Opened 3 months ago

Last modified 7 weeks ago

## #288 new defect

# Float exponent is not compatible with other dialects (VW, Squeak, Dolphin, ...)

Reported by: | Owned by: | ||
---|---|---|---|

Priority: | major | Milestone: | |

Component: | default | Keywords: | Float exponent |

Cc: | Also affects CVS HEAD (eXept version): | no |

### Description

Currently, exponent and mantissa are defined in term of frexp() function.

They return a significand in interval [0.5,1.0) and exponent accordingly.

Other dialects return the significand in interval [1.0,2.0).

Because they are defined in term of ilogb()/scalb() function.

Thus 1.0 exponent is 1 in stx and 0 in other dialects, making port of some library more difficult than necessary.

I would rather use ilogb() and scalb() for the exponent because they also work if ever the radix of Float implementation is not 2.

There is no sender of exponent/mantissa in base image, so this should be a safe change.

### Change History (4)

### comment:1 Changed 3 months ago by

### comment:2 Changed 3 months ago by

Hi,

nice to see you here!

Now I must admit that I never touched this code nor I feel an expert in FP. What you say makes sense - I have no problem changing `#exponent`

and `#mantissa`

(or any other FP-related code given the change is justified - your word is enough :-) - and accompanied with tests).

Can you provide some tests to star with, or/and (even better) a proposal for a fix?

### comment:3 Changed 7 weeks ago by

Beware, there are two sets of standards:

IEEE 754 defines:

float = sign * significand * (radix raisedTo: exponent)

where 1 <= significand, significand < radix for normalized floats

ISO/IEC 10967 defines:

float = sign * fraction * (radix raisedTo: exponent)

where radix reciprocal <= fraction, fraction < 1.

So obviously the exponent differ by 1 in the two models!

For ieee754, exponent is (self abs log: Float radix) floor (the ilogb function)

For ISO-10967, exponent is (self abs log: Float radix) floor + 1. (the frexp)

See for example ISO/IEC 10967 (Language Independent Arithmetic (LIA))

http://www.open-std.org/jtc1/sc22/wg11/docs/n507.pdf draft for part I

http://www.open-std.org/JTC1/SC22/WG11/docs/n462.pdf draft for part II

But in both models, the mininum and maximum exponent should fit definitions of emin and emax:

page 54 of first draft (10967-1) for IEEE754 model

page 64 of first draft for second model

Float fminNormalized exponent = Float emin.

Float fmax exponent = Float emax.

Float fminDenormalized exponent = (Float emin - Float precision + 1).

Float fminNormalized predecessor exponent = (Float emin - 1).

We must either implement one model, or the other, but not something inconsistent.

Squeak and VW are implementing significand/exponent and have emin=-1022 emax=1023 for double precision, thus my suggestion, even if it does not fit ISO-10967 definition...

### comment:4 Changed 7 weeks ago by

BTW, mantissa is ambiguous See https://en.wikipedia.org/wiki/Mantissa

It could be interpreted as fraction part of logarithm!

For this reason, significand is the preferred term nowadays.

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More over, exponents also should be compatible with emin and emax