• C1 LoopInvariantCodeMotion


    从GVN开始

    C1在build_hir的时候会构造GlobalValueNumbering:

    GlobalValueNumbering::GlobalValueNumbering(IR* ir)
      : _compilation(ir->compilation())
      , _current_map(NULL)
      , _value_maps(ir->linear_scan_order()->length(), ir->linear_scan_order()->length(), NULL)
      , _has_substitutions(false)
    {
      TRACE_VALUE_NUMBERING(tty->print_cr("****** start of global value numbering"));
    
      ShortLoopOptimizer short_loop_optimizer(this);
    
      BlockList* blocks = ir->linear_scan_order();
      int num_blocks = blocks->length();
    
      BlockBegin* start_block = blocks->at(0);
      assert(start_block == ir->start() && start_block->number_of_preds() == 0 && start_block->dominator() == NULL, "must be start block");
      assert(start_block->next()->as_Base() != NULL && start_block->next()->next() == NULL, "start block must not have instructions");
    
      // method parameters are not linked in instructions list, so process them separateley
      for_each_state_value(start_block->state(), value,
         assert(value->as_Local() != NULL, "only method parameters allowed");
         set_processed(value); // !!!!!!!!value加入invariant集,即函数形参
      );
    
      // initial, empty value map with nesting 0
      set_value_map_of(start_block, new ValueMap());
      // !!!!!!遍历所有bblock
      for (int i = 1; i < num_blocks; i++) {
        BlockBegin* block = blocks->at(i);
        TRACE_VALUE_NUMBERING(tty->print_cr("**** processing block B%d", block->block_id()));
    
        int num_preds = block->number_of_preds();
        assert(num_preds > 0, "block must have predecessors");
    
        BlockBegin* dominator = block->dominator();
        assert(dominator != NULL, "dominator must exist");
        assert(value_map_of(dominator) != NULL, "value map of dominator must exist");
    
        // create new value map with increased nesting
        _current_map = new ValueMap(value_map_of(dominator));
    
        if (num_preds == 1 && !block->is_set(BlockBegin::exception_entry_flag)) {
          assert(dominator == block->pred_at(0), "dominator must be equal to predecessor");
          // nothing to do here
    
        } else if (block->is_set(BlockBegin::linear_scan_loop_header_flag)) {
          // block has incoming backward branches -> try to optimize short loops
          // !!!!!!!如果是bblock循环头,那么执行ShortLoopOptimizer
          if (!short_loop_optimizer.process(block)) {
            // loop is too complicated, so kill all memory loads because there might be
            // stores to them in the loop
            current_map()->kill_memory();
          }
    
        } else {
          // only incoming forward branches that are already processed
          for (int j = 0; j < num_preds; j++) {
            BlockBegin* pred = block->pred_at(j);
            ValueMap* pred_map = value_map_of(pred);
    
            if (pred_map != NULL) {
              // propagate killed values of the predecessor to this block
              current_map()->kill_map(value_map_of(pred));
            } else {
              // kill all memory loads because predecessor not yet processed
              // (this can happen with non-natural loops and OSR-compiles)
              current_map()->kill_memory();
            }
          }
        }
    
        // phi functions are not linked in instructions list, so process them separateley
        for_each_phi_fun(block, phi,
          set_processed(phi);
        );
    
        TRACE_VALUE_NUMBERING(tty->print("value map before processing block: "); current_map()->print());
    
        // visit all instructions of this block
        for (Value instr = block->next(); instr != NULL; instr = instr->next()) {
          // check if instruction kills any values
          instr->visit(this);
          // perform actual value numbering
          substitute(instr);
        }
    
        // remember value map for successors
        set_value_map_of(block, current_map());
      }
    
      if (_has_substitutions) {
        SubstitutionResolver resolver(ir);
      }
    
      TRACE_VALUE_NUMBERING(tty->print("****** end of global value numbering. "); ValueMap::print_statistics());
    }
    

    这里做的事情也很简单,遍历所有bblock,如果发现某个bblock是循环头,那么执行ShortLoopOptimizer

    到ShortLoopOptimizer

    bool ShortLoopOptimizer::process(BlockBegin* loop_header) {
      TRACE_VALUE_NUMBERING(tty->print_cr("** loop header block"));
    
      _too_complicated_loop = false;
      _loop_blocks.clear();
      _loop_blocks.append(loop_header);
    
      for (int i = 0; i < _loop_blocks.length(); i++) {
        BlockBegin* block = _loop_blocks.at(i);
        TRACE_VALUE_NUMBERING(tty->print_cr("processing loop block B%d", block->block_id()));
    
        if (block->is_set(BlockBegin::exception_entry_flag)) {
          // this would be too complicated
          return false;
        }
    
        // add predecessors to worklist
        for (int j = block->number_of_preds() - 1; j >= 0; j--) {
          BlockBegin* pred = block->pred_at(j);
    
          if (pred->is_set(BlockBegin::osr_entry_flag)) {
            return false;
          }
    
          ValueMap* pred_map = value_map_of(pred);
          if (pred_map != NULL) {
            current_map()->kill_map(pred_map);
          } else if (!_loop_blocks.contains(pred)) {
            if (_loop_blocks.length() >= ValueMapMaxLoopSize) {
              return false;
            }
            _loop_blocks.append(pred);
          }
        }
    
        // use the instruction visitor for killing values
        for (Value instr = block->next(); instr != NULL; instr = instr->next()) {
          instr->visit(this);
          if (_too_complicated_loop) {
            return false;
          }
        }
      }
    
      bool optimistic = this->_gvn->compilation()->is_optimistic();
    
      if (UseLoopInvariantCodeMotion && optimistic) {
        LoopInvariantCodeMotion code_motion(this, _gvn, loop_header, &_loop_blocks);
      }
    
      TRACE_VALUE_NUMBERING(tty->print_cr("** loop successfully optimized"));
      return true;
    }
    

    ShortLoopOptimizer实际是个ValueNumberingVisitor,从上面的代码上不难看出,它做的事情是,遍历所有bblock,搜集循环的bblock,然后visit的所有instr检查他们是否不能做Value numbering(比如a kill了b,b就不能用来value numbering替换其他instr)。最后ShortLoopOptimizer执行LoopInvariantCodeMotion

    最后LoopInvariantCodeMotion

    LoopInvariantCodeMotion::LoopInvariantCodeMotion(ShortLoopOptimizer *slo, GlobalValueNumbering* gvn, BlockBegin* loop_header, BlockList* loop_blocks)
      : _gvn(gvn), _short_loop_optimizer(slo), _insertion_point(NULL), _state(NULL), _insert_is_pred(false) {
    
      TRACE_VALUE_NUMBERING(tty->print_cr("using loop invariant code motion loop_header = %d", loop_header->block_id()));
      TRACE_VALUE_NUMBERING(tty->print_cr("** loop invariant code motion for short loop B%d", loop_header->block_id()));
    
      BlockBegin* insertion_block = loop_header->dominator();
      if (insertion_block->number_of_preds() == 0) {
        return;  // only the entry block does not have a predecessor
      }
    
      assert(insertion_block->end()->as_Base() == NULL, "cannot insert into entry block");
      _insertion_point = insertion_block->end()->prev();
      _insert_is_pred = loop_header->is_predecessor(insertion_block);
    
      BlockEnd *block_end = insertion_block->end();
      _state = block_end->state_before();
    
      if (!_state) {
        // If, TableSwitch and LookupSwitch always have state_before when
        // loop invariant code motion happens..
        assert(block_end->as_Goto(), "Block has to be goto");
        _state = block_end->state();
      }
    
      // the loop_blocks are filled by going backward from the loop header, so this processing order is best
      assert(loop_blocks->at(0) == loop_header, "loop header must be first loop block");
      process_block(loop_header);
      for (int i = loop_blocks->length() - 1; i >= 1; i--) {
        process_block(loop_blocks->at(i));
      }
    }
    

    LoopInvariantCodeMotion遍历之前搜集的所有属于循环的bblock,再process_block这些bblock:

    
    void LoopInvariantCodeMotion::process_block(BlockBegin* block) {
      TRACE_VALUE_NUMBERING(tty->print_cr("processing block B%d", block->block_id()));
    
      Instruction* prev = block;
      Instruction* cur = block->next();
      // !!!!!遍历bblock的所有instr
      while (cur != NULL) {
        // determine if cur instruction is loop invariant
        // only selected instruction types are processed here
        bool cur_invariant = false;
        //!!!!!!看当前instr是否是invariant instr
        if (cur->as_Constant() != NULL) {
          cur_invariant = !cur->can_trap();
        } else if (cur->as_ArithmeticOp() != NULL || cur->as_LogicOp() != NULL || cur->as_ShiftOp() != NULL) {
          assert(cur->as_Op2() != NULL, "must be Op2");
          Op2* op2 = (Op2*)cur;
          cur_invariant = !op2->can_trap() && is_invariant(op2->x()) && is_invariant(op2->y());
        } else if (cur->as_LoadField() != NULL) {
          LoadField* lf = (LoadField*)cur;
          // deoptimizes on NullPointerException
          cur_invariant = !lf->needs_patching() && !lf->field()->is_volatile() && !_short_loop_optimizer->has_field_store(lf->field()->type()->basic_type()) && is_invariant(lf->obj()) && _insert_is_pred;
        } else if (cur->as_ArrayLength() != NULL) {
          ArrayLength *length = cur->as_ArrayLength();
          cur_invariant = is_invariant(length->array());
        } else if (cur->as_LoadIndexed() != NULL) {
          LoadIndexed *li = (LoadIndexed *)cur->as_LoadIndexed();
          cur_invariant = !_short_loop_optimizer->has_indexed_store(as_BasicType(cur->type())) && is_invariant(li->array()) && is_invariant(li->index()) && _insert_is_pred;
        }
        // !!!!!!如果是invariant
        if (cur_invariant) {
          // perform value numbering and mark instruction as loop-invariant
          // !!!!!先尝试value numbering
          _gvn->substitute(cur);
    
          if (cur->as_Constant() == NULL) {
            // ensure that code for non-constant instructions is always generated
            cur->pin();
          }
    
          // remove cur instruction from loop block and append it to block before loop
          // !!!!!!再将invariant instr从当前bblock移动到循环的前面去,这就是code motion了
          Instruction* next = cur->next();
          Instruction* in = _insertion_point->next();
          _insertion_point = _insertion_point->set_next(cur);
          cur->set_next(in);
    
          //  Deoptimize on exception
          cur->set_flag(Instruction::DeoptimizeOnException, true);
    
          //  Clear exception handlers
          cur->set_exception_handlers(NULL);
    
          TRACE_VALUE_NUMBERING(tty->print_cr("Instruction %c%d is loop invariant", cur->type()->tchar(), cur->id()));
          TRACE_VALUE_NUMBERING(cur->print_line());
    
          if (cur->state_before() != NULL) {
            cur->set_state_before(_state->copy());
          }
          if (cur->exception_state() != NULL) {
            cur->set_exception_state(_state->copy());
          }
    
          cur = prev->set_next(next);
        } else {
          prev = cur;
          cur = cur->next();
        }
      }
    }
    

    process_block也很直白,它遍历当前bblock的所有instr,如果发现instr是invariant,就把它移动到循环的前面去。

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  • 原文地址:https://www.cnblogs.com/kelthuzadx/p/15718359.html
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