Re: [Maxima-discuss] scale-float with sbcl & ccl

[Stavros M. <mac...@gm...>]

RJF,

Agreed that the Common Lisp spec didn't specify the behavior of floats
completely, leaving it to the underlying implementation.
I'd hope that denormalized 64-bit IEEE 754 floats are implemented correctly
and consistently on common modern processors (x86, ARM).
Are you claiming that there are "nuances" in the spec or
implementation of *scale-float
*that mean that it can't be consistent across Common Lisp + CPU
combinations?

          -s

On Fri, Dec 5, 2025 at 6:52 PM Richard Fateman <fa...@gm...> wrote:

> There are nuances in the IEEE standard that are not widely used and
> sometimes
> were left out of the software support (for laziness, lack of
> understanding, uncertainty
> of whether this would catch on?) and
> were, certainly in the past, left out of some hardware. (vendors would say
> they support
> the IEEE format, not mentioning lack of support of some operations.)
> Denormalized numbers are certainly in the category of not-widely-used.
> Common Lisp Standard support for floats was deliberately underspecified --
> IEEE floats
> were new at the time.
>
> Curious --  is there a call for this in some important place, say in a
> math library routine??
>
> RJF
>
> On Fri, Dec 5, 2025 at 3:39 PM Raymond Toy <toy...@gm...> wrote:
>
>>
>> On 12/5/25 3:24 PM, David Scherfgen wrote:
>>
>> This is a known bug in SBCL: https://bugs.launchpad.net/sbcl/+bug/2000178
>>
>> Almost 3 years old.  I guess it's not important enough to anyone.
>>
>>
>> Raymond Toy <toy...@gm...> schrieb am Fr., 5. Dez. 2025, 16:00:
>>
>>>
>>> On 12/5/25 10:21 AM, Barton Willis via Maxima-discuss wrote:
>>>
>>>
>>> For subnormal floats,  the SBCL CL function scale-float is possibly
>>> buggy:
>>>
>>> SBCL 2.4.7:
>>>
>>> Define Maxima function that calls the CL function scale-float
>>>
>>> (%i1) :lisp(defun $scale_float (x m) (scale-float x m))
>>>
>>> (%i1) x : 2.0^(-1024);
>>> (%o1)                       5.562684646268004e-309
>>>
>>> This is a SBCL bug, I think:  surely scaling by zero should be an
>>> identity?
>>>
>>> (%i2) scale_float(x,0);
>>> (%o2)                       2.7813423231340017e-308
>>>
>>> (%i3) x : 2.0^(-1023);
>>> (%o3)                       1.1125369292536007e-308
>>>
>>> This is kinda surprising.  Cmucl gets this right.  So does ecl.  Kinda
>>> funny too; it's not obvious what's happening here.  `integer-decode-float`
>>> for the scaled result is `#x14000000000000` and an exponent of -1074,
>>> instead of the expected `#x10000000000000` with exponent -1076.
>>>
>>>
>>> _______________________________________________
>>> Maxima-discuss mailing list
>>> Max...@li...
>>> https://lists.sourceforge.net/lists/listinfo/maxima-discuss
>>>
>>
>>
>> _______________________________________________
>> Maxima-discuss mailing list
>> Max...@li...
>> https://lists.sourceforge.net/lists/listinfo/maxima-discuss
>>
> _______________________________________________
> Maxima-discuss mailing list
> Max...@li...
> https://lists.sourceforge.net/lists/listinfo/maxima-discuss
>