If you are computing both the sine and cosine of an angle, then you will be twice as quick if you call sincos() than if you call cos() and sin() independently:
#include
int main()
{
double a,b,c;
a=1.0;
for (int i=0;i<100000000;i++) { b=sin(a); c=cos(a); }
}
$ cc -O sc.c -lm
$ timex ./a.out
real 19.13
vs
#include
int main()
{
double a,b,c;
a=1.0;
for (int i=0;i<100000000;i++) { sincos(a,&b,&c); }
}
$ cc -O sc.c -lm
$ timex ./a.out
real 9.80