Pattern matcher inefficiency for basic constructor patterns #6941
Description
The code for all constructor patterns suffers from two inefficiencies. First, there is an unnecessary null-check (note that isInstanceOf will succeed only if scrutinee is non-null). Second, unused fields are still copied into local variables, even if the variable is a wildcard.
Consider this program
object Test {
def foo(xs: List[Int]) = xs match {
case x :: _ => x
}
def bar(xs: List[Int]) = xs match {
case xs: ::[Int] => xs.head
}
}The two methods should produce equally efficient code. Yet foo takes 48 bytes where bar only takes 34. I have not measured the performance impact of the additional 14 bytes. But in any case 34 vs 48 means that bar can be inlined whereas foo cannot.
Here's the javap output I am seeing:
Compiled from "patmatcode.scala"
public final class Test$ extends java.lang.Object{
public static final Test$ MODULE$;
public static {};
Code:
0: new #2; //class Test$
3: invokespecial #12; //Method "<init>":()V
6: return
public int foo(scala.collection.immutable.List);
Code:
0: aload_1
1: astore_2
2: aload_2
3: instanceof #16; //class scala/collection/immutable/$colon$colon
6: ifeq 40
9: aload_2
10: checkcast #16; //class scala/collection/immutable/$colon$colon
13: astore_3
14: aload_3
15: ifnull 40
18: aload_3
19: invokevirtual #20; //Method scala/collection/immutable/$colon$colon.hd$1:()Ljava/lang/Object;
22: invokestatic #26; //Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
25: istore 4
27: aload_3
28: invokevirtual #30; //Method scala/collection/immutable/$colon$colon.tl$1:()Lscala/collection/immutable/List;
31: astore 5
33: iload 4
35: istore 6
37: iload 6
39: ireturn
40: new #32; //class scala/MatchError
43: dup
44: aload_2
45: invokespecial #35; //Method scala/MatchError."<init>":(Ljava/lang/Object;)V
48: athrow
public int bar(scala.collection.immutable.List);
Code:
0: aload_1
1: astore_2
2: aload_2
3: instanceof #16; //class scala/collection/immutable/$colon$colon
6: ifeq 26
9: aload_2
10: checkcast #16; //class scala/collection/immutable/$colon$colon
13: astore_3
14: aload_3
15: invokevirtual #49; //Method scala/collection/immutable/$colon$colon.head:()Ljava/lang/Object;
18: invokestatic #26; //Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
21: istore 4
23: iload 4
25: ireturn
26: new #32; //class scala/MatchError
29: dup
30: aload_2
31: invokespecial #35; //Method scala/MatchError."<init>":(Ljava/lang/Object;)V
34: athrow
}