On 03/13/2014 05:02 PM, Paul Sandoz wrote: > On Mar 13, 2014, at 5:26 PM, Andrew Haley <a...@redhat.com> wrote: >> >>>>> 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? >>> >>> We can arguably implement it faster [2] >> >> I doubt that very much. In fact, I would say that it is almost >> certainly not true. HotSpot usually knows, for example, when both >> offsets are zero and can generate better code for machines with strict >> alignment. >> >> And also, Unsafe has the same speed problems with unaligned data >> that an intrinsic would have. > > I meant i can *implement* a comparison solution using Unsafe faster > than i can implement a more general solution using intrinsics.
I see. Yes, what you said is clear, now I read it again. :-) > I presume intrinsics might be of great advantage if SIMD > instructions can be leveraged? if not would it be reasonable to > assume that for common 0 offset case the two solutions might be > similar in terms of performance? That's hard to say for sure. It might well be so, but I suspect you'll hit memor bandwidth limits in both cases. > Yeah, but i think that could take longer and there is also the > Arrays 2.0 work to consider, the scope is considerably larger than > providing an efficient comparison method for byte[] arrays. OK, so I think I understand now: you can do what you need without having to drag in HotSpot engineers. But Unsafe only works really efficiently when it is intrinsified. > It is possible to tackle this simple use-case with an API tweak > (ByteBuffer.compareUnsigned) and revisit the implementation if/when > instincts for arrays get into 9. That is a small but useful step in > the right direction. Ok, fair point. Andrew.
