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[Caml-list] Weird behavior with nan's and min/max
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Date: -- (:)
From: Yaron Minsky <yminsky@c...>
Subject: Re: [Caml-list] Weird behavior with nan's and min/max
Well, finding the problem was a bit easier than I'd expected.

# min 3 4

causes a segfault.

> I tried a solution based on Xavier's suggestion, and it now looks to me
> like Xavier's suggsetion (or my implementation) is unsafe.  Here's the
> code I tried:
>
> let is_obj_nan x =
>   Obj.tag (Obj.repr x) = Obj.double_tag &&
>   (let f = (Obj.magic x : float) in not (f = f))
>
> let min x y =
>   if is_obj_nan x then x
>   else if is_obj_nan y then y
>   else Pervasives.min x y
>
> let max x y =
>   if is_obj_nan x then x
>   else if is_obj_nan y then y
>   else Pervasives.max x y;;
>
> The resulting code segfaulted on me, and the segfault went away when I
> went back to the ordinary min and max.  Does anyone have a thought on
> this?  Is this use of Obj safe or not?
>
> I'm still trying to debug the actual location of the segfault, but the
> lack of stacktraces makes it a bit harder....
>
> y
>
>>> Now here's the weird bit.  I decided I wanted a polymorphic comparison
>>> that wouldn't have this problem.  But this is a little harder than it
>>> seems, since it turns out that specialized float version of equality is
>>> different from the polymorphic version.
>>
>> Yes, it's a long-standing bug for which we haven't yet a good
>> solution.  More exactly, there are two problematic solutions:
>>
>> 1- Fix polymorphic equality so that it behaves like IEEE equality on
>> floats,
>> i.e. it always returns false when one of its arguments is NaN.
>> The problem is that this breaks the implication
>>         x == y  imply  x = y
>> and thus the current implementation of polymorphic equality needs to
>> be made less efficient.  Currently, x = y starts by testing x == y
>> and returns true if the pointer equality holds.  But this could be the
>> wrong result according to the new specification, since x can contain
>> an NaN somewhere.  Hence, polymorphic equality would have to traverse
>> its two arguments even when they are physically the same.  The
>> performance impact of this change on real programs is unknown.
>>
>> 2- As J M Skaller proposed, change the behavior of polymorphic
>> equality and its version specialized to floats so that nan = nan
>> and nan <> x if x <> nan.  Similar changes need to be done on the
>> <>, <= and >= tests for consistency.  IEEE comparisons would then have
>> to
>> be
>> provided as separate primitives.  This preserves the implication
>> x == y ==> x = y.  But the machine code generated for =, <>, <= and >=
>> over floats will have to be a lot less efficient than it is now, since
>> all processors implement float comparisons as per IEEE.
>>
>> Coming back to your proposed workaround:
>>
>>> # let raw_min = min
>>> val raw_min : 'a -> 'a -> 'a = <fun>
>>> # let min x y =
>>>   if not (y = y) then y
>>>   else if not (x = x) then x
>>>   else raw_min x y
>>> ;;
>>
>> A way to make this work would be to replace the "not (x = x)" tests
>> by calls to the following function (of type 'a -> bool):
>>
>> let is_obj_nan x =
>>   Obj.tag (Obj.repr x) = Obj.double_tag &&
>>   (let f = (Obj.magic x : float) in not (f = f))
>>
>> Not pretty, I agree.
>>
>> - Xavier Leroy
>>
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