RFR(XS): 8228367: [lworld] Constant, null-free inline type fields should be treated as flattenable

[Tobias Hartmann]

Hi Rémi,

On 19.07.19 17:02, Remi Forax wrote:
> The scalarization has nothing to do with the fact of being flattenable or not,
> the flattenability is a storage property so for value in fields or array elements, not for value on the stack.
> 
> Being a null-free inline type is enough, you do not have to check if the fields are flattenable or not.

Yes, sure. But this change is about VM/JIT internal code that attempts scalarization when creating
the C2 IR for loading a constant inline type field with an inline type holder. And here the term
"flattenable" is currently used interchangeably with "null-free". That's actually not specific to
the JIT, but just how the VM propagates inline type field specific information internally.

Basically, we have three relevant field states:
(1) !flattenable && !flattened => !null-free, !flat, !inline type field
(2)  flattenable && !flattened =>  null-free, !flat,  inline type field
(3)  flattenable &&  flattened =>  null-free,  flat,  inline type field

The JIT will attempt scalarization for (2) and (3).

This simplification will change as soon as there is a possibility to declare inline type fields that
are null-free and !flattenable (or !null-free and flattenable, once we have support for null-default
inline classes).

Best regards,
Tobias