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

[Barton W. <wi...@un...>]

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 also wrong:

(%i4) scale_float(x,0);
(%o4)                       3.337610787760802e-308

(%i5) x : 2.0^(-1022);
(%o5)                       2.2250738585072014e-308

Finally this is OK:

(%i6) scale_float(x,0);
(%o6)                       2.2250738585072014e-308

With CCL 1.13

(%i1) :lisp(defun $scale_float (x m) (scale-float x m))


This is a recently  reported CCL bug
(%i1) x : 2.0^(-1024);

Maxima encountered a Lisp error:

 FLOATING-POINT-OVERFLOW detected
performing * on (1.3407807929942597E+154 1.3407807929942597E+154)

A silly workaround for this bug:

(%i2) x : 2.0^(-1023)/2;
(%o2)                       5.562684646268003E-309

OK:

(%i3) scale_float(x,0);
(%o3)                       5.562684646268003E-309

OK:

(%i4) x : 2.0^(-1023);
(%o4)                       1.1125369292536007E-308

OK:

(%i5) scale_float(x,0);
(%o5)                       1.1125369292536007E-308