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

[Peter Levart]

On 03/10/2015 08:02 PM, Andrew Haley wrote:
> The new algorithm does an N-way branch, always loading and storing
> subwords according to their natural alignment.  So, if the address is
> random and the size is long it will access 8 bytes 50% of the time, 4
> shorts 25% of the time, 2 ints 12.5% of the time, and 1 long 12.5% of
> the time.  So, for every random load/store we have a 4-way branch.


...so do you think it would be better if the order of checks in if/else 
chain:

  972     public final long getLongUnaligned(Object o, long offset) {
  973         if ((offset & 7) == 0) {
  974             return getLong(o, offset);
  975         } else if ((offset & 3) == 0) {
  976             return makeLong(getInt(o, offset),
  977                             getInt(o, offset + 4));
  978         } else if ((offset & 1) == 0) {
  979             return makeLong(getShort(o, offset),
  980                             getShort(o, offset + 2),
  981                             getShort(o, offset + 4),
  982                             getShort(o, offset + 6));
  983         } else {
  984             return makeLong(getByte(o, offset),
  985                             getByte(o, offset + 1),
  986                             getByte(o, offset + 2),
  987                             getByte(o, offset + 3),
  988                             getByte(o, offset + 4),
  989                             getByte(o, offset + 5),
  990                             getByte(o, offset + 6),
  991                             getByte(o, offset + 7));
  992         }
  993     }


...was reversed:

if ((offset & 1) == 1) {
     // bytes
} else if ((offset & 2) == 2) {
     // shorts
} else if ((offset & 4) == 4) {
     // ints
} else {
     // longs
}


...or are JIT+CPU smart enough and there would be no difference?


Peter