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<p style="margin: 0px 0px 1.2em !important;">On 30/10/2024 21:49,
Maurizio Cimadamore wrote:</p>
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<blockquote type="cite" cite="mid:c95ed331-461e-46e2-a05f-1ec7e2ef0197@oracle.com">What
I would like to do in the meantime is (when I have some
cycles) to write a benchmark which tests memory segment in a
loop with random access (e.g. using random offsets). And then
evaluate the costs associated with the various approaches in
isolation and see if anything pops up. I have a couple of
ideas on how to possibly improve the story for the unbounded
MAX_VALUE access -- but I'd like to see where we are w.r.t.
random access in vanilla Java 24 first.
</blockquote>
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<p style="margin: 0px 0px 1.2em !important;">I find this email
from a year ago or so interesting:</p>
<p style="margin: 0px 0px 1.2em !important;"><a href="https://mail.openjdk.org/pipermail/panama-dev/2023-July/019478.html" class="moz-txt-link-freetext">https://mail.openjdk.org/pipermail/panama-dev/2023-July/019478.html</a></p>
<p style="margin: 0px 0px 1.2em !important;">Some kind of issue <em>was
indeed</em> identified in there. Surely, adding an extra <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;">< 0</code>
check in all memory segment accessors shouldn’t do much, but the
numbers suggested that the patched code was almost 2x as fast
(although not as fast as Unsafe).</p>
<p style="margin: 0px 0px 1.2em !important;">Then I had the other
idea to adapt var handles (the one you are currently using):</p>
<p style="margin: 0px 0px 1.2em !important;"><a href="https://mail.openjdk.org/pipermail/panama-dev/2023-July/019487.html" class="moz-txt-link-freetext">https://mail.openjdk.org/pipermail/panama-dev/2023-July/019487.html</a></p>
<p style="margin: 0px 0px 1.2em !important;">Which, in retrospect,
was perhaps going too far. Sure, the numbers in the synthetic
benchmark looked very very good… but such an approach does add
problems when it comes to the shape and simplicity of the
generated code. There’s an allocation in every hot path, and a
call to reinterpret (which is restricted, so there’s a check for
that too). All that stuff is not normally on the critical path
for memory access, but the trick of putting it inside the var
handle adaptation code makes it part of the hot path, which I
think solves some problems (it’s fast!) and creates some new
ones (it’s brittle!).</p>
<p style="margin: 0px 0px 1.2em !important;">Hence my suggestion
to go back a little, and see what we can do to speed up access
for a segment created with:</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); -moz-text-size-adjust: none;">MemorySegment.NULL.reinterpret(Long.MAX_VALUE)
</code></pre>
<p style="margin: 0px 0px 1.2em !important;">(which, as Ron
correctly points out, might not mean <em>exactly as fast as
Unsafe</em>)</p>
<p style="margin: 0px 0px 1.2em !important;">Maurizio</p>
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