Hi Uwe,
I was planning to add a bug for this feature but it seems that Michael was
faster than me,
https://bugs.openjdk.java.net/browse/JDK-8156915
regards,
Rémi
----- Mail original -----
> De: "Uwe Schindler" <uschind...@apache.org>
> À: "Michael Haupt" <michael.ha...@oracle.com>, "Core-Libs-Dev"
> <core-libs-dev@openjdk.java.net>
> Envoyé: Jeudi 12 Mai 2016 14:34:34
> Objet: RE: MethodHandle for array length
>
> Hi Michael,
>
> > > Am 11.05.2016 um 21:35 schrieb Uwe Schindler <uschind...@apache.org>:
> > > With Java 9 this gets a bit worse: There is no "easy way" with the
> > MethodHanldes class to generate a MethodHandles.countedLoop() on all
> > elements of an array:
> > >
> > > public static MethodHandle countedLoop(MethodHandle iterations,
> > MethodHandle init, MethodHandle body) [new in Java 9]
> >
> > this isn't a remedy when you need the index in each iteration, but you can
> > generate a loop over all elements of an array using iteratedLoop(), as
> > illustrated by this test from LoopCombinatorTest:
> >
> > @Test
> > public static void testIterateSum() throws Throwable {
> > // Integer[] a = new Integer[]{1,2,3,4,5,6}; int sum = 0; for (int e :
> > a) { sum
> > += e; } return sum; => 21
> > MethodHandle loop =
> > MethodHandles.iteratedLoop(Iterate.MH_sumIterator, Iterate.MH_sumInit,
> > Iterate.MH_sumStep);
> > assertEquals(Iterate.MT_sum, loop.type());
> > assertEquals(21, loop.invoke(new Integer[]{1, 2, 3, 4, 5, 6}));
> > }
> >
> > ... where MH_sumIterator is a handle to this method:
> >
> > static Iterator<Integer> sumIterator(Integer[] a) {
> > return Arrays.asList(a).iterator();
> > }
>
> Of course this works, too.
>
> My proposal or question here was more about the API inconsistency in general.
> You can do a lot here and there, you can implement effective or
> non-effective loops with MethodHandles, but still the following holds true:
> java.lang.invoke.MethodHandles is missing an accessor for "array.length",
> which is not understandable. The "countedLoop" here was just an example use
> case (if that one is great or not does not matter), it was just posted to
> actually show the API and how it *could* be used.
>
> The real example from our script engine was not related to loops, it was an
> actual invokedynamic callsite implementation where it was not possible to
> figure out how to get a method handle to the very special arraylength byte
> code, aka "array.length javac sugar". Everything else to handle arrays is
> there at one single place, but the array length is missing and you get very
> annoyed. If you are not so familiar to bytecode and the mechanics behind,
> you have a very hard time to find out how to do this: the intuitive
> solutions does not work: "array.length" -> aha it’s a field, so let's try
> Lookup.findGetter(long[].class, "length", int.class) -> does not work!
>
> What is so hard to accept the proposal to have
> MethodHandles.arrayLengthGetter, implemented in the most effective way (as a
> methodhandle that solely calls the special bytecode "arraylength")? In fact
> thais is like a 10 liner to implement + Javadocs. No need to delay a
> release, there are still changes in JDK 9 going on that are far more risky
> than this.
>
> Best,
> Uwe
>
> > Best,
> >
> > Michael
> >
> > --
> >
> > <http://www.oracle.com/>
> > Dr. Michael Haupt | Principal Member of Technical Staff
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>
