John Phillips skrev:
Ideas that force the floating point value out of the register and into cache will, on average cost more than an order of magnitude in time increase for the comparison (forcing to main memory is far, far worse). This isn't acceptable for anyone who needs performance from the comparison.
Reducing the bits used in the mantissa for the registers can be far worse. Most accuracy guarantees made for the calculations assume the existence of guard bits in the register. Without them, the calculation loses significance faster than expected. This is not acceptable for anyone who needs high precision.
In general, these are difficult and subtle problems you are approaching when you try to compare close floating point numbers. The past is littered with examples of how not to do it, and doing it well is very fiddly. It is possible to find all of the details for this, but it should only be done if someone wants to put in the time and effort to find out the best available methods. "This is probably good enough" ideas almost never are in this setting.
Well, problem one would be to find one that has the knowledge to find the datails. John Maddock gave two examples: one where we accapted wrong values and one where we rejected correct values. I found the latter somewhat better (because the invariant of bounded_float would be preserved), and if we are able to get that behavior consistently, I would be quite happy. So maybe most of our problems will be fixed by using >= and <= instead of < and >? -Thorsten