On Tue, 25 May 2021 14:13:55 GMT, Rémi Forax <fo...@openjdk.org> wrote:
> > Thanks Evgeny, I'll take a look.
> > @forax, do you mean why there is "0" in:
> > 11: invokedynamic #13, 0
> > ?
>
> Not this one, the one on the stack.
>
> 7: iconst_0 <---- this zero
> 8: istore_3
> 9: aload_2
> 10: iload_3
> 11: invokedynamic #13, 0 // InvokeDynamic
> #0:typeSwitch:(Ljava/lang/Object;I)I
>
> Why the descriptor is (Ljava/lang/Object;I)I instead of (Ljava/lang/Object;)I,
> what is the semantics associated to that integer ?
The reason for this integer (which is not a constant in the case of this
switch) is to restart the matching in case guards fail to "match". Considering
the example here:
class Example {
void example(Object o) {
switch (o) {
case String s && s.length() == 0 ->
System.out.println("1st case");
case String s && s.length() == 1 -> // line 6
System.out.println("2nd case"); // line 7
case String s -> // line 8
System.out.println("3rd case"); // line 9
default -> // line 10
System.out.println("default case"); // line 11
}
}
}
If `o` is `String`, then the first call to indy will be `indy[...](o, 0)`,
returning `0`. Then the guard will be evaluated `s.length() == 0`. If the
length is not zero, the local variable 3 will be reassigned to `1`(bytecode
index 58, 59) and the whole switch is restarted - just this time, the matching
in the indy will start at index `1`, not `0`, i.e. `indy[...](o, 1)`. And so
on. I believe there is a text explaining the meaning of the parameter in the
javadoc of the bootstrap, and in TransPatterns in javac.
>
> > The "0" is an artifact of how invokedynamic is represented in the classfile
> > (invokedynamic, 2 bytes of constant pool reference, byte 0, byte 0) - javap
> > shows the value of the first zero byte. That is probably not needed
> > anymore, but there is nothing special in this patch, I think - all
> > invokedynamic calls look like this, AFAIK.
>
> I know that a little to well, i'm one of the guys behind the design of indy :)
-------------
PR: https://git.openjdk.java.net/jdk/pull/3863
