# [Vm-dev] vm crash when using rairedTo: with fractions

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Thu Aug 10 20:42:43 UTC 2017

```2017-08-10 22:19 GMT+02:00 Andrei Chis <chisvasileandrei at gmail.com>:

>
> Hi Nicolas,
>
> Thanks for the info. Indeed sending #asFloat to the operand leads to a
> correct behaviour.
>
> Just then is there a need for this special use case for handling Fractions
> if it can lead to such problematic behaviour? Would the logarithmic way
> ((aNumber * self ln) exp) not be enough? On this example the computation
> takes a few minutes before crashing the image.
>
>
Having (1/1000 raisedTo: 2/3) = (1/100)  is a nice thing and we want to
keep it.
But once we detect that we can't have an exact result, we should adopt a
more efficient strategy.

There are corner cases where we want to avoid overflow/underflow in
intermediate values, such as (1<<2000 + 1) raisedTo: 1/200 or ((1<<2000 +
1) reciprocal raisedTo: 1/200, so we must not convert asFloat too soon.
But since ln already handles those edge cases (at least in Squeak, I have
to check Pharo), ((aNumber * self ln) exp) is a good approximation (a few
ulp off, depending on the quality of underlying libm).

Nicolas

> Cheers,
> Andrei
>
> On Thu, Aug 10, 2017 at 9:27 PM, Nicolas Cellier <
> nicolas.cellier.aka.nice at gmail.com> wrote:
>
>>
>> Hi Andrei,
>> indeed, the method does not scale well...
>>
>> If the result is an exact Fraction, then it will answer the Fraction.
>> Else if not exact, it will be converted to a Float.
>>
>> The problem is that it will try to answer nearest Float with a rather
>> naive algorithm.
>> Moreover, computing the nthRoot: first, then raising the result to the
>> power of numerator will cumulate rounding errors.
>> So trying to get a very accurate nthRoot: first in case of Float result
>> is not a good strategy anyway.
>>
>> Why the VM crashes exactly is another problem and remains to see, we'd
>> prefer an Exception.
>>
>> As a workaround, I suggest sending asFloat to the receiver and/or operand.
>>
>>
>> 2017-08-10 12:02 GMT+02:00 Andrei Chis <chisvasileandrei at gmail.com>:
>>
>>>
>>> Hi,
>>>
>>> I was executing this code  '(2009/2000) ** (3958333/100000)' with the
>>> Pharo6.1 distribution and the vm crashed with she stack attached below.
>>> Tried it on both mac and windows 10.
>>> Seems that #raisedTo: has a special case for fractions that ends up
>>> calling #nthRoot: like '2009 nthRoot: 100000' leading to the crash.
>>>
>>> Cheers,
>>> Andrei
>>>
>>>
>>> 0xaddeac M LargePositiveInteger(Integer)>quo: 0x314093e8: a(n)
>>> LargePositiveInteger
>>> 0xaddec8 M LargePositiveInteger(LargeInteger)>quo: 0x314093e8: a(n)
>>> LargePositiveInteger
>>> 0xaddee8 M LargePositiveInteger(Integer)>// 0x314093e8: a(n)
>>> LargePositiveInteger
>>> 0xaddf04 M LargePositiveInteger(LargeInteger)>// 0x314093e8: a(n)
>>> LargePositiveInteger
>>> a(n) LargePositiveInteger
>>> a(n) LargePositiveInteger
>>> 0xaddfb4 I Fraction>nthRoot: 0x4f9a940: a(n) Fraction
>>> 0xaddfd8 I Fraction(Number)>raisedTo: 0x4f9a940: a(n) Fraction
>>> 0xaddffc I Fraction(Number)>** 0x4f9a940: a(n) Fraction
>>> 0xade018 M UndefinedObject>DoIt 0x5fe5d00: a(n) UndefinedObject
>>> 0xade048 I OpalCompiler>evaluate 0x4f9a998: a(n) OpalCompiler
>>> 0xade074 I RubSmalltalkEditor>evaluate:andDo: 0x305e5878: a(n)
>>> RubSmalltalkEditor
>>> 0xade09c I RubSmalltalkEditor>highlightEvaluateAndDo: 0x305e5878: a(n)
>>> RubSmalltalkEditor
>>> 0x3062fdc8: a(n) GLMMorphicPharoScri
>>> enderer
>>> 0xade0d8 I MorphicAlarm(MessageSend)>value 0x4f9ab20: a(n) MorphicAlarm
>>> 0xade0f4 M MorphicAlarm>value: 0x4f9ab20: a(n) MorphicAlarm
>>> 0xade114 M WorldState>triggerAlarmsBefore: 0x71bb5e0: a(n) WorldState
>>> 0xade140 M WorldState>runLocalStepMethodsIn: 0x71bb5e0: a(n) WorldState
>>> 0xade164 M WorldState>runStepMethodsIn: 0x71bb5e0: a(n) WorldState
>>> 0xade180 M WorldMorph>runStepMethods 0x6ab7778: a(n) WorldMorph
>>> 0xade198 M WorldState>doOneCycleNowFor: 0x71bb5e0: a(n) WorldState
>>> 0xade1b4 M WorldState>doOneCycleFor: 0x71bb5e0: a(n) WorldState
>>> 0xade1d0 M WorldMorph>doOneCycle 0x6ab7778: a(n) WorldMorph
>>> 0xade1e8 M WorldMorph class>doOneCycle 0x6a9f960: a(n) WorldMorph class
>>> 0xade200 M [] in MorphicUIManager>spawnNewProcess 0x2cc88718: a(n)
>>> MorphicUIManager
>>> 0xade220 I [] in BlockClosure>newProcess 0x2f178150: a(n) BlockClosure
>>>
>>>
>>
>>
>
>
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