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jvmbytecodejavacjikes

Why does javac checkcast arrays twice?

Examining bytecode, I've noticed javac seems to duplicate checkcast instructions when casting to array types.

Cast.java:

    class Cast {
      void test(Object a) {
        Object[] b = (Object[]) a;
      }
    }

javap disassembly of the javac compiled version

    void test(java.lang.Object);
      Code:
       0:   aload_1
       1:   checkcast   #2; //class "[Ljava/lang/Object;"
       4:   checkcast   #2; //class "[Ljava/lang/Object;"
       7:   astore_2
       8:   return

Testing jikes shows the expected single cast

    void test(java.lang.Object);
      Code:
       0:   aload_1
       1:   checkcast   #10; //class "[Ljava/lang/Object;"
       4:   astore_2
       5:   return

checkcast is supposed to raise an exception if the object can't be treated as the requested type and otherwise does nothing, so I don't see why it might help to double the cast. I haven't looked at the JDK sources to see how it's produced, and if that helps explain the why (maybe it's meant as a hint).

Solution

  • It is a known bug of javac. But it is mostly harmless.