Float Exp Bias Constants

Proposal

Constants that specify the bias used by the encoding of the exponent for the given float type.

Problem statement

To actually figure out what the exponent of a float is given its binary representation you need to know the bias of the exponent.

Motivating examples or use cases

Implementing a function like ilog2, custom floating point conversions.

Solution sketch

impl f64 {
     pub const EXPONENT_BIAS: u64 = 1024;
}

Alternatives

Implementing functions that set or return the exponent. Much less versatile and can result in unecessary moves between fpu and regular registers.

Links and related work

#753

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start with the proposed impl f64 entry point, then read related issue #753 and the linked feature lifecycle guidance. This is an API proposal awaiting libs-team review, so completion is not yet defined beyond reaching a decision on the proposed constants.