InvocationCounter クラス

JIT コンパイル処理のためのクラス. JIT コンパイルを開始するための閾値を管理する (See: here, here and here for details).

(JIT コンパイルは実行される回数が多いコードにのみ適用される. InvocationCounter クラスはそのための閾値と実行回数を管理し, 実行回数が閾値に達したかどうかを判定する)

    ((cite: hotspot/src/share/vm/interpreter/invocationCounter.hpp))
    // InvocationCounters are used to trigger actions when a limit (threshold) is reached.
    // For different states, different limits and actions can be defined in the initialization
    // routine of InvocationCounters.
    //
    // Implementation notes: For space reasons, state & counter are both encoded in one word,
    // The state is encoded using some of the least significant bits, the counter is using the
    // more significant bits. The counter is incremented before a method is activated and an
    // action is triggered when when count() > limit().

    class InvocationCounter VALUE_OBJ_CLASS_SPEC {

使われ方(Usage)

インスタンスの格納場所(where its instances are stored)

    ((cite: hotspot/src/share/vm/oops/methodOop.hpp))
      InvocationCounter _invocation_counter;         // Incremented before each activation of the method - used to trigger frequency-based optimizations
      InvocationCounter _backedge_counter;           // Incremented before each backedge taken - used to trigger frequencey-based optimizations

    ((cite: hotspot/src/share/vm/oops/methodDataOop.hpp))
      // How many invocations has this MDO seen?
      // These counters are used to determine the exact age of MDO.
      // We need those because in tiered a method can be concurrently
      // executed at different levels.
      InvocationCounter _invocation_counter;
      // Same for backedges.
      InvocationCounter _backedge_counter;

内部構造(Internal structure)

内部には実行回数をカウントする unsigned int フィールドを保持している.

    ((cite: hotspot/src/share/vm/interpreter/invocationCounter.hpp))
     private:                             // bit no: |31  3|  2  | 1 0 |
      unsigned int _counter;              // format: [count|carry|state]

また, 内部には 3種類の int 変数を保持している. これらに JIT コンパイル処理の閾値が格納されている.

    ((cite: hotspot/src/share/vm/interpreter/invocationCounter.hpp))
      static int InterpreterInvocationLimit;        // CompileThreshold scaled for interpreter use
      static int InterpreterBackwardBranchLimit;    // A separate threshold for on stack replacement
      static int InterpreterProfileLimit;           // Profiling threshold scaled for interpreter use

備考(Notes)

なお, 閾値の値は CompileThreshold オプション, InterpreterProfilePercentage オプション, 及び OnStackReplacePercentage オプションで変更できる.

    ((cite: hotspot/src/share/vm/interpreter/invocationCounter.cpp))
      InterpreterInvocationLimit = CompileThreshold << number_of_noncount_bits;
      InterpreterProfileLimit = ((CompileThreshold * InterpreterProfilePercentage) / 100)<< number_of_noncount_bits;

      // When methodData is collected, the backward branch limit is compared against a
      // methodData counter, rather than an InvocationCounter.  In the former case, we
      // don't need the shift by number_of_noncount_bits, but we do need to adjust
      // the factor by which we scale the threshold.
      if (ProfileInterpreter) {
        InterpreterBackwardBranchLimit = (CompileThreshold * (OnStackReplacePercentage - InterpreterProfilePercentage)) / 100;
      } else {
        InterpreterBackwardBranchLimit = ((CompileThreshold * OnStackReplacePercentage) / 100) << number_of_noncount_bits;
      }

備考(Notes)

なお, OSR に関連しそうな BackEdgeThreshold というオプションもあるが, どこからも使われていない...

    ((cite: hotspot/src/share/vm/runtime/globals.hpp))
      product_pd(intx, BackEdgeThreshold,                                       \
              "Interpreter Back edge threshold at which an OSR compilation is invoked")\

InvocationCounter クラス