Value Types: crash when System.arrayCopy sets null in null-restricted array

[theresa-m]

Related to #20223 I also tried the following to see whether or not I can copy null into a null-restricted array. This test results in a segmentation error.

java --enable-preview --add-exports java.base/jdk.internal.vm.annotation=ALL-UNNAMED --add-exports java.base/jdk.internal.value=ALL-UNNAMED -XX:ValueTypeFlatteningThreshold=99999 -XX:+EnableArrayFlattening -Xint TestNullRestrictedArray

        static private void test1(int x) {
           SomeValueClass1[] array1 = (SomeValueClass1[])ValueClass.newArrayInstance(NullRestrictedCheckedType.of(SomeValueClass1.class), ARRAY_SIZE);
           SomeValueClass1[] array2 = new SomeValueClass1[ARRAY_SIZE];
                
                for (int i=0; i < ARRAY_SIZE; i++) {
                        array1[i] = new SomeValueClass1(i*x);
                        array2[i] = null;
                }
                
                System.arraycopy(array2, 0, array1, 0, ARRAY_SIZE);
        }

Changes from #20250 are needed to reproduce this.

[github-actions]

Issue Number: 20253
Status: Open
Recommended Components: comp:vm, comp:test, comp:gc
Recommended Assignees: hangshao0, babsingh, gacholio

[dmitripivkine]

Speculating, but please be aware it is possible we do not have proper implementation for some cases for Flattened Arrays copy. Traditional array copy functions might have deal with hardcoded element sizes 1, 2, 4 , 8. Copy of different size element byte by byte might be not sufficient due possible object references embedded to the element. Coping object reference should be done size-atomic. Also it might require triggering of GC barriers. Please do not hesitate to ask if there is one of this cases or you need help with investigation.

[hangshao0]

There is a comment in the code:

openj9/runtime/vm/createramclass.cpp

Lines 3455 to 3458 in e425dbf

	/* TODO restrict this flag to be set only for null-restricted
	* arrays once value type command line tests are updated.
	*/
	arrayFlags |= J9ClassIsFlattened;

The flag J9ClassIsFlattened is expected to be removed from nullable array.

[theresa-m]

Oh yea... that was me wasn't it 😅. Thanks, I'll start there.

[github-actions]

Issue Number: 20253
Status: Closed
Actual Components: comp:vm, project:valhalla
Actual Assignees: No one :(
PR Assignees: theresa-m