[Vm-dev] vm problem on cog and stack spur

[Nicolas Cellier]

Thanks David.

By running the snippet, instrumenting erf, printing each and every
intermediate value to a text file, and diffing:
{
(10 asArbitraryPrecisionFloatNumBits: 33) erf.
(10 asArbitraryPrecisionFloatNumBits: 34) erf.
}

I reduced the failing test to:

 a :=
1598335257761788022467377781654101148543282249044465229239888363328190330275719997501596724768507889233831388734160190922469363547795602076820594918.
b := 49612.
self assert: a - ((a quo: b)*b) < b

So it must be some nasty sign promotion (or lack of?) in division.
Normally, this could be simulated, but with signedness, I'm never sure,
maybe my next step will be a breakpoint in xcode/gdb...


Curiously, (a rem: b) is correct !

2016-03-27 17:36 GMT+02:00 David T. Lewis <lewis at mail.msen.com>:

>
> I confirm the same results for your ArbitraryPrecisionFloat example below,
> testing on Linux with a 64 bit interpreter VM and 32 bit image.
>
>   Image
>   -----
>   /home/lewis/squeak/Squeak4.6/squeak.37.image
>   Squeak4.6
>   latest update: #15723
>   Current Change Set: Unnamed2
>   Image format 6504 (32 bit)
>
>   Virtual Machine
>   ---------------
>   /usr/local/lib/squeak/4.15.4-3634/squeakvm
>   Squeak4.5 of 10 December 2015 [latest update: #1195]
>   Unix built on Mar 25 2016 13:27:51 Compiler: 4.9.2
>   platform sources revision 3634
>   VMMaker versionString 4.15.4
>
> Dave
>
>
> On Sun, Mar 27, 2016 at 03:25:58PM +0200, Nicolas Cellier wrote:
> >
> > Hi Levente, thank you.
> > Here is a follow up:
> >
> > I was able to reproduce the same symptom on the classical interpreter VM.
> > <rant> It's just that for MacOSX, there is no recent VM available, and
> that
> > the MacOS build is somehow abandonware, so it took much more time than
> > necessary :( </rant>
> >
> > So the changes that have been integrated on Interpreter Vm in july 2014
> and
> > that I thought correct were not.
> > It's certainly related to a LargeInteger plugin primitive.
> > Those are triggered when nbits of magnitude > 64.
> > (otherwise, <= 64 bits, the LargeInteger numbered primitives do their
> job).
> >
> > Now the job is to produce/isolate an elementary failing LargeInteger
> test.
> > But you need to generate the VM from VMMaker.oscog head for COG/STACK
> > V3/SPUR 32/64 bits
> > or use a sufficiently recent interpreter VM (> july 2014) to trigger such
> > failure.
> >
> >
> > --------------------------------------------
> >
> > Here is my starting point:
> >
> > If you load ArbitraryPrecisionFloat and Tests from
> > http://www.squeaksource.com/ArbitraryPrecisionFl
> > then you'll see this strange pattern:
> >
> > (53 to: 70) collect: [:i |
> >     i -> (10 asArbitraryPrecisionFloatNumBits: i) erf ]
> >
> >  {53->(ArbitraryPrecisionFloat readFrom: '0.9999998981732067' readStream
> > numBits: 53) .
> >  54->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 54) .
> >  55->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 55) .
> >  56->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 56) .
> >  57->(ArbitraryPrecisionFloat readFrom: '0.99999989817320666' readStream
> > numBits: 57) .
> >  58->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 58) .
> >  59->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 59) .
> >  60->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 60) .
> >  61->(ArbitraryPrecisionFloat readFrom: '0.9999998981732066655'
> readStream
> > numBits: 61) .
> >  62->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 62) .
> >  63->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 63) .
> >  64->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 64) .
> >  65->(ArbitraryPrecisionFloat readFrom: '0.99999989817320666564'
> readStream
> > numBits: 65) .
> >  66->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 66) .
> >  67->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 67) .
> >  68->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 68) .
> >  69->(ArbitraryPrecisionFloat readFrom: '0.999999898173206665646'
> > readStream numBits: 69) .
> >  70->(ArbitraryPrecisionFloat readFrom: '1.0' readStream numBits: 70)}
> >
> > The value seems incorrect every four numbits
> > The exact value is near 1- 2.0e-45, and given the insufficient precision
> > requested, the result should be rounded to 1.
> > We can confirm with online high precision erf calculator like found for
> > example with http://keisan.casio.com/exec/system/1180573449
> >
> > Or we can check with
> > (10 asArbitraryPrecisionFloatNumBits: 200) erf (ArbitraryPrecisionFloat
> > readFrom:
> '0.999999999999999999999999999999999999999999997911512416237455'
> > readStream numBits: 200)
> >
> > -----------------------------
> >
> > Note that when I change the LargeInteger digitLength to 32bits instead
> of 8
> > (my own VM branch) then the problem magically disappear.
> >
> > I plan to introduce these change in the VMMaker.oscog, so I can as well
> > ignore the problem and proceed, however I much prefer to isolate it, and
> > understand it for gain of knowledge :)
> >
> >
> > 2016-03-27 4:27 GMT+02:00 Levente Uzonyi <leves at caesar.elte.hu>:
> >
> > >
> > > Hi Nicolas,
> > >
> > > If you give us a bit more detailed instructions about what to test,
> then
> > > we might be able to help you get this sorted out.
> > >
> > > Levente
> > >
> > >
>
>
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