Hi Eliot,
Thanks for your reply,
On 24-Nov-23 12:18:17 AM, "Eliot Miranda" eliot.miranda@gmail.com wrote:
On Thu, Nov 23, 2023 at 4:37 AM Jaromir Matas mail@jaromir.net wrote:
Hi Eliot,
There seems to be a substantial difference between the two approaches when running the enclosed test:
- In my uglier solution (see down below): If you remove the `self
error` part from #cannotReturn and run the test, one assertion fails, if you remove the `pc: nil` bit the other assertion fails and the test won't crash the system.
- If you do the same with your solution the test crashes the system
when you remove the pc := nil bit and I cant't figure out why.
As far as I understand it, nilling the pc is essential to prevent the context continuing. If the pc is not nilled then the execution machinery will resume the context after the return instruction, which is wrong, even if valid bytecodes follow the pc.
Can you understand the reason? It looks like adding the `push: #whatever` before returning from #cannotReturn:to: "fixes" your solution. Is it that the VM should do the push before returning control to the image and placing the #cannotReturn context on top of the stack?
Pushing anything onto the stack makes no sense. It is nothing to do with the return instruction, nothing to do with normal execution of a cannot return error. IMO, it should be done. Given that my solution works if the pc is nilled I don't understand what you're trying to aschieve.
Well, more than anything else I'm trying to understand; in this case why pushing something makes such a difference. Never mind, I hoped it could have been something obvious for you but if not I'll keep trying :) Thanks, J
Context >> cannotReturn: result to: homeContext "The receiver tried to return result to homeContext that cannot be returned from. Capture the return pc in a BlockCannotReturn. Nil the pc to prevent repeat attempts and/or invalid continuation. Answer the result of raising the exception." | exception | exception := BlockCannotReturn new. exception result: result; deadHome: homeContext; pc: self previousPc. pc := nil. self push: nil. "<-------- this helps -------"
Maybe, but it makes no sense.
^exception signal
Best, Jaromir
Context >> cannotReturn: result
closureOrNil ifNotNil: [^ self cannotReturn: result to: self home
sender; push: pc; pc: nil]. self error: 'Computation has been terminated!'
On 21-Nov-23 6:40:32 PM, "Eliot Miranda" <eliot.miranda@gmail.com> wrote: >Hi Jaromir, > >>On Nov 20, 2023, at 11:51 PM, Jaromir Matas <mail@jaromir.net> >>wrote: >> >> >>Hi Eliot, >>Very elegant! Now I finally got what you meant exactly :) Thanks. >> >>Two questions: >>1. in order for the enclosed test to work I'd need an Error instead >>of Processor debugWithTitle:full: call in #cannotReturn:. Otherwise >>I don't know how to catch a plain invocation of the Debugger: >> >>cannotReturn: result >> >> closureOrNil ifNotNil: [^ self cannotReturn: result to: self >>home sender]. >> self error: 'Computation has been terminated!' > >Much nicer. > >>2. We are capturing a pc of self which is completely different >>context from homeContext indeed. > >Right. The return is attempted from a specific return bytecode in a >specific block. This is the coordinate of the return that cannot be >made. This is the relevant point of origin of the cannot return >exception. > >Why the return fails is another matter: >- the home context’s sender is a dead context (cannot be resumed) >- the home context’s sender is nil (home already returned from) >- the block activation’s home is nil rather than a context (should >not happen) > >But in all these cases the pc of the home context is immaterial. The >hike is being returned through/from, rather than from; the home’s pc >is not relevant. > >>Maybe we could capture self in the exception too to make it more >>clear/explicit what is going on: what context the captured pc is >>actually associated with. Just a thought... > >Yes, I like that. I also like the idea of somehow passing the block >activation’s pc to the debugger so that the relevant return >expression is highlighted in the debugger. > >> >>Thanks again, >>Jaromir > >You’re welcome. I love working in this part of the system. Thanks >for dragging me there. I’m in a slump right now and appreciate the >fellowship. > >>------ Original Message ------ >>From "Eliot Miranda" <eliot.miranda@gmail.com> >>To "Jaromir Matas" <mail@jaromir.net> >>Cc squeak-dev@lists.squeakfoundation.org >>Date 11/21/2023 2:17:21 AM >>Subject Re: Re[2]: [squeak-dev] Re: Resuming on BlockCannotReturn >>exception >> >>>Hi Jaromir, >>> >>> see Kernel-eem.1535 for what I was suggesting. This example now >>>has an exception with the right pc value in it: >>> >>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect. ex resume]] >>>fork >>> >>>The fix is simply >>> >>>Context>>cannotReturn: result to: homeContext >>> "The receiver tried to return result to homeContext that cannot >>>be returned from. >>> Capture the return pc in a BlockCannotReturn. Nil the pc to >>>prevent repeat >>> attempts and/or invalid continuation. Answer the result of >>>raising the exception." >>> >>> | exception | >>> exception := BlockCannotReturn new. >>> exception >>> result: result; >>> deadHome: homeContext; >>> pc: self previousPc. >>> pc := nil. >>> ^exception signal >>> >>> >>>The VM crash is now avoided. The debugger displays the method, but >>>does not highlight the offending pc, which is no big deal. A >>>suitable defaultHandler for B lockCannotReturn may be able to get >>>the debugger to highlight correctly on opening. Try the following >>>examples: >>> >>>[[^1] on: BlockCannotReturn do: #resume] fork. >>> >>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect. ex resume]] >>>fork >>> >>>[[^1] value] fork. >>> >>>They al; seem to behave perfectly acceptably to me. Does this fix >>>work for you? >>> >>>On Fri, Nov 17, 2023 at 3:14 PM Jaromir Matas <mail@jaromir.net> >>>wrote: >>>>Hi Eliot, >>>> >>>>How about to nil the pc just before making the return: >>>>``` >>>>Context >> #cannotReturn: result >>>> >>>> self push: self pc. "backup the pc for the sake of debugging" >>>> closureOrNil ifNotNil: [^self cannotReturn: result to: self >>>>home sender; pc: nil]. >>>> Processor debugWithTitle: 'Computation has been terminated!' >>>>translated full: false >>>>``` >>>>The nilled pc should not even potentially interfere with the >>>>#isDead now. >>>> >>>>I hope this is at least a step in the right direction :) >>>> >>>>However, there's still a problem when debugging the resumption of >>>>#cannotReturn because the encoders expect a reasonable index. I >>>>haven't figured out yet where to place a nil check - #step, >>>>#stepToSendOrReturn... ? >>>> >>>>Thanks again, >>>>Jaromir >>>> >>>> >>>>------ Original Message ------ >>>>From "Eliot Miranda" <eliot.miranda@gmail.com> >>>>To "Jaromir Matas" <mail@jaromir.net> >>>>Date 11/17/2023 8:36:50 PM >>>>Subject Re: [squeak-dev] Re: Resuming on BlockCannotReturn >>>>exception >>>> >>>>>Hi Jaromir, >>>>> >>>>>>On Nov 17, 2023, at 7:05 AM, Jaromir Matas <mail@jaromir.net> >>>>>>wrote: >>>>>> >>>>>> >>>>>>Eliot, hi again, >>>>>> >>>>>>Please disregard my previous comment about nilling the contexts >>>>>>that have returned. We are indeed talking about the context >>>>>>directly under the #cannotReturn context which is totally >>>>>>different from the home context in another thread that's gone. >>>>>> >>>>>>I may still be confused but would nilling the pc of the context >>>>>>directly under the cannotReturn context help? Here's what I >>>>>>mean: >>>>>>``` >>>>>>Context >> #cannotReturn: result >>>>>> >>>>>> closureOrNil ifNotNil: [^self pc: nil; cannotReturn: result >>>>>>to: self home sender]. >>>>>> Processor debugWithTitle: 'Computation has been terminated!' >>>>>>translated full: false. >>>>>>``` >>>>>>Instead of crashing the VM invokes the debugger with the >>>>>>'Computation has been terminated!' message. >>>>>> >>>>>>Does this make sense? >>>>> >>>>>Nearly. But it loses the information on what the pc actually is, >>>>>and that’s potentially vital information. So IMO the ox should >>>>>only be nilled between the BlockCannotReturn exception being >>>>>created and raised. >>>>> >>>>>[But if you try this don’t be surprised if it causes a few >>>>>temporary problems. It looks to me that without a little >>>>>refactoring this could easily cause an infinite recursion around >>>>>the sending of isDead. I’m sure you’ll be able to fix the code >>>>>to work correctly] >>>>> >>>>>>Thanks, >>>>>>Jaromir >>>>>> >>>>>> >>>>>>------ Original Message ------ >>>>>>From "Jaromir Matas" <mail@jaromir.net> >>>>>>To "Eliot Miranda" <eliot.miranda@gmail.com>; "The >>>>>>general-purpose Squeak developers list" >>>>>><squeak-dev@lists.squeakfoundation.org> >>>>>>Date 11/17/2023 10:15:17 AM >>>>>>Subject [squeak-dev] Re: Resuming on BlockCannotReturn exception >>>>>> >>>>>>>Hi Eliot, >>>>>>> >>>>>>> >>>>>>> >>>>>>>------ Original Message ------ >>>>>>>From "Eliot Miranda" <eliot.miranda@gmail.com> >>>>>>>To "Jaromir Matas" <mail@jaromir.net> >>>>>>>Cc "The general-purpose Squeak developers list" >>>>>>><squeak-dev@lists.squeakfoundation.org> >>>>>>>Date 11/16/2023 11:52:45 PM >>>>>>>Subject Re: Re[2]: [squeak-dev] Re: Resuming on >>>>>>>BlockCannotReturn exception >>>>>>> >>>>>>>>Hi Jaromir, >>>>>>>> >>>>>>>>On Thu, Nov 16, 2023 at 2:22 PM Jaromir Matas >>>>>>>><mail@jaromir.net> wrote: >>>>>>>>>Hi Nicolas, Eliot, >>>>>>>>> >>>>>>>>>here's what I understand is happening (see the enclosed >>>>>>>>>screenshot): >>>>>>>>> >>>>>>>>>1) we fork a new process to evaluate [^1] >>>>>>>>>2) the new process evaluates [^1] which means instruction 18 >>>>>>>>>is being evaluated, hence pc points to instruction 19 now >>>>>>>>>3) however, the home context where ^1 should return to is >>>>>>>>>gone by this time (the process that executed the fork has >>>>>>>>>already returned - notice the two up arrows in the debugger >>>>>>>>>screenshot) >>>>>>>>>4) the VM can't finish the instruction and returns control to >>>>>>>>>the image via placing the #cannotReturn: context on top of >>>>>>>>>the [^1] context >>>>>>>>>5) #cannotReturn: evaluation results in signalling the BCR >>>>>>>>>exception which is then handled by the #resume handler >>>>>>>>> (in our debugged case the [:ex | self halt. ex resume] >>>>>>>>>handler) >>>>>>>>>6) ex resume is evaluated, however, this means requesting the >>>>>>>>>VM to evaluate instruction 19 of the [^1] context - which is >>>>>>>>>past the last instruction of the context and the crash ensues >>>>>>>>> >>>>>>>>>I wonder whether such situations could/should be prevented >>>>>>>>>inside the VM or whether such an expectation is wrong for >>>>>>>>>some reason. >>>>>>>> >>>>>>>>As Nicolas says, IMO this is best done at the image level. >>>>>>>> >>>>>>>>It could be prevented in the VM, but at great cost, and only >>>>>>>>partially. The performance issue is that the last bytecode in >>>>>>>>a method is not marked in any way, and that to determine the >>>>>>>>last bytecode the bytecodes must be symbolically evaluated >>>>>>>>from the start of the method. See implementors of endPC at >>>>>>>>the image level (which defer to the method trailer) and >>>>>>>>implementors of endPCOf: in the VMMaker code. Doing this every >>>>>>>>time execution commences is prohibitively expensive. The >>>>>>>>"only partially" issue is that following the return >>>>>>>>instruction may be other valid bytecodes, but these are not a >>>>>>>>continuation. >>>>>>>> >>>>>>>> >>>>>>>>Consider the following code in some block: >>>>>>>> [self expression ifTrue: >>>>>>>> [^1]. >>>>>>>> ^2 >>>>>>>> >>>>>>>>The bytecodes for this are >>>>>>>> pushReceiver >>>>>>>> send #expression >>>>>>>> jumpFalse L1 >>>>>>>> push 1 >>>>>>>> methodReturnTop >>>>>>>>L1 >>>>>>>> push 2 >>>>>>>> methodReturnTop >>>>>>>> >>>>>>>>Clearly if expression is true these should be *no* >>>>>>>>continuation in which ^2 is executed. >>>>>>> >>>>>>>Well, in that case there's a bug because the computation in the >>>>>>>following example shouldn't continue past the [^1] block but it >>>>>>>silently does: >>>>>>>`[[true ifTrue: [^ 1]] on: BlockCannotReturn do: #resume ] >>>>>>>fork` >>>>>>> >>>>>>>The bytecodes are >>>>>>> push true >>>>>>> jumpFalse L1 >>>>>>> push 1 >>>>>>> returnTop >>>>>>>L1 >>>>>>> push nil >>>>>>> blockReturn >>>>>>> >>>>>>> >>>>>>> >>>>>>>> >>>>>>>>So even if the VM did try and detect whether the return was at >>>>>>>>the last block method, it would only work for special cases. >>>>>>>> >>>>>>>> >>>>>>>>It seems to me the issue is simply that the context that >>>>>>>>cannot be returned from should be marked as dead (see >>>>>>>>Context>>isDead) by setting its pc to nil at some point, >>>>>>>>presumably after copying the actual return pc into the >>>>>>>>BlockCannotReturn exception, to avoid ever trying to resume >>>>>>>>the context. >>>>>>> >>>>>>>Does this mean, in other words, that every context that returns >>>>>>>should nil its pc to avoid being "wrongly" reused/executed in >>>>>>>the future, which concerns primarily those being referenced >>>>>>>somewhere hence potentially executable in the future, is that >>>>>>>right? >>>>>>>Hypothetical question: would nilling the pc during returns >>>>>>>"fix" the example? >>>>>>>Thanks a lot for helping me understand this. >>>>>>>Best, >>>>>>>Jaromir >>>>>>> >>>>>>> >>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> >>>>>>>>>Thanks, >>>>>>>>>Jaromir >>>>>>>>> >>>>>>>>><bdxuqalu.png> >>>>>>>>> >>>>>>>>>------ Original Message ------ >>>>>>>>>From "Eliot Miranda" <eliot.miranda@gmail.com> >>>>>>>>>To "Jaromir Matas" <mail@jaromir.net>; "The general-purpose >>>>>>>>>Squeak developers list" >>>>>>>>><squeak-dev@lists.squeakfoundation.org> >>>>>>>>>Date 11/16/2023 6:48:43 PM >>>>>>>>>Subject Re: [squeak-dev] Re: Resuming on BlockCannotReturn >>>>>>>>>exception >>>>>>>>> >>>>>>>>>>Hi Jaromir, >>>>>>>>>> >>>>>>>>>>>On Nov 16, 2023, at 3:23 AM, Jaromir Matas >>>>>>>>>>><mail@jaromir.net> wrote: >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>Hi Nicloas, >>>>>>>>>>>No no, I don't have any practical scenario in mind, I'm >>>>>>>>>>>just trying to understand why the VM is implemented like >>>>>>>>>>>this, whether there were a reason to leave this possibility >>>>>>>>>>>of a crash, e.g. it would slow down the VM to try to >>>>>>>>>>>prevent such a dumb situation (who would resume from BCR in >>>>>>>>>>>his right mind? :) ) - or perhaps I have overlooked some >>>>>>>>>>>good reason to even keep this behavior in the VM. That's >>>>>>>>>>>all. >>>>>>>>>> >>>>>>>>>>Let’s first understand what’s really happening. Presumably >>>>>>>>>>at tone point a context is resumed those pc is already at >>>>>>>>>>the block return bytecode (effectively, because it crashes >>>>>>>>>>in JITted code, but I bet the stack vm will crash also, but >>>>>>>>>>not as cleanly - it will try and execute the bytes in the >>>>>>>>>>encoded method trailer). So which method actually sends >>>>>>>>>>resume, and to what, and what state is resume’s receiver >>>>>>>>>>when resume is sent? >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>Thanks for your reply. >>>>>>>>>>>Regards, >>>>>>>>>>>Jaromir >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>------ Original Message ------ >>>>>>>>>>>From "Nicolas Cellier" <nicolas.cellier.aka.nice@gmail.com> >>>>>>>>>>>To "Jaromir Matas" <mail@jaromir.net>; "The general-purpose >>>>>>>>>>>Squeak developers list" >>>>>>>>>>><squeak-dev@lists.squeakfoundation.org> >>>>>>>>>>>Date 11/16/2023 7:20:20 AM >>>>>>>>>>>Subject Re: [squeak-dev] Resuming on BlockCannotReturn >>>>>>>>>>>exception >>>>>>>>>>> >>>>>>>>>>>>Hi Jaromir, >>>>>>>>>>>>Is there a scenario where it would make sense to resume a >>>>>>>>>>>>BlockCannotReturn? >>>>>>>>>>>>If not, I would suggest to protect at image side and >>>>>>>>>>>>override #resume. >>>>>>>>>>>> >>>>>>>>>>>>Le mer. 15 nov. 2023, 23:42, Jaromir Matas >>>>>>>>>>>><mail@jaromir.net> a écrit : >>>>>>>>>>>>>Hi Eliot, Christoph, All, >>>>>>>>>>>>> >>>>>>>>>>>>>It's known the following example crashes the VM. Is this >>>>>>>>>>>>>an intended behavior or a "tolerated bug"? >>>>>>>>>>>>> >>>>>>>>>>>>>`[[^ 1] on: BlockCannotReturn do: #resume] fork` >>>>>>>>>>>>> >>>>>>>>>>>>>I understand why it crashes: the non-local return has >>>>>>>>>>>>>nowhere to return to and so resuming the computation >>>>>>>>>>>>>leads to a crash. But why not raise another BCR exception >>>>>>>>>>>>>to prevent the crash? Potential infinite loop? Perhaps >>>>>>>>>>>>>I'm just missing the purpose of this behavior... >>>>>>>>>>>>> >>>>>>>>>>>>>Thanks for an explanation. >>>>>>>>>>>>> >>>>>>>>>>>>>Best, >>>>>>>>>>>>>Jaromir >>>>>>>>>>>>> >>>>>>>>>>>>>-- >>>>>>>>>>>>> >>>>>>>>>>>>>Jaromir Matas >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>-- >>>>>>>>_,,,^..^,,,_ >>>>>>>>best, Eliot >>>>>><Context-cannotReturn.st> >>> >>> >>>-- >>>_,,,^..^,,,_ >>>best, Eliot >><ProcessTest-testResumeAfterBCR.st>
-- _,,,^..^,,,_ best, Eliot