Updated SoV documents

[Frederic Parain]

I haven’t mentioned bytecodes that are using CONSTANT_Class_info indirectly,
all the get/put and invoke bytecodes which are using CONSTANT_Class_info
through CONSTANT_Methodref_info and CONSTANT_Fieldref_info. Anyway, they
don’t need ReferenceType either.

Fred


> On Apr 3, 2020, at 13:08, Frederic Parain <frederic.parain at oracle.com> wrote:
> 
> Hi,
> 
> This is a good question.
> 
> The ReferenceType descriptor was added to make the distinction between the two VM-level
> type sharing the same class name. The CONSTANT_Class_info is used by the following bytecodes:
>  - ldc
>  - new
>  - defaultvalue
>  - anewarray
>  - multianewarray
>  - checkcast
>  - instanceof
> 
> I’m not seeing any case now where a ReferenceType would be needed. All these bytecodes can
> work properly with the old fashioned CONSTANT_Class_info. For bytecodes like new, defaultvalue,
> anewarray, multianewarray, the class is loaded, so verifications are performed at runtime
> (for instance, new can throw an InstantiationError if the loaded type is an inline type),
> and layout information are available to allocate structures likes arrays.
> 
> The Q-signature is used as a marker for inline types (null-free/immutable/identity-free),
> but also as a pre-loading and eager-loading signal. The places where it is used as such a
> signal are the descriptors in field_info and method_info, which point directly to an
> Utf8 entry.
> 
> CONSTANT_Class_info is also used for the this_class and super_class fields at the classfile
> top-level structure. In both cases, there’s no need to have a ReferenceType: no distinction
> is needed, and these types are loaded anyway, so verification is performed on loaded types.
> 
> Unless I’ve missed something, I would say that we are clear to return CONSTANT_Class_info
> to its pre-LW2 format, supporting only "binary class or interface name”.
> 
> Fred
> 
> 
>> On Apr 2, 2020, at 12:26, Brian Goetz <brian.goetz at oracle.com> wrote:
>> 
>> Hi Tobi;
>> 
>> I don't have an answer for you offhand, but the current "eclair" model eliminates one of the reasons we needed the new syntax for C_Class -- that a class name could refer to two VM-level types.  This is no longer the case; each VM type corresponds to exactly one classfile.  
>> 
>> The reason I can't give you an unqualified yes at this point is that I am not sure whether we still need to include a "Q" somewhere as a preload signal.  When Q descriptors appear in certain places (e.g., field declarations), they generate load order constraints, and in other places, they generate nullity assumptions.  
>> 
>> I will leave it to Frederic to answer whether we have run out of reasons to need anything other than binary class names in C_Class structures.  (It would be nice if we have!)
>> 
>> On 4/1/2020 10:19 AM, Tobi Ajila wrote:
>>> Hi Brian 
>>> 
>>> Thanks for the updated SoV docs. In section 4, it mentions:
>>> 
>>>> In most cases, such as field descriptors and method descriptors, uses of C.ref is translated as LC$ref;, uses of C.val is translated as QC$val;, 
>>> 
>>> In the LW2 spec the `name_index` in `CONSTANT_Class_info` structures could refer to "binary class or interface name" as well as "ReferenceType descriptors" which referenced UTF8s with 'Q' descriptors. In LW2 inline-types were both nullable and null-free so it was necessary to have ReferenceType descriptors in order make a distinction in CONSTANT_Class_info structures for things like allocating arrays. With the new model inline-types can only be null-free, so will the CONSTANT_Class_info structures be limited to binary class or interface names? or will ReferenceType descriptors be used for inline-types?
>>> 
>>> --Tobi
>>> 
>>> "valhalla-spec-experts" <valhalla-spec-experts-bounces at openjdk.java.net> wrote on 2020/03/27 03:59:38 PM:
>>> 
>>>> From: Brian Goetz <brian.goetz at oracle.com>
>>>> To: valhalla-spec-experts <valhalla-spec-experts at openjdk.java.net>
>>>> Date: 2020/03/27 03:59 PM
>>>> Subject: [EXTERNAL] Updated SoV documents
>>>> Sent by: "valhalla-spec-experts" <valhalla-spec-experts-
>>>> bounces at openjdk.java.net>
>>>> 
>>>> I've updated the SoV documents, including the new sections on VM 
>>>> model and translation:
>>>> 
>>>>    http://cr.openjdk.java.net/~briangoetz/valhalla/sov/01-background.html
>>>> 
>>> 
>>> 
>> 
>