Unsafe access and aot-image building

[Vojin Jovanovic]

Hi Stefan,

What fails is the verification that use to avoid accidental SEGFAULTS 
that requires the `fieldOffset` to be recomputed which is not the case 
here. What you did is correct, but our verification is too narrow to 
figure that out. We will find a way to loosen this verification step for 
the future.

For now, you can disable offset verification by adding

   -H:-ThrowUnsafeOffsetErrors

to your image build command.

Hope this helps.

Cheers,

  -- Vojin

On 6/15/17 10:52 AM, Stefan Marr wrote:
> 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
>