compiler/dex/mir_optimization.cc

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "compiler_internals.h"
#include "local_value_numbering.h"
#include "dataflow_iterator-inl.h"

namespace art {

static unsigned int Predecessors(BasicBlock* bb) {
  return bb->predecessors->Size();
}

/* Setup a constant value for opcodes thare have the DF_SETS_CONST attribute */
void MIRGraph::SetConstant(int32_t ssa_reg, int value) {
  is_constant_v_->SetBit(ssa_reg);
  constant_values_[ssa_reg] = value;
}

void MIRGraph::SetConstantWide(int ssa_reg, int64_t value) {
  is_constant_v_->SetBit(ssa_reg);
  constant_values_[ssa_reg] = Low32Bits(value);
  constant_values_[ssa_reg + 1] = High32Bits(value);
}

void MIRGraph::DoConstantPropogation(BasicBlock* bb) {
  MIR* mir;

  for (mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
    int df_attributes = oat_data_flow_attributes_[mir->dalvikInsn.opcode];

    DecodedInstruction *d_insn = &mir->dalvikInsn;

    if (!(df_attributes & DF_HAS_DEFS)) continue;

    /* Handle instructions that set up constants directly */
    if (df_attributes & DF_SETS_CONST) {
      if (df_attributes & DF_DA) {
        int32_t vB = static_cast<int32_t>(d_insn->vB);
        switch (d_insn->opcode) {
          case Instruction::CONST_4:
          case Instruction::CONST_16:
          case Instruction::CONST:
            SetConstant(mir->ssa_rep->defs[0], vB);
            break;
          case Instruction::CONST_HIGH16:
            SetConstant(mir->ssa_rep->defs[0], vB << 16);
            break;
          case Instruction::CONST_WIDE_16:
          case Instruction::CONST_WIDE_32:
            SetConstantWide(mir->ssa_rep->defs[0], static_cast<int64_t>(vB));
            break;
          case Instruction::CONST_WIDE:
            SetConstantWide(mir->ssa_rep->defs[0], d_insn->vB_wide);
            break;
          case Instruction::CONST_WIDE_HIGH16:
            SetConstantWide(mir->ssa_rep->defs[0], static_cast<int64_t>(vB) << 48);
            break;
          default:
            break;
        }
      }
      /* Handle instructions that set up constants directly */
    } else if (df_attributes & DF_IS_MOVE) {
      int i;

      for (i = 0; i < mir->ssa_rep->num_uses; i++) {
        if (!is_constant_v_->IsBitSet(mir->ssa_rep->uses[i])) break;
      }
      /* Move a register holding a constant to another register */
      if (i == mir->ssa_rep->num_uses) {
        SetConstant(mir->ssa_rep->defs[0], constant_values_[mir->ssa_rep->uses[0]]);
        if (df_attributes & DF_A_WIDE) {
          SetConstant(mir->ssa_rep->defs[1], constant_values_[mir->ssa_rep->uses[1]]);
        }
      }
    }
  }
  /* TODO: implement code to handle arithmetic operations */
}

void MIRGraph::PropagateConstants() {
  is_constant_v_ = new (arena_) ArenaBitVector(arena_, GetNumSSARegs(), false);
  constant_values_ = static_cast<int*>(arena_->NewMem(sizeof(int) * GetNumSSARegs(), true,
                                                      ArenaAllocator::kAllocDFInfo));
  AllNodesIterator iter(this, false /* not iterative */);
  for (BasicBlock* bb = iter.Next(); bb != NULL; bb = iter.Next()) {
    DoConstantPropogation(bb);
  }
}

/* Advance to next strictly dominated MIR node in an extended basic block */
static MIR* AdvanceMIR(BasicBlock** p_bb, MIR* mir) {
  BasicBlock* bb = *p_bb;
  if (mir != NULL) {
    mir = mir->next;
    if (mir == NULL) {
      bb = bb->fall_through;
      if ((bb == NULL) || Predecessors(bb) != 1) {
        mir = NULL;
      } else {
      *p_bb = bb;
      mir = bb->first_mir_insn;
      }
    }
  }
  return mir;
}

/*
 * To be used at an invoke mir.  If the logically next mir node represents
 * a move-result, return it.  Else, return NULL.  If a move-result exists,
 * it is required to immediately follow the invoke with no intervening
 * opcodes or incoming arcs.  However, if the result of the invoke is not
 * used, a move-result may not be present.
 */
MIR* MIRGraph::FindMoveResult(BasicBlock* bb, MIR* mir) {
  BasicBlock* tbb = bb;
  mir = AdvanceMIR(&tbb, mir);
  while (mir != NULL) {
    int opcode = mir->dalvikInsn.opcode;
    if ((mir->dalvikInsn.opcode == Instruction::MOVE_RESULT) ||
        (mir->dalvikInsn.opcode == Instruction::MOVE_RESULT_OBJECT) ||
        (mir->dalvikInsn.opcode == Instruction::MOVE_RESULT_WIDE)) {
      break;
    }
    // Keep going if pseudo op, otherwise terminate
    if (opcode < kNumPackedOpcodes) {
      mir = NULL;
    } else {
      mir = AdvanceMIR(&tbb, mir);
    }
  }
  return mir;
}

static BasicBlock* NextDominatedBlock(BasicBlock* bb) {
  if (bb->block_type == kDead) {
    return NULL;
  }
  DCHECK((bb->block_type == kEntryBlock) || (bb->block_type == kDalvikByteCode)
      || (bb->block_type == kExitBlock));
  bb = bb->fall_through;
  if (bb == NULL || (Predecessors(bb) != 1)) {
    return NULL;
  }
  DCHECK((bb->block_type == kDalvikByteCode) || (bb->block_type == kExitBlock));
  return bb;
}

static MIR* FindPhi(BasicBlock* bb, int ssa_name) {
  for (MIR* mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
    if (static_cast<int>(mir->dalvikInsn.opcode) == kMirOpPhi) {
      for (int i = 0; i < mir->ssa_rep->num_uses; i++) {
        if (mir->ssa_rep->uses[i] == ssa_name) {
          return mir;
        }
      }
    }
  }
  return NULL;
}

static SelectInstructionKind SelectKind(MIR* mir) {
  switch (mir->dalvikInsn.opcode) {
    case Instruction::MOVE:
    case Instruction::MOVE_OBJECT:
    case Instruction::MOVE_16:
    case Instruction::MOVE_OBJECT_16:
    case Instruction::MOVE_FROM16:
    case Instruction::MOVE_OBJECT_FROM16:
      return kSelectMove;
    case Instruction::CONST:
    case Instruction::CONST_4:
    case Instruction::CONST_16:
      return kSelectConst;
    case Instruction::GOTO:
    case Instruction::GOTO_16:
    case Instruction::GOTO_32:
      return kSelectGoto;
    default:;
  }
  return kSelectNone;
}

int MIRGraph::GetSSAUseCount(int s_reg) {
  return raw_use_counts_.Get(s_reg);
}


/* Do some MIR-level extended basic block optimizations */
bool MIRGraph::BasicBlockOpt(BasicBlock* bb) {
  if (bb->block_type == kDead) {
    return true;
  }
  int num_temps = 0;
  LocalValueNumbering local_valnum(cu_);
  while (bb != NULL) {
    for (MIR* mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
      // TUNING: use the returned value number for CSE.
      local_valnum.GetValueNumber(mir);
      // Look for interesting opcodes, skip otherwise
      Instruction::Code opcode = mir->dalvikInsn.opcode;
      switch (opcode) {
        case Instruction::CMPL_FLOAT:
        case Instruction::CMPL_DOUBLE:
        case Instruction::CMPG_FLOAT:
        case Instruction::CMPG_DOUBLE:
        case Instruction::CMP_LONG:
          if ((cu_->disable_opt & (1 << kBranchFusing)) != 0) {
            // Bitcode doesn't allow this optimization.
            break;
          }
          if (mir->next != NULL) {
            MIR* mir_next = mir->next;
            Instruction::Code br_opcode = mir_next->dalvikInsn.opcode;
            ConditionCode ccode = kCondNv;
            switch (br_opcode) {
              case Instruction::IF_EQZ:
                ccode = kCondEq;
                break;
              case Instruction::IF_NEZ:
                ccode = kCondNe;
                break;
              case Instruction::IF_LTZ:
                ccode = kCondLt;
                break;
              case Instruction::IF_GEZ:
                ccode = kCondGe;
                break;
              case Instruction::IF_GTZ:
                ccode = kCondGt;
                break;
              case Instruction::IF_LEZ:
                ccode = kCondLe;
                break;
              default:
                break;
            }
            // Make sure result of cmp is used by next insn and nowhere else
            if ((ccode != kCondNv) &&
                (mir->ssa_rep->defs[0] == mir_next->ssa_rep->uses[0]) &&
                (GetSSAUseCount(mir->ssa_rep->defs[0]) == 1)) {
              mir_next->dalvikInsn.arg[0] = ccode;
              switch (opcode) {
                case Instruction::CMPL_FLOAT:
                  mir_next->dalvikInsn.opcode =
                      static_cast<Instruction::Code>(kMirOpFusedCmplFloat);
                  break;
                case Instruction::CMPL_DOUBLE:
                  mir_next->dalvikInsn.opcode =
                      static_cast<Instruction::Code>(kMirOpFusedCmplDouble);
                  break;
                case Instruction::CMPG_FLOAT:
                  mir_next->dalvikInsn.opcode =
                      static_cast<Instruction::Code>(kMirOpFusedCmpgFloat);
                  break;
                case Instruction::CMPG_DOUBLE:
                  mir_next->dalvikInsn.opcode =
                      static_cast<Instruction::Code>(kMirOpFusedCmpgDouble);
                  break;
                case Instruction::CMP_LONG:
                  mir_next->dalvikInsn.opcode =
                      static_cast<Instruction::Code>(kMirOpFusedCmpLong);
                  break;
                default: LOG(ERROR) << "Unexpected opcode: " << opcode;
              }
              mir->dalvikInsn.opcode = static_cast<Instruction::Code>(kMirOpNop);
              mir_next->ssa_rep->num_uses = mir->ssa_rep->num_uses;
              mir_next->ssa_rep->uses = mir->ssa_rep->uses;
              mir_next->ssa_rep->fp_use = mir->ssa_rep->fp_use;
              mir_next->ssa_rep->num_defs = 0;
              mir->ssa_rep->num_uses = 0;
              mir->ssa_rep->num_defs = 0;
            }
          }
          break;
        case Instruction::GOTO:
        case Instruction::GOTO_16:
        case Instruction::GOTO_32:
        case Instruction::IF_EQ:
        case Instruction::IF_NE:
        case Instruction::IF_LT:
        case Instruction::IF_GE:
        case Instruction::IF_GT:
        case Instruction::IF_LE:
        case Instruction::IF_EQZ:
        case Instruction::IF_NEZ:
        case Instruction::IF_LTZ:
        case Instruction::IF_GEZ:
        case Instruction::IF_GTZ:
        case Instruction::IF_LEZ:
          if (bb->taken->dominates_return) {
            mir->optimization_flags |= MIR_IGNORE_SUSPEND_CHECK;
            if (cu_->verbose) {
              LOG(INFO) << "Suppressed suspend check on branch to return at 0x" << std::hex << mir->offset;
            }
          }
          break;
        default:
          break;
      }
      // Is this the select pattern?
      // TODO: flesh out support for Mips and X86.  NOTE: llvm's select op doesn't quite work here.
      // TUNING: expand to support IF_xx compare & branches
      if (!(cu_->compiler_backend == kPortable) && (cu_->instruction_set == kThumb2) &&
          ((mir->dalvikInsn.opcode == Instruction::IF_EQZ) ||
          (mir->dalvikInsn.opcode == Instruction::IF_NEZ))) {
        BasicBlock* ft = bb->fall_through;
        DCHECK(ft != NULL);
        BasicBlock* ft_ft = ft->fall_through;
        BasicBlock* ft_tk = ft->taken;

        BasicBlock* tk = bb->taken;
        DCHECK(tk != NULL);
        BasicBlock* tk_ft = tk->fall_through;
        BasicBlock* tk_tk = tk->taken;

        /*
         * In the select pattern, the taken edge goes to a block that unconditionally
         * transfers to the rejoin block and the fall_though edge goes to a block that
         * unconditionally falls through to the rejoin block.
         */
        if ((tk_ft == NULL) && (ft_tk == NULL) && (tk_tk == ft_ft) &&
            (Predecessors(tk) == 1) && (Predecessors(ft) == 1)) {
          /*
           * Okay - we have the basic diamond shape.  At the very least, we can eliminate the
           * suspend check on the taken-taken branch back to the join point.
           */
          if (SelectKind(tk->last_mir_insn) == kSelectGoto) {
              tk->last_mir_insn->optimization_flags |= (MIR_IGNORE_SUSPEND_CHECK);
          }
          // Are the block bodies something we can handle?
          if ((ft->first_mir_insn == ft->last_mir_insn) &&
              (tk->first_mir_insn != tk->last_mir_insn) &&
              (tk->first_mir_insn->next == tk->last_mir_insn) &&
              ((SelectKind(ft->first_mir_insn) == kSelectMove) ||
              (SelectKind(ft->first_mir_insn) == kSelectConst)) &&
              (SelectKind(ft->first_mir_insn) == SelectKind(tk->first_mir_insn)) &&
              (SelectKind(tk->last_mir_insn) == kSelectGoto)) {
            // Almost there.  Are the instructions targeting the same vreg?
            MIR* if_true = tk->first_mir_insn;
            MIR* if_false = ft->first_mir_insn;
            // It's possible that the target of the select isn't used - skip those (rare) cases.
            MIR* phi = FindPhi(tk_tk, if_true->ssa_rep->defs[0]);
            if ((phi != NULL) && (if_true->dalvikInsn.vA == if_false->dalvikInsn.vA)) {
              /*
               * We'll convert the IF_EQZ/IF_NEZ to a SELECT.  We need to find the
               * Phi node in the merge block and delete it (while using the SSA name
               * of the merge as the target of the SELECT.  Delete both taken and
               * fallthrough blocks, and set fallthrough to merge block.
               * NOTE: not updating other dataflow info (no longer used at this point).
               * If this changes, need to update i_dom, etc. here (and in CombineBlocks).
               */
              if (opcode == Instruction::IF_NEZ) {
                // Normalize.
                MIR* tmp_mir = if_true;
                if_true = if_false;
                if_false = tmp_mir;
              }
              mir->dalvikInsn.opcode = static_cast<Instruction::Code>(kMirOpSelect);
              bool const_form = (SelectKind(if_true) == kSelectConst);
              if ((SelectKind(if_true) == kSelectMove)) {
                if (IsConst(if_true->ssa_rep->uses[0]) &&
                    IsConst(if_false->ssa_rep->uses[0])) {
                    const_form = true;
                    if_true->dalvikInsn.vB = ConstantValue(if_true->ssa_rep->uses[0]);
                    if_false->dalvikInsn.vB = ConstantValue(if_false->ssa_rep->uses[0]);
                }
              }
              if (const_form) {
                // "true" set val in vB
                mir->dalvikInsn.vB = if_true->dalvikInsn.vB;
                // "false" set val in vC
                mir->dalvikInsn.vC = if_false->dalvikInsn.vB;
              } else {
                DCHECK_EQ(SelectKind(if_true), kSelectMove);
                DCHECK_EQ(SelectKind(if_false), kSelectMove);
                int* src_ssa =
                    static_cast<int*>(arena_->NewMem(sizeof(int) * 3, false,
                                                     ArenaAllocator::kAllocDFInfo));
                src_ssa[0] = mir->ssa_rep->uses[0];
                src_ssa[1] = if_true->ssa_rep->uses[0];
                src_ssa[2] = if_false->ssa_rep->uses[0];
                mir->ssa_rep->uses = src_ssa;
                mir->ssa_rep->num_uses = 3;
              }
              mir->ssa_rep->num_defs = 1;
              mir->ssa_rep->defs =
                  static_cast<int*>(arena_->NewMem(sizeof(int) * 1, false,
                                                   ArenaAllocator::kAllocDFInfo));
              mir->ssa_rep->fp_def =
                  static_cast<bool*>(arena_->NewMem(sizeof(bool) * 1, false,
                                                    ArenaAllocator::kAllocDFInfo));
              mir->ssa_rep->fp_def[0] = if_true->ssa_rep->fp_def[0];
              // Match type of uses to def.
              mir->ssa_rep->fp_use =
                  static_cast<bool*>(arena_->NewMem(sizeof(bool) * mir->ssa_rep->num_uses, false,
                                                    ArenaAllocator::kAllocDFInfo));
              for (int i = 0; i < mir->ssa_rep->num_uses; i++) {
                mir->ssa_rep->fp_use[i] = mir->ssa_rep->fp_def[0];
              }
              /*
               * There is usually a Phi node in the join block for our two cases.  If the
               * Phi node only contains our two cases as input, we will use the result
               * SSA name of the Phi node as our select result and delete the Phi.  If
               * the Phi node has more than two operands, we will arbitrarily use the SSA
               * name of the "true" path, delete the SSA name of the "false" path from the
               * Phi node (and fix up the incoming arc list).
               */
              if (phi->ssa_rep->num_uses == 2) {
                mir->ssa_rep->defs[0] = phi->ssa_rep->defs[0];
                phi->dalvikInsn.opcode = static_cast<Instruction::Code>(kMirOpNop);
              } else {
                int dead_def = if_false->ssa_rep->defs[0];
                int live_def = if_true->ssa_rep->defs[0];
                mir->ssa_rep->defs[0] = live_def;
                int* incoming = reinterpret_cast<int*>(phi->dalvikInsn.vB);
                for (int i = 0; i < phi->ssa_rep->num_uses; i++) {
                  if (phi->ssa_rep->uses[i] == live_def) {
                    incoming[i] = bb->id;
                  }
                }
                for (int i = 0; i < phi->ssa_rep->num_uses; i++) {
                  if (phi->ssa_rep->uses[i] == dead_def) {
                    int last_slot = phi->ssa_rep->num_uses - 1;
                    phi->ssa_rep->uses[i] = phi->ssa_rep->uses[last_slot];
                    incoming[i] = incoming[last_slot];
                  }
                }
              }
              phi->ssa_rep->num_uses--;
              bb->taken = NULL;
              tk->block_type = kDead;
              for (MIR* tmir = ft->first_mir_insn; tmir != NULL; tmir = tmir->next) {
                tmir->dalvikInsn.opcode = static_cast<Instruction::Code>(kMirOpNop);
              }
            }
          }
        }
      }
    }
    bb = NextDominatedBlock(bb);
  }

  if (num_temps > cu_->num_compiler_temps) {
    cu_->num_compiler_temps = num_temps;
  }
  return true;
}

void MIRGraph::NullCheckEliminationInit(struct BasicBlock* bb) {
  if (bb->data_flow_info != NULL) {
    bb->data_flow_info->ending_null_check_v =
        new (arena_) ArenaBitVector(arena_, GetNumSSARegs(), false, kBitMapNullCheck);
  }
}

/* Collect stats on number of checks removed */
void MIRGraph::CountChecks(struct BasicBlock* bb) {
  if (bb->data_flow_info != NULL) {
    for (MIR* mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
      if (mir->ssa_rep == NULL) {
        continue;
      }
      int df_attributes = oat_data_flow_attributes_[mir->dalvikInsn.opcode];
      if (df_attributes & DF_HAS_NULL_CHKS) {
        checkstats_->null_checks++;
        if (mir->optimization_flags & MIR_IGNORE_NULL_CHECK) {
          checkstats_->null_checks_eliminated++;
        }
      }
      if (df_attributes & DF_HAS_RANGE_CHKS) {
        checkstats_->range_checks++;
        if (mir->optimization_flags & MIR_IGNORE_RANGE_CHECK) {
          checkstats_->range_checks_eliminated++;
        }
      }
    }
  }
}

/* Try to make common case the fallthrough path */
static bool LayoutBlocks(struct BasicBlock* bb) {
  // TODO: For now, just looking for direct throws.  Consider generalizing for profile feedback
  if (!bb->explicit_throw) {
    return false;
  }
  BasicBlock* walker = bb;
  while (true) {
    // Check termination conditions
    if ((walker->block_type == kEntryBlock) || (Predecessors(walker) != 1)) {
      break;
    }
    BasicBlock* prev = walker->predecessors->Get(0);
    if (prev->conditional_branch) {
      if (prev->fall_through == walker) {
        // Already done - return
        break;
      }
      DCHECK_EQ(walker, prev->taken);
      // Got one.  Flip it and exit
      Instruction::Code opcode = prev->last_mir_insn->dalvikInsn.opcode;
      switch (opcode) {
        case Instruction::IF_EQ: opcode = Instruction::IF_NE; break;
        case Instruction::IF_NE: opcode = Instruction::IF_EQ; break;
        case Instruction::IF_LT: opcode = Instruction::IF_GE; break;
        case Instruction::IF_GE: opcode = Instruction::IF_LT; break;
        case Instruction::IF_GT: opcode = Instruction::IF_LE; break;
        case Instruction::IF_LE: opcode = Instruction::IF_GT; break;
        case Instruction::IF_EQZ: opcode = Instruction::IF_NEZ; break;
        case Instruction::IF_NEZ: opcode = Instruction::IF_EQZ; break;
        case Instruction::IF_LTZ: opcode = Instruction::IF_GEZ; break;
        case Instruction::IF_GEZ: opcode = Instruction::IF_LTZ; break;
        case Instruction::IF_GTZ: opcode = Instruction::IF_LEZ; break;
        case Instruction::IF_LEZ: opcode = Instruction::IF_GTZ; break;
        default: LOG(FATAL) << "Unexpected opcode " << opcode;
      }
      prev->last_mir_insn->dalvikInsn.opcode = opcode;
      BasicBlock* t_bb = prev->taken;
      prev->taken = prev->fall_through;
      prev->fall_through = t_bb;
      break;
    }
    walker = prev;
  }
  return false;
}

/* Combine any basic blocks terminated by instructions that we now know can't throw */
bool MIRGraph::CombineBlocks(struct BasicBlock* bb) {
  // Loop here to allow combining a sequence of blocks
  while (true) {
    // Check termination conditions
    if ((bb->first_mir_insn == NULL)
        || (bb->data_flow_info == NULL)
        || (bb->block_type == kExceptionHandling)
        || (bb->block_type == kExitBlock)
        || (bb->block_type == kDead)
        || ((bb->taken == NULL) || (bb->taken->block_type != kExceptionHandling))
        || (bb->successor_block_list.block_list_type != kNotUsed)
        || (static_cast<int>(bb->last_mir_insn->dalvikInsn.opcode) != kMirOpCheck)) {
      break;
    }

    // Test the kMirOpCheck instruction
    MIR* mir = bb->last_mir_insn;
    // Grab the attributes from the paired opcode
    MIR* throw_insn = mir->meta.throw_insn;
    int df_attributes = oat_data_flow_attributes_[throw_insn->dalvikInsn.opcode];
    bool can_combine = true;
    if (df_attributes & DF_HAS_NULL_CHKS) {
      can_combine &= ((throw_insn->optimization_flags & MIR_IGNORE_NULL_CHECK) != 0);
    }
    if (df_attributes & DF_HAS_RANGE_CHKS) {
      can_combine &= ((throw_insn->optimization_flags & MIR_IGNORE_RANGE_CHECK) != 0);
    }
    if (!can_combine) {
      break;
    }
    // OK - got one.  Combine
    BasicBlock* bb_next = bb->fall_through;
    DCHECK(!bb_next->catch_entry);
    DCHECK_EQ(Predecessors(bb_next), 1U);
    MIR* t_mir = bb->last_mir_insn->prev;
    // Overwrite the kOpCheck insn with the paired opcode
    DCHECK_EQ(bb_next->first_mir_insn, throw_insn);
    *bb->last_mir_insn = *throw_insn;
    bb->last_mir_insn->prev = t_mir;
    // Use the successor info from the next block
    bb->successor_block_list = bb_next->successor_block_list;
    // Use the ending block linkage from the next block
    bb->fall_through = bb_next->fall_through;
    bb->taken->block_type = kDead;  // Kill the unused exception block
    bb->taken = bb_next->taken;
    // Include the rest of the instructions
    bb->last_mir_insn = bb_next->last_mir_insn;
    /*
     * If lower-half of pair of blocks to combine contained a return, move the flag
     * to the newly combined block.
     */
    bb->terminated_by_return = bb_next->terminated_by_return;

    /*
     * NOTE: we aren't updating all dataflow info here.  Should either make sure this pass
     * happens after uses of i_dominated, dom_frontier or update the dataflow info here.
     */

    // Kill bb_next and remap now-dead id to parent
    bb_next->block_type = kDead;
    block_id_map_.Overwrite(bb_next->id, bb->id);

    // Now, loop back and see if we can keep going
  }
  return false;
}

/* Eliminate unnecessary null checks for a basic block. */
bool MIRGraph::EliminateNullChecks(struct BasicBlock* bb) {
  if (bb->data_flow_info == NULL) return false;

  /*
   * Set initial state.  Be conservative with catch
   * blocks and start with no assumptions about null check
   * status (except for "this").
   */
  if ((bb->block_type == kEntryBlock) | bb->catch_entry) {
    temp_ssa_register_v_->ClearAllBits();
    if ((cu_->access_flags & kAccStatic) == 0) {
      // If non-static method, mark "this" as non-null
      int this_reg = cu_->num_dalvik_registers - cu_->num_ins;
      temp_ssa_register_v_->SetBit(this_reg);
    }
  } else if (bb->predecessors->Size() == 1) {
    BasicBlock* pred_bb = bb->predecessors->Get(0);
    temp_ssa_register_v_->Copy(pred_bb->data_flow_info->ending_null_check_v);
    if (pred_bb->block_type == kDalvikByteCode) {
      // Check to see if predecessor had an explicit null-check.
      MIR* last_insn = pred_bb->last_mir_insn;
      Instruction::Code last_opcode = last_insn->dalvikInsn.opcode;
      if (last_opcode == Instruction::IF_EQZ) {
        if (pred_bb->fall_through == bb) {
          // The fall-through of a block following a IF_EQZ, set the vA of the IF_EQZ to show that
          // it can't be null.
          temp_ssa_register_v_->SetBit(last_insn->ssa_rep->uses[0]);
        }
      } else if (last_opcode == Instruction::IF_NEZ) {
        if (pred_bb->taken == bb) {
          // The taken block following a IF_NEZ, set the vA of the IF_NEZ to show that it can't be
          // null.
          temp_ssa_register_v_->SetBit(last_insn->ssa_rep->uses[0]);
        }
      }
    }
  } else {
    // Starting state is intersection of all incoming arcs
    GrowableArray<BasicBlock*>::Iterator iter(bb->predecessors);
    BasicBlock* pred_bb = iter.Next();
    DCHECK(pred_bb != NULL);
    temp_ssa_register_v_->Copy(pred_bb->data_flow_info->ending_null_check_v);
    while (true) {
      pred_bb = iter.Next();
      if (!pred_bb) break;
      if ((pred_bb->data_flow_info == NULL) ||
          (pred_bb->data_flow_info->ending_null_check_v == NULL)) {
        continue;
      }
      temp_ssa_register_v_->Intersect(pred_bb->data_flow_info->ending_null_check_v);
    }
  }

  // Walk through the instruction in the block, updating as necessary
  for (MIR* mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
    if (mir->ssa_rep == NULL) {
        continue;
    }
    int df_attributes = oat_data_flow_attributes_[mir->dalvikInsn.opcode];

    // Mark target of NEW* as non-null
    if (df_attributes & DF_NON_NULL_DST) {
      temp_ssa_register_v_->SetBit(mir->ssa_rep->defs[0]);
    }

    // Mark non-null returns from invoke-style NEW*
    if (df_attributes & DF_NON_NULL_RET) {
      MIR* next_mir = mir->next;
      // Next should be an MOVE_RESULT_OBJECT
      if (next_mir &&
          next_mir->dalvikInsn.opcode == Instruction::MOVE_RESULT_OBJECT) {
        // Mark as null checked
        temp_ssa_register_v_->SetBit(next_mir->ssa_rep->defs[0]);
      } else {
        if (next_mir) {
          LOG(WARNING) << "Unexpected opcode following new: " << next_mir->dalvikInsn.opcode;
        } else if (bb->fall_through) {
          // Look in next basic block
          struct BasicBlock* next_bb = bb->fall_through;
          for (MIR* tmir = next_bb->first_mir_insn; tmir != NULL;
            tmir =tmir->next) {
            if (static_cast<int>(tmir->dalvikInsn.opcode) >= static_cast<int>(kMirOpFirst)) {
              continue;
            }
            // First non-pseudo should be MOVE_RESULT_OBJECT
            if (tmir->dalvikInsn.opcode == Instruction::MOVE_RESULT_OBJECT) {
              // Mark as null checked
              temp_ssa_register_v_->SetBit(tmir->ssa_rep->defs[0]);
            } else {
              LOG(WARNING) << "Unexpected op after new: " << tmir->dalvikInsn.opcode;
            }
            break;
          }
        }
      }
    }

    /*
     * Propagate nullcheck state on register copies (including
     * Phi pseudo copies.  For the latter, nullcheck state is
     * the "and" of all the Phi's operands.
     */
    if (df_attributes & (DF_NULL_TRANSFER_0 | DF_NULL_TRANSFER_N)) {
      int tgt_sreg = mir->ssa_rep->defs[0];
      int operands = (df_attributes & DF_NULL_TRANSFER_0) ? 1 :
          mir->ssa_rep->num_uses;
      bool null_checked = true;
      for (int i = 0; i < operands; i++) {
        null_checked &= temp_ssa_register_v_->IsBitSet(mir->ssa_rep->uses[i]);
      }
      if (null_checked) {
        temp_ssa_register_v_->SetBit(tgt_sreg);
      }
    }

    // Already nullchecked?
    if ((df_attributes & DF_HAS_NULL_CHKS) && !(mir->optimization_flags & MIR_IGNORE_NULL_CHECK)) {
      int src_idx;
      if (df_attributes & DF_NULL_CHK_1) {
        src_idx = 1;
      } else if (df_attributes & DF_NULL_CHK_2) {
        src_idx = 2;
      } else {
        src_idx = 0;
      }
      int src_sreg = mir->ssa_rep->uses[src_idx];
        if (temp_ssa_register_v_->IsBitSet(src_sreg)) {
          // Eliminate the null check
          mir->optimization_flags |= MIR_IGNORE_NULL_CHECK;
        } else {
          // Mark s_reg as null-checked
          temp_ssa_register_v_->SetBit(src_sreg);
        }
     }
  }

  // Did anything change?
  bool changed = !temp_ssa_register_v_->Equal(bb->data_flow_info->ending_null_check_v);
  if (changed) {
    bb->data_flow_info->ending_null_check_v->Copy(temp_ssa_register_v_);
  }
  return changed;
}

void MIRGraph::NullCheckElimination() {
  if (!(cu_->disable_opt & (1 << kNullCheckElimination))) {
    DCHECK(temp_ssa_register_v_ != NULL);
    AllNodesIterator iter(this, false /* not iterative */);
    for (BasicBlock* bb = iter.Next(); bb != NULL; bb = iter.Next()) {
      NullCheckEliminationInit(bb);
    }
    PreOrderDfsIterator iter2(this, true /* iterative */);
    bool change = false;
    for (BasicBlock* bb = iter2.Next(change); bb != NULL; bb = iter2.Next(change)) {
      change = EliminateNullChecks(bb);
    }
  }
  if (cu_->enable_debug & (1 << kDebugDumpCFG)) {
    DumpCFG("/sdcard/4_post_nce_cfg/", false);
  }
}

void MIRGraph::BasicBlockCombine() {
  PreOrderDfsIterator iter(this, false /* not iterative */);
  for (BasicBlock* bb = iter.Next(); bb != NULL; bb = iter.Next()) {
    CombineBlocks(bb);
  }
  if (cu_->enable_debug & (1 << kDebugDumpCFG)) {
    DumpCFG("/sdcard/5_post_bbcombine_cfg/", false);
  }
}

void MIRGraph::CodeLayout() {
  if (cu_->enable_debug & (1 << kDebugVerifyDataflow)) {
    VerifyDataflow();
  }
  AllNodesIterator iter(this, false /* not iterative */);
  for (BasicBlock* bb = iter.Next(); bb != NULL; bb = iter.Next()) {
    LayoutBlocks(bb);
  }
  if (cu_->enable_debug & (1 << kDebugDumpCFG)) {
    DumpCFG("/sdcard/2_post_layout_cfg/", true);
  }
}

void MIRGraph::DumpCheckStats() {
  Checkstats* stats =
      static_cast<Checkstats*>(arena_->NewMem(sizeof(Checkstats), true,
                                              ArenaAllocator::kAllocDFInfo));
  checkstats_ = stats;
  AllNodesIterator iter(this, false /* not iterative */);
  for (BasicBlock* bb = iter.Next(); bb != NULL; bb = iter.Next()) {
    CountChecks(bb);
  }
  if (stats->null_checks > 0) {
    float eliminated = static_cast<float>(stats->null_checks_eliminated);
    float checks = static_cast<float>(stats->null_checks);
    LOG(INFO) << "Null Checks: " << PrettyMethod(cu_->method_idx, *cu_->dex_file) << " "
              << stats->null_checks_eliminated << " of " << stats->null_checks << " -> "
              << (eliminated/checks) * 100.0 << "%";
    }
  if (stats->range_checks > 0) {
    float eliminated = static_cast<float>(stats->range_checks_eliminated);
    float checks = static_cast<float>(stats->range_checks);
    LOG(INFO) << "Range Checks: " << PrettyMethod(cu_->method_idx, *cu_->dex_file) << " "
              << stats->range_checks_eliminated << " of " << stats->range_checks << " -> "
              << (eliminated/checks) * 100.0 << "%";
  }
}

bool MIRGraph::BuildExtendedBBList(struct BasicBlock* bb) {
  if (bb->visited) return false;
  if (!((bb->block_type == kEntryBlock) || (bb->block_type == kDalvikByteCode)
      || (bb->block_type == kExitBlock))) {
    // Ignore special blocks
    bb->visited = true;
    return false;
  }
  // Must be head of extended basic block.
  BasicBlock* start_bb = bb;
  extended_basic_blocks_.push_back(bb);
  bool terminated_by_return = false;
  // Visit blocks strictly dominated by this head.
  while (bb != NULL) {
    bb->visited = true;
    terminated_by_return |= bb->terminated_by_return;
    bb = NextDominatedBlock(bb);
  }
  if (terminated_by_return) {
    // This extended basic block contains a return, so mark all members.
    bb = start_bb;
    while (bb != NULL) {
      bb->dominates_return = true;
      bb = NextDominatedBlock(bb);
    }
  }
  return false; // Not iterative - return value will be ignored
}


void MIRGraph::BasicBlockOptimization() {
  if (!(cu_->disable_opt & (1 << kBBOpt))) {
    DCHECK_EQ(cu_->num_compiler_temps, 0);
    ClearAllVisitedFlags();
    PreOrderDfsIterator iter2(this, false /* not iterative */);
    for (BasicBlock* bb = iter2.Next(); bb != NULL; bb = iter2.Next()) {
      BuildExtendedBBList(bb);
    }
    // Perform extended basic block optimizations.
    for (unsigned int i = 0; i < extended_basic_blocks_.size(); i++) {
      BasicBlockOpt(extended_basic_blocks_[i]);
    }
  }
  if (cu_->enable_debug & (1 << kDebugDumpCFG)) {
    DumpCFG("/sdcard/6_post_bbo_cfg/", false);
  }
}

}  // namespace art