Re: Optimize three-valued comparison between integers

[Paul Eggert]

On 7/23/20 3:15 PM, Bruno Haible wrote:
> +  AH_VERBATIM([micro_optimizations],
> +[/* _GL_CMP (n1, n2) performs a three-valued comparison on n1 vs. n2.
> +   It returns
> +     1  if n1 > n2
> +     0  if n1 == n2
> +     -1 if n1 < n2

While looking into using this in Emacs I noticed that _GL_CMP (A, B) currently 
returns 0 if either A or B is a NaN. I suggest that we change the documentation 
comment for _GL_CMP to say specifically that the result is indeterminate if 
either argument is a NaN, to make it clearer that callers should not expect the 
result to be 0 in this case.

Another possibility would be to document that the result is 0 in this case, but 
I don't see how that would be useful, as macros like _GL_CMP don't work if 
arguments are not taken from a total order.

Still another possibility would be to restrict _GL_CMP's arguments to integers, 
but I could see uses of that macro on non-NaN floating-point values (though 
Emacs doesn't happen to have any such uses now).