Unsafe access and aot-image building

[Stefan Marr]

Hi:

I am experimenting with using SVM/the AOT image building from GraalVM 0.24 for SOMns.

One difficulty is that I am not aware of much publicly available information.
So, I am kind of flying blind here.

The next challenge is that SOMns still uses my own object model, which is implemented with unsafe operations. And, I’d really like to keep it, if at all possible.

With a bit of guidance, I got so far that I get the following error:

error: Field AnalysisField<StorageLocation$ObjectDirectStorageLocation.fieldOffset accessed: false reads: true written: true> is used as an offset in an unsafe operation, but no value recomputation found.

And the `fieldOffset` is the offset used by the unsafe access.

In code [1] this looks like this:

public static final class ObjectDirectStorageLocation {

    private final long fieldOffset;

    public ObjectDirectStorageLocation(final ObjectLayout layout, final SlotDefinition slot,
        final int objFieldIdx) {
      super(layout, slot);
      fieldOffset = SObject.getObjectFieldOffset(objFieldIdx);
    }



The underlying problem is that this of course depends on functionality to determine the object offset, roughly something like this:

  private static long FIRST_OBJECT_FIELD_OFFSET = getFirstObjectFieldOffset();
  private static final long OBJECT_FIELD_LENGTH = getObjectFieldLength();
  public static final int NUM_OBJECT_FIELDS    = 5;

  public static long getObjectFieldOffset(final int fieldIndex) {
    assert 0 <= fieldIndex && fieldIndex < NUM_OBJECT_FIELDS;
    return FIRST_OBJECT_FIELD_OFFSET + fieldIndex * 8 /* OBJECT_FIELD_LENGTH */;
  }

`getFirstObjectFieldOffset()` is normally using reflection, but Peter’s help, I used the svm.jar to build a replacement class, which is supposed to make sure that the image will use the correct offset for the object access at run time:

@TargetClass(SObject.class)
public final class SObjectReplacement {
  @RecomputeFieldValue(kind = Kind.FieldOffset, declClass = SMutableObject.class, name = "field1")
  @Alias
  private static long FIRST_OBJECT_FIELD_OFFSET;
}

However, when I do this, it still has cause the very same error.
I am sure that SVM picks up the SObjectReplacement class, because it throws an error if the class is not marked final.

Any help, pointers, and suggestions would be very welcome.
I suppose I am missing something to make the analysis happy with the replacement. But I have no clue what that might be.
Do I need to do something with the `fieldOffset` field itself, instead of the fields it relies indirectly on?

Thanks
Stefan



[1] https://github.com/smarr/SOMns/blob/master/src/som/interpreter/objectstorage/StorageLocation.java#L152

-- 
Stefan Marr
Johannes Kepler Universität Linz
http://stefan-marr.de/research/