algorithm - Optimizing C code, Horner's polynomial evaluation -

i'm trying learn how optimize code (i'm learning c), , in 1 of books there's problem optimizing horner's method evaluation polynomials. i'm little lost on how approach problem. i'm not great @ recognizing needs optimizing.

any advice on how make function run faster appreciated.

thanks

 double polyh(double a[], double x, int degree) {         long int i;         double result = a[degree];         (i = degree-1; >= 0; i--)              result = a[i] + x*result;         return result;     } 

you need profile code test whether proposed optimizations help. example, may case declaring i long int rather int slows function on machine, on other hand may make no difference on machine might make difference on others, etc. anyway, there's no reason declare i long int when degree int, changing won't hurt. (but still profile!)

horner's rule supposedly optimal in terms of number of multiplies , adds required evaluate polynomial, don't see can it. 1 thing might (profile!) changing test i>=0 i!=0. of course, loop doesn't run enough times, you'll have add line below loop take care of final case.

alternatively use do { ... } while (--i) construct. (or do { ... } while (i--)? figure out.)

you might not need i, using degree instead not save observable amount of time , make code harder debug, it's not worth it.

another thing might (i doubt it, profile!) breaking arithmetic expression inside loop , playing around order, like

for (...) {   result *= x;   result += a[i]; } 

which may reduce need temporary variables/registers. try out.