[jdk8u-dev] RFR: 8310026: [8u] make java_lang_String::hash_code consistent across platforms

[Andrew Dinn]

On Mon, 10 Jul 2023 09:06:06 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> `java_lang_String::hash_code` produces inconsistent results on different platforms, when `s` is `char*`.  This is because on some platforms `char` is signed, while on others unsigned (resulting in `char` to be either zero-extended or sign-extended, when cast to `unsigned int`). This causes 1 tier1 test failure on aarch64.
>> 
>> Details:
>> This was discovered by examining one failing test (from tier1) present on aarch64 builds:
>> `test/serviceability/sa/jmap-hashcode/Test8028623.java`
>> Test was introduced by [JDK-8028623](https://bugs.openjdk.org/browse/JDK-8028623). However fix done there does not work on aarch64. Code was later fixed (newer jdks) in [hotspot part](https://github.com/openjdk/jdk11u-dev/commit/7af927f9c10923b61f746eb6e566bcda853dd95a) of [JDK-8141132](https://bugs.openjdk.org/browse/JDK-8141132) (JEP 254: Compact Strings).
>> 
>> Fix:
>> Fixed by backporting very small portion of JDK-8141132.
>> 
>> Testing:
>> tier1 (x86, x86_64, aarch64): OK (tested by GH and in rhel-8 aarch64 VM)
>
> hotspot/agent/src/share/classes/sun/jvm/hotspot/utilities/Hashtable.java line 66:
> 
>> 64:     // Emulate the unsigned int in java_lang_String::hash_code
>> 65:     while (len-- > 0) {
>> 66:       h = 31*h + (0xFFL & buf[s]);
> 
> `Byte.toUnsignedInt()` would be clearer.

yes, but that would also be different to upstream jdk11u

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PR Review Comment: https://git.openjdk.org/jdk8u-dev/pull/336#discussion_r1258017598