RFR: 8345120: A likely bug in StringSupport::chunkedStrlenShort [v8]

[Maurizio Cimadamore]

On Mon, 2 Dec 2024 15:40:18 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to rewrite the `StringSupport::chunkedStrlen*`  methods, fixing a bug in the `short_strlen` variant for odd offsets (`offset % 2 != 0`).
>> 
>> This PR also improves performance on modern hardware, as there is no need for pre-looping alignment. Removing this improves performance by about 30% for larger strings.
>> 
>> It passes the `jdk_foreign` test suit. Also, `tier1` through `tier3` passes on various platforms and configurations.
>> 
>> Base:
>> 
>> 
>> Benchmark                               (size)  Mode  Cnt    Score   Error  Units
>> InternalStrLen.changedElementQuad            1  avgt   30    2.057 ? 0.012  ns/op
>> InternalStrLen.changedElementQuad            4  avgt   30    3.776 ? 0.031  ns/op
>> InternalStrLen.changedElementQuad           16  avgt   30    6.690 ? 0.060  ns/op
>> InternalStrLen.changedElementQuad          251  avgt   30   48.581 ? 0.764  ns/op
>> InternalStrLen.changedElementQuad         1024  avgt   30  196.188 ? 3.484  ns/op
>> InternalStrLen.chunkedDouble                 1  avgt   30    1.903 ? 0.013  ns/op
>> InternalStrLen.chunkedDouble                 4  avgt   30    3.446 ? 0.025  ns/op
>> InternalStrLen.chunkedDouble                16  avgt   30    5.759 ? 0.062  ns/op
>> InternalStrLen.chunkedDouble               251  avgt   30   26.892 ? 0.141  ns/op
>> InternalStrLen.chunkedDouble              1024  avgt   30   72.940 ? 1.562  ns/op
>> InternalStrLen.chunkedSingle                 1  avgt   30    1.897 ? 0.015  ns/op
>> InternalStrLen.chunkedSingle                 4  avgt   30    5.357 ? 0.560  ns/op
>> InternalStrLen.chunkedSingle                16  avgt   30    3.821 ? 0.052  ns/op
>> InternalStrLen.chunkedSingle               251  avgt   30   19.482 ? 0.190  ns/op
>> InternalStrLen.chunkedSingle              1024  avgt   30   38.938 ? 0.411  ns/op
>> InternalStrLen.chunkedSingleMisaligned       1  avgt   30    2.230 ? 0.147  ns/op
>> InternalStrLen.chunkedSingleMisaligned       4  avgt   30    5.424 ? 0.688  ns/op
>> InternalStrLen.chunkedSingleMisaligned      16  avgt   30    9.573 ? 0.063  ns/op
>> InternalStrLen.chunkedSingleMisaligned     251  avgt   30   22.242 ? 0.182  ns/op
>> InternalStrLen.chunkedSingleMisaligned    1024  avgt   30   45.442 ? 0.252  ns/op
>> InternalStrLen.elementByteMisaligned         1  avgt   30    1.616 ? 0.041  ns/op
>> InternalStrLen.elementByteMisaligned         4  avgt   30    2.982 ? 0.018  ns/op
>> InternalStrLen.elementByteMisaligned        16  avgt   30    8.662 ? 0.085  ns/op
>> InternalStrLe...
>
> Per Minborg 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 12 additional commits since the last revision:
> 
>  - Add test and change exception type for empty segments
>  - Merge branch 'master' into simplify-string-support
>  - Improve docs
>  - Add zero detection method for quad words
>  - Rename method
>  - Add test for segments with odd byte at the end
>  - Fix imports
>  - Move back methods to StringSupport
>  - Clean up
>  - Add int method
>  - ... and 2 more: https://git.openjdk.org/jdk/compare/2646bede...ae1acf5f

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/22451#pullrequestreview-2473196320