RFR: 8349637: Integer.numberOfLeadingZeros outputs incorrectly in certain cases [v5]

[Tobias Hartmann]

On Mon, 17 Feb 2025 05:13:07 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> Hi all,
>> This is a fix for a miscompile in the AVX2 implementation of `CountLeadingZerosV` for int types. Currently, the implementation turns ints into floats, in order to calculating the leading zeros based on the exponent part of the float. Unfortunately, floats can only accurately represent integers up to 2^24. After that, multiple integer values can map onto the same floating point value. The issue manifests when an int is converted to a floating point representation that is higher than it, crossing a bit boundary. As an example, `(float)0x01FFFFFF == (float)0x02000000`, but `lzcnt(0x01FFFFFF) == 7` and `lzcnt(0x02000000) == 6`. The values are incorrectly rounded up.
>> 
>> This patch fixes the issue by masking the input in the cases where it is larger than 2^24, to set the low bits to 0. Removing these bits prevents the accidental rounding behavior. I've added these cases to`TestNumberOfContinuousZeros`, and removed the set random seed so that it can produce random inputs to test with.
>> 
>> Reviews would be appreciated!
>
> Jasmine Karthikeyan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into fix-8349637
>  - Comments from review, add exhaustive test
>  - Improve explanation of logic
>  - Comments from code review
>  - Fix CountLeadingZerosV miscompile on AVX2

Fix and tests look good to me. Thanks!

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23579#pullrequestreview-2629717631