JVMS changes for JEP 401

[Dan Smith]

On Jan 29, 2024, at 9:06 AM, Remi Forax <forax at univ-mlv.fr> wrote:

----- Original Message -----
From: "Clement Cherlin" <ccherlin at gmail.com>
To: "Valhalla Expert Group Observers" <valhalla-spec-observers at openjdk.org>
Cc: "daniel smith" <daniel.smith at oracle.com>, "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
Sent: Monday, January 29, 2024 4:30:01 PM
Subject: Re: JVMS changes for JEP 401

On Tue, Jan 23, 2024 at 7:11 PM Dan Smith <daniel.smith at oracle.com> wrote:

I've posted a revised spec change document for JEP 401, Value Classes and
Objects, here:

https://cr.openjdk.org/~dlsmith/jep401/jep401-20240116/specs/value-objects-jvms.html

[snip]

- The 4.9.2 rule about ACC_STRICT is hard to parse, will be rephrased somehow

- 2.4 should say more about "sameness" as it relates to "identity"

On Tue, Jan 23, 2024 at 8:15 PM - <liangchenblue at gmail.com> wrote:

Hi Dan,
Thanks for sharing this document! It looks great!
There's a typo in section 2.4, where the definition of "element type" for an
array type is accidentally removed.

The reference to the "start of object construction" can probably be changed to
"before the super-call," taken from John's "Constructive Classes" essay. [1]
The 4.9.2 rule is correct;

Unless I'm mistaken, the rule in 4.9.2 is not correct.

From the proposal:
An instance field of this declared in the current class with its ACC_STRICT flag
set may be assigned by a putfield instruction occurring in an instance
initialization method only if another instance initialization method has not
yet been called.

As I understand the current design intent, a constructor that calls
'this' is forbidden from assigning to a strict field, because that
will lead to a duplicate assignment in a constructor that calls
'super'.

This language appears to say the opposite. It seems to state that only
the first constructor of the current class can assign the class's
strict instance fields. If that is the case, then further constructors
(including the one that calls 'super') are forbidden from assigning to
strict instance fields.

Hello,
the problem is not duplicate assignment, as you said that part of the spec allows that,
the problem is assignment of final fields after a call to super() or this() because when the assignment occurs, 'this' may have already leaked to another context.

Right.

There is a *language* rule that final fields must not be assigned more than once. (Specifically, they must be definitely unassigned before their assignment; we'll add a rule that final fields must be definitely assigned before a 'this()' call.)

In the *JVM*, the only new thing we're guarding against is assignments after the constructor call. All assignments before a constructor call, duplicate or not, are allowed.
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