RFR: 8339874: Avoid duplicate checking of trailing slash in ZipFile.getZipEntry

[Lance Andersen]

On Tue, 10 Sep 2024 18:35:52 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

> Please review this PR which speeds up `ZipFile.getZipEntry` by removing slash-checking logic which is already taking place during lookup in `ZipFile.Source.getEntryPos`. 
> 
> `ZipFile.Source.getEntryPos` includes logic to match a lookup for "name" against a directory entry "name/" (with a trailing slash). However, only the CEN position is currently returned, so `ZipFile.getZipEntry` needs to re-read the name from the CEN and determine if a trailing slash needs to be appended to the name of the returned `ZipEntry`.
> 
> By letting `ZipFile.Source.getEntryPos` return the resolved name along with the CEN position (in a new record `EntryPos`), `ZipFile.getZipEntry` can now instead use the already resolved name. 
> 
> This results in a nice ~18% speedup in the `ZipFileGetEntry.getEntryHit` micro:
> 
> Baseline:
> 
> 
> Benchmark                    (size)  Mode  Cnt   Score   Error  Units
> ZipFileGetEntry.getEntryHit     512  avgt   15  63.713 ? 2.645  ns/op
> ZipFileGetEntry.getEntryHit    1024  avgt   15  67.405 ? 1.474  ns/op
> 
> 
> PR:
> 
> 
> Benchmark                    (size)  Mode  Cnt   Score   Error  Units
> ZipFileGetEntry.getEntryHit     512  avgt   15  52.027 ? 2.669  ns/op
> ZipFileGetEntry.getEntryHit    1024  avgt   15  55.211 ? 1.169  ns/op
> 
> 
> This purely a cleanup and optimization PR, no functional tests are changed or added.

Some initial thoughts but overall it seems to be a beneficial change.

src/java.base/share/classes/java/util/zip/ZipCoder.java line 161:

> 159:     }
> 160: 
> 161:     protected boolean hasTrailingSlash(byte[] a, int end) {

Can you explain the need for this

src/java.base/share/classes/java/util/zip/ZipFile.java line 700:

> 698:         }
> 699:         if (clen != 0) {
> 700:             int nlen = CENNAM(cen, pos);

If this does not make a huge performance difference, I would keep this together with the init of elen and clen on line 692.

While there are not many  entry comments encountered, I do expect extra fields to somewhat common

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PR Review: https://git.openjdk.org/jdk/pull/20939#pullrequestreview-2293604516
PR Review Comment: https://git.openjdk.org/jdk/pull/20939#discussion_r1752629986
PR Review Comment: https://git.openjdk.org/jdk/pull/20939#discussion_r1752620013