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2.1.3 Extended Floats

CMUCL also has an extension to support double-double-float type. This float format provides extended precision of about 31 decimal digits, with the same exponent range as double-float. It is completely integrated into CMUCL, and can be used just like any other floating-point object, including arrays, complex double-double-float’s, and special functions. With appropriate declarations, no boxing is needed, just like single-float and double-float.

The exponent marker for a double-double float number is “W”, so “1.234w0” is a double-double float number.

Note that there are a few shortcomings with double-double-float’s:

• There are no equivalents to most-positive-double-float, double-float-positive-infinity, etc. This is because these are not really well defined for double-double-float’s.
• Underflow and overflow may be prematurely signaled. This is due to how double-double-float’s are implemented.
• Basic arithmetic operations are inlined, so the code size is fairly large.
• double-double-float arithmetic is quite a bit slower than double-float since there is no hardware support for this type.
• The constant pi is still a double-float instead of a double-double-float. Use ext:dd-pi if you want a double-double-float value for \pi.

float: extensions:double-double-float ¶

The double-double-float type. It is in the EXTENSIONS package.

Constant: extensions:dd-pi ¶

A double-double-float approximation to \pi.