Fastest way to clamp a real (fixed/floating point) value?

Question

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).

Answer 1

Both GCC and clang generate beautiful assembly for the following simple, straightforward, portable code:

double clamp(double d, double min, double max) {   const double t = d < min ? min : d;   return t > max ? max : t; } 

> gcc -O3 -march=native -Wall -Wextra -Wc++-compat -S -fverbose-asm clamp_ternary_operator.c

GCC-generated assembly:

maxsd   %xmm0, %xmm1    # d, min movapd  %xmm2, %xmm0    # max, max minsd   %xmm1, %xmm0    # min, max ret 

> clang -O3 -march=native -Wall -Wextra -Wc++-compat -S -fverbose-asm clamp_ternary_operator.c

Clang-generated assembly:

maxsd   %xmm0, %xmm1 minsd   %xmm1, %xmm2 movaps  %xmm2, %xmm0 ret 

Three instructions (not counting the ret), no branches. Excellent.

This was tested with GCC 4.7 and clang 3.2 on Ubuntu 13.04 with a Core i3 M 350. On a side note, the straightforward C++ code calling std::min and std::max generated the same assembly.

This is for doubles. And for int, both GCC and clang generate assembly with five instructions (not counting the ret) and no branches. Also excellent.

I don't currently use fixed-point, so I will not give an opinion on fixed-point.