RFR: 8203352: Improve java implementation of Integer/Long.numberOfLeadingZeros

[Ivan Gerasimov]

Hi Claes!


On 5/18/18 3:51 AM, Claes Redestad wrote:
> Hi,
>
> while there are C2 intrinsics on most platforms providing access to 
> specialized hardware instructions, e.g., lzcnt on Intel, optimizing 
> the java implementations of Integer/Long.numberOfLeadingZeros can 
> still be worthwhile, especially if it also helps C1 and implicitly 
> startup/warmup. This implementation wins slightly (5-25%) over the 
> baseline in all tested optimization modes (-Xint, 
> -XX:TieredStopAtLevel=1-3), as well as on C2 if the intrinsics are 
> disabled.
>
> Webrev: http://cr.openjdk.java.net/~redestad/8203352/open.00/
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8203352
>
> Correctness is checked by existing tests, mainly 
> test/jdk/java/lang/Integer|Long/BitTwiddle.java
>
I think the Long version needs some adjustment.
The old code correctly handled the case when 32nd bit is the highest bit 
set.
However, the in the new code such a value will be cast to a negative 
int, and all the checks at lines 1774-1777 will fail.

We may want to add such test case to the existing tests.

With kind regards,
Ivan

> /Claes
>
>

-- 
With kind regards,
Ivan Gerasimov