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?
And here's another option, using bitfields: https://godbolt.org/z/Y7asaMczz GCC seems to like this better because it's able to figure out that the exponent and the significand are adjacent in memory, so the shift+or can be optimized out.
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: