Valhalla EG meeting Sept 27 - and Sept 13 minutes

[Karen Kinnear]

Reminder - meeting TODAY, Sept 27 9am PT/noon ET/
dial-in:  https://oracle.zoom.us/j/5249803466 <https://oracle.zoom.us/j/5249803466>

AI: Karen send Dan H example in which a BSM can be used for both indy and condy
AI: Dan Smith version of Condy JVMS changes (not an appendix
AI: nestmates JVMS comments: http://cr.openjdk.java.net/~dlsmith/private-access.html
     Remi proposal: rename attribute to “NestHost"
AI: Karen: write-up preparation relative to selection
AI: Karen - ask Paul progress on VarHandles and atomicity relative to Value Types

Question for EG:

Doug, Charlie - does it work for you if we provide MVT as Early Access Binaries instead of part of 18.3?


Attendees: Remi, Frederic, Brian, Dan H, Karen

1. JDK version numbers relative to class file versions
Would be useful to find a way to document the mapping for users

2. Delivery vehicles
Discussion of making MVT available via an Early Access Binary vs. part of 18.3 experimentally

pros and cons:
1. late in the cycle to be adding to 18.3, specifically relative to JVMS/JCP process
2. with EA binary we can make this available sooner than 18.3, and update it more often in response to requests
and prototype progress
3. EA is less cost to productize and support
4. - the risk of not being in 18.3 is that we will get fewer adopters
   - the hoped for benefit of an EA is that we will get more of the advanced early adopters who can give us valuable
     feedback and use cases
5. No 18.3 features are dependent on MVT (note: pattern matching is not dependent on MVT, and not targeted for 18.3)
    current goal for features: Local Variable Type Inference and Condy (not experimental) in 18.3
    goal: nestmates early in the 18.9 cycle to shake it out (meeting attendees ok with delaying nestmates)
4. As an EA binary, we are ok with having a javac with MVT support behind a flag
5. Benefit of 18.3 experimental: 2nd level customer, i.e. user of Scala or JRuby could enable this with a flag
    - the theory is that those trying the experimental compiler would be able to download an experimental JDK binary to try it on
6. Time to get an experimental feature through JCP process - into experimental appendix with disclaimers
    - Brian working with JCP to reduce lag time, see JSR 383 for 18.3
7. IBM can find a way to do external binaries
    - both IBM and Hotspot are working on internal processes to make this happen
    - we will keep in contact on the target date - not expected to be exactly in sync
    - Hotspot expect ~ 4-6 weeks from Sept 13

3. ConstantDynamic concerns:
   1. BootstrapCallInfo - i.e. lazy resolution - IBM less confident (Hotspot agree)
   Remi wants to continue the discussion
   There are ways to handle > 251 arguments with varargs, rather than requiring BSCI
   channeling John: one of the goals of lazy resolution is to allow only evaluating a subset of constants that may succeed

   2. Question - can a condy and an indy share the same BSM?
       In practice yes - if the BSM specifies a java.lang.Object for the MethodType returned, then it can return a java.lang.Class for the condy
       and a CallSite for the invoke dynamic. The verifier does not check the BSM MethodType, deliberately. The verifier does minimal checking
       for BSMs deliberately, according to Remi - this provides requested flexibility for dynamic languages

    3. Remi - we want condy without java.lang.invoke, and we want it to work during vm bootstrapping.
    

4. Nestmates
  1. Question: how do we handle invokeinterface for methods in java.lang.Object?
AI: Karen to email details of invocation including corner cases - relative to the specification changes

   2. Lazy loading - what should reflection do?
      Request from last meeting to lazily load the nest top so as to not break current nested class examples that work today (e.g. someone deletes the outer class)
      Proposal:
           get list of Nestmates: model on getAnnotations - if you can’t find a nest member, skip it, do not throw an exception
[ed. note: what if you can not find the nest top?]
           check if two types are nestmates: throw LinkageError if nest top can not be found

   3. Attribute renaming: Proposal “NestHost” from last time -> tell Dan Smith

5. MVT 
    1. Remi: OutOfMemoryError if 15 million element array of ValueTypes
    If fail due to OOME, could consider falling back to allocating in the heap, this would be slower
    Remi: prefer OOME to not performant
    note: choosing to flatten is an implementation detail which could be architecture dependent, depend on atomicity requirements, etc. - no user control

    2. Atomic operations
       Karen: request changing VarHandles to require declaration site atomicity and consistency across byte code and VarHandle access relative to non-tearing
       Remi - less than 64 bits no need to declare
       AI: Karen ask Paul to work with Doug on this
       Remi: please do not reuse volatile, do not use atomic - want e.g. untearable

optional flattening work in progress - should be there in a couple of weeks