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Comments on Uniform floating point from unsigned integer

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Uniform floating point from unsigned integer

+4
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How can I turn uniform samples of unsigned n-bit (n=16/32/64) integers (over the whole range of integers) into uniform samples of n-bit IEEE floats in [0, 1]? By uniform, I mean the continuous uniform distribution. All bounds are inclusive.

It needs to be accurate enough for stats/ML.

I'm using the StableHLO API, but an answer in e.g. C with equivalent maths and bit ops would be trivial to translate. Note this API can bitcast naively to float (i.e. reinterpret bits as they are as a float), and cast to float using the (approx) numerical value.

NB. I assume I can scale [0, 1] to [a, b] as samples * (a - b) + b since that's what XLA does (though I will have to be careful about overflow).

History

4 comment threads

Undeleted because of timing: an answer was being added as the question was being deleted. (1 comment)
Definition of "uniform" (12 comments)
Uniform over what ranges, exactly? (4 comments)
The most naive "bitcast" (7 comments)
Uniform over what ranges, exactly?
Karl Knechtel‭ wrote 7 months ago

To make my own objection explicit:

Do you mean that the input integer, respectively the output float, should be uniform over a specific range (defined ahead of time) which the type is capable of representing? Or that it should be uniform over the entire range represented by the type (e.g. 0..65535 for 16-bit unsigned)? Or exactly what? And how should endpoints be handled? Some examples of expected input and output would be helpful, even if e.g. they use rationals rather than exact floating-point values to demonstrate what should be approximated.

deleted user wrote 7 months ago

What do you mean by endpoints? I've now explained inclusive bounds. I don't know what examples would make sense because I don't know what the float values would look like.

Karl Knechtel‭ wrote 7 months ago

I think that's good enough for now, and I've reopened it. But I mean even simple examples: if we map integers less than 4 to floats less than 1, should [0, 1, 2, 3] map to [0/3, 1/3, 2/3, 3/3] (float approximations thereof), or is it really mapping [0, 1, 2, 3) to [0., 0.25, 0.5, 0.75), or just what?

The problem with talking about "the whole range of possible floats" is that the space of IEEE754 floats is not uniform, even disregarding special values. The space between large values is greater than the space between small values.

deleted user wrote 7 months ago

I've narrowed the question to U[0, 1]