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

[Peter Levart]

On 03/12/2015 10:04 PM, Peter Levart wrote:
> ... putLongUnaligned in the style of above getLongUnaligned is more 
> tricky with current code structure. But there may be a middle ground 
> (or a sweet spot):
>
>
>     public final void putLongUnaligned(Object o, long offset, long x) {
>         if (((int) offset & 1) == 1) {
>             putLongParts(o, offset,
>                 (byte) (x >>> 0),
>                 (short) (x >>> 8),
>                 (short) (x >>> 24),
>                 (short) (x >>> 40),
>                 (byte) (x >>> 56));
>         } else if (((int) offset & 2) == 2) {
>             putLongParts(o, offset,
>                 (short)(x >>> 0),
>                 (int)(x >>> 16),
>                 (short)(x >>> 48));
>         } else if (((int) offset & 4) == 4) {
>             putLongParts(o, offset,
>                 (int)(x >> 0),
>                 (int)(x >>> 32));
>         } else {
>             putLong(o, offset, x);
>         }
>     }
>
>
> ...this has the same number of branches, but less instructions. You 
> also need the following two:


At least on Intel (with -XX:-UseUnalignedAccesses) above code 
(Unaligned2) is not any faster then your code (Unaligned) according to a 
JMH random-access test. Neither is the reversal of if/else branches 
(Unaligned1). Unaligned3 is switch-based variant (just get) and is 
slowest. Your variant seems to be the fastest by a hair:

Benchmark                               Mode   Samples Score  Score 
error    Units
j.t.UnalignedTest.getLongUnaligned      avgt 5       16.375        
0.837    ns/op
j.t.UnalignedTest.getLongUnaligned1     avgt 5       18.340        
0.617    ns/op
j.t.UnalignedTest.getLongUnaligned2     avgt 5       16.784        
0.969    ns/op
j.t.UnalignedTest.getLongUnaligned3     avgt 5       19.634        
0.871    ns/op
j.t.UnalignedTest.putLongUnaligned      avgt 5       15.521        
0.589    ns/op
j.t.UnalignedTest.putLongUnaligned1     avgt 5       16.676        
1.042    ns/op
j.t.UnalignedTest.putLongUnaligned2     avgt 5       16.394        
3.028    ns/op


Regards, Peter

Peter

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