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

[Raymond T. <toy...@gm...>]

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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