Proposal: Large arrays (original) (raw)

james lowden jl0235 at yahoo.com
Tue Mar 24 10:54:55 PDT 2009

Ideally, I would like to see the ability to declare and manipulate arrays with long indicies exactly the same way that we currently do with ints. Unfortunately, this introduces the need to have the standard array's length field become a long, which immediately breaks a whole lot of existing code; thus I went with proposing a totally new array type.

-JL-

--- On Tue, 3/24/09, Reinier Zwitserloot <reinier at zwitserloot.com> wrote:

From: Reinier Zwitserloot <reinier at zwitserloot.com> Subject: Re: Proposal: Large arrays To: jl0235 at yahoo.com Cc: coin-dev at openjdk.java.net Date: Tuesday, March 24, 2009, 11:53 AM This change has serious impact on the JVM, and the language support for java is clearly a patchy hack: The 'best' solution (however unattainable that may be) is to just ensure that arrays can be long-indexed, and forego the double bracket notation. You also forgot to mention you need rather a lot of work in the reflection API, because you're introducing a new magic type here (there are the primitives, arrays, and objects. You're adding a new one: large arrays).

I'm opposed to any solution where a clearly superior one that is similar is obvious, and not clearly impossible to implement (even if it will be implemented in java8 or beyond). If your proposal was somehow compatible with such a future, that'd be okay, I guess, but it isn't: If this double-bracket feature is added, then the extra type will never go away. If you're willing to eat up 20 of the precious opcode space, you can instead propose something that is much simpler for all involved: Any attempt to use arrays with a long instead of an int will work fine, it'll just use a different opcodes. Internally, there are such things as large arrays, but you can index into them using an int, and you can index into a plain array with a long (with the usual result if your long has a non-zero upper int: AIOOBException). For most other things including reflection, there is no discernable difference between large and non-large arrays. You can't grow arrays, so I don't really understand why you'd want a distinction in the first place. That's just implementation detail leaking through. --Reinier Zwitserloot

On Mar 24, 2009, at 17:35, james lowden wrote: > > Proposal: Large arrays > > AUTHOR(S): > > J. Lowden > > VERSION: > > 1.0 Initial version. > > > OVERVIEW > > FEATURE SUMMARY: > > Java arrays are accessed via 32-bit ints, resulting in a maximum theoretical array size of 2147483647 elements. While > > this is enough for most uses, some applications that need to handle large sequential data sets in memory would benefit > > from the ability to work with arrays indexed using 64-bit indices. As "simply" changing the current array syntax to use > > longs as indices rather than ints would have the potential to break a large amount of existing code, I'm not proposing > > doing any such thing. Instead, I'd like to see a separate but parallel array feature for creating and accessing both > > types of arrays. A rather boring example, which simply fills a byte array with hexadecimal "FF" values, is shown below: > > //int-indexed array > byte [] foo = new byte [200000]; > for (int i = 0; i < foo.length; i++)_ _> foo [i] = (byte) 0xff; > > //long-indexed array > byte [[]] foo2 = new byte [[20000000000L]]; > for (long i = 0; i < foo2.length; i++)_ _> foo2 [i] = (byte) 0xff; > > Syntax for declaration, instantation, instanceof and casting would use doubled square brackets to differentiate it from > > current array access. Single brackets can still be used for access/mutation as the compiler should have sufficient > > information to decide whether it can treat a specific reference as an int-indexed or long-indexed ("large") array. The length field would be a long rather than an int. Like standard arrays, large arrays would derive directly from > > java.lang.Object. > > > MAJOR ADVANTAGE: > > Java gains the ability to easily work with large sequential data sets. > > > MAJOR BENEFIT: > > Declaring extremely large arrays simply isn't possible right now; in cases where such a structure is desirable, something > > has to be hacked together. For applications dealing with large data sets, the current upper limit on array size is constricting, and will only grow more so as 64-bit systems continue to become more common and RAM less expensive. > > > MAJOR DISADVANTAGE: > > This can't be implemented well solely via a compiler patch; existing VMs would likely need to be changed to support this > > concept natively; new VM instructions parallel to the existing array instructions would likely be required. Also, a class > > parallel to java.util.Arrays for performing standard operations on large arrays would likely be desirable (although > > presumably fairly simple to implement.) > > > ALTERNATIVES: > > Build your own class for storing long-indexes sequences, possibly as an array-or-arrays. > > > EXAMPLES > > SIMPLE EXAMPLE: > > // calculate the first 3 billion fibonacci numbers and store them in an array > > long [[]] fib = new long [[3000000000L]]; > fib [0] = 0; > fib [1] = 0; > for (long i = 2; i < fib.length; i++)_ _> fib [i] = fib [i - 1] + fib [i - 2]; > > > ADVANCED EXAMPLE: > > // this doesn't really do anything particular useful, but does serve to show how casting and instanceof are handled > byte [] foo = new byte [400000]; > byte [[]] blah = new byte [[40000000000L]]; > Object temp = Math.random () < 0.5 ? foo : blah;_ _> if (foo instanceof byte [[]]) > System.out.println (((byte [[]]) foo).length); > else > System.out.println (((byte []) foo).length); > > > DETAILS > > SPECIFICATION: > > Syntax-wise, the following expressions become legal: > > SomeType [[]] foo; // declares foo as a long-indexed "large" array of SomeType, > // where sometype is either a primitive or reference type > > foo = new SomeType [[somelongvalue]]; // instantiates a large array of length > // somelongvalue, which is either a long > // or some type that can upconvert or unbox > // to a long > > long somelong = foo.length; // large arrays will have a "length" field, which is > // a long indicating the number of elements in the array > > foo instanceof SomeType [[]] // returns true if foo is a large array > // of SomeType, false otherwise > > (SomeType [[]]) foo // casts foo to a large array of SomeType. If foo isn't > // a large array of SomeType or a subclass of SomeType, > // throws a ClassCastException > > > Large arrays are not castable or assignable to or from int-indexed arrays of the same element type. > > int [[]] p = new int [40000]; // compiler error > int [] p = new int [[760000]]; // compiler error > int [] foo = (int []) (new int [[4040404040L]]); // throws ClassCastException > int [[]] foo = (int [[]]) (new int [4040]); // throws ClassCastException > > > Canonical class names for large arrays would be generated in a similar fashion to those for standard arrays, but > > using the opposite square bracket. The following table shows some examples. > > declared type class name > int [[]] ]I > int [[]][[]] ]]I > Object [[]] ]Ljava.lang.Object; > > > The same set of indicator character (i.e., 'I' for int, 'L' for reference types, etc.) as are currently used for arrays > > would be used for large arrays. > > A set of additional VM instructions parallel to the current array instructions would be added to support this feature. > > The following table shows the current instruction, the proposed instruction for large arrays, and any semantic > > differences (reference http://java.sun.com/docs/books/jvms/secondedition/html/Instructions2.doc.html): > > array instruction proposed instruction differences > aaload lxaaload index value becomes a long > aastore lxaastore index value becomes a long > anewarray lxanewarray count value becomes a long > arraylength lxarraylength return value becomes a long > baload lxbaload index value becomes a long > bastore lxbastore index value becomes a long > caload lxcaload index value becomes a long > castore lxcastore index value becomes a long > daload lxdaload index value becomes a long > dastore lxdastore index value becomes a long > faload lxfaload index value becomes a long > fastore lxfastore index value becomes a long > iaload lxiaload index value becomes a long > iastore lxiastore index value becomes a long > laload lxlaload index value becomes a long > lastore lxlastore index value becomes a long > multianewarray lxmultianewarray countN values become longs > newarray lxnewarray count value becomes a long > saload lxsaload index value becomes a long > sastore lxsastore index value becomes a long > > > COMPILATION: > > Compilation would be parallel to the present mechanism for compiling arrays, but would output the lx* VM instructions listed above for large arrays instead of the current array instructions. > > TESTING: > > Any test sufficient to test support for current arrays should work for this as well. > > LIBRARY SUPPORT: > > It would probably be desirable to create large array versions of the various java.util.Arrays methods, whether that be done by adding methods to the existing java.util.Arrays class or putting them somewhere new. At some point in the future, large java.util.List might be a desirable library feature, but that is beyond the scope of this proposal. > > REFLECTIVE APIS: > > An additional class (LargeArray or something of that sort) would need to be added to the java.lang.reflect package. This would have a similar set of methods and constructors to java.lang.reflect.Array, but with long "index" and "length" parameters where appropriate. > > OTHER CHANGES: > > VM needs to support the above-listed large array instructions. > > > MIGRATION: > > Existing "hacks" to provide this kind of behavior could be replaced with large arrays. > > > COMPATIBILITY > > BREAKING CHANGES: > > None. This feature simple doesn't exist; the proposed declaration/instantation syntax currently results in a compiler error. > > > EXISTING PROGRAMS: > > No changes. > > > REFERENCES > > EXISTING BUGS: > > 4880587 (64-Bit indexing for arrays) > > > URL FOR PROTOTYPE (optional): > > None. > > > >

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