DominatorTreeWrapperPass *DTWP =
            PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
      DT = &DTWP->getDomTree();
      DT->recalculate(*L->getHeader()->getParent());
    }

    // Simplify any new induction variables in the partially unrolled loop.
    if (SE && !CompletelyUnroll) {
      SmallVector<WeakVH, 16> DeadInsts;
      simplifyLoopIVs(L, SE, LPM, DeadInsts);

      // Aggressively clean up dead instructions that simplifyLoopIVs already
      // identified. Any remaining should be cleaned up below.
      while (!DeadInsts.empty())
        if (Instruction *Inst =
            dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
          RecursivelyDeleteTriviallyDeadInstructions(Inst);
    }
  }
  // At this point, the code is well formed.  We now do a quick sweep over the
  // inserted code, doing constant propagation and dead code elimination as we
  // go.
  const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
  for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
       BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
    for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
      Instruction *Inst = I++;

      if (isInstructionTriviallyDead(Inst))
        (*BB)->getInstList().erase(Inst);
      else if (Value *V = SimplifyInstruction(Inst))
        if (LI->replacementPreservesLCSSAForm(Inst, V)) {
          Inst->replaceAllUsesWith(V);
          (*BB)->getInstList().erase(Inst);
        }
    }

  NumCompletelyUnrolled += CompletelyUnroll;
  ++NumUnrolled;

  Loop *OuterL = L->getParentLoop();
  // Remove the loop from the LoopPassManager if it's completely removed.
  if (CompletelyUnroll && LPM != nullptr)
    LPM->deleteLoopFromQueue(L);

  // If we have a pass and a DominatorTree we should re-simplify impacted loops
  // to ensure subsequent analyses can rely on this form. We want to simplify
  // at least one layer outside of the loop that was unrolled so that any
  // changes to the parent loop exposed by the unrolling are considered.
  if (PP && DT) {
    if (!OuterL && !CompletelyUnroll)
      OuterL = L;
    if (OuterL) {
      DataLayoutPass *DLP = PP->getAnalysisIfAvailable<DataLayoutPass>();
      const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
      simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ nullptr, SE, DL, AC);

      // LCSSA must be performed on the outermost affected loop. The unrolled
      // loop's last loop latch is guaranteed to be in the outermost loop after
      // deleteLoopFromQueue updates LoopInfo.
      Loop *LatchLoop = LI->getLoopFor(Latches.back());
      if (!OuterL->contains(LatchLoop))
        while (OuterL->getParentLoop() != LatchLoop)
          OuterL = OuterL->getParentLoop();

      formLCSSARecursively(*OuterL, *DT, LI, SE);
    }
  }

  return true;
}
Beispiel #7
0
// Sinks \p I from the loop \p L's preheader to its uses. Returns true if
// sinking is successful.
// \p LoopBlockNumber is used to sort the insertion blocks to ensure
// determinism.
static bool sinkInstruction(Loop &L, Instruction &I,
                            const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
                            const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber,
                            LoopInfo &LI, DominatorTree &DT,
                            BlockFrequencyInfo &BFI) {
  // Compute the set of blocks in loop L which contain a use of I.
  SmallPtrSet<BasicBlock *, 2> BBs;
  for (auto &U : I.uses()) {
    Instruction *UI = cast<Instruction>(U.getUser());
    // We cannot sink I to PHI-uses.
    if (dyn_cast<PHINode>(UI))
      return false;
    // We cannot sink I if it has uses outside of the loop.
    if (!L.contains(LI.getLoopFor(UI->getParent())))
      return false;
    BBs.insert(UI->getParent());
  }

  // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
  // BBs.size() to avoid expensive computation.
  // FIXME: Handle code size growth for min_size and opt_size.
  if (BBs.size() > MaxNumberOfUseBBsForSinking)
    return false;

  // Find the set of BBs that we should insert a copy of I.
  SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
      findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI);
  if (BBsToSinkInto.empty())
    return false;

  // Copy the final BBs into a vector and sort them using the total ordering
  // of the loop block numbers as iterating the set doesn't give a useful
  // order. No need to stable sort as the block numbers are a total ordering.
  SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
  SortedBBsToSinkInto.insert(SortedBBsToSinkInto.begin(), BBsToSinkInto.begin(),
                             BBsToSinkInto.end());
  std::sort(SortedBBsToSinkInto.begin(), SortedBBsToSinkInto.end(),
            [&](BasicBlock *A, BasicBlock *B) {
              return *LoopBlockNumber.find(A) < *LoopBlockNumber.find(B);
            });

  BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
  // FIXME: Optimize the efficiency for cloned value replacement. The current
  //        implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
  for (BasicBlock *N : SortedBBsToSinkInto) {
    if (N == MoveBB)
      continue;
    // Clone I and replace its uses.
    Instruction *IC = I.clone();
    IC->setName(I.getName());
    IC->insertBefore(&*N->getFirstInsertionPt());
    // Replaces uses of I with IC in N
    for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
      Use &U = *UI++;
      auto *I = cast<Instruction>(U.getUser());
      if (I->getParent() == N)
        U.set(IC);
    }
    // Replaces uses of I with IC in blocks dominated by N
    replaceDominatedUsesWith(&I, IC, DT, N);
    DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
                 << '\n');
    NumLoopSunkCloned++;
  }
  DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
  NumLoopSunk++;
  I.moveBefore(&*MoveBB->getFirstInsertionPt());

  return true;
}
Beispiel #8
0
/// For every instruction from the worklist, check to see if it has any uses
/// that are outside the current loop.  If so, insert LCSSA PHI nodes and
/// rewrite the uses.
bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
                                    DominatorTree &DT, LoopInfo &LI) {
    SmallVector<Use *, 16> UsesToRewrite;
    SmallSetVector<PHINode *, 16> PHIsToRemove;
    PredIteratorCache PredCache;
    bool Changed = false;

    // Cache the Loop ExitBlocks across this loop.  We expect to get a lot of
    // instructions within the same loops, computing the exit blocks is
    // expensive, and we're not mutating the loop structure.
    SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;

    while (!Worklist.empty()) {
        UsesToRewrite.clear();

        Instruction *I = Worklist.pop_back_val();
        BasicBlock *InstBB = I->getParent();
        Loop *L = LI.getLoopFor(InstBB);
        if (!LoopExitBlocks.count(L))
            L->getExitBlocks(LoopExitBlocks[L]);
        assert(LoopExitBlocks.count(L));
        const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];

        if (ExitBlocks.empty())
            continue;

        // Tokens cannot be used in PHI nodes, so we skip over them.
        // We can run into tokens which are live out of a loop with catchswitch
        // instructions in Windows EH if the catchswitch has one catchpad which
        // is inside the loop and another which is not.
        if (I->getType()->isTokenTy())
            continue;

        for (Use &U : I->uses()) {
            Instruction *User = cast<Instruction>(U.getUser());
            BasicBlock *UserBB = User->getParent();
            if (PHINode *PN = dyn_cast<PHINode>(User))
                UserBB = PN->getIncomingBlock(U);

            if (InstBB != UserBB && !L->contains(UserBB))
                UsesToRewrite.push_back(&U);
        }

        // If there are no uses outside the loop, exit with no change.
        if (UsesToRewrite.empty())
            continue;

        ++NumLCSSA; // We are applying the transformation

        // Invoke instructions are special in that their result value is not
        // available along their unwind edge. The code below tests to see whether
        // DomBB dominates the value, so adjust DomBB to the normal destination
        // block, which is effectively where the value is first usable.
        BasicBlock *DomBB = InstBB;
        if (InvokeInst *Inv = dyn_cast<InvokeInst>(I))
            DomBB = Inv->getNormalDest();

        DomTreeNode *DomNode = DT.getNode(DomBB);

        SmallVector<PHINode *, 16> AddedPHIs;
        SmallVector<PHINode *, 8> PostProcessPHIs;

        SmallVector<PHINode *, 4> InsertedPHIs;
        SSAUpdater SSAUpdate(&InsertedPHIs);
        SSAUpdate.Initialize(I->getType(), I->getName());

        // Insert the LCSSA phi's into all of the exit blocks dominated by the
        // value, and add them to the Phi's map.
        for (BasicBlock *ExitBB : ExitBlocks) {
            if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
                continue;

            // If we already inserted something for this BB, don't reprocess it.
            if (SSAUpdate.HasValueForBlock(ExitBB))
                continue;

            PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),
                                          I->getName() + ".lcssa", &ExitBB->front());

            // Add inputs from inside the loop for this PHI.
            for (BasicBlock *Pred : PredCache.get(ExitBB)) {
                PN->addIncoming(I, Pred);

                // If the exit block has a predecessor not within the loop, arrange for
                // the incoming value use corresponding to that predecessor to be
                // rewritten in terms of a different LCSSA PHI.
                if (!L->contains(Pred))
                    UsesToRewrite.push_back(
                        &PN->getOperandUse(PN->getOperandNumForIncomingValue(
                                               PN->getNumIncomingValues() - 1)));
            }

            AddedPHIs.push_back(PN);

            // Remember that this phi makes the value alive in this block.
            SSAUpdate.AddAvailableValue(ExitBB, PN);

            // LoopSimplify might fail to simplify some loops (e.g. when indirect
            // branches are involved). In such situations, it might happen that an
            // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
            // create PHIs in such an exit block, we are also inserting PHIs into L2's
            // header. This could break LCSSA form for L2 because these inserted PHIs
            // can also have uses outside of L2. Remember all PHIs in such situation
            // as to revisit than later on. FIXME: Remove this if indirectbr support
            // into LoopSimplify gets improved.
            if (auto *OtherLoop = LI.getLoopFor(ExitBB))
                if (!L->contains(OtherLoop))
                    PostProcessPHIs.push_back(PN);
        }

        // Rewrite all uses outside the loop in terms of the new PHIs we just
        // inserted.
        for (Use *UseToRewrite : UsesToRewrite) {
            // If this use is in an exit block, rewrite to use the newly inserted PHI.
            // This is required for correctness because SSAUpdate doesn't handle uses
            // in the same block.  It assumes the PHI we inserted is at the end of the
            // block.
            Instruction *User = cast<Instruction>(UseToRewrite->getUser());
            BasicBlock *UserBB = User->getParent();
            if (PHINode *PN = dyn_cast<PHINode>(User))
                UserBB = PN->getIncomingBlock(*UseToRewrite);

            if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
                // Tell the VHs that the uses changed. This updates SCEV's caches.
                if (UseToRewrite->get()->hasValueHandle())
                    ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
                UseToRewrite->set(&UserBB->front());
                continue;
            }

            // Otherwise, do full PHI insertion.
            SSAUpdate.RewriteUse(*UseToRewrite);
        }

        // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
        // to post-process them to keep LCSSA form.
        for (PHINode *InsertedPN : InsertedPHIs) {
            if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
                if (!L->contains(OtherLoop))
                    PostProcessPHIs.push_back(InsertedPN);
        }

        // Post process PHI instructions that were inserted into another disjoint
        // loop and update their exits properly.
        for (auto *PostProcessPN : PostProcessPHIs) {
            if (PostProcessPN->use_empty())
                continue;

            // Reprocess each PHI instruction.
            Worklist.push_back(PostProcessPN);
        }

        // Keep track of PHI nodes that we want to remove because they did not have
        // any uses rewritten.
        for (PHINode *PN : AddedPHIs)
            if (PN->use_empty())
                PHIsToRemove.insert(PN);

        Changed = true;
    }
    // Remove PHI nodes that did not have any uses rewritten.
    for (PHINode *PN : PHIsToRemove) {
        assert (PN->use_empty() && "Trying to remove a phi with uses.");
        PN->eraseFromParent();
    }
    return Changed;
}
Beispiel #9
0
Datei: Writer.cpp Projekt: 8l/lld
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::createSections() {
  // .interp needs to be on the first page in the output file.
  if (needsInterpSection())
    OutputSections.push_back(Out<ELFT>::Interp);

  SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSectionBase<ELFT> *> Map;

  std::vector<OutputSectionBase<ELFT> *> RegularSections;

  for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
    for (InputSectionBase<ELFT> *C : F->getSections()) {
      if (isDiscarded(C))
        continue;
      const Elf_Shdr *H = C->getSectionHdr();
      uintX_t OutFlags = H->sh_flags & ~SHF_GROUP;
      // For SHF_MERGE we create different output sections for each sh_entsize.
      // This makes each output section simple and keeps a single level
      // mapping from input to output.
      typename InputSectionBase<ELFT>::Kind K = C->SectionKind;
      uintX_t EntSize = K != InputSectionBase<ELFT>::Merge ? 0 : H->sh_entsize;
      uint32_t OutType = H->sh_type;
      if (OutType == SHT_PROGBITS && C->getSectionName() == ".eh_frame" &&
          Config->EMachine == EM_X86_64)
        OutType = SHT_X86_64_UNWIND;
      SectionKey<ELFT::Is64Bits> Key{getOutputSectionName(C->getSectionName()),
                                     OutType, OutFlags, EntSize};
      OutputSectionBase<ELFT> *&Sec = Map[Key];
      if (!Sec) {
        switch (K) {
        case InputSectionBase<ELFT>::Regular:
          Sec = new (SecAlloc.Allocate())
              OutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
          break;
        case InputSectionBase<ELFT>::EHFrame:
          Sec = new (EHSecAlloc.Allocate())
              EHOutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
          break;
        case InputSectionBase<ELFT>::Merge:
          Sec = new (MSecAlloc.Allocate())
              MergeOutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
          break;
        }
        OutputSections.push_back(Sec);
        RegularSections.push_back(Sec);
      }
      switch (K) {
      case InputSectionBase<ELFT>::Regular:
        static_cast<OutputSection<ELFT> *>(Sec)
            ->addSection(cast<InputSection<ELFT>>(C));
        break;
      case InputSectionBase<ELFT>::EHFrame:
        static_cast<EHOutputSection<ELFT> *>(Sec)
            ->addSection(cast<EHInputSection<ELFT>>(C));
        break;
      case InputSectionBase<ELFT>::Merge:
        static_cast<MergeOutputSection<ELFT> *>(Sec)
            ->addSection(cast<MergeInputSection<ELFT>>(C));
        break;
      }
    }
  }

  Out<ELFT>::Bss = static_cast<OutputSection<ELFT> *>(
      Map[{".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE, 0}]);

  Out<ELFT>::Dynamic->PreInitArraySec = Map.lookup(
      {".preinit_array", SHT_PREINIT_ARRAY, SHF_WRITE | SHF_ALLOC, 0});
  Out<ELFT>::Dynamic->InitArraySec =
      Map.lookup({".init_array", SHT_INIT_ARRAY, SHF_WRITE | SHF_ALLOC, 0});
  Out<ELFT>::Dynamic->FiniArraySec =
      Map.lookup({".fini_array", SHT_FINI_ARRAY, SHF_WRITE | SHF_ALLOC, 0});

  auto AddStartEnd = [&](StringRef Start, StringRef End,
                         OutputSectionBase<ELFT> *OS) {
    if (OS) {
      Symtab.addSyntheticSym(Start, *OS, 0);
      Symtab.addSyntheticSym(End, *OS, OS->getSize());
    } else {
      Symtab.addIgnoredSym(Start);
      Symtab.addIgnoredSym(End);
    }
  };

  AddStartEnd("__preinit_array_start", "__preinit_array_end",
              Out<ELFT>::Dynamic->PreInitArraySec);
  AddStartEnd("__init_array_start", "__init_array_end",
              Out<ELFT>::Dynamic->InitArraySec);
  AddStartEnd("__fini_array_start", "__fini_array_end",
              Out<ELFT>::Dynamic->FiniArraySec);

  for (OutputSectionBase<ELFT> *Sec : RegularSections)
    addStartStopSymbols(Sec);

  // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
  // static linking the linker is required to optimize away any references to
  // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
  // to avoid the undefined symbol error.
  if (!isOutputDynamic())
    Symtab.addIgnoredSym("__tls_get_addr");

  // If the "_end" symbol is referenced, it is expected to point to the address
  // right after the data segment. Usually, this symbol points to the end
  // of .bss section or to the end of .data section if .bss section is absent.
  // The order of the sections can be affected by linker script,
  // so it is hard to predict which section will be the last one.
  // So, if this symbol is referenced, we just add the placeholder here
  // and update its value later.
  if (Symtab.find("_end"))
    Symtab.addAbsoluteSym("_end", DefinedAbsolute<ELFT>::End);

  // If there is an undefined symbol "end", we should initialize it
  // with the same value as "_end". In any other case it should stay intact,
  // because it is an allowable name for a user symbol.
  if (SymbolBody *B = Symtab.find("end"))
    if (B->isUndefined())
      Symtab.addAbsoluteSym("end", DefinedAbsolute<ELFT>::End);

  // Scan relocations. This must be done after every symbol is declared so that
  // we can correctly decide if a dynamic relocation is needed.
  for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
    for (InputSectionBase<ELFT> *C : F->getSections()) {
      if (isDiscarded(C))
        continue;
      if (auto *S = dyn_cast<InputSection<ELFT>>(C))
        scanRelocs(*S);
      else if (auto *S = dyn_cast<EHInputSection<ELFT>>(C))
        if (S->RelocSection)
          scanRelocs(*S, *S->RelocSection);
    }
  }

  std::vector<DefinedCommon<ELFT> *> CommonSymbols;
  std::vector<SharedSymbol<ELFT> *> SharedCopySymbols;
  for (auto &P : Symtab.getSymbols()) {
    SymbolBody *Body = P.second->Body;
    if (auto *U = dyn_cast<Undefined<ELFT>>(Body))
      if (!U->isWeak() && !U->canKeepUndefined())
        reportUndefined<ELFT>(Symtab, *Body);

    if (auto *C = dyn_cast<DefinedCommon<ELFT>>(Body))
      CommonSymbols.push_back(C);
    if (auto *SC = dyn_cast<SharedSymbol<ELFT>>(Body))
      if (SC->needsCopy())
        SharedCopySymbols.push_back(SC);

    if (!includeInSymtab<ELFT>(*Body))
      continue;
    if (Out<ELFT>::SymTab)
      Out<ELFT>::SymTab->addSymbol(Body);

    if (isOutputDynamic() && includeInDynamicSymtab(*Body))
      Out<ELFT>::DynSymTab->addSymbol(Body);
  }
  addCommonSymbols(CommonSymbols);
  addSharedCopySymbols(SharedCopySymbols);

  // This order is not the same as the final output order
  // because we sort the sections using their attributes below.
  if (Out<ELFT>::SymTab)
    OutputSections.push_back(Out<ELFT>::SymTab);
  OutputSections.push_back(Out<ELFT>::ShStrTab);
  if (Out<ELFT>::StrTab)
    OutputSections.push_back(Out<ELFT>::StrTab);
  if (isOutputDynamic()) {
    OutputSections.push_back(Out<ELFT>::DynSymTab);
    if (Out<ELFT>::GnuHashTab)
      OutputSections.push_back(Out<ELFT>::GnuHashTab);
    if (Out<ELFT>::HashTab)
      OutputSections.push_back(Out<ELFT>::HashTab);
    OutputSections.push_back(Out<ELFT>::Dynamic);
    OutputSections.push_back(Out<ELFT>::DynStrTab);
    if (Out<ELFT>::RelaDyn->hasRelocs())
      OutputSections.push_back(Out<ELFT>::RelaDyn);
    if (Out<ELFT>::RelaPlt && Out<ELFT>::RelaPlt->hasRelocs())
      OutputSections.push_back(Out<ELFT>::RelaPlt);
    // This is a MIPS specific section to hold a space within the data segment
    // of executable file which is pointed to by the DT_MIPS_RLD_MAP entry.
    // See "Dynamic section" in Chapter 5 in the following document:
    // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
    if (Config->EMachine == EM_MIPS && !Config->Shared) {
      Out<ELFT>::MipsRldMap = new (SecAlloc.Allocate())
          OutputSection<ELFT>(".rld_map", SHT_PROGBITS, SHF_ALLOC | SHF_WRITE);
      Out<ELFT>::MipsRldMap->setSize(ELFT::Is64Bits ? 8 : 4);
      Out<ELFT>::MipsRldMap->updateAlign(ELFT::Is64Bits ? 8 : 4);
      OutputSections.push_back(Out<ELFT>::MipsRldMap);
    }
  }

  // We add the .got section to the result for dynamic MIPS target because
  // its address and properties are mentioned in the .dynamic section.
  if (!Out<ELFT>::Got->empty() ||
      (isOutputDynamic() && Config->EMachine == EM_MIPS))
    OutputSections.push_back(Out<ELFT>::Got);
  if (Out<ELFT>::GotPlt && !Out<ELFT>::GotPlt->empty())
    OutputSections.push_back(Out<ELFT>::GotPlt);
  if (!Out<ELFT>::Plt->empty())
    OutputSections.push_back(Out<ELFT>::Plt);

  std::stable_sort(OutputSections.begin(), OutputSections.end(),
                   compareSections<ELFT>);

  for (unsigned I = 0, N = OutputSections.size(); I < N; ++I) {
    OutputSections[I]->SectionIndex = I + 1;
    HasRelro |= (Config->ZRelro && isRelroSection(OutputSections[I]));
  }

  for (OutputSectionBase<ELFT> *Sec : OutputSections)
    Out<ELFT>::ShStrTab->add(Sec->getName());

  // Finalizers fix each section's size.
  // .dynamic section's finalizer may add strings to .dynstr,
  // so finalize that early.
  // Likewise, .dynsym is finalized early since that may fill up .gnu.hash.
  Out<ELFT>::Dynamic->finalize();
  if (isOutputDynamic())
    Out<ELFT>::DynSymTab->finalize();

  // Fill other section headers.
  for (OutputSectionBase<ELFT> *Sec : OutputSections)
    Sec->finalize();

  // If we have a .opd section (used under PPC64 for function descriptors),
  // store a pointer to it here so that we can use it later when processing
  // relocations.
  Out<ELFT>::Opd = Map.lookup({".opd", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC, 0});
}