<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body dir="auto">
<div dir="ltr"></div>
<div dir="ltr">Generally speaking, when trading off complexity against performance in interpreted code paths, performance always loses. We only add complexity after the less complex solution has proven itself to be a measurable problem for real workloads. </div>
<div dir="ltr"><br>
<blockquote type="cite">On Oct 9, 2024, at 6:43 PM, Mat Carter <Matthew.Carter@microsoft.com> wrote:<br>
<br>
</blockquote>
</div>
<blockquote type="cite">
<div dir="ltr">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style>@font-face { font-family: "Cambria Math"; }
@font-face { font-family: Calibri; }
@font-face { font-family: Aptos; }
p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm; font-size: 12pt; font-family: Aptos, sans-serif; }
a:link, span.MsoHyperlink { color: rgb(5, 99, 193); text-decoration: underline; }
.MsoChpDefault { font-size: 10pt; }
@page WordSection1 { size: 612pt 792pt; margin: 72pt; }
div.WordSection1 { page: WordSection1; }</style>
<div class="WordSection1">
<p class="MsoNormal">Is there a place to encode calls to the VM in the interpreted methods other than the method prologs? </p>
<p class="MsoNormal">I've recently discovered the interpreted method adaptors which seems like a candidate, but there's
</p>
<p class="MsoNormal">no examples of the adaptors calling into the VM.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Follows is the rational as to why I'm looking for an alternative to the method prologs</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The AOTEndTrainingOnMethodEntry feature [1] introduces calls from java methods into the
</p>
<p class="MsoNormal">VM (upcalls) when specific methods are entered. The methods are identified via a pattern in a
</p>
<p class="MsoNormal">similar manner to the CompileOnly option.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Following the initial PR review we’re looking at removing the knowledge of this AOT feature from
</p>
<p class="MsoNormal">the compilers/interpreter and introducing a more generic system (RuntimeUpcalls) that can be
</p>
<p class="MsoNormal">used by other parts of the VM [2]. In building out the RuntimeUpcalls system we've come across
</p>
<p class="MsoNormal">an inefficiency that isn't an immediate problem for this feature, but should another feature use this
</p>
<p class="MsoNormal">new system then it's less than optimal.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Interpreted code uses a shared method prolog (there are 8 variants for 'regular' methods [more
</p>
<p class="MsoNormal">for some special math/zip methods]), the AOTEndTrainingOnMethodEntry feature introduces a
</p>
<p class="MsoNormal">further 5 prolog types. When there is a single upcall (eg. AOTEndTrainingOnMethodEntry) to the VM
</p>
<p class="MsoNormal">everything is efficient.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The inefficiency issue arises as soon as there are two or more upcalls; which upcalls relate to
</p>
<p class="MsoNormal">which methods is contained within the RuntimeUpcalls system. When the interpreter examines
</p>
<p class="MsoNormal">the method flags they only indicate whether there are any upcalls (but not how many or which ones).
</p>
<p class="MsoNormal">As the interpreter can't encode which upcalls should be called in the prolog (without an explosion
</p>
<p class="MsoNormal">of new runtime generated prologs), it needs to call the RuntimeUpcalls system which in turn iterates</p>
<p class="MsoNormal">over the upcalls and calls the appropriate ones; the problem is that during that iteration the methods
</p>
<p class="MsoNormal">need to be compared against the pattern. So we either pay a memory cost to cache the method to
</p>
<p class="MsoNormal">upcall relationships or we pay a performance cost to repeatedly test the method against the pattern.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This is not a problem for C1 and C2 as we pay this cost only when the methods are compiled and
</p>
<p class="MsoNormal">create the multiple upcalls in those methods, eliminating the need for pattern matching by the
</p>
<p class="MsoNormal">RuntimeUpcalls system during method execution</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks in advance </p>
<p class="MsoNormal">Mat</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[1] <a href="https://github.com/openjdk/leyden/pull/21">https://github.com/openjdk/leyden/pull/21</a></p>
<p class="MsoNormal">[2] <a href="https://github.com/macarte/leyden/pull/2">https://github.com/macarte/leyden/pull/2</a></p>
</div>
<div><image.png></div>
</div>
</blockquote>
</body>
</html>