RFR: 8073093: AARCH64: C2 generates poor code for ByteBuffer accesses

[Dean Long]

On 2/13/2015 3:04 PM, christos at zoulas.com wrote:
> On Feb 13,  2:52pm, dean.long at oracle.com (Dean Long) wrote:
> -- Subject: Re: RFR: 8073093: AARCH64: C2 generates poor code for ByteBuffer
>
> | My understanding is that whether or not aarch64 allows unaligned=20
> | accesses is based on a
> | system setting, so this change is too simplistic.  I would prefer that=20
> | this was controlled with
> | something more flexible, like "sun.cpu.unaligned".
>
> So does x86_64 and you can ask the CPU if it is enabled... I am not sure
> if a variable setting makes sense because if alignment is required you
> get a signal (BUS error -- hi linux, SEGV), or incorrect results.
>
> christos

So it sounds like we need to determine if unaligned accesses are 
supported during startup,
in a platform-specific way.  This could be exposed through a property 
like I suggested,
or perhaps a new Unsafe method.

Regarding x86_64, there may be places in the JVM that already assume 
unaligned accesses
are allowed, so disabling them may completely break the JVM until those 
assumptions
are fixed.

dl