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<p>Hi Eliot,</p>
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<p>thanks for your answer!</p>
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<p>Just to be sure: What class do you intend to lookup <span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">#doesNotUnderstand: </span>for
if the message not understood's lookupClass is ProtoObject superclass (i.e. nil)? Are you thinking of ProtoObject again? This would break the existing rule "If a message is sent to super but not understood, send #doesNotUnderstand: to super as well" (*), or
am I misinterpreting this? Shouldn't you rather signal some special selector in this case?</p>
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<p>(*) I'm not sure about this, but I think this results from the Bluebook, page 589. According to the implementation given there, the VM had to crash with a recursion error if a nil class is reached while looking up #doesNotUnderstand:. Because the Bluebook
states that the existence of a #doesNotUnderstand: implementation is guaranteed, I would also find it reasonable to have the VM crashed if you actually make a call to super from a method compiled on ProtoObject.</p>
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<p>What do you think? :-)</p>
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<p>Best,</p>
<p>Christoph</p>
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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 Eliot Miranda <eliot.miranda@gmail.com><br>
<b>Gesendet:</b> Sonntag, 6. September 2020 04:49:54<br>
<b>An:</b> The general-purpose Squeak developers list<br>
<b>Betreff:</b> Re: [squeak-dev] Two new questions about VM machinery :-)</font>
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<div dir="ltr">Hi Christophe, Hi Tobias,<br>
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<div dir="ltr" class="gmail_attr">On Sat, Sep 5, 2020 at 4:10 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, hi Eliot,</p>
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<p>just for curiosity, I have two new questions <span style="font-family:Calibri,Helvetica,sans-serif,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols;font-size:16px">about the VM machinery</span> (which
are far away from praxis but close to my interests):</p>
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<p><span style="font-size:12pt"><b>1.</b></span><span style="font-size:12pt"><b> super on ProtoObject</b></span></p>
<p><span style="font-size:12pt">What would you expect the following to return?</span></p>
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<div>x := Compiler evaluate: 'super identityHash' for: ProtoObject new.</div>
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<div>thisContext objectClass: x.</div>
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I would have guessed that either the VM crashes or ProtoObject >> #<span>cannotInterpret: is called.</span></div>
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<div>My guess before looking at the code was that the send provoked a doesNotUnderstand: and hence the expression returned the result of raising a MessageNotUnderstood exception. But looking at the code I see that at least in the JIT there is undefined behavior
in looking up a message in a nil class (the superclass of ProtoObject is nil). So thank you for this. I'll have to firm up the behaviour to ensure a doesNotUnderstand: is the result.</div>
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<div id="gmail-m_-3938398495572026058Item.MessagePartBody">But actually, it answers you the ProtoObject instance, which I find very interesting because it means that if a message cannot be looked up, it is simply and silently skipped.</div>
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<div>Which is a bug. The only reasonable thing to do here (IMO) is for the VM to send doesNotUnderstnnd:.</div>
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<div id="gmail-m_-3938398495572026058Item.MessagePartBody">If you debug the call instead (Context class >> #runSimulated:), <span>Context >> #send:to:with:lookupIn: raises a nil DNU error, which makes more sense but unfortunately is not consistent with the
original VM behavior.</span></div>
<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>Maybe the Compiler should forbid any call to super from ProtoObject instances at all (more formally: if the receiver's class's superclass is nil)? Or should we adjust the simulation code?</span></div>
<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>(By the way: If you do the same in Squeak.js, you'll get an infinite recursion :D)</span></div>
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<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span><b>2. Mirror primitives</b></span></div>
<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>What is the reason for primitives such as 75 (#identityHash) or 78/139 (#nextInstance/#nextObject) not being mirrored in Context?</span></div>
<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>Was this simply forgotten, or don't we have the claim to mirror any "essential" primitive without it actually being needed by the simulation machinery?</span></div>
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<div>They're not needed by the execution simulation machinery.</div>
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<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span style="font-size:12pt">As always, looking forward to your interesting explanations! :-)</span><span style="font-family:Arial,Helvetica,sans-serif;font-size:small;color:rgb(34,34,34)"> </span></div>
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<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>Best,</span></div>
<div id="gmail-m_-3938398495572026058Item.MessagePartBody"><span>Christoph</span></div>
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<div>Cheers </div>
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<div>_,,,^..^,,,_<br>
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<div>best, Eliot</div>
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