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<p style="margin: 0px 0px 1.2em !important;">On 24/03/2023 19:07,
Glavo wrote:</p>
<div class="markdown-here-exclude">
<blockquote type="cite" cite="mid:CAJL5A3=E+1pD7AKffx7pSMksq6jGWhL8B39XidPbO0+b8MW8zg@mail.gmail.com">
<div>2. StructLayout must manually specify all padding</div>
<div><br>
</div>
<blockquote style="margin:0 0 0 40px;border:none;padding:0px">
<div>Can we provide a convenient method for automatically
padding between fields based on alignment?</div>
<div>The current structLayout method is annoying for
situations where you need to manually simulate the layout
of a C struct.</div>
</blockquote>
<div><br>
</div>
</blockquote>
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<p style="margin: 0px 0px 1.2em !important;">I'd like to return to
this topic, since we have spent some more time thinking about
it.</p>
<p style="margin: 0px 0px 1.2em !important;">First, we have added
extra eager checks (as also suggested by Michael) so that
non-sensical layouts are ruled out at creation:</p>
<p style="margin: 0px 0px 1.2em !important;"><a href="https://git.openjdk.org/panama-foreign/pull/824" class="moz-txt-link-freetext">https://git.openjdk.org/panama-foreign/pull/824</a></p>
<p style="margin: 0px 0px 1.2em !important;">On the topic of
adding a more user-friendly struct factory, I am less convinced
that such an API really belongs in MemoryLayout. First, memory
layouts are fairly general, and can be used to model things that
are <b>not</b> C structs. Consider this:</p>
<pre style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-java" 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;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block !important;display: block; overflow-x: auto; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 248) none repeat scroll 0% 0%; -moz-text-size-adjust: none;"> structLayout(JAVA_LONG, JAVA_INT)
</code></pre>
<p style="margin: 0px 0px 1.2em !important;">This layout is valid,
and you can dereference both struct fields w/o issues using var
handles. That said, the size of this layout (12) is not a
multiple of its alignment (8), so if you nest this layout inside
a sequence layout, you would get an IllegalArgumentException.
But you might never need to create an array of these things, in
which case a memory layout such as the one above is perfectly
sensible. The C language, on the other hand, enforce specific
constraints on its structs - one such constraint is that all
struct layouts must have a size that is a multiple of their
alignment. Which means that the above struct layout does not
correspond to any C struct declaration. <i>And that’s ok</i>.</p>
</div>
On top of that, even if C structs and FFM struct layouts agreed
100%, it would still make sense for memory layouts to expose a “raw”
struct layout factory, for clients (maybe tools?) wanting to
validate the correctness of their assumptions. <br>
<div class="markdown-here-wrapper" data-md-url="" style="" markdown-here-wrapper-content-modified="true"> <br>
<p style="margin: 0px 0px 1.2em !important;">So, for the time
being, I think it would be better to keep the FFM API the way it
is, and have all the various layout factories act transparently
in a “what you see is what you get” manner. The existing
factories are meant to be primitive building blocks, and their
primary goal is to be predictable, general and easy to
understand. (After all, any non-primitive factory we might add
in the future <i>needs</i> to be explained in terms of some
simpler factory).<br>
</p>
<p style="margin: 0px 0px 1.2em !important;">Does this mean that
there will never be higher-level functionalities? Not
necessarily, even though such functionalities might well end up
falling outside the scope of JEP 442 (and its predecessor JEPs).
Some work Per did in this area [1, 2] shows that there are
functionalities that are useful when working with layouts, even
though we might not necessarily have found the best place where
to put them. Also, I believe at some point there will be some
story for <em>transforming</em> a layout into another, so again
there’s a question of where to put some canned transforms (e.g.
to swap endianness of all value layouts in a layout). I think a
“higher-level” factory for creating C-like layouts belong in the
same place as these functionalities. They are nice-to-have, but
not “primitive” enough, I think, to deserve a spot in the
MemoryLayout class (evidence of that is that it is fairly easy
to build such a factory externally [3]).</p>
<p style="margin: 0px 0px 1.2em !important;">Maurizio</p>
<p style="margin: 0px 0px 1.2em !important;">[1] - <a href="https://git.openjdk.org/panama-foreign/pull/715" class="moz-txt-link-freetext">https://git.openjdk.org/panama-foreign/pull/715</a><br>
[2] - <a href="https://github.com/openjdk/panama-foreign/pull/695" class="moz-txt-link-freetext">https://github.com/openjdk/panama-foreign/pull/695</a><br>
[3] - <a href="https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/src/java.base/share/classes/jdk/internal/foreign/Utils.java#L223" class="moz-txt-link-freetext">https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/src/java.base/share/classes/jdk/internal/foreign/Utils.java#L223</a></p>
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