Why use abs() or fabs() instead of conditional negation?

Question

In C/C++, why should one use abs() or fabs() to find the absolute value of a variable without using the following code?

int absoluteVal         


        

Answer 1

Assuming that the compiler won't be able to determine that both abs() and conditional negation are attempting to achieve the same goal, conditional negation compiles to a compare instruction, a conditional jump instruction, and a move instruction, whereas abs() either compiles to an actual absolute value instruction, in instruction sets that support such a thing, or a bitwise and that keeps everthing the same, except for the sign bit. Every instruction above is typically 1 cycle, so using abs() is likely to be at least as fast, or faster than conditional negation (since the compiler might still recognize that you are attempting to calculate an absolute value when using the conditional negation, and generate an absolute value instruction anyway). Even if there is no change in the compiled code, abs() is still more readable than conditional negation.