ADDITIONAL SYSTEM INFORMATION :
Windows 11 (can't select above)
Affects OpenJDK 24 and 25

A DESCRIPTION OF THE PROBLEM :
I believe it's caused by this change:
JDK-8320448
I found a bug issue:
JDK-8360271

I'd like to add a simple Java reproducer for the problem.
My workaround is to disable 'EnableX86ECoreOpts'

REGRESSION : Last worked in version 23

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Please run the JUnit test below

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
As the test expects: 9
ACTUAL -
Actual is -1 if the intrinsic was used (I assume)

---------- BEGIN SOURCE ----------
public class ReproStringBugTest {

    @BeforeClass // if this line is commented out, the tests are good
    public static void warmup() {
        for (int i = 0; i < 10_000; i++) { // could also just specify -Xcomp VM option but thats much much slower
            "Sher\ufffflockSherlock".indexOf("Sherlock");
            "Sher\ufffflockSherlock".indexOf("Sherlock", 0);
        }
    }

    @Test
    public void repro1() {
        // any char above 0xff seems to trigger this error
        Assert.assertEquals(9, "Sher\ufffflockSherlock".indexOf("Sherlock"));
    }

    @Test
    public void repro2() {
        Assert.assertEquals(9, "Sher\ufffflockSherlock".indexOf("Sherlock", 0));
    }
}

---------- END SOURCE ----------