RFR: 8283892: Compress and expand bits

[Alan Bateman]

On Tue, 5 Apr 2022 22:05:19 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> Add support to compress bits and expand bits of `int` and `long` values, see Hacker's Delight (2nd edition), section 7.4.
> 
> Compressing or expanding bits of an `int` or `long` value can be composed to enable general permutations, and the "sheep and goats" operation (SAG) see Hacker's Delight (2nd edition), section 7.7. SAG can be used to perform a stable binary radix sort.
> 
> The compress and expand functionality maps efficiently to hardware instructions, such as `PEXT` and `PDEP` on x86 hardware. Thus the implementations can be very efficient on supporting hardware. Intrinsification will occur in a separate PR.
> 
> This [paper](https://arxiv.org/pdf/1706.00990.pdf) investigates the beneficial performance impact of the `PDEP` instruction, and by extension the `expand` method, when applied to the implementation of a bit-vector select operation in succinct data structures (for example `select(r)` returns the position of the `r`th 1).
> 
> Testing-wise the approach take is three fold:
> 1. Tests compared against simple implementations that are easy to read and verify against the JDK implementations (which later will also be made intrinsic). To compensate all tests are also run flipping the test methods and the methods under test.
> 2. Tests composed of compress and expand and vice versa.
> 3. Tests with known mask patterns, whose expected values are easily derived from the inputs.

src/java.base/share/classes/java/lang/Integer.java line 1775:

> 1773:      * the specified bit mask.
> 1774:      * <p>
> 1775:      * For each one-bit value of the mask, {@code mb} say, from least

A minor comments is that "For each one-bit value of the mask mb ...." might be a bit better, otherwise I think these methods and their javadoc looks good. If it comes up then these methods could include an example in the javadoc as they aren't hard once you see an example.

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PR: https://git.openjdk.java.net/jdk/pull/8115