SPARC <was> Re: RFR 8151163 All Buffer implementations should leverage Unsafe unaligned accessors

[Andrew Haley]

On 03/30/2016 10:36 AM, Paul Sandoz wrote:

> Unsafe.getLongUnaligned needs to perform three alignment checks
> before accessing bytes and then optionally perform a byte
> swap. Bits.getLong always access bytes and composing using the
> defined endianness and requires no additional byte swap.
> 
> Since SPARC is Big Endian and buffers are by default Big Endian i
> can rule any byte swapping, but it potentially could increase the
> regression if Little Endian is chosen [2].
> 
> When access is performed in loops this can cost, as the alignment
> checks are not hoisted out. Theoretically could for regular 2, 4, 8
> strides through the buffer contents. For such cases alignment of the
> base address can be checked. Not sure how complicated that would be
> to support.

Going back to the "Unsafe.{get,put}-X-Unaligned performance" and
"Unsafe.{get,put}-X-Unaligned; Efficient array comparison intrinsics"
discussion a year ago, the rationale behind the way things are was to
enable auto-vectorization.  We certainly can do things with more
complex checks at the Java level, but IMVHO vectorization is the right
way to fix it.

Andrew.