Unsafe.{get,put}-X-Unaligned performance

[Peter Levart]

On 03/12/2015 07:37 PM, Andrew Haley wrote:
> On 03/12/2015 05:15 PM, Peter Levart wrote:
>> ...or are JIT+CPU smart enough and there would be no difference?
> C2 always orders things based on profile counts, so there is no
> difference.  Your suggestion would be better for interpreted code
> and I guess C1 also, so I agree it is worthwhile.
>
> Thanks,
> Andrew.
>

What about the following variant (or similar with ifs in case switch is 
sub-optimal):

     public final long getLongUnaligned(Object o, long offset) {
         switch ((int) offset & 7) {
             case 1:
             case 5: return
                 (toUnsignedLong(getByte(o, offset)) << pickPos(56, 0)) |
                 (toUnsignedLong(getShort(o, offset + 1)) << pickPos(48, 
8)) |
                 (toUnsignedLong(getInt(o, offset + 3)) << pickPos(32, 
24)) |
                 (toUnsignedLong(getByte(o, offset + 7)) << pickPos(56, 
56));
             case 2:
             case 6: return
                 (toUnsignedLong(getShort(o, offset)) << pickPos(48, 0)) |
                 (toUnsignedLong(getInt(o, offset + 2)) << pickPos(32, 
16)) |
                 (toUnsignedLong(getShort(o, offset + 6)) << pickPos(48, 
48));
             case 3:
             case 7: return
                 (toUnsignedLong(getByte(o, offset)) << pickPos(56, 0)) |
                 (toUnsignedLong(getInt(o, offset + 1)) << pickPos(32, 8)) |
                 (toUnsignedLong(getShort(o, offset + 5)) << pickPos(48, 
40)) |
                 (toUnsignedLong(getByte(o, offset + 7)) << pickPos(56, 
56));
             case 4: return
                 (toUnsignedLong(getInt(o, offset)) << pickPos(32, 0)) |
                 (toUnsignedLong(getInt(o, offset + 4)) << pickPos(32, 32));
             case 0:
             default: return
                 getLong(o, offset);
         }
     }


...it may have more branches, but less instructions in average per call.



Peter