Lexicographic array comparators

[Paul Sandoz]

On Feb 13, 2015, at 2:16 PM, Andrew Haley <aph at redhat.com> wrote:

> On 02/10/2015 07:48 PM, Martin Buchholz wrote:
> 
>> People will continue to want to access byte arrays (and direct byte
>> buffers) with C-like performance, and are currently using Unsafe to
>> do so.  Hard to fix for real.  Endianness and unaligned access are
>> both non-portable aspects.  People don't want to pay for bounds
>> checking and especially not for alignment checking of indexes known
>> to be aligned.  Lexicographic comparison is only one use case (that
>> happened to be important performance wise for guava).
> 
> I've been looking at the code HotSpot generates for
> DirectByteBuffer.getXXX() and it's actually pretty good.  In many
> cases C2 optimizes away the endianness, alignment, and bounds checks,
> and nicely inlines and unrolls everything.  Sure, HotSpot can't always
> remove bounds checks, but that's a fairly small price to pay, IMO.
> 
> getXXX() methods in HeapByteBuffers, however, aren't intrinsified, so
> access to data items larger than bytes is byte-at-a-time and pretty
> horrible.  It wouldn't be a huge job to fix this by adding a few
> Unsafe.getUnalignedX intrinsics, and then I think we'd have C-like
> performance.  Whether it makes sense to do this rather than add a new
> API with a bunch of array methods to do the accesses directly is hard
> to say.  Doing it in HeapByteBuffer has the advantage of reqiring no
> API changes.  We could get it done in the JDK9 timeframe.
> 

John suggested that a general array mis-match method could be achieved with carefully written Java code [1], and slightly hidden within that is the comment "FIXME: Add Unsafe.getLongMisaligned to avoid this cutout".

I am inclined to keep pushing on this array mis-match functionality and it's usages for existing and new stuff in Arrays as well as trying to leverage component pieces in other areas like you suggest.

Paul.

[1] https://bugs.openjdk.java.net/browse/JDK-8044082?focusedCommentId=13608657&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13608657