scala多重分配效率

Question

多个赋值(例如val (x, y) = (1, 2))在运行时的效率低于相应的单个赋值(val x = 1; val y = 2)吗？

我可以想象答案是肯定的,因为scala可能需要构建中间元组.它是否正确？

如果我有一个额外的元组,例如val tup = (1, 2) 现在,它会更有效率:

(一个) val (x, y) = tup

要么

(b)中 val x = tup._1; val y = tup._2

还是他们一样？

与前一个示例的不同之处在于不再需要分配RHS.

Answer 1

您可以使用:javapscala 2.9 REPL 的新功能:

scala> class A { val (a, b) = (1, 2) }
scala> :javap -c A
Compiled from "<console>"
public class A extends java.lang.Object implements scala.ScalaObject{
...
public A();
  Code:
   0:   aload_0
   1:   invokespecial   #22; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   new #24; //class scala/Tuple2$mcII$sp
   8:   dup
   9:   iconst_1
   10:  iconst_2
   11:  invokespecial   #27; //Method scala/Tuple2$mcII$sp."<init>":(II)V
   14:  astore_1
   15:  aload_1
   16:  ifnull  68
   19:  aload_1
   20:  astore_2
   21:  new #24; //class scala/Tuple2$mcII$sp
   24:  dup
   25:  aload_2
   26:  invokevirtual   #33; //Method scala/Tuple2._1:()Ljava/lang/Object;
   29:  invokestatic    #39; //Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
   32:  aload_2
   33:  invokevirtual   #42; //Method scala/Tuple2._2:()Ljava/lang/Object;
   36:  invokestatic    #39; //Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
   39:  invokespecial   #27; //Method scala/Tuple2$mcII$sp."<init>":(II)V
   42:  putfield    #44; //Field x$1:Lscala/Tuple2;
   45:  aload_0
   46:  aload_0
   47:  getfield    #44; //Field x$1:Lscala/Tuple2;
   50:  invokevirtual   #47; //Method scala/Tuple2._1$mcI$sp:()I
   53:  putfield    #14; //Field a:I
   56:  aload_0
   57:  aload_0
   58:  getfield    #44; //Field x$1:Lscala/Tuple2;
   61:  invokevirtual   #50; //Method scala/Tuple2._2$mcI$sp:()I
   64:  putfield    #16; //Field b:I
   67:  return
   68:  new #52; //class scala/MatchError
   71:  dup
   72:  aload_1
   73:  invokespecial   #55; //Method scala/MatchError."<init>":(Ljava/lang/Object;)V
   76:  athrow

}

scala> class B { val a = 1; val b = 2 }
scala> :javap -c B
Compiled from "<console>"
public class B extends java.lang.Object implements scala.ScalaObject{
...
public B();
  Code:
   0:   aload_0
   1:   invokespecial   #20; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   iconst_1
   6:   putfield    #12; //Field a:I
   9:   aload_0
   10:  iconst_2
   11:  putfield    #14; //Field b:I
   14:  return

}

所以我猜答案是元组版本更慢.我想知道为什么有拳击正在进行,不应该与元组的专业化一起消失？!