Unsafe.{get,put}-X-Unaligned; Efficient array comparison intrinsics

[Ulf Zibis]

Hi Andrew,

I like this approach very much. Since long time I had a similar idea on my ToDo list.

Thanks,
-Ulf

Am 23.02.2015 um 19:13 schrieb Andrew Haley:
> I've been kicking around a few ideas for Unsafe access methods for
> unaligned access to byte arrays and buffers in order to provide
> "whatever second-best mechanism the platform offers".  These would
> provide the base for fast lexicographic array comparisons, etc.
>
> https://bugs.openjdk.java.net/browse/JDK-8044082
>
> If the platform supports unaligned memory accesses, the implementation
> of {get,put}-X-Unaligned is obvious and trivial for both C1 and C2.
> It gets interesting when we want to provide efficient unaligned
> methods on machines with no hardware support.
>
> We could provide compiler intrinsics which do when we need on such
> machines.  However, I think this wouldn't deliver the best results.
> >From the experiments I've done, the best implementation is to write
> the access methods in Java and allow HotSpot to optimize them.  While
> this seemed a bit counter-intuitive to me, it's best because C2 has
> profile data that it can work on.  In many cases I suspect that data
> read and written from a byte array will be aligned for their type and
> C2 will take advantage of this, relegating the misaligned access to an
> out-of-line code path as appropriate.  Also, these methods have the
> additional benefit that they are always atomic as long as the data are
> naturally aligned.
>
> This does result in rather a lot of code for the methods for all sizes
> and endiannesses, but none of it will be used on machines with
> unaligned hardware support except in the interpreter.  (Perhaps the
> interpreter too could have intrinsics?)
>
> I have changed HeapByteBuffer to use these methods, with a major
> performance improvement.  I've also provided Unsafe methods to query
> endianness and alignment support.
>
> Webrevs at http://cr.openjdk.java.net/~aph/unaligned.hotspot.1/
> http://cr.openjdk.java.net/~aph/unaligned.jdk.1/
>
> Andrew.
>