[patterns-instanceof-primitive] RFR: 8303374: Compiler Implementation for Primitive types in patterns, instanceof, and switch [v2]

[Raffaello Giulietti]

On Fri, 10 Mar 2023 00:01:28 GMT, Michael Hixson <duke at openjdk.org> wrote:

>> Aggelos Biboudis has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update int_float, long_float, long_double
>>   
>>   Co-authored-by: Angelos Bimpoudis <angelos.bimpoudis at oracle.com>
>>   Co-authored-by: Raffaello Giulietti <raffaello.giulietti at oracle.com>
>
> src/java.base/share/classes/java/lang/runtime/ExactnessMethods.java line 178:
> 
>> 176:                 (64 - (Long.numberOfLeadingZeros(n) +
>> 177:                         Long.numberOfTrailingZeros(n))) ;
>> 178:     }
> 
> Are you interested in optimizing the performance of these methods at this time?
> 
> For example, in this method:
> * You can remove the `Long.MIN_VALUE` check.  That value "accidentally" works out with the rest of the logic; it has no leading zeros and 63 trailing zeros.  This optimization assumes that `Long.MIN_VALUE` isn't common enough to deserve its own fast path (at the expense of every other input), which seems true.
> * If you assume that most inputs to this method are "small" integers that do fit in doubles (which I think is true), you can optimize for them by checking trailing zeros lazily.
>   ```java
>   n = Math.abs(n);
>   int z = Long.numberOfLeadingZeros(n);
>   return z > 10 || z + Long.numberOfTrailingZeros(n) > 10;
>   ```
> 
> I can share benchmarks if that's useful.  (I needed this method years ago, and I wrote benchmarks back then.)
> 
> I'm assuming these methods are invoked whenever an expression like `longValue instanceof double` is evaluated, and that these methods aren't intrinsified (yet), so they seem worth optimizing.  But I could be mistaken.

@michaelhixson The more compact variant given above in the comment by @biboudis, compiles to minimal machine code. There's hardly any need for an intrinsic.

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PR: https://git.openjdk.org/amber/pull/91