Hi Lars,<br><br><div class="gmail_quote">On Fri, May 17, 2013 at 9:36 AM, Lars Wassermann <span dir="ltr"><<a href="mailto:lars.wassermann@googlemail.com" target="_blank">lars.wassermann@googlemail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <br>Hello VM-dev,<br>
I habe problems understanding what exactly happens when I step through:<br>
(ContextPart contextOn: Error do: []) halt<br>
in a workspace.<br>
When I step over the parenthesis in the debugger, everything works kind of like expected (the context of contextOn:do: misses one temporary but the returned value is what the methods description promised). When I step into or through the code, an error is raised when executing bytecode 54 of #contextOn:do:, because the temps-array illegally has been poped (by bytecode 53).<br>
<br>
My question now is:<br>
Why does the normal vm not only break when runing this code, but also return the correct value (the first element of the last temporary).<br>
I appended a graphic trying to visualize what happens up to the miracle of three skipped bytecodes.<br></blockquote><div><br></div><div>Ugh. I hate this method. Actually I hate ContextPart>>jump, which is the really evil method.</div>
<div><br></div><div>So let's take a look at the methods:</div><div><br></div><div>ContextPasrt class>>contextOn: exceptionClass do: block</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"Create an #on:do: context that is ready to return from executing its receiver"</div>
<div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>| ctxt chain |</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>ctxt := thisContext.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>[chain := thisContext sender cut: ctxt. ctxt jump]</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>on: exceptionClass</div><div><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>do: block.</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>"jump above will resume here without unwinding chain"</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>^ chain</div><div><br>
</div><div>answers a context that is an activation of on:do: ready to catch the supplied exception. It is used by ContextPart>>runUntilErrorOrReturnFrom: to insert an exception handler for UnhandledError immediately beneath the current context being executed by the debugger.</div>
<div><br></div><div>The on:do: argument is a little unclear. It is easier to understand if it reads</div><div><br></div><div><div>ContextPasrt class>>contextOn: exceptionClass do: block</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"Create an #on:do: context that is ready to return from executing its receiver"</div>
<div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>| ctxt onDoContext |</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>ctxt := thisContext.</div><div><span class="Apple-style-span"><span class="Apple-tab-span" style="white-space:pre"> </span>[</span>onDoContext<span class="Apple-style-span"> := thisContext sender.</span></div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span> onDoContext cut: ctxt.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span> ctxt jump]</div><div><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>on: exceptionClass</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>do: block.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"jump above will resume here without unwinding chain"</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>^ onDoContext</div></div><div><br></div><div>So the idea is to run until in the on:do:'s receiver (the [onDoContext := ...] block), grab the sender (the on:do: activation), remove intervening contexts (onDoContext cut: ctxt) so that on:do:'s handlerActive doesn't get set, resume executing in the contextOn:do: context (ctxt jump) and answer the onDoContext.</div>
<div><br></div><div>The horror is in ContextPart>>jump:</div><div><br></div><div><div>ContextPart>>jump</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"Abandon thisContext and resume self instead (using the same current process). You may want to save thisContext's sender before calling this so you can jump back to it.</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>Self MUST BE a top context (ie. a suspended context or a abandoned context that was jumped out of). A top context already has its return value on its stack (see Interpreter>>primitiveSuspend and other suspending primitives).</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>thisContext's sender is converted to a top context (by pushing a nil return value on its stack) so it can be jump back to."</div><div><br></div><div>
<span class="Apple-tab-span" style="white-space:pre"> </span>| top |</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"Make abandoned context a top context (has return value (nil)) so it can be jumped back to"</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>thisContext sender push: nil.</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>"Pop self return value then return it to self (since we jump to self by returning to it)"</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>stackp = 0 ifTrue: [self stepToSendOrReturn].</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>stackp = 0 ifTrue: [self push: nil]. "must be quick return self/constant"</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>top := self pop.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>thisContext privSender: self.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>^ top</div>
</div><div><br></div><div>This works by returning to self (a MethodContext), but return pushes a result. So the code pops the top thing off the stack and returns it. This is evil. If the stack is empty then this will pop the receiver, return it and push it back in the return (!!). If the stack contains only temps then the last temp is popped and pushed by the return (!!). This is what you're seeing. The stack's last temp is an indirection vector. It gets popped off the stack by</div>
<span class="Apple-tab-span" style="white-space:pre"> </span>top := self pop.</div><div class="gmail_quote">at which point any attempt to look at self's temps in a debugger will cause an error because the indirection vector is no longer there. It then gets pushed back by=</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>^ top</div><div class="gmail_quote"><br></div><div class="gmail_quote">Horrible. It all depends on being able to create a temporary invalid execution state in a context.</div>
<div class="gmail_quote"><br></div><div class="gmail_quote">A better way to do this is by process switch, avoiding the return. For example, in my basic block profiler I handle an unknownBytecode error caused by executing the unknownBytecode that overwrites the bytecode at the beginning of each basic block, replaces the unknownBytecode with the correct bytecode, and continues:</div>
<div class="gmail_quote"><br></div><div class="gmail_quote"><div class="gmail_quote">ContextPart>>unusedBytecode</div><div class="gmail_quote"><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>"Handle unusedBytecode by replacing the bytecode with the</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> correct one found in the coverage property and continuing.</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> Continue via wait/signal since return would push a result."</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>| coverage semaphore process |</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>self assert: (method at: pc) = method encoderClass unusedBytecode.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>coverage := method propertyValueAt: #coverage.</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>self assert: coverage notNil.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>self assert: (coverage includesKey: pc).</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>semaphore := Semaphore new.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>process := Processor activeProcess.</div><div class="gmail_quote"><br></div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>[method</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>at: pc</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>put: (coverage removeKey: pc).</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> process suspendedContext unwindTo: self.</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> process suspendedContext: self.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> semaphore signal] fork.</div><div class="gmail_quote"><br></div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>semaphore wait</div>
</div><div class="gmail_quote"><br></div><div class="gmail_quote">So that could become something like</div><div class="gmail_quote"><br></div><div class="gmail_quote"><div class="gmail_quote">jump</div><div class="gmail_quote">
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>"Abandon thisContext and resume self instead (using the same current process). You may want to save thisContext's sender before calling this so you can jump back to it.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>Self MUST BE a top context (ie. a suspended context or a abandoned context that was jumped out of). A top context already has its return value on its stack (see Interpreter>>primitiveSuspend and other suspending primitives)."</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>| semaphore process |</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>semaphore := Semaphore new.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>process := Processor activeProcess.</div><div class="gmail_quote"><br></div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>[process suspendedContext unwindTo: self.</div>
<div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> process suspendedContext: self.</div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span> semaphore signal] fork.</div>
<div class="gmail_quote"><br></div><div class="gmail_quote"><span class="Apple-tab-span" style="white-space:pre"> </span>semaphore wait</div><div><br></div><div>but perhaps the process switch will introduce other problems. I don't know.</div>
<div><br></div><div>Hope this helps.</div><div><br></div></div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
I replicated the behavior in Squeak 4.0 and 4.4 images on interpreter VMs 3.7.7, 4.10, and a newer cog VM.<br>
<br>
<br>
A fix to the problem of stepping and comprehension as well as the missing temp in the frame would be changing the method to:<br>
<br>
contextOn: exceptionClass do: block<br>
"Create an #on:do: context that is ready to return from executing its receiver"<br>
<br>
| ctxt chain |<br>
ctxt := thisContext.<br>
[chain := thisContext sender cut: ctxt.<br>
ctxt push: nil. ctxt jump] on: exceptionClass do: block.<br>
"jump above will resume here without unwinding chain"<br>
^ chain<br>
<br>
The difference is that after cutting off ctxt, we ensure that it is a top context by pushing a meaningless value. But this still doesn't help me understanding the original behaviour.<br>
<br>
Thank you and all the best<br>
Lars<br>
<br></blockquote></div><br><br clear="all"><div><br></div>-- <br>best,<div>Eliot</div>