Unsafe: efficiently comparing two byte arrays

[Andrew Haley]

On 03/13/2014 12:49 PM, Paul Sandoz wrote:
> On Mar 13, 2014, at 1:19 PM, Andrew Haley <aph at redhat.com> wrote:
>> On 03/13/2014 11:57 AM, Paul Sandoz wrote:
>>> Now i am in two minds to whether to add ByteBuffer.compareUnsigned or add Arrays.compareUnsigned.
>>>
>>> An explosion of methods on Arrays for all types (plus to/from versions) would be annoying.
>>
>> Surely it's a no brainer?  All we have to do is add the intrinsics for
>> ByteBuffers, then we get fast ByteBuffers and fast array comparisons too.
>> I don't see the problem.
> 
> For byte[] comparison, I don't think optimal long access is
> sufficient on it's own due to alignment issues. It would be nice if
> we can avoid burdening users with alignment issues to ensure the
> main comparison loop is efficient.

But alignment issues are a feature of the hardware, not of software.
The problem right now is that ByteBuffers use a byte-by-byte
comparison of longs, even when it isn't needed, because there aren't
the intrinsics.

> A quick solution is to leverage Unsafe within a
> ByteBuffer.compareUnsigned method. In fact i believe Unsafe could be
> used (as Aleksey says in [1]) for put/get as well, which i presume
> is likely to be much easier/quicker to implement. Might be a good
> first step until a more superior intrinsics solution is implemented?

I still don't get it, sorry.  What can Unsafe do that ByteBuffer
intrinsics can't do?

Andrew.