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<p style="margin: 0px 0px 1.2em !important;">Hi John</p>
<p style="margin: 0px 0px 1.2em !important;">On 18/01/2024 19:38,
John Bossons wrote:</p>
<p style="margin: 0px 0px 1.2em !important;"></p>
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<p></p>
<blockquote type="cite" cite="mid:CACvEv_wHnpkrQ3vq1thJRig_mGLskPFO_JH+6XD+M8yfDUxQKw@mail.gmail.com">Why
complicate the specification with an implicit constructor that
a developer will never explicitly invoke? Why permit a
developer to 'opt in' to non-atomic?</blockquote>
<p></p>
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<p style="margin: 0px 0px 1.2em !important;"></p>
<p style="margin: 0px 0px 1.2em !important;">I believe you are
coming at this from the wrong angle. The implicit constructor
is, at its core, a mechanism for the class to advertise that it
has a zero instance (as the JEP calls it [1]).</p>
<p style="margin: 0px 0px 1.2em !important;">This means that
whenever a default value of the class has to be materialized
it’s as if the impliict constructor was called.</p>
<p style="margin: 0px 0px 1.2em !important;">This are some of the
ways this can happen (there’s probably more):</p>
<ul style="margin: 1.2em 0px;padding-left: 2em;">
<li style="margin: 0.5em 0px;">when you allocate arrays</li>
<li style="margin: 0.5em 0px;">when you create a <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">MethodHandle</code>
which returns the zero value of a given class (see <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">MethodHandles::zero</code>)</li>
<li style="margin: 0.5em 0px;">if we ever provide a syntax to
say <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">Foo.default</code>/<code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">Foo.zero</code></li>
</ul>
<p style="margin: 0px 0px 1.2em !important;">In other words, once
a value class has an associated zero-instance, developers can
materialize that zero-instance in many different ways. Because
of that, the point you raise that implicit constructors make you
declare a constructor that you don’t want your clients to invoke
is a bit off the mark, because clients can maetrialize
zero-instances anyway. Actually, advertising it as an API point
makes it quite clear: the maintainer of that value class has to
be prepared to handle cases where all fields are zeroed - that’s
part of its API contract.</p>
<p style="margin: 0px 0px 1.2em !important;">Zooming back a bit,
you seem to favor a model where the cost is pushed to every read
of an array element, or flattenable field. Reads are (way) more
common that writes, which is why the current proposal only lets
you build array types (and flattenable fields) which make sense
given the value type under consideration. If you want to build a
non-null container, fine, but the type of the container must
better support the “all zero” configuration, since that’s the
paintbrush the JVM will use to paint the container bits. This
means that, array creation of a null-restricted value is the
same as for a regular identity class (all bits zeros) and, since
the value class explicitly declared its support for the
zero-instance configuration, no check is required on reads.</p>
<p style="margin: 0px 0px 1.2em !important;">And, zooming back
even more (now we’re up in the clouds :-) ), the model we’re
trying to achieve here is one where you get more (flattening)
guarantees, by progressively giving up other guarantees. For
example: </p>
<ul style="margin: 1.2em 0px;padding-left: 2em;">
<li style="margin: 0.5em 0px;">
<p style="margin: 0px 0px 1.2em !important;margin: 0.5em 0px !important;">opt-out
of identity (use <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">value</code>)
-> enables better flattening/scalarization (on the
stack), allow flattening (but with some extra bits to handle
nulls)</p>
<ul style="margin: 1.2em 0px;padding-left: 2em;margin: 0px; padding-left: 1em;">
<li style="margin: 0.5em 0px;">opt-out of zero-instance
protection (use implicit constructors) -> removes the
extra null state from the flattened storage</li>
<li style="margin: 0.5em 0px;">opt-out of nullability (use
null-restricted types) -> allows flattening of
fields/array elements</li>
<li style="margin: 0.5em 0px;">opt-out of atomic updates
(use non-atomic) -> allows flattening of types that are
bigger than N (where N defined by the JVM)</li>
</ul>
<p style="margin: 0px 0px 1.2em !important;margin: 0.5em 0px !important;">You
can see this as a series of step that takes you, smoothly,
from an identity class such as <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">java.util.ArrayList</code>,
down to something simpler and simpler, until you get to <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">int</code>,
a primitive type (which, not coincidentally, has given up
exactly the same guarantees listed above!). That is, maximum
flattening (e.g. int-like) is not a simple on/off switch,
but, rather, a property that emerges when the treatment of
the value being flattened can be relaxed along <em>all</em>
the axis shown above. This model is described, in much
better words than the ones I used here, in Brian’s document
[2].</p>
</li>
</ul>
<p style="margin: 0px 0px 1.2em !important;">Maurizio</p>
<p style="margin: 0px 0px 1.2em !important;">[1] - <a href="https://openjdk.org/jeps/8316779" class="moz-txt-link-freetext">https://openjdk.org/jeps/8316779</a><br>
[2] - <a href="https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/state-of-valhalla/02-object-model.md" class="moz-txt-link-freetext">https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/state-of-valhalla/02-object-model.md</a></p>
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