DeadCodeEliminationPass.cpp

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/* src/vm/jit/compiler2/DeadCodeEliminationPass.cpp - DeadCodeEliminationPass

   Copyright (C) 2013
   CACAOVM - Verein zur Foerderung der freien virtuellen Maschine CACAO

   This file is part of CACAO.

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License as
   published by the Free Software Foundation; either version 2, or (at
   your option) any later version.

   This program is distributed in the hope that it will be useful, but
   WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
   02110-1301, USA.

*/

#include "vm/jit/compiler2/DeadCodeEliminationPass.hpp"
#include "vm/jit/compiler2/PassManager.hpp"
#include "vm/jit/compiler2/JITData.hpp"
#include "vm/jit/compiler2/PassUsage.hpp"
#include "vm/jit/compiler2/MethodC2.hpp"
#include "toolbox/logging.hpp"
#include "vm/jit/compiler2/Instruction.hpp"
#include "vm/jit/compiler2/SSAPrinterPass.hpp"
#include "vm/jit/compiler2/BasicBlockSchedulingPass.hpp"
#include "vm/jit/compiler2/ScheduleEarlyPass.hpp"
#include "vm/jit/compiler2/InstructionMetaPass.hpp"

#define DEBUG_NAME "compiler2/DeadCodeEliminationPass"

STAT_DECLARE_GROUP(compiler2_stat)
STAT_REGISTER_SUBGROUP(compiler2_deadcodeeliminationpass_stat,
    "deadcodeeliminationpass","deadcodeeliminationpass",compiler2_stat)
STAT_REGISTER_GROUP_VAR(std::size_t,num_dead_nodes,0,"# dead nodes",
    "number of removed dead nodes",compiler2_deadcodeeliminationpass_stat)
STAT_REGISTER_GROUP_VAR(std::size_t,num_remaining_nodes,0,"# remaining nodes",
    "number of nodes not removed",compiler2_deadcodeeliminationpass_stat)

namespace cacao {
namespace jit {
namespace compiler2 {

// TODO: maybe we should make this a flag in the Instruction class
bool is_control_flow_inst(Instruction *inst) {
    return inst->get_opcode() == Instruction::BeginInstID
            || inst->to_EndInst();
}

bool DeadCodeEliminationPass::run(JITData &JD) {
    Method *M = JD.get_Method();

    // this work list is used by the algorithm to store the instructions which
    // have to be reconsidered. at the beginning it therefore contains all
    // instructions.
    Method::InstructionListTy workList(M->begin(), M->end());

    // will be used to look up whether an instruction is currently contained in the
    // worklist to avoid inserting an instruction which is already in the list.
    InstBoolMapTy inWorkList;
    for (Method::const_iterator i = workList.begin(), e = workList.end();
            i != e; i++) {
        inWorkList[*i] = true;
    }

    // will be used to mark instructions as dead.
    InstBoolMapTy dead;

    // used to track for each instruction the number of its users which are
    // already dead
    InstIntMapTy deadUsers;

    Method::InstructionListTy liveInstructions;
    Method::InstructionListTy deadInstructions;

    while (!workList.empty()) {
        Instruction *I = workList.front();
        workList.pop_front();
        inWorkList[I] = false;

        // here we inspect whether the node that has just been picked from
        // the work list is dead
        if (I->user_size() == deadUsers[I] && !I->has_side_effects()
                && !is_control_flow_inst(I) && I->rdep_size() == 0) {
            dead[I] = true;
            InstBoolMapTy visited;

            // insert the dead instructions in the order they should be deleted
            deadInstructions.push_back(I);

            for (Instruction::OperandListTy::const_iterator
                    i = I->op_begin(), e = I->op_end(); i != e; i++) {
                Instruction *Op = (*i)->to_Instruction();
                
                if (Op && !visited[Op]) {
                    visited[Op] = true;
                    deadUsers[Op]++;

                    if (!inWorkList[Op]) {
                        workList.push_back(Op);
                        inWorkList[Op] = true;
                    }
                }
            }
        }
    }
    
    // collect the living instructions (i.e., the instructions which are not dead)
    for (Method::const_iterator i = M->begin(), e = M->end(); i != e; i++) {
        Instruction *I = *i;
        if (dead[I]) {
            LOG("Dead: " << I << nl);
        } else {
            liveInstructions.push_back(I);
            LOG("Live: " << I << nl);
        }
    }

    STATISTICS(num_dead_nodes = deadInstructions.size());
    STATISTICS(num_remaining_nodes = liveInstructions.size());

    M->replace_instruction_list(liveInstructions.begin(), liveInstructions.end());

    // delete the dead instructions
    while (!deadInstructions.empty()) {
        Instruction *I = deadInstructions.front();
        deadInstructions.pop_front();
        delete I;
    }

    return true;
}

// pass usage
PassUsage& DeadCodeEliminationPass::get_PassUsage(PassUsage &PU) const {
    PU.add_requires<InstructionMetaPass>();
    PU.add_schedule_before<ScheduleEarlyPass>();
    PU.add_schedule_before<SSAPrinterPass>();
    return PU;
}

// the address of this variable is used to identify the pass
char DeadCodeEliminationPass::ID = 0;

// register pass
static PassRegistry<DeadCodeEliminationPass> X("DeadCodeEliminationPass");

} // end namespace compiler2
} // end namespace jit
} // end namespace cacao


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