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

[Vitaly Davidovich]

Isn't the C2 intrinsic just reading the value starting at the specified
offset directly (when unaligned access is supported) and not doing the
branching?

On Thu, Mar 12, 2015 at 1:15 PM, Peter Levart <peter.levart at gmail.com>
wrote:

>
>
> 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
>
>