generic specialization design discussion

Remi Forax forax at
Mon Apr 8 21:55:59 UTC 2019

in that case, direct class ? 


> De: "Brian Goetz" <brian.goetz at>
> À: "Kevin Bourrillion" <kevinb at>
> Cc: "valhalla-spec-experts" <valhalla-spec-experts at>
> Envoyé: Lundi 8 Avril 2019 22:45:46
> Objet: Re: generic specialization design discussion

> And the opposite of “inline” is “indirect”. ref, interface, and null-adjoined
> types are _indirect_. Indirect classes are passed by pointer, do not get
> flattened, get erased, are nullable.

>> On Apr 8, 2019, at 4:20 PM, Brian Goetz < [ mailto:brian.goetz at |
>> brian.goetz at ] > wrote:

>> Refining this:

>> - inline classes are inlinable (duh)
>> - reference classes, interfaces, and nullable-projections of zero-default inline
>> classes are not inlinable

>> (We later use these to say: Instantiation of generics with non-inlinable types
>> are erased.)

>> That seems not terrible, except for the “nullable projection of zero-default
>> inline classes” part, which is both a mouthful and has “inline” in it. Perhaps
>> calling these something like “null-adjoined types” (to reflect the fact that
>> we’re cramming a null into a type whose value set doesn’t naturally contain
>> null) makes that slightly better. So:

>> - inline classes are inlinable
>> - reference classes, interfaces, and null-adjoined types are not inlinable

>>> On Apr 8, 2019, at 3:06 PM, Brian Goetz < [ mailto:brian.goetz at |
>>> brian.goetz at ] > wrote:

>>> A related issue is that we want a word for describing which types are routinely
>>> flattened in layouts, and specialized in generics (the criteria are the same).
>>> Currently:

>>> - References and nullable projections of values (V?) are erased and not
>>> flattened
>>> - Values (zero-default and null-default) are specializable and flattenable

>>> (This thread is for terminology; if you have questions about the above claims,
>>> make a new thread, or better, wait for the more detailed writeup explaining why
>>> this is.)

>>> We need words for these two things too.

>>>> On Apr 8, 2019, at 2:58 PM, Brian Goetz < [ mailto:brian.goetz at |
>>>> brian.goetz at ] > wrote:

>>>> Yes, that’s a promising direction. And this is surely the motivation why the C#
>>>> folks picked “struct”; they wanted to carry the connotation that this is a
>>>> structure that is inlined. Problem is, the word “struct” is already so heavily
>>>> polluted by what it means in C. So perhaps something like:

>>>> inline class V { … }

>>>> This says than a V can be inlined into things that contain a V — other classes
>>>> and arrays. It also kind of suggests that this thing has no intrinsic identity.

>>>> A possible downside of this choice is that one might mistake it for meaning “its
>>>> methods are inlined”. Which is actually a little true, in that the methods are
>>>> implicitly static and therefore more amenable to dynamic inlining. So that
>>>> might actually be OK.

>>>> Others?

>>>>> On Apr 8, 2019, at 2:25 PM, Kevin Bourrillion < [ mailto:kevinb at |
>>>>> kevinb at ] > wrote:

>>>>> I'd suggest the name should in some way allude to the inline/compact/flat memory
>>>>> layout, because that is the distinguishing feature of these new things compared
>>>>> to anything else you can do in Java. And it is what people should be thinking
>>>>> about as they decide whether a new class should use this.

>>>>> On Mon, Apr 8, 2019 at 10:02 AM Brian Goetz < [ mailto:brian.goetz at |
>>>>> brian.goetz at ] > wrote:

>>>>>> The slide deck contains a list of terminology. I’d like to posit that the most
>>>>>> confusion-reducing thing we could do is come up with another word for value
>>>>>> types/classes/instances, since the word “value” is already used to describe
>>>>>> primitives and references themselves. This is a good time to see if there are
>>>>>> better names available.

>>>>>> So for this thread only, we’re turning on the syntax light to discuss what might
>>>>>> be a better name for the abstraction currently known as “value classes”.

>>>>>>> On Mar 29, 2019, at 12:08 PM, John Rose < [ mailto:john.r.rose at |
>>>>>> > john.r.rose at ] > wrote:

>>>>>> > This week I gave some presentations of my current thinking
>>>>>> > about specializations to people (from Oracle and IBM) gathered
>>>>>> > in Burlington. Here it is FTR. If you read it you will find lots
>>>>>> > of questions, as well as requirements and tentative answers.

>>>>>>> [ |
>>>>>> > ]

>>>>>> > This is a checkpoint. I have more tentative answers on the
>>>>>> > drawing board that didn't fit into the slide deck. Stay tuned.

>>>>>> > — John

>>>>> --
>>>>> Kevin Bourrillion | Java Librarian | Google, Inc. | [ mailto:kevinb at |
>>>>> kevinb at ]

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