Hi Christoph,
thanks again for these examples. I really look forward to trying to crack them :)
In the meantime, I wonder if you could help me cleanup my recent Inbox changesets:
If you're happy with Kernel-jar.1555, Kernel-jar.1554 and Kernel-jar.1553 they can be merged. These are related to the problems you observed in the Simulation Studio. All of them are IMO simple bugs that need fixing.
This one: Kernel-jar.1550 is a bugfix too - if you're ok with it, it's ready for merging.
Kernel-jar.1552, Kernel-jar.1545, and Tools-jar.1240 can be moved to Treated.
As for Kernel-jar.1551, I have a better version I'm looking forward to showing you :)
Sorry for throwing all this at you; many thanks for helping me to clean this up.
Best regards, Jaromir
On 24-Feb-24 10:38:37 PM, christoph.thiede@student.hpi.uni-potsdam.de wrote:
Hi Jaromir, all,
just found another oddity with debugging non-local returns:
Debug it in a workspace, then select the second method from the stack in the debugger (CompiledMethod>>valueWithReceiver:arguments:) and press Through:
[sender:=thisContextswapSender:nil. ^1]value.
Expected: A BlockCannotReturn error Actual: The method has returned 1!
It seems that the VM "checks" the validity of the entire stack up to the sender-to-return-to while the simulator essentially just uses "self home sender". I wonder what's the best way to fix this. Insert something like this at the beginning of Context>>#return:from:?
newSender:=selffindContextSuchThat:[:ea|ea==aSender].
Or would this break sideways returns for any scenario I currently don't see? I have to confess I do not know whether and when we support them at all ...
Best, Christoph
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On 2024-02-01T17:27:21+00:00, mail@jaromir.net wrote:
Hi Christoph,
I still owe you an explanation of the mechanics of the bug (detailed description for future reference - especially for me):
On 13-Jan-24 9:52:19 PM,
christoph.thiede(a)student.hpi.uni-potsdam.de
wrote:
Hi Jaromir,
do you have any idea why the former behavior would also stop when
the
context activates a new method?
The key to understanding the issue with #runUntilErrorOrReturnFrom:
in
this particular example is that before returning, when stepping until the guard contexts inserted by #runUntilErrorOrReturnFrom: are gone,
the
stepping finalizes the execution of #resumeEvaluating:through: and it finally terminates all contexts including the guard context inserted
by
#runUntilErrorOrReturnFrom: which will satisfiy the condition `ctxt isDead` at the end of #runUntilErrorOrReturnFrom: BUT #resumeEvaluating:through: still has to execute `aBlock value` which will become the intermediate point where contexts switch (stack top context changes), hence #stepToCalleeOrNil returns and the above mentioned condition is checked - resulting in the observed premature return from #runUntilErrorOrReturnFrom:. If we replace #stepToCalleeOrNil with #stepToSenderOrNil the stepping
in
#runUntilErrorOrReturnFrom: will only stop when the stack goes down which is exactly was was intended. (i.e. the bug manifests in
#stepOver
but it's a general deficiency in #runUntilErrorOrReturnFrom:)
Otherwise, I agree that seeing that #resume:through: context in the debugger is probably not required in this situation.
Best, Christoph
PS: Here's another bug if you haven't it on your radar already: In
the
same expression ([^2] ensure: []), step through, through, over so
you
end in FullBlockClosure(BlockClosure)>>ensure:. Step over, over,
over,
over to move beyond aBlock value. At the last step, you will get another BCR in a second debugger. :-)
Yeah, a nice one. I've already wondered why... I'll investigate.
Thanks
for the push :)
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On 2024-01-09T19:46:15+00:00, mail(a)jaromir.net wrote:
Hi Christoph, all
I've just sent a minor fix to the Inbox I missed previously - Kernel-jar.1550.
if you debug and do step through to ^2 and then step over in [^2] ensure: [] the debugger incorrectly stops at #resume:through:.
This is an old issue predating any of my changes. I'm sending the
fix
as
part of this thread because it's closely related (but
independent).
I think it can be safely merged.
Thanks, Jaromir
On 30-Dec-23 6:15:25 PM,
christoph.thiede(a)student.hpi.uni-potsdam.de
wrote:
Hi Jaromir, hi all,
finally I have found the time to review these suggestions. Kernel-jar.1537, Kernel-jar.1538, and Kernel-jar.1539 look
excellent
to
me as well. Clear, straightforward, useful. :-) I have merged
them
into
the trunk via Kernel-ct.1545.
Regarding DebuggerTests>>test16HandleSimulationError, I have
patched
it
via ToolsTests-ct.125. Nothing to rack your brains over:
"thisContext
pc: nil" just mimicks any kind of unhandled error inside the
simulator
- since we now gently handle this via #cannotReturn:, I just
replaced
it with "thisContext pc: false". :-) Sorry for not clarifying
that
earlier and letting you speculate.
Thanks for your work, and I already wish you a happy new year!
Best, Christoph
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On 2023-11-29T13:31:09+00:00, mail(a)jaromir.net wrote:
Hi Marcel,
> [myself] whether the patch would have been necessary should
the
#return:from: had been fixed then
Nonsense, I just mixed it up with another issue :)
On 29-Nov-23 1:51:21 PM, "Jaromir Matas" <mail(a)jaromir.net>
wrote:
>Thanks Marcel! This test somehow slipped my attention :) > >The test can no longer work as is. It takes advantage of the
erroneous
>behavior of #return:from: in the sense that if you simulate > > thisContext pc: nil > >it'll happily return to a dead context (i.e. to thisContext
from
#pc:
>nil context) - which is not what the VM does during runtime.
It
should
>immediately raise an illegal return exception not only
during
runtime
>but also during simulation. > >The test mentions a patch for an infinite debugger chain >(http://forum.world.st/I-broke-the-debugger-td5110752.html).
I
wonder
>whether the problem could have something to do with this
simulation
bug
>in return:from:; and a terrible idea occurred to me whether
the
patch
>would have been necessary should the #return:from: had been
fixed
then
>;O > >We may potentially come up with more examples like this,
even in
the
>trunk, where the bug from #return:from: propagated and was
taken
>advantage of. I've found and fixed
#runUntilErrorOrReturnFrom:
but
more
>can still be surviving undetected... > >I'd place the test into #expectedFailures for now but maybe
it's
time
>to remove it; Christoph should decide :) > >Thanks again, >Jaromir > > >On 29-Nov-23 10:28:38 AM, "Taeumel, Marcel via Squeak-dev" ><squeak-dev(a)lists.squeakfoundation.org> wrote: > >>Hi Jaromir -- >> >>Looks good. Still, what about that
#test16HandleSimulationError
now?
>>:-) It is failing with your changes ... how would you adapt
it?
>> >> >> >>Best, >>Marcel >>>Am 28.11.2023 01:29:39 schrieb Jaromir Matas
<mail(a)jaromir.net>:
>>> >>>Hi Eliot, Marcel, all, >>> >>>I've sent a fix Kernel-jar.1539 to the Inbox that solves
the
>>>remaining bit of the chain of bugs described in the
previous
post.
>>>All tests are green now and I think the root cause has
been
found
and
>>>fixed. >>> >>>In this last bit I've created a version of stepToCallee
that
would
>>>identify a potential illegal return to a nil sender and
avoid
it.
>>> >>>Now this example can be debugged without any problems: >>> >>>[[self halt. ^ 1] on: BlockCannotReturn do: #resume ] fork >>> >>>If you're happy with the solution in Kernel-jar.1539, >>>Kernel-jar.1538, Kernel-jar.1537 and the test in
KernelTests-jar.447,
>>>could you please double-check and merge, please? (And
remove
>>>Kernel-mt.1534 and Tools-jar.1240 from the Inbox) >>> >>>Best, >>>Jaromir >>> >>> >>> >>>On 27-Nov-23 12:09:37 AM, "Jaromir Matas"
<mail(a)jaromir.net>
wrote:
>>> >>>>Hi Eliot, Christoph, all >>>> >>>>It looks like there are some more skeletons in the closet
:/
>>>> >>>>If you run this example >>>> >>>>[[self halt. ^ 1] on: BlockCannotReturn do: [:ex | ex
resume]
]
fork
>>>> >>>>and step over halt and then step over ^1 you get a
nonsensical
error
>>>>as a result of decoding nil as an instruction. >>>> >>>>It turns out that the root cause is in the #return:from:
method:
it
>>>>only checks whether aSender is dead but ignores the
possibility
that
>>>>aSender sender may be nil or dead in which cases the VM
also
>>>>responds with sending #cannotReturn, hence I assume the
simulator
>>>>should do the same. In addition, the VM nills the pc in
such
>>>>scenario, so I added the same functionality here too: >>>> >>>>Context >> return: value from: aSender >>>> "For simulation. Roll back self to aSender and return
value
>>>>from it. Execute any unwind blocks on the way. ASSUMES
aSender is
>>>>a sender of self" >>>> >>>> | newTop | >>>> newTop := aSender sender. >>>> (aSender isDead or: [newTop isNil or: [newTop isDead]])
ifTrue:
>>>> "<--------- this is extended ------" >>>> [^self pc: nil; send: #cannotReturn: to: self with: >>>>{value}]. "<------ pc: nil is added ----" >>>> (self findNextUnwindContextUpTo: newTop) ifNotNil: >>>> "Send #aboutToReturn:through: with nil as the second >>>>argument to avoid this bug: >>>> Cannot #stepOver '^2' in example '[^2] ensure: []'. >>>> See
http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html
http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html"
>>>> [^self send: #aboutToReturn:through: to: self with:
{value.
>>>>nil}]. >>>> self releaseTo: newTop. >>>> newTop ifNotNil: [newTop push: value]. >>>> ^newTop >>>> >>>>In order for this to work #cannotReturn: has to be
modified
as in
>>>>Kernel-jar.1537: >>>> >>>>Context >> cannotReturn: result >>>> >>>> closureOrNil ifNotNil: [^ self cannotReturn: result to:
self
>>>>home sender]. >>>> self error: 'Computation has been terminated!' >>>>"<----------- this has to be an Error -----" >>>> >>>>Then it almost works except when you keep stepping over
in
the
>>>>example above, you get an MNU error on `self previousPc`
in
>>>>#cannotReturn:to:` with your solution of the VM crash. If
you
don't
>>>>mind I've amended your solution and added the final
context
where
>>>>the computation couldn't return along with the pc: >>>> >>>>Context >> cannotReturn: result to: homeContext >>>> "The receiver tried to return result to homeContext that
cannot
>>>>be returned from. >>>> Capture the return context/pc in a BlockCannotReturn.
Nil
the pc
>>>>to prevent repeat >>>> attempts and/or invalid continuation. Answer the result
of
>>>>raising the exception." >>>> >>>> | exception previousPc | >>>> exception := BlockCannotReturn new. >>>> previousPc := pc ifNotNil: [self previousPc]. "<-----
here's
a
>>>>fix ----" >>>> exception >>>> result: result; >>>> deadHome: homeContext; >>>> finalContext: self; "<----- here's the new state, if >>>>that's fine ----" >>>> pc: previousPc. >>>> pc := nil. >>>> ^exception signal >>>> >>>>Unfortunately, this is still not the end of the story:
there
are
>>>>situations where #runUntilErrorOrReturnFrom: places the
two
guard
>>>>contexts below the bottom context. And that is a problem
because
>>>>when the method tries to remove the two guard contexts
before
>>>>returning at the end it uses #stepToCalee to do the job
but
this
>>>>unforotunately was (ab)using the above bug in
#return:from: -
I'll
>>>>explain: #return:from: didn't check whether aSender
sender
was
nil
>>>>and as a result it allowed to simulate a return to a "nil
context"
>>>>which was then (ab)used in the clean-up via #stepToCalee
in
the
>>>>#runUntilErrorOrReturnFrom:. >>>> >>>>When I fixed the #return:from: bug, the
#runUntilErrorOrReturnFrom:
>>>>cleanup of the guard contexts no longer works in that
very
special
>>>>case where the guard contexts are below the bottom
context.
There's
>>>>one case where this is being used: #terminateAggresively
by
>>>>Christoph. >>>> >>>>If I'm right with this analysis, the
#runUntilErrorOrReturnFrom:
>>>>should get fixed too but I'll be away now for a few days
and
I
won't
>>>>be able to respond. If you or Christoph had a chance to
take
a
look
>>>>at Kernel-jar.1538 and Kernel-jar.1537 I'd be very
grateful.
I
hope
>>>>this super long message at least makes some sense :) >>>>Best, >>>>Jaromir >>>> >>>>[1] Kernel-jar.1538, Kernel-jar.1537 >>>>[2] KernelTests-jar.447 >>>> >>>> >>>>PS: Christoph, >>>> >>>>With Kernel-jar.1538 + Kernel-jar.1537 your example >>>> >>>>process := >>>> [(c := thisContext) pc: nil. >>>> 2+3] newProcess. >>>>process runUntil: [:ctx | ctx selector = #cannotReturn:]. >>>>self assert: process suspendedContext sender sender = c. >>>>self assert: process suspendedContext arguments = {c}. >>>> >>>>works fine, I've just corrected your first assert. >>>> >>>> >>>> >>>> >>>> >>>>On 21-Nov-23 6:40:32 PM, "Eliot Miranda"
<eliot.miranda(a)gmail.com>
>>>>wrote: >>>> >>>>>Hi Jaromir, >>>>> >>>>>>On Nov 20, 2023, at 11:51 PM, Jaromir Matas
<mail(a)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(a)gmail.com> >>>>>>To "Jaromir Matas" <mail(a)jaromir.net> >>>>>>Cc squeak-dev(a)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(a)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(a)gmail.com> >>>>>>>>To "Jaromir Matas" <mail(a)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(a)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(a)jaromir.net> >>>>>>>>>>To "Eliot Miranda" <eliot.miranda(a)gmail.com>;
"The
>>>>>>>>>>general-purpose Squeak developers list" >>>>>>>>>><squeak-dev(a)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(a)gmail.com> >>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net> >>>>>>>>>>>Cc "The general-purpose Squeak developers list" >>>>>>>>>>><squeak-dev(a)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(a)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(a)gmail.com> >>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The
general-purpose
>>>>>>>>>>>>>Squeak developers list" >>>>>>>>>>>>><squeak-dev(a)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(a)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(a)gmail.com> >>>>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The >>>>>>>>>>>>>>>general-purpose Squeak developers list" >>>>>>>>>>>>>>><squeak-dev(a)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(a)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>