ADDITIONAL SYSTEM INFORMATION :
21-ea build 21-ea+19-1566

A DESCRIPTION OF THE PROBLEM :
JVMTI operations related to tags are unusably slow now. Some timing data for tagging and iterating 10_000_000 objects:

19 build 19.0.2+7-44

setTag: 1969 ms
getTag: 743 ms
iterate tagged: 437 ms
iterate all: 499 ms

20 build 20.0.1+9-29

setTag: 2852 ms
getTag: 1851 ms
iterate tagged: 1496 ms
iterate all: 1585 ms

21-ea build 21-ea+19-1566

setTag: 4135765 ms
getTag: 4118573 ms
iterate tagged: 9287657 ms
iterate all: 9495863 ms

JVMTI tag map already got up to 3 times slower in JDK 20, probably due to JDK-8256072.

When reverting the JDK-8292741 changes, the timings are similar to JDK-20.

A fast (optimally concurrent) tag map is extremely important for profilers and other kind of JVMTI agents.

REGRESSION : Last worked in version 20.0.1

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
With the attached sources and JAVA_HOME set to the JDK to be tested:

g++ -shared -I$JAVA_HOME/include -I$JAVA_HOME/include/linux -fPIC TagMapTest.cpp -o libTagMapTest.so

$JAVA_HOME/bin/javac TagMapTest.java

LD_LIBRARY_PATH=. $JAVA_HOME/bin/java -agentlib:TagMapTest TagMapTest

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Timings like in JDK 19 or at least JDK 20.
ACTUAL -
Extremely slow timings

---------- BEGIN SOURCE ----------
TagMapTest.cpp -----------------------------------------------
#include <jvmti.h>
#include <cstdlib>
#include <cstring>

namespace {
    jlong nextTag = 1;
    jvmtiEnv *jvmti = nullptr;

    void checkJvmti(int code, const char* message) {
        if (code != JVMTI_ERROR_NONE) {
            printf("Error %s: %d\n", message, code);
            abort();
        }
    }

    jvmtiIterationControl JNICALL heapObjectCallback(jlong class_tag, jlong size, jlong* tag_ptr, void* user_data) {
        if (*tag_ptr == 0) {
            *tag_ptr = nextTag++;
        }
        return JVMTI_ITERATION_CONTINUE;
    }
}

extern "C" JNIEXPORT void JNICALL Java_TagMapTest_setTag(JNIEnv* jni_env, jclass clazz, jobject object) {
    checkJvmti(jvmti->SetTag(object, nextTag++), "could not set tag");
}

extern "C" JNIEXPORT jlong JNICALL Java_TagMapTest_getTag(JNIEnv* jni_env, jclass clazz, jobject object) {
    jlong tag;
    checkJvmti(jvmti->GetTag(object, &tag), "could not get tag");
    return tag;
}

extern "C" JNIEXPORT void JNICALL Java_TagMapTest_iterate(JNIEnv* jni_env, jclass clazz, jboolean tagged) {
    checkJvmti(jvmti->IterateOverHeap(tagged ? JVMTI_HEAP_OBJECT_TAGGED : JVMTI_HEAP_OBJECT_EITHER, &heapObjectCallback, nullptr), "could not iterate");
}

extern "C" JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
    if (vm->GetEnv(reinterpret_cast<void **>(&jvmti), JVMTI_VERSION) != JNI_OK || !jvmti) {
        printf("Could not initialize JVMTI\n");
        abort();
    }
    jvmtiCapabilities capabilities;
    memset(&capabilities, 0, sizeof(capabilities));
    capabilities.can_tag_objects = 1;
    checkJvmti(jvmti->AddCapabilities(&capabilities), "adding capabilities");
    printf("Loaded agent\n");
    fflush(stdout);
    return JVMTI_ERROR_NONE;
}
-------------------------------------------------
TagMapTest.java -------------------------------
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class TagMapTest {
    private static final List<TagMapTest> items = new ArrayList<>();

    private static native void setTag(Object object);
    private static native long getTag(Object object);
    private static native void iterate(boolean tagged);

    public static void main(String[] args) {
        System.loadLibrary("TagMapTest");
        for (int i = 0; i < 10_000_000; i++) {
            items.add(new TagMapTest());
        }

        long startTime = System.nanoTime();
        for (TagMapTest item : items) {
            setTag(item);
        }
        System.out.println("setTag: " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");

        startTime = System.nanoTime();
        for (TagMapTest item : items) {
            getTag(item);
        }
        System.out.println("getTag: " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");

        startTime = System.nanoTime();
        iterate(true);
        System.out.println("iterate tagged: " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");

        startTime = System.nanoTime();
        iterate(false);
        System.out.println("iterate all: " + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");
    }
}
---------------------------------------------
---------- END SOURCE ----------

FREQUENCY : always