Proposal for Decimal64 and Decimal128 value-based classes

[Raffaello Giulietti]

Hello,

I'm experimenting with an implementation of Decimal64 and Decimal128, 
two standard IEEE 754-2019 formats supporting, well, decimal floating 
point numbers of up to 16 resp 34 decimal digits.

While BigDecimal can simulate most finite number computations on these 
formats by passing MathContext.DECIMAL64 and MathContext.DECIMAL128, 
resp., to the arithmetic operations, this approach has some shortcomings 
wrt to the standard:
* Like the binary formats (float and double in Java), the standard 
mandates signed zeroes, signed infinities and NaNs for the decimal 
formats as well, while BigDecimal has neither.
* BigDecimal is not a final class, so is not (and can never be) value-based.
* BigDecimal needs support from BigInteger (another non final, non 
value-based class).

On the other hand, classes Decimal64 and Decimal128:
* Are value-based and can be declared to be primitive in a future 
version of OpenJDK, once Valhalla is completed.
* Require 12 and 20 bytes, resp, of storage in the Valhalla model of 
primitive objects and have no references to other identity or primitive 
objects.
* Support the signed zeroes, the infinities and the NaNs.
* Currently support the 4 fundamental operations (+ - * /) according to 
the standard and will support remainder, square root, comparisons, etc. 
and the conversions listed in the standard.
* Could be extended to support the (rather cumbersome) exceptions 
handling of the standard.
* There's no plan to support transcendental functions, as the aim is to 
have numerical classes for business applications, not scientific or 
engineering purposes.
* Are faster than the (incomplete) simulation with BigDecimal, 1x-5x in 
the current incarnations.


I would be glad to contribute the code to the OpenJDK provided there is 
a genuine interest from the community and a reviewer willing to sponsor 
my effort. (I guess the best candidate for this role would be Joe Darcy 
from what I can read in this mailing list.)


Greetings
Raffaello