Unsafe.{get,put}-X-Unaligned; Efficient array comparison intrinsics

[Andrew Haley]

On 02/24/2015 03:40 PM, Peter Levart wrote:
> On 02/24/2015 02:48 PM, Andrew Haley wrote:
>>>> private static final boolean IS_UNALIGNED = theUnsafe.unalignedAccess();
>>>>
>>>> public void putIntUnaligned(Object o, long offset, int x) { if (IS_UNALIGNED || (offset & 3) == 0) { putInt(o, offset, x); } else if (byteOrder == BIG_ENDIAN) { putIntB(o, offset, x); } else { putIntL(o, offset, x); } }
>> Yes.  It certainly could be done like this but I think C1 doesn't do
>> the optimization to remove the IS_UNALIGNED test, so we'd still want
>> the C1 builtins.  Perhaps we could do without the C2 builtins but they
>> cost very little, they save C2 a fair amount of work, and they remove
>> the vagaries of inlining.  I take your point about the interpreter,
>> though.
>>
> 
> What about if you make unalignedAccess() and getByteOrder() static 
> methods in Unsafe (they are safe aren't they?) and then do the following:
> 
> public abstract class Unsafe {
> ...
> private static final Unsafe theUnsafe =
>    unalignedAccess()
>      ? (getByteOrder() ? new UnsafeUB() : new UnsafeUL())
>      : (getByteOrder() ? new UnsafeAB() : new UnsafeAL());
> ...
>      public abstract int getIntUnaligned(Object o, long offset);
> ...
>    private static final class UnsafeUB extends Unsafe { ... }
>    private static final class UnsafeUL extends Unsafe { ... }
>    private static final class UnsafeAB extends Unsafe { ... }
>    private static final class UnsafeAL extends Unsafe { ... }
> 
> 
> There will be only one runtime Unsafe sub-type ever observed in a 
> particular VM.

Oh, that's very nice.

Thanks,
Andrew.