MethodHandle for array length

Wed May 18 08:11:22 UTC 2016

Hi Uwe,

it's a different issue, but still: https://bugs.openjdk.java.net/browse/JDK-8157225

Best,

Michael

> Am 13.05.2016 um 12:14 schrieb Uwe Schindler <uschindler at apache.org>:
> 
> Hi,
> 
> One addition, maybe add to issue:
> 
> If this was added, jdk.dynalink module could use it, too - this was mentioned by Attila already: http://hg.openjdk.java.net/jdk9/dev/nashorn/file/4b118e012ac4/src/jdk.dynalink/share/classes/jdk/dynalink/beans/BeanLinker.java
> 
> Uwe
> 
> -----
> Uwe Schindler
> uschindler at apache.org 
> ASF Member, Apache Lucene PMC / Committer
> Bremen, Germany
> http://lucene.apache.org/
> 
>> -----Original Message-----
>> From: Uwe Schindler [mailto:uschindler at apache.org]
>> Sent: Friday, May 13, 2016 11:55 AM
>> To: 'Remi Forax' <forax at univ-mlv.fr>
>> Cc: 'Michael Haupt' <michael.haupt at oracle.com>; 'Core-Libs-Dev' <core-libs-
>> dev at openjdk.java.net>
>> Subject: RE: MethodHandle for array length
>> 
>> Hi,
>> 
>> OK, thanks for creating an issue!
>> 
>> Uwe
>> 
>> -----
>> Uwe Schindler
>> uschindler at apache.org
>> ASF Member, Apache Lucene PMC / Committer
>> Bremen, Germany
>> http://lucene.apache.org/
>> 
>>> -----Original Message-----
>>> From: Remi Forax [mailto:forax at univ-mlv.fr]
>>> Sent: Friday, May 13, 2016 10:37 AM
>>> To: Uwe Schindler <uschindler at apache.org>
>>> Cc: Michael Haupt <michael.haupt at oracle.com>; Core-Libs-Dev <core-libs-
>>> dev at openjdk.java.net>
>>> Subject: Re: MethodHandle for array length
>>> 
>>> 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" <uschindler at apache.org>
>>>> À: "Michael Haupt" <michael.haupt at oracle.com>, "Core-Libs-Dev"
>> <core-
>>> libs-dev at 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
>>> <uschindler at 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

-- 

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