path: root/include/llvm/Transforms/Utils/SSAUpdaterImpl.h

//===-- SSAUpdaterImpl.h - SSA Updater Implementation -----------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file provides a template that implements the core algorithm for the
// SSAUpdater and MachineSSAUpdater.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
#define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ValueHandle.h"

namespace llvm {

class CastInst;
class PHINode;
template<typename T> class SSAUpdaterTraits;

template<typename UpdaterT>
class SSAUpdaterImpl {
private:
  UpdaterT *Updater;

  typedef SSAUpdaterTraits<UpdaterT> Traits;
  typedef typename Traits::BlkT BlkT;
  typedef typename Traits::ValT ValT;
  typedef typename Traits::PhiT PhiT;

  /// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
  /// The predecessors of each block are cached here since pred_iterator is
  /// slow and we need to iterate over the blocks at least a few times.
  class BBInfo {
  public:
    BlkT *BB;          // Back-pointer to the corresponding block.
    ValT AvailableVal; // Value to use in this block.
    BBInfo *DefBB;     // Block that defines the available value.
    int BlkNum;        // Postorder number.
    BBInfo *IDom;      // Immediate dominator.
    unsigned NumPreds; // Number of predecessor blocks.
    BBInfo **Preds;    // Array[NumPreds] of predecessor blocks.
    PhiT *PHITag;      // Marker for existing PHIs that match.

    BBInfo(BlkT *ThisBB, ValT V)
      : BB(ThisBB), AvailableVal(V), DefBB(V ? this : 0), BlkNum(0), IDom(0),
      NumPreds(0), Preds(0), PHITag(0) { }
  };

  typedef DenseMap<BlkT*, ValT> AvailableValsTy;
  AvailableValsTy *AvailableVals;

  SmallVectorImpl<PhiT*> *InsertedPHIs;

  typedef SmallVectorImpl<BBInfo*> BlockListTy;
  typedef DenseMap<BlkT*, BBInfo*> BBMapTy;
  BBMapTy BBMap;
  BumpPtrAllocator Allocator;

public:
  explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
                          SmallVectorImpl<PhiT*> *Ins) :
    Updater(U), AvailableVals(A), InsertedPHIs(Ins) { }

  /// GetValue - Check to see if AvailableVals has an entry for the specified
  /// BB and if so, return it.  If not, construct SSA form by first
  /// calculating the required placement of PHIs and then inserting new PHIs
  /// where needed.
  ValT GetValue(BlkT *BB) {
    SmallVector<BBInfo*, 100> BlockList;
    BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);

    // Special case: bail out if BB is unreachable.
    if (BlockList.size() == 0) {
      ValT V = Traits::GetUndefVal(BB, Updater);
      (*AvailableVals)[BB] = V;
      return V;
    }

    FindDominators(&BlockList, PseudoEntry);
    FindPHIPlacement(&BlockList);
    FindAvailableVals(&BlockList);

    return BBMap[BB]->DefBB->AvailableVal;
  }

  /// BuildBlockList - Starting from the specified basic block, traverse back
  /// through its predecessors until reaching blocks with known values.
  /// Create BBInfo structures for the blocks and append them to the block
  /// list.
  BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) {
    SmallVector<BBInfo*, 10> RootList;
    SmallVector<BBInfo*, 64> WorkList;

    BBInfo *Info = new (Allocator) BBInfo(BB, 0);
    BBMap[BB] = Info;
    WorkList.push_back(Info);

    // Search backward from BB, creating BBInfos along the way and stopping
    // when reaching blocks that define the value.  Record those defining
    // blocks on the RootList.
    SmallVector<BlkT*, 10> Preds;
    while (!WorkList.empty()) {
      Info = WorkList.pop_back_val();
      Preds.clear();
      Traits::FindPredecessorBlocks(Info->BB, &Preds);
      Info->NumPreds = Preds.size();
      if (Info->NumPreds == 0)
        Info->Preds = 0;
      else
        Info->Preds = static_cast<BBInfo**>
          (Allocator.Allocate(Info->NumPreds * sizeof(BBInfo*),
                              AlignOf<BBInfo*>::Alignment));

      for (unsigned p = 0; p != Info->NumPreds; ++p) {
        BlkT *Pred = Preds[p];
        // Check if BBMap already has a BBInfo for the predecessor block.
        typename BBMapTy::value_type &BBMapBucket =
          BBMap.FindAndConstruct(Pred);
        if (BBMapBucket.second) {
          Info->Preds[p] = BBMapBucket.second;
          continue;
        }

        // Create a new BBInfo for the predecessor.
        ValT PredVal = AvailableVals->lookup(Pred);
        BBInfo *PredInfo = new (Allocator) BBInfo(Pred, PredVal);
        BBMapBucket.second = PredInfo;
        Info->Preds[p] = PredInfo;

        if (PredInfo->AvailableVal) {
          RootList.push_back(PredInfo);
          continue;
        }
        WorkList.push_back(PredInfo);
      }
    }

    // Now that we know what blocks are backwards-reachable from the starting
    // block, do a forward depth-first traversal to assign postorder numbers
    // to those blocks.
    BBInfo *PseudoEntry = new (Allocator) BBInfo(0, 0);
    unsigned BlkNum = 1;

    // Initialize the worklist with the roots from the backward traversal.
    while (!RootList.empty()) {
      Info = RootList.pop_back_val();
      Info->IDom = PseudoEntry;
      Info->BlkNum = -1;
      WorkList.push_back(Info);
    }

    while (!WorkList.empty()) {
      Info = WorkList.back();

      if (Info->BlkNum == -2) {
        // All the successors have been handled; assign the postorder number.
        Info->BlkNum = BlkNum++;
        // If not a root, put it on the BlockList.
        if (!Info->AvailableVal)
          BlockList->push_back(Info);
        WorkList.pop_back();
        continue;
      }

      // Leave this entry on the worklist, but set its BlkNum to mark that its
      // successors have been put on the worklist.  When it returns to the top
      // the list, after handling its successors, it will be assigned a
      // number.
      Info->BlkNum = -2;

      // Add unvisited successors to the work list.
      for (typename Traits::BlkSucc_iterator SI =
             Traits::BlkSucc_begin(Info->BB),
             E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
        BBInfo *SuccInfo = BBMap[*SI];
        if (!SuccInfo || SuccInfo->BlkNum)
          continue;
        SuccInfo->BlkNum = -1;
        WorkList.push_back(SuccInfo);
      }
    }
    PseudoEntry->BlkNum = BlkNum;
    return PseudoEntry;
  }

  /// IntersectDominators - This is the dataflow lattice "meet" operation for
  /// finding dominators.  Given two basic blocks, it walks up the dominator
  /// tree until it finds a common dominator of both.  It uses the postorder
  /// number of the blocks to determine how to do that.
  BBInfo *IntersectDominators(BBInfo *Blk1, BBInfo *Blk2) {
    while (Blk1 != Blk2) {
      while (Blk1->BlkNum < Blk2->BlkNum) {
        Blk1 = Blk1->IDom;
        if (!Blk1)
          return Blk2;
      }
      while (Blk2->BlkNum < Blk1->BlkNum) {
        Blk2 = Blk2->IDom;
        if (!Blk2)
          return Blk1;
      }
    }
    return Blk1;
  }

  /// FindDominators - Calculate the dominator tree for the subset of the CFG
  /// corresponding to the basic blocks on the BlockList.  This uses the
  /// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey
  /// and Kennedy, published in Software--Practice and Experience, 2001,
  /// 4:1-10.  Because the CFG subset does not include any edges leading into
  /// blocks that define the value, the results are not the usual dominator
  /// tree.  The CFG subset has a single pseudo-entry node with edges to a set
  /// of root nodes for blocks that define the value.  The dominators for this
  /// subset CFG are not the standard dominators but they are adequate for
  /// placing PHIs within the subset CFG.
  void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) {
    bool Changed;
    do {
      Changed = false;
      // Iterate over the list in reverse order, i.e., forward on CFG edges.
      for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
             E = BlockList->rend(); I != E; ++I) {
        BBInfo *Info = *I;
        BBInfo *NewIDom = 0