[foreign-memaccess+abi] RFR: 8313238: Improve the performance of strlen for null-terminated strings [v3]

[ExE Boss]

On Mon, 7 Aug 2023 10:48:24 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to use native calls for determining the string length for C-type strings. 
>> 
>> Smaller segments (<1024 bytes) is using a trivial call whereas all other segments are using a normal call. We might consider always using regular calls if we think most segments are unbound anyhow.
>> 
>> The PR also contains a number of new tests, one of which requires a large heap size to run. It is likely that this latter test will not run in most test environments. In such cases, it is silently ignored.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Break out native invocations in separate methods

Changes requested by ExE-Boss at github.com (no known OpenJDK username).

src/java.base/share/classes/jdk/internal/foreign/StringSupport.java line 192:

> 190:         // iterate until overflow (String can only hold a byte[], whose length can be expressed as an int)
> 191:         for (int offset = 0; offset >= 0; offset += 2) {
> 192:             short curr = segment.get(JAVA_SHORT, offset);

The old version intentionally used `JAVA_BYTE` here, as `strlen` and `strnlen` is for byte‑encoded strings (e.g.: **UTF‑8**):
Suggestion:

        for (int offset = 0; offset >= 0; offset++) {
            short curr = segment.get(JAVA_BYTE, offset);

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PR Review: https://git.openjdk.org/panama-foreign/pull/857#pullrequestreview-1570598897
PR Review Comment: https://git.openjdk.org/panama-foreign/pull/857#discussion_r1289226078