Re: fake floating point number

[Sam S. <sd...@gn...>]

Bruno,

Bruno Haible wrote:
> 
> 
>> I still think that it is wrong to roundtrip through lisp when I am
>> comparing C integers with C floats, so the correct expression is
>>
>> if (elt2.machine_float != elt1) goto no;
> 
> This expression will not work. CPUs have no instructions for comparing
> a single-float with an uint32. The compiler will either turn this
> expression into
>           ((uint32)elt2.machine_float != elt1)
> or into   (elt2.machine_float != (float)elt1)

the latter is what C does.

> In the first case, it produces the wrong result when elt2 is >= 2^32
> and elt1 is 2^32-1 or 2^31-1. In the second case, it produces wrong
> results when elt2 = 2^30 and elt1 = 2^30 + 1.

this is fine.

[1]> (setq a (ash 1 30))
1073741824
[2]> (1+ (float a))
1.0737418E9
[3]> (= (float a) (1+ (float a)))
T
[4]> (= a (1+ (float a)))
T
[5]> (equalp a (1+ (float a)))
T

i.e., returning T is correct.
(elt2.machine_float != (float)elt1) is exactly what I meant.
(and that's what C does).