[PATCH] 8217561 : X86: Add floating-point Math.min/max intrinsics, approval request

[Andrew Haley]

On 2/18/19 4:09 PM, B. Blaser wrote:
> Yes and no, simply try your example with unfavourable data:
> 
> @BenchmarkMode(Mode.AverageTime)
> @OutputTimeUnit(TimeUnit.NANOSECONDS)
> @State(Scope.Thread)
> public class FpMinMaxIntrinsics {
>     private static final int COUNT = 1000;
> 
>     private float[] floats = new float[COUNT];
> 
>     private Random r = new Random();
> 
>     @Setup
>     public void init() {
>         for (int i=0; i<COUNT; i++) {
>             if (i % 2 == 0)
>                 floats[i] = r.nextFloat();
>             else
>                 floats[i] = -0.0f;
>         }
>     }
> 
>     @Benchmark
>     public float fMinReduce() {
>         float result = Float.MAX_VALUE;
> 
>         for (int i=0; i<COUNT; i++)
>             result = Math.min(result, floats[i]);
> 
>         return result;
>     }
> }
> 
> With the intrinsic:
> 
> Benchmark                      Mode  Cnt     Score   Error  Units
> FpMinMaxIntrinsics.fMinReduce  avgt       2386.708          ns/op
> 
> 
> Without:
> 
> Benchmark                      Mode  Cnt      Score   Error  Units
> FpMinMaxIntrinsics.fMinReduce  avgt       14042.155          ns/op
> 
> 
> The execution time of the intrinsic will always be stable and you'll
> never have such performance drop-down.

This message is hard to interpret. The question is whether we should
be sure that adding an intrinsic will help Java performance. I don't
know what your point is. Of course there will be some cases where an
intrinsic actually helps, even in the case of a reduction. Whoever
said otherwise? The question is whether, on average or in typical,
such an intrinsic will make Java better. We have not answered that
question, but we are still talking about pushing this intrinsic.

Intel themselves are quite clear in their optimization guide that the
MIN/MAX instructions should be used only when the branch isn't
predictable. We have profile data when generating code in C2, so we
know enough to make an intelligent choice.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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