Question on the inline type flattening decision

[Frederic Parain]

Hi Xiaohong,


Thank you for reporting this. It looks like a bug to me, specifying 
-XX:InlineFieldMaxFlatSize=0 should disable all field flattening, 
including final fields.

Do you want to fill a bug report or do you want me to take care of it?


Best Regards,


Fred


On 6/30/23 3:04 AM, Xiaohong Gong wrote:
>
> Hi,
>
> I guess this is the right place to ask this question related to the 
> flattening decision on inline type fields?
>
> I met an issue when I was running the tests under 
> “hotspot/jtreg/compiler/valhalla/inlinetypes” with 
> “-XX:InlineFieldMaxFlatSize=0”. The intention is doing some testing by 
> forcing the inline type fields not be flattened.  And I debugged some 
> C2 code in `inlinetypenode.cpp` like `InlineTypeNode::load()`.  But I 
> found that the code path has no difference, which means this flag 
> cannot work and the field is flattened as without this flag.
>
> And then, I checked the relative code 
> https://github.com/openjdk/valhalla/blob/lworld/src/hotspot/share/classfile/fieldLayoutBuilder.cpp#L759 
> to find out the reason. It checks the three necessary conditions (i.e. 
> “the code size comparison with `InlineFieldMaxSize`”, “atomic” or 
> “volatile” fields) firstly, which is the right behavior I think. But 
> then the result is “||” with the `final` access flag. Which means if 
> the field is `final`, it will be flattened anyway regardless of the 
> necessary three limitations.
>
> So is this the expected behavior? Does this mean the inline type field 
> can always be flattened if it is declared with `final`? But I didn’t 
> find any descriptions on the flattening decision related to the 
> `final` flag.
>
> Many thanks if any help on this!
>
> Best Regards,
>
> Xiaohong
>