C或C ++中的一元运算符，用于数字的二次幂

Question

Given a double x , it is known that it is more efficient to use x*x instead of pow(x,2) . Imagine for simplicity that we have to calculate the square root of x : as it is a unary operation, for this purpose we have sqrt(x) . Now, also raising x to the second power is a unary operation, but we have nothing like (as far as I know) pow2(x) .

I implemented my own pow2 as:

inline double pow2(double a){return a*a;}

which should be still better than pow(a,2) , but it is based on the * operator that is not unary. How to implement a genuine unary implementation of pow2 ? Would it be the most efficient way to obtain the second power of a double ?

NOTE: I am aware of the fact that every real power of a positive real is an unary operation and that it is nonsense to define an infinite number of pow2 , pow3 , pow3.14 ... from the practical point of view I'm very happy with pow(double, double) .

Answer 1

"it is more efficient to use x*x instead of pow(x,2) "

Not certainly more efficient. It might be the same. C allows analyze-ability of such functions like pow() and both may emit the same code.

A compiler may not analyze your pow2() and create sub-optimal code as compared to pow(a,2) .

If truly concerned, profile your code. Yet what is best on one platform may differ on others.

How to implement a genuine unary implementation of pow2?

inline double pow2(double a){return a*a;} is OK.

Would it be the most efficient way to obtain the second power of a double?

"most efficient" --> I suggest no function, just x*x .

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Also note that C allows FP to evaluate at higher precision that required. Research FLT_EVL_METHOD . The goal for most efficient way to obtain the second power of a double with a function may defeat overall performance.