Assuming normalized numbers (which should be the case) of the same sign, operator< is equivalent (NaN notwithstanding) to comparing the two integers
eeeeeeee... mmmmmmmmmm...
I'm not quite sure whether zero needs to be a special case here. There's special case logic in operator< for it, but I'm not positive it's needed.
The current operator< is this: https://godbolt.org/z/Tj7Me5aW3 This is a sketch of what I was thinking about: https://godbolt.org/z/9M1cevYef I haven't run this against the test suite, so it might not be quite correct, but it looks correct to me. :-) It assumes that zeroes are normalized. The current implementation seems to not assume that; it handles 0e+7, but since the _fast types are supposed to be normalized, this should never happen, should it?
This makes me wonder whether { uintNN_t exp_and_mant; bool sign; } wouldn't be a better representation for _fast, although that would depend on how important comparisons are, performance-wise.
Should look somewhat like this: https://godbolt.org/z/bqWKxbdjd