Unsafe: efficiently comparing two byte arrays

[Martin Buchholz]

Are word-size reads in ByteBuffer actually intrinsified?  I could find no
evidence for that.  Perhaps this is an important missing optimization for
hotspot to make?

I see DirectByteBuffer, but not garden-variety ByteBuffer, using Unsafe.
share/classes/java/nio/Direct-X-Buffer.java.template

Probably should be investigated using disassembly of jitted code, which I
have never done.

---

I've been on both sides of the Unsafe-using fence.  On the one hand I
helped implement LexicographicalComparator, and on the other dissuade
others from adding even more usages of Unsafe.


On Thu, Feb 27, 2014 at 9:23 AM, Paul Sandoz <paul.sandoz at oracle.com> wrote:

>
> On Feb 27, 2014, at 4:12 PM, Andrew Haley <aph at redhat.com> wrote:
>
> > On 02/27/2014 03:05 PM, Paul Sandoz wrote:
> >>
> >> On Feb 27, 2014, at 3:52 PM, Andrew Haley <aph at redhat.com> wrote:
> >>
> >>> Hi,
> >>>
> >>> On 02/26/2014 03:42 PM, Paul Sandoz wrote:
> >>>
> >>>> A common reason why Unsafe is used is to more efficiently compare two
> unsigned byte arrays, viewing those byte arrays as long arrays.
> >>>
> >>> Why not simply provide ByteBuffer.forArray(byte[]) ?  Then we'd have
> all the ByteBuffer intrinsics for put() and get() operations.
> >>
> >> Good point, even if it does require the creation of two ByteBuffer
> instances.
> >>
> >> An implementation of Arrays.compare could do that if things were
> suitably intrinsified, which i am not sure is the case as of today.
> >>
> >> I will do some measurements and report back.
> >
> > Unless something has been done in HotSpot pretty recently, the
> > ByteBuffer intrinsics are not as performat as they might be, so the
> > results might be a little underwhelming.
> >
>
> Yes.
>
> For this benchmark:
>
>   http://cr.openjdk.java.net/~psandoz/unsafe/ByteArrayCompareTest.java
>
> Here are results for comparing two byte arrays of size N containing the
> same content, on my mac book:
>
>
> http://cr.openjdk.java.net/~psandoz/unsafe/byte-array-compare-results.txt
>
> The unsafe test is about ~3/4 x faster than the safe test. And the buffer
> test is slower than the safe test.
>
> I also included a variant of the unsafe test that uses Long.reverseBytes;
> the results are similar to the unsafe test. I believe reverseBytes is
> intrinsified as is numberOfTrailingZeros (i tried to disable it with
> -XX:DisableIntrinsic=_reverseBytes_l but it did not make any great
> different to the results, which makes me think i configured the option
> incorrectly).
>
> Paul.
>