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

[Richard F. <fa...@gm...>]

I'm thinking mostly of trap and exception handling by interrupts not being
uniform in language implementation.  No specific items at the moment.


On Sat, Dec 6, 2025, 8:47 AM Stavros Macrakis <mac...@gm...> wrote:

> 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.
>>>>
>>>>
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>>>
>>>
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