I am always interested in some of the subtle effects that coding can have on not just the behavior of code, but also its performance. Recently, I stumbled across some benchmarking code – I cannot recall how I actually found it. It was not written in C, so I translated it.<\/p>\n
In the process, I realized that a literal translation might not be ideal …<\/p>\n
The code is to do with Mandelbrot sets. My first C translation looked like this:<\/p>\n
int mandelbrot(float a)<\/pre>\n{<\/pre>\nfloat b;<\/pre>\nint iterations = 50;<\/pre>\nint i;<\/pre>\nb = a;<\/pre>\nfor (i=1; i<=iterations; i++)<\/pre>\n{<\/pre>\nif (abs(a) > 2)<\/pre>\nreturn (i - 1);<\/pre>\na = pow(a, 2) + b;<\/pre>\n}<\/pre>\nreturn (iterations);<\/pre>\n}<\/pre>\n <\/p>\n
I then saw a couple of optimizations that I could make:<\/p>\n
#define ITERATIONS 50<\/pre>\nint mandelbrot(float a)<\/pre>\n{<\/pre>\nfloat b;<\/pre>\nint i;<\/pre>\nb = a;<\/pre>\nfor (i=1; i<=ITERATIONS; i++)<\/pre>\n{<\/pre>\nif (abs(a) > 2)<\/pre>\nreturn (i - 1);<\/pre>\na = a * a + b;<\/pre>\n}<\/pre>\nreturn (ITERATIONS);<\/pre>\n}<\/pre>\n <\/p>\n
I made it clear that iterations<\/strong> was a constant. I could have simply qualified it with const<\/strong>, but I chose to use #define<\/strong>. Call me old fashioned if you like. Both would most likely produce the same result. A good compiler may well optimize this without my help.<\/p>\nSince the code was translated from a language that included an exponentiation operator, using the pow()<\/strong> library function seemed logical, as C has no such operator. As a<\/strong> is simply squared, multiplication seemed better. Again, a smart compiler may well have addressed this.<\/p>\nI then saw another possible enhancement that might be effective:<\/p>\n
#define ITERATIONS 50<\/pre>\nint mandelbrot(float a)<\/pre>\n{<\/pre>\nfloat b;<\/pre>\nint i;<\/pre>\nb = a;<\/pre>\nfor (i=1; i<=ITERATIONS; i++)<\/pre>\n{<\/pre>\nif (abs(b) > 2)<\/pre>\nreturn (i - 1);<\/pre>\nb = b * b + a;<\/pre>\n}<\/pre>\nreturn (ITERATIONS);<\/pre>\n}<\/pre>\n <\/p>\n