Is there a more efficient way to clamp real numbers than using if statements or ternary operators? I want to do this both for doubles and for a 32-bit fixpoint implementation (16.16). I’m not asking for code that can handle both cases; they will be handled in separate functions.
Obviously, I can do something like:
double clampedA; double a = calculate(); clampedA = a > MY_MAX ? MY_MAX : a; clampedA = a < MY_MIN ? MY_MIN : a; or
double a = calculate(); double clampedA = a; if(clampedA > MY_MAX) clampedA = MY_MAX; else if(clampedA < MY_MIN) clampedA = MY_MIN; The fixpoint version would use functions/macros for comparisons.
This is done in a performance-critical part of the code, so I’m looking for an as efficient way to do it as possible (which I suspect would involve bit-manipulation)
EDIT: It has to be standard/portable C, platform-specific functionality is not of any interest here. Also, MY_MIN and MY_MAX are the same type as the value I want clamped (doubles in the examples above).
For the 16.16 representation, the simple ternary is unlikely to be bettered speed-wise.
And for doubles, because you need it standard/portable C, bit-fiddling of any kind will end badly.
Even if a bit-fiddle was possible (which I doubt), you’d be relying on the binary representation of doubles. THIS (and their size) IS IMPLEMENTATION-DEPENDENT.
Possibly you could ‘guess’ this using sizeof(double) and then comparing the layout of various double values against their common binary representations, but I think you’re on a hiding to nothing.
The best rule is TELL THE COMPILER WHAT YOU WANT (ie ternary), and let it optimise for you.
EDIT: Humble pie time. I just tested quinmars idea (below), and it works – if you have IEEE-754 floats. This gave a speedup of about 20% on the code below. IObviously non-portable, but I think there may be a standardised way of asking your compiler if it uses IEEE754 float formats with a #IF…?