On Mon, 10 Jul 2023 17:07:28 GMT, Joe Darcy <da...@openjdk.org> wrote:
>> LGTM; I assume the comment about aarch64 perf was a 10% improvement.
>
>> Here's some explanation for the recent commits I've added since @RogerRiggs
>> reviewed this PR.
>>
>> 1. Since `BigInteger.hashCode()` is unspecified, we can change it.
>> Again: I think that the proposed implementation is no worse in hashing
>> quality than the current one; if you disagree, please voice your concerns.
>>
>>
>> FWIW, we can keep the existing `BigInteger.hashCode()` values whilst still
>> improving the implementation, using JDK-internal support:
>>
>> ```
>> @Override
>> public int hashCode() {
>> return ArraysSupport.vectorizedHashCode(mag, 0, mag.length, 1,
>> ArraysSupport.T_INT) * signum;
>> }
>> ```
>>
>> AFAIU, such an implementation would always yield exactly the same values the
>> current (i.e. mainline) implementation does. But that could be a little
>> slower than the original proposal, especially for a smaller BigInteger.
>>
>> The key thing that allows to keep the current hash-code values in the above
>> implementation is that `1` argument, which is the initial hash-code value,
>> which cannot be specified in `Arrays.hashCode`. Unfortunately, we don't have
>> mid-layer methods in between Arrays.hashCode and
>> ArraysSupport.vectorizedHashCode like that of Arrays.mismatch and
>> ArraysSupport.vectorizedMismatch. It's either all the null check but
>> short-circuits or unconditional vectorization but the initial value. I
>> wonder if we could consider `ArraysSupport.hashCode($type[] array, int
>> fromIndex, int length, int initialValue)` overloads, which could be useful
>> beyond BigInteger, as I've already seen in JDK. Contributors to
>> ArraysSupport, @PaulSandoz, @ChrisHegarty, @cl4es, @stuart-marks; thoughts?
>>
>> 2. Maybe surprisingly, but we don't have microbenchmarks for
>> BigInteger's equals, hashCode, and compareTo. While I don't know how often
>> people use the former two methods, I reckon, the latter method is essential.
>> Anyway, I added benchmarks to cover all three. Note: benchmark for hashCode
>> shows only its performance, not its hashing quality. Again: if you think the
>> current version is in any way worse than the mainline version, please voice
>> your concerns.
>>
>>
>> AFAIK, the biggest consumer of BigInteger in JDK is security area. So, I
>> assume a good way to judge this change is to run security benchmarks to make
>> sure they haven't slipped.
>
> I would be in favor of keeping the current hash behavior for now, even if it
> is updated in subsequent work. Users have had more than two decades to
> become, accidentally, reliant on the hash algorithms.
@jddarcy would you be okay with the change and the test in
[d087a59](https://github.com/openjdk/jdk/pull/14630/commits/d087a592c8996e90a5754f023c2eac3a194d007f)?
-------------
PR Comment: https://git.openjdk.org/jdk/pull/14630#issuecomment-1644134205
