[Vm-dev] [Pharo-dev] eventual crashes pharo vm

henry henry at callistohouse.club
Thu Aug 17 04:04:21 UTC 2017

To let you know what I tried, trying to learn about this system as I dig into it, I added isInteger to the #blockedSelectors of NearERef. This crashed the squeak vm when I ran the test, and as I look into how blockedSelectors is used it smashes the resolver. What seems to be needed is a list of immediateSelectors, though that may grow to be a sizeable list. This causes me to think there must be a different approach between async and sync code.

- HH

> -------- Original Message --------
> Subject: Re: [Vm-dev] [Pharo-dev] eventual crashes pharo vm
> Local Time: August 16, 2017 11:46 PM
> UTC Time: August 17, 2017 3:46 AM
> From: henry at callistohouse.club
> To: Eliot Miranda <eliot.miranda at gmail.com>
> Squeak Virtual Machine Development Discussion <vm-dev at lists.squeakfoundation.org>, Pharo Development List <pharo-dev at lists.pharo.org>, Marcus Denker <marcus.denker at inria.fr>
> Hi Eliot,
> I have disabled that test for the time being. It will require some deeper thought regarding immediate selectors, I think, and I am deep into another area right now. As mustBeBoolean is from inside the VM, a different approach may be the right solution. A part of me thinks autocoercion between msg sending (async) and msg calling (sync) is what is needed, but I want continuation-based VatSemaphores to prevent a liveness lock on the event loop to support. Again, I have not thought deep enough in this area.
> Thanks again for your help,
> - HH
>> -------- Original Message --------
>> Subject: Re: [Pharo-dev] eventual crashes pharo vm
>> Local Time: August 16, 2017 10:32 PM
>> UTC Time: August 17, 2017 2:32 AM
>> From: eliot.miranda at gmail.com
>> To: henry <henry at callistohouse.club>
>> Marcus Denker <marcus.denker at inria.fr>, Pharo Development List <pharo-dev at lists.pharo.org>, Squeak Virtual Machine Development Discussion <vm-dev at lists.squeakfoundation.org>
>> Hi Henry,
>> On Wed, Aug 16, 2017 at 6:33 PM, henry <henry at callistohouse.club> wrote:
>>> That is good news, that it is due to this code doing funniness than a VM issue. This code trying to bring asynchrony within a synchronous environment brings new issues.
>>> What do you think that right solution is to the issue of a call expected to be immediate, change out to go eventual until the arguments resolve?
>> I'm not informed enough to know.  One could implement mustBeBoolean in the ERef hierarchy and resolve the promise before going on.  One could rely on the mustBeBooleanMagic: if one wanted a fully lazy system.  I don't know the trade-offs between the two.  I do know that while the miustBeBooleanMagic: solution is cool and fun it is extremely slow.  So if performance is an issue use the first approach.
>>> How can it be structured correctly on the stack without generic functions? I think with the double dispatch of an eventual but I have not spend much time in this particular area.
>> Yes that's an issue  There is already a problem with #==.  It needs to be symmetric for correctness.
>>> Preventing the vm from crashing would be a good interim step but even here I am not sure how to go about crafting a solution.
>> Well, the issue is simply that the wrong pc is chosen for the continuation after the mustBeBoolean.  I'm sure the right answer is straight-forward to obtain.
>>> Thank you for investigating this.
>> You're welcome.
>>> - HH
>>> On Wed, Aug 16, 2017 at 20:46, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>>>> Hi Henry, Hi Marcus,
>>>> On Sun, Aug 13, 2017 at 5:08 AM, henry <henry at callistohouse.club> wrote:
>>>>> Hi all. I was testing with this eventual_test package and it blows up the pharo 6.1 vm. I'd welcome pointers
>>>>> http://www.squeaksource.com/TurquoiseTesting.html
>>>>> - HH
>>>> I took a look at this and I think you've found a bug in the mustBeBooleanMagic: code.  What's happening is a mustBeBoolean in Integer>>* due to evaluating
>>>>     10 * 42 eventual
>>>> in RefsTest>>testFailureArithmeticPrimitivesWithPromiseArgument
>>>> Since 42 eventual is a NearERef the SmallInteger>>* primitive fails and does ^super * anInteger (where anInteger is the NearERef).  So that evaluates Integer>>*
>>>> Integer>>* aNumber
>>>> "Refer to the comment in Number * "
>>>> aNumber isInteger ifTrue:
>>>> [^ self digitMultiply: aNumber
>>>> neg: self negative ~~ aNumber negative].
>>>> ^ aNumber adaptToInteger: self andSend: #*
>>>> aNumber, being a NearERef, answers a PromiseERef for the isInteger send, and this provokes a mustBeBoolean for the isInteger ifTrue: [...
>>>> After the mustBeBooleanMagic: the stack looks wrong. The activation of Integer>>*, which is about to do
>>>>     ^ aNumber adaptToInteger: self andSend: #*
>>>> does not have enough items on the stack.  Instead of containing
>>>>     a NearERef (for 42)
>>>>     10
>>>>      #*
>>>> it contains
>>>>     a PromiseERef (for 42 eventual isInteger)
>>>> and the send of #adaptToInteger:andSend: ends up taking more form the stack than the VM can handle and it crashes.  The bug appears to be with the use of sendNode irInstruction nextBytecodeOffsetAfterJump in Object>>mustBeBooleanMagic: since execution should resume at bytecode 55 below, but does so at bytecode 57
>>>> 41 <10> pushTemp: 0
>>>> 42 <D0> send: isInteger
>>>> 43 <AC 09> jumpFalse: 54
>>>> 45 <70> self
>>>> 46 <10> pushTemp: 0
>>>> 47 <70> self
>>>> 48 <D1> send: negative
>>>> 49 <10> pushTemp: 0
>>>> 50 <D1> send: negative
>>>> 51 <E2> send: ~~
>>>> 52 <F3> send: digitMultiply:neg:
>>>> 53 <7C> returnTop
>>>> 54 <10> pushTemp: 0
>>>> 55 <70> self
>>>> 56 <24> pushConstant: #*
>>>> 57 <F5> send: adaptToInteger:andSend:
>>>> 58 <7C> returnTop
>>>> So the positioning of the context's pc must be before any argument marshaling for the next send, not simply the send itself.
>>>> Put a breakpoint at the end of Object>>mustBeBooleanMagic: and add initlaPC and resumePC temporaries at the beginning and capture them via
>>>>     initialPC := context pc.
>>>> at the beginning and then
>>>> context pc: (resumePC := sendNode irInstruction nextBytecodeOffsetAfterJump)
>>>> to see what I'm seeing.
>>>> Phew.  Glad it's not a VM bug :-)
>>>> HTH
>>>> _,,,^..^,,,_
>>>> best, Eliot
>> --
>> _,,,^..^,,,_
>> best, Eliot
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