And of course a cousin

    ((1 << 63) negated * -1)

2012/8/29 Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com>:
> Do you want an other one ?
>
> Try
>     (1 << 63) negated / -1
> for fun
>
> Nicolas
>
> 2012/8/28 Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com>:
>> Sorry to post here, this bounced at vm-dev
>> But I think it's important
>>
>> Nicolas
>>
>>
>> ---------- Forwarded message ----------
>> From: Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com>
>> Date: 2012/8/28
>> Subject: doesn't Primitive 33 primitiveQuoLargeIntegers relies on C
>> Undefined Behaviour ? uncover a BUG in Primitive 32
>> primitiveDivLargeIntegers and maybe some others
>> To: vm-dev-request at lists.squeakfoundation.org
>>
>>
>> (1 << 63) negated is converted to signed long long
>> 0x8000000000000000LL by signed64BitValueOf:
>>
>> So far so good...
>> Then in primitiveQuoLargeIntegers, we will try to take absolute value
>> of this beast...
>>
>> integerRcvr > 0 ifTrue: [
>>                 integerArg > 0
>>                         ifTrue: [result := integerRcvr // integerArg]
>>                         ifFalse: [result := 0 - (integerRcvr // (0 -
>> integerArg))].
>>         ] ifFalse: [
>>                 integerArg > 0
>>                         ifTrue: [result := 0 - ((0 - integerRcvr) //
>> integerArg)]
>>                         ifFalse: [result := (0 - integerRcvr) // (0 -
>> integerArg)].
>>         ].
>>
>> That is translated straightfully in C...
>>
>>     sqLong integerArg;
>>     sqLong integerRcvr;
>>     sqLong result;
>>
>> snip...
>>
>>         if (integerRcvr > 0) {
>>                 if (integerArg > 0) {
>>                         result = integerRcvr / integerArg;
>>                 }
>>                 else {
>>                         result = 0 - (integerRcvr / (0 - integerArg));
>>                 }
>>         }
>>         else {
>>                 if (integerArg > 0) {
>>                         result = 0 - ((0 - integerRcvr) / integerArg);
>>                 }
>>                 else {
>>                         result = (0 - integerRcvr) / (0 - integerArg);
>>                 }
>>         }
>>
>> Isn't this relying on UB? like
>> http://stackoverflow.com/questions/2539178/why-is-abs0x80000000-0x80000000
>> At worse, I would expect 0 -  0x8000000000000000LL to be converted to
>> itself  0x8000000000000000LL...
>> So can someone explain why this works?
>>
>>     ((1 << 63) negated quo: 2) = (1 << 62) negated
>>
>> I guess it must be some compiler optimization, like:
>>
>>     0 - (0-a)/b -> 0 - (0/b-a/b) -> a/b
>>
>> But then, why doing the sign checks at all?
>> Isn't it just result = integerRcvr / integerArg; whatever the signs?
>>
>> Maybe the compiler is able to just do that, but it sounds like obfuscation...
>>
>> Anyway, that won't work in primitiveDivLargeIntegers, i would expect:
>>     ((1 << 63) negated // -2) =  (1 << 62)
>> But I get
>>     ((1 << 63) negated // -2) =  (1 << 62) negated
>>
>> Ah Ah, it looks like a bug this time, no Compiler optimization did save us
>>
>> We have several solutions...
>> - let signed64BitValueOf: (1 << 63) negated fail
>> - protect each and every negation of those
>> - perform most arithmetic with unsigned, handle sign separately
>>
>> Nicolas