Found this when testing JDK 25:

$ CONF=linux-x86_64-server-fastdebug make images test TEST=java/lang/String/IndexOf.java TEST_VM_OPTS=-XX:-CompactStrings

...

    FAILED: 80 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaddddd[ad]
(UU)(T3) Trying repeaded needle[2] in haystack[87] at offset[82]
    FAILED: 81 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaadddd[ad]
(UU)(T3) Trying repeaded needle[2] in haystack[87] at offset[83]
    FAILED: 82 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaddd[ad]
(UU)(T3) Trying repeaded needle[2] in haystack[87] at offset[84]
    FAILED: 83 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaadd[ad]
STDERR:
java.lang.RuntimeException: IndexOf test failed.
at IndexOf.main(IndexOf.java:143)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
at java.base/java.lang.reflect.Method.invoke(Method.java:565)
at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
at java.base/java.lang.Thread.run(Thread.java:1474)


I believe only the subtests with -XX:+EnableX86ECoreOpts are failing, because they enter the AVX2-enabled stub added by JDK-8320448:

#ifdef COMPILER2
  if ((UseAVX == 2) && EnableX86ECoreOpts) {
    generate_string_indexof(StubRoutines::_string_indexof_array);
  }
#endif

If I disable that block, the test starts to pass. The failure was caught on AMD Zen 3 machines. I have not tested any Xeons. I doubt it is architecture-specific issue.