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<p>Hi Bert,</p>
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<p>thanks for the helpful answer. So there is no chance to use a first-class object for the method dictionary - unless I set the instance variable methodDictionary to nil and force the VM to send #cannotInterpret: for every message? :-)</p>
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<div><font size="3" color="black"><span style="font-size:12pt"><a href="http://www.hpi.de/" target="_blank" rel="noopener noreferrer" id="LPNoLP"><font size="2"><span id="LPlnk909538"><font color="#757B80"></font></span></font></a></span></font></div>
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<div class="_rp_T4" id="Item.MessagePartBody">Your SqueakJS sources are indeed very helpful for studying a VM implementation. Great work!</div>
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<div class="_rp_T4" id="Item.MessagePartBody">Best,</div>
<div class="_rp_T4" id="Item.MessagePartBody">Christoph</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>Von:</b> Squeak-dev <squeak-dev-bounces@lists.squeakfoundation.org> im Auftrag von Bert Freudenberg <bert@freudenbergs.de><br>
<b>Gesendet:</b> Montag, 24. Februar 2020 23:21:44<br>
<b>An:</b> The general-purpose Squeak developers list<br>
<b>Betreff:</b> Re: [squeak-dev] How does the VM deal with method dictionaries?</font>
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<div dir="ltr" class="gmail_attr">On Mon, Feb 24, 2020 at 5:14 AM Thiede, Christoph <<a href="mailto:Christoph.Thiede@student.hpi.uni-potsdam.de">Christoph.Thiede@student.hpi.uni-potsdam.de</a>> wrote:<br>
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<p>Hi all,</p>
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<p>one curious question:</p>
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<div>c := Object newSubclass.</div>
<div>c compile: 'foo ^#foo'.</div>
<div>c methodDict: (ObjectTracer on: c methodDict).</div>
<div>c flushCache.</div>
<div>o := c new.</div>
<div>o foo.</div>
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<p>I would have expected to get a notification from the ObjectTracer when executing the last line, because somehow the execution machinery must resolve the method to be executed. But actually, what I get is an MNU (Object1 >> #foo).</p>
<p>For comparison: If I skip the ObjectTracer line, the example works and #foo is returned.</p>
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<span style="font-size:small"></span>The VM assumes the object in the methodDict slot of a class is a MethodDictionary. It will look for a method with the #foo selector, and if it does not find that, it will invoke #doesNotUnderstand instead.</div>
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There is only one special value the VM recognizes instead, and that is nil. If a nil methodDict is encountered, the VM will send #cannotInterpret:.</div>
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This lets the image intercept the VM's method lookup. The default handler for cannotInterpret: invokes the class's #lookupSelector: method and retries the message send if found.</div>
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This was used for example to swap out classes to disk when memory was still expensive. The class is replaced (via become:) by a stub that has a nil in the slot that normally is the method dict. So when a message is sent to an instance of a swapped-out class,
the cannotInterpret: mechanism loads the actual class from disc, replaces the stub, and then continues execution as if nothing ever happened.</div>
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I find looking at the SqueakJS sources quite instructive, since they are not obscured by all the optimizations in the OpenSmalltalk VM. Here's the relevant part, <span style="background-color:rgb(255,251,221);color:rgb(111,66,193);font-family:SFMono-Regular,Consolas,"Liberation Mono",Menlo,monospace;font-size:12px;white-space:pre">findSelectorInClass()</span> which
is invoked from <span style="color:rgb(111,66,193);font-family:SFMono-Regular,Consolas,"Liberation Mono",Menlo,monospace;font-size:12px;white-space:pre">send()</span></div>
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<a href="https://github.com/bertfreudenberg/SqueakJS/blob/13581caeb489875c4a39a78aa9d95866cc8892a5/vm.interpreter.js#L619">https://github.com/bertfreudenberg/SqueakJS/blob/13581caeb489875c4a39a78aa9d95866cc8892a5/vm.interpreter.js#L619</a><br>
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Have fun!</div>
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- Bert -</div>
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