C++ TerminatorInst::getNumSuccessors方法代码示例

// Propagate existing explicit probabilities from either profile data or
// 'expect' intrinsic processing.
bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
  TerminatorInst *TI = BB->getTerminator();
  if (TI->getNumSuccessors() == 1)
    return false;
  if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
    return false;

  MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
  if (!WeightsNode)
    return false;

  // Ensure there are weights for all of the successors. Note that the first
  // operand to the metadata node is a name, not a weight.
  if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
    return false;

  // Build up the final weights that will be used in a temporary buffer, but
  // don't add them until all weihts are present. Each weight value is clamped
  // to [1, getMaxWeightFor(BB)].
  uint32_t WeightLimit = getMaxWeightFor(BB);
  SmallVector<uint32_t, 2> Weights;
  Weights.reserve(TI->getNumSuccessors());
  for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
    ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
    if (!Weight)
      return false;
    Weights.push_back(
      std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
  }
  assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
    BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);

  return true;
}

/// \brief Generate branch weight metadata for all branches in \p F.
///
/// For every branch instruction B in \p F, we compute the weight of the
/// target block for each of the edges out of B. This is the weight
/// that we associate with that branch.
///
/// TODO - This weight assignment will most likely be wrong if the
/// target branch has more than two predecessors. This needs to be done
/// using some form of flow propagation.
///
/// Once all the branch weights are computed, we emit the MD_prof
/// metadata on B using the computed values.
///
/// \param F The function to query.
bool SampleProfile::emitAnnotations(Function &F) {
  bool Changed = false;
  FunctionProfile &FProfile = Profiles[F.getName()];
  unsigned FirstLineno = inst_begin(F)->getDebugLoc().getLine();
  MDBuilder MDB(F.getContext());

  // Clear the block weights cache.
  FProfile.BlockWeights.clear();

  // When we find a branch instruction: For each edge E out of the branch,
  // the weight of E is the weight of the target block.
  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    BasicBlock *B = I;
    TerminatorInst *TI = B->getTerminator();
    if (TI->getNumSuccessors() == 1)
      continue;
    if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
      continue;

    SmallVector<uint32_t, 4> Weights;
    unsigned NSuccs = TI->getNumSuccessors();
    for (unsigned I = 0; I < NSuccs; ++I) {
      BasicBlock *Succ = TI->getSuccessor(I);
      uint32_t Weight =
          computeBlockWeight(Succ, FirstLineno, FProfile.BodySamples);
      Weights.push_back(Weight);
    }

    TI->setMetadata(llvm::LLVMContext::MD_prof,
                    MDB.createBranchWeights(Weights));
    Changed = true;
  }

  return Changed;
}

/// \brief Calculate edge weights for successors lead to unreachable.
///
/// Predict that a successor which leads necessarily to an
/// unreachable-terminated block as extremely unlikely.
bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
  TerminatorInst *TI = BB->getTerminator();
  if (TI->getNumSuccessors() == 0) {
    if (isa<UnreachableInst>(TI))
      PostDominatedByUnreachable.insert(BB);
    return false;
  }

  SmallVector<unsigned, 4> UnreachableEdges;
  SmallVector<unsigned, 4> ReachableEdges;

  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
    if (PostDominatedByUnreachable.count(*I))
      UnreachableEdges.push_back(I.getSuccessorIndex());
    else
      ReachableEdges.push_back(I.getSuccessorIndex());
  }

  // If all successors are in the set of blocks post-dominated by unreachable,
  // this block is too.
  if (UnreachableEdges.size() == TI->getNumSuccessors())
    PostDominatedByUnreachable.insert(BB);

  // Skip probabilities if this block has a single successor or if all were
  // reachable.
  if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
    return false;

  // If the terminator is an InvokeInst, check only the normal destination block
  // as the unwind edge of InvokeInst is also very unlikely taken.
  if (auto *II = dyn_cast<InvokeInst>(TI))
    if (PostDominatedByUnreachable.count(II->getNormalDest())) {
      PostDominatedByUnreachable.insert(BB);
      // Return false here so that edge weights for InvokeInst could be decided
      // in calcInvokeHeuristics().
      return false;
    }

  if (ReachableEdges.empty()) {
    BranchProbability Prob(1, UnreachableEdges.size());
    for (unsigned SuccIdx : UnreachableEdges)
      setEdgeProbability(BB, SuccIdx, Prob);
    return true;
  }

  BranchProbability UnreachableProb(UR_TAKEN_WEIGHT,
                                    (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
                                        UnreachableEdges.size());
  BranchProbability ReachableProb(UR_NONTAKEN_WEIGHT,
                                  (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
                                      ReachableEdges.size());

  for (unsigned SuccIdx : UnreachableEdges)
    setEdgeProbability(BB, SuccIdx, UnreachableProb);
  for (unsigned SuccIdx : ReachableEdges)
    setEdgeProbability(BB, SuccIdx, ReachableProb);

  return true;
}

void CheckInserter::insertCycleChecks(Function &F) {
  IdentifyBackEdges &IBE = getAnalysis<IdentifyBackEdges>();

  for (Function::iterator B1 = F.begin(); B1 != F.end(); ++B1) {
    TerminatorInst *TI = B1->getTerminator();
    for (unsigned j = 0; j < TI->getNumSuccessors(); ++j) {
      BasicBlock *B2 = TI->getSuccessor(j);
      unsigned BackEdgeID = IBE.getID(B1, B2);
      if (BackEdgeID != (unsigned)-1) {
        assert(BackEdgeID < MaxNumBackEdges);
        BasicBlock *BackEdgeBlock = BasicBlock::Create(
            F.getContext(),
            "backedge_" + B1->getName() + "_" + B2->getName(),
            &F);
        CallInst::Create(CycleCheck,
                         ConstantInt::get(IntType, BackEdgeID),
                         "",
                         BackEdgeBlock);
        // BackEdgeBlock -> B2
        // Fix the PHINodes in B2.
        BranchInst::Create(B2, BackEdgeBlock);
        for (BasicBlock::iterator I = B2->begin();
             B2->getFirstNonPHI() != I;
             ++I) {
          PHINode *PHI = cast<PHINode>(I);
          // Note: If B2 has multiple incoming edges from B1 (e.g. B1 terminates
          // with a SelectInst), its PHINodes must also have multiple incoming
          // edges from B1. However, after adding BackEdgeBlock and essentially
          // merging the multiple incoming edges from B1, there will be only one
          // edge from BackEdgeBlock to B2. Therefore, we need to remove the
          // redundant incoming edges from B2's PHINodes.
          bool FirstIncomingFromB1 = true;
          for (unsigned k = 0; k < PHI->getNumIncomingValues(); ++k) {
            if (PHI->getIncomingBlock(k) == B1) {
              if (FirstIncomingFromB1) {
                FirstIncomingFromB1 = false;
                PHI->setIncomingBlock(k, BackEdgeBlock);
              } else {
                PHI->removeIncomingValue(k, false);
                --k;
              }
            }
          }
        }
        // B1 -> BackEdgeBlock
        // There might be multiple back edges from B1 to B2. Need to replace
        // them all.
        for (unsigned j2 = j; j2 < TI->getNumSuccessors(); ++j2) {
          if (TI->getSuccessor(j2) == B2) {
            TI->setSuccessor(j2, BackEdgeBlock);
          }
        }
      }
    }
  }
}

/// \brief Calculate edge weights for edges leading to cold blocks.
///
/// A cold block is one post-dominated by  a block with a call to a
/// cold function.  Those edges are unlikely to be taken, so we give
/// them relatively low weight.
///
/// Return true if we could compute the weights for cold edges.
/// Return false, otherwise.
bool BranchProbabilityInfo::calcColdCallHeuristics(BasicBlock *BB) {
  TerminatorInst *TI = BB->getTerminator();
  if (TI->getNumSuccessors() == 0)
    return false;

  // Determine which successors are post-dominated by a cold block.
  SmallVector<unsigned, 4> ColdEdges;
  SmallVector<unsigned, 4> NormalEdges;
  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
    if (PostDominatedByColdCall.count(*I))
      ColdEdges.push_back(I.getSuccessorIndex());
    else
      NormalEdges.push_back(I.getSuccessorIndex());

  // If all successors are in the set of blocks post-dominated by cold calls,
  // this block is in the set post-dominated by cold calls.
  if (ColdEdges.size() == TI->getNumSuccessors())
    PostDominatedByColdCall.insert(BB);
  else {
    // Otherwise, if the block itself contains a cold function, add it to the
    // set of blocks postdominated by a cold call.
    assert(!PostDominatedByColdCall.count(BB));
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
      if (CallInst *CI = dyn_cast<CallInst>(I))
        if (CI->hasFnAttr(Attribute::Cold)) {
          PostDominatedByColdCall.insert(BB);
          break;
        }
  }

  // Skip probabilities if this block has a single successor.
  if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
    return false;

  if (NormalEdges.empty()) {
    BranchProbability Prob(1, ColdEdges.size());
    for (unsigned SuccIdx : ColdEdges)
      setEdgeProbability(BB, SuccIdx, Prob);
    return true;
  }

  BranchProbability ColdProb(CC_TAKEN_WEIGHT,
                             (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
                                 ColdEdges.size());
  BranchProbability NormalProb(CC_NONTAKEN_WEIGHT,
                               (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
                                   NormalEdges.size());

  for (unsigned SuccIdx : ColdEdges)
    setEdgeProbability(BB, SuccIdx, ColdProb);
  for (unsigned SuccIdx : NormalEdges)
    setEdgeProbability(BB, SuccIdx, NormalProb);

  return true;
}

unsigned
llvm::SplitAllCriticalEdges(Function &F,
                            const CriticalEdgeSplittingOptions &Options) {
  unsigned NumBroken = 0;
  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    TerminatorInst *TI = I->getTerminator();
    if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
        if (SplitCriticalEdge(TI, i, Options))
          ++NumBroken;
  }
  return NumBroken;
}

unsigned
llvm::SplitAllCriticalEdges(Function &F,
                            const CriticalEdgeSplittingOptions &Options) {
  unsigned NumBroken = 0;
  for (BasicBlock &BB : F) {
    TerminatorInst *TI = BB.getTerminator();
    if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
        if (SplitCriticalEdge(TI, i, Options))
          ++NumBroken;
  }
  return NumBroken;
}

// runOnFunction - Loop over all of the edges in the CFG, breaking critical
// edges as they are found.
//
bool BreakCriticalEdges::runOnFunction(Function &F) {
  bool Changed = false;
  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    TerminatorInst *TI = I->getTerminator();
    if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
        if (SplitCriticalEdge(TI, i, this)) {
          ++NumBroken;
          Changed = true;
        }
  }

  return Changed;
}

/*
 * Verify if a given value has references after a call site.
 */
bool DeadStoreEliminationPass::isRefAfterCallSite(Value* v, CallSite &CS) {
  BasicBlock* CSBB = CS.getInstruction()->getParent();

  // Collect basic blocks to inspect
  std::vector<BasicBlock*> BBToInspect;
  std::set<BasicBlock*> BBToInspectSet;
  BBToInspect.push_back(CSBB);
  BBToInspectSet.insert(CSBB);
  for (unsigned int i = 0; i < BBToInspect.size(); ++i) {
    BasicBlock* BB = BBToInspect.at(i);
    TerminatorInst* terminator = BB->getTerminator();
    if (terminator && terminator->getNumSuccessors() > 0) {
      unsigned numSuccessors = terminator->getNumSuccessors();
      for (unsigned i = 0; i < numSuccessors; ++i) {
        // Collect successors
        BasicBlock* successor = terminator->getSuccessor(i);
        if (!BBToInspectSet.count(successor)) {
          BBToInspect.push_back(successor);
          BBToInspectSet.insert(successor);
        }
      }
    }
  }

  // Inspect if any instruction after CS references v
  AliasAnalysis::Location loc(v, getPointerSize(v, *AA), NULL);
  for (unsigned int i = 0; i < BBToInspect.size(); ++i) {
    BasicBlock* BB = BBToInspect.at(i);
    BasicBlock::iterator I = BB->begin();
    if (BB == CSBB) {
      Instruction* callInst = CS.getInstruction();
      Instruction* inst;
      do {
        inst = I;
        ++I;
      } while (inst != callInst);
    }
    for (BasicBlock::iterator IE = BB->end(); I != IE; ++I) {
      Instruction* inst = I;
      DEBUG(errs() << "Verifying if instruction " << *inst << " refs " << *v << ": ");
      AliasAnalysis::ModRefResult mrf = AA->getModRefInfo(inst, loc);
      DEBUG(errs() << mrf << "\n");
      if (mrf == AliasAnalysis::Ref || mrf == AliasAnalysis::ModRef) {
        return true;
      }
    }
  }
  return false;
}

// Assign the scaled count values to the BB with multiple out edges.
void PGOUseFunc::setBranchWeights() {
  // Generate MD_prof metadata for every branch instruction.
  DEBUG(dbgs() << "\nSetting branch weights.\n");
  for (auto &BB : F) {
    TerminatorInst *TI = BB.getTerminator();
    if (TI->getNumSuccessors() < 2)
      continue;
    if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
      continue;
    if (getBBInfo(&BB).CountValue == 0)
      continue;

    // We have a non-zero Branch BB.
    const UseBBInfo &BBCountInfo = getBBInfo(&BB);
    unsigned Size = BBCountInfo.OutEdges.size();
    SmallVector<uint64_t, 2> EdgeCounts(Size, 0);
    uint64_t MaxCount = 0;
    for (unsigned s = 0; s < Size; s++) {
      const PGOUseEdge *E = BBCountInfo.OutEdges[s];
      const BasicBlock *SrcBB = E->SrcBB;
      const BasicBlock *DestBB = E->DestBB;
      if (DestBB == nullptr)
        continue;
      unsigned SuccNum = GetSuccessorNumber(SrcBB, DestBB);
      uint64_t EdgeCount = E->CountValue;
      if (EdgeCount > MaxCount)
        MaxCount = EdgeCount;
      EdgeCounts[SuccNum] = EdgeCount;
    }
    setProfMetadata(M, TI, EdgeCounts, MaxCount);
  }
}

// Partition blocks in an outer/inner loop pair into blocks before and after
// the loop
static bool partitionOuterLoopBlocks(Loop *L, Loop *SubLoop,
                                     BasicBlockSet &ForeBlocks,
                                     BasicBlockSet &SubLoopBlocks,
                                     BasicBlockSet &AftBlocks,
                                     DominatorTree *DT) {
  BasicBlock *SubLoopLatch = SubLoop->getLoopLatch();
  SubLoopBlocks.insert(SubLoop->block_begin(), SubLoop->block_end());

  for (BasicBlock *BB : L->blocks()) {
    if (!SubLoop->contains(BB)) {
      if (DT->dominates(SubLoopLatch, BB))
        AftBlocks.insert(BB);
      else
        ForeBlocks.insert(BB);
    }
  }

  // Check that all blocks in ForeBlocks together dominate the subloop
  // TODO: This might ideally be done better with a dominator/postdominators.
  BasicBlock *SubLoopPreHeader = SubLoop->getLoopPreheader();
  for (BasicBlock *BB : ForeBlocks) {
    if (BB == SubLoopPreHeader)
      continue;
    TerminatorInst *TI = BB->getTerminator();
    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
      if (!ForeBlocks.count(TI->getSuccessor(i)))
        return false;
  }

  return true;
}

/// DeleteDeadBlock - Delete the specified block, which must have no
/// predecessors.
void llvm::DeleteDeadBlock(BasicBlock *BB) {
  assert((pred_begin(BB) == pred_end(BB) ||
         // Can delete self loop.
         BB->getSinglePredecessor() == BB) && "Block is not dead!");
  TerminatorInst *BBTerm = BB->getTerminator();
  
  // Loop through all of our successors and make sure they know that one
  // of their predecessors is going away.
  for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
    BBTerm->getSuccessor(i)->removePredecessor(BB);
  
  // Zap all the instructions in the block.
  while (!BB->empty()) {
    Instruction &I = BB->back();
    // If this instruction is used, replace uses with an arbitrary value.
    // Because control flow can't get here, we don't care what we replace the
    // value with.  Note that since this block is unreachable, and all values
    // contained within it must dominate their uses, that all uses will
    // eventually be removed (they are themselves dead).
    if (!I.use_empty())
      I.replaceAllUsesWith(UndefValue::get(I.getType()));
    BB->getInstList().pop_back();
  }
  
  // Zap the block!
  BB->eraseFromParent();
}

void CleanupBasicBlock(BasicBlock *BB){
    std::vector<llvm::BasicBlock *>removedSuccessorsOfMore;
    
    if(BB->getParent()){
        TerminatorInst *BBTerm = BB->getTerminator();
        // Loop through all of our successors and make sure they know that one
        // of their predecessors is going away.
        for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i){
            BBTerm->getSuccessor(i)->removePredecessor(BB);
        }
        
        // Zap all the instructions in the block.
        while (!BB->empty()) {
            Instruction &I = BB->back();
            // If this instruction is used, replace uses with an arbitrary value.
            // Because control flow can't get here, we don't care what we replace the
            // value with.  Note that since this block is unreachable, and all values
            // contained within it must dominate their uses, that all uses will
            // eventually be removed (they are themselves dead).
            if (!I.use_empty())
            I.replaceAllUsesWith(UndefValue::get(I.getType()));
            BB->getInstList().pop_back();
            I.dropAllReferences();
        }
        
        BB->dropAllReferences();
        BB->eraseFromParent();
    }
}

//------------------------------------------------------------------------------
void changeBlockTarget(BasicBlock *block, BasicBlock *newTarget,
                       unsigned int branchIndex) {
  TerminatorInst *terminator = block->getTerminator();
  assert(terminator->getNumSuccessors() &&
         "The target can be change only if it is unique");
  terminator->setSuccessor(branchIndex, newTarget);
}

MDNode *Loop::getLoopID() const {
  MDNode *LoopID = nullptr;
  if (isLoopSimplifyForm()) {
    LoopID = getLoopLatch()->getTerminator()->getMetadata(LoopMDName);
  } else {
    // Go through each predecessor of the loop header and check the
    // terminator for the metadata.
    BasicBlock *H = getHeader();
    for (block_iterator I = block_begin(), IE = block_end(); I != IE; ++I) {
      TerminatorInst *TI = (*I)->getTerminator();
      MDNode *MD = nullptr;

      // Check if this terminator branches to the loop header.
      for (unsigned i = 0, ie = TI->getNumSuccessors(); i != ie; ++i) {
        if (TI->getSuccessor(i) == H) {
          MD = TI->getMetadata(LoopMDName);
          break;
        }
      }
      if (!MD)
        return nullptr;

      if (!LoopID)
        LoopID = MD;
      else if (MD != LoopID)
        return nullptr;
    }
  }
  if (!LoopID || LoopID->getNumOperands() == 0 ||
      LoopID->getOperand(0) != LoopID)
    return nullptr;
  return LoopID;
}