Induction Variable Optimizations


for(j=1;j<5;++j)
 fprintf(stdout,"%d\n",j*3);

Addition is cheaper than multiplication, so the compiler can convert this fragment to the one bellow.


int s = 0;
for(j=1;j<5;++j)
{
 s += 3
 fprintf(stdout,"%d\n",s);
}

A temporary variable has been introduced in order to eliminate the multiplications.

It's not nearly as costly as you think it is. Stop trying to microoptimize useless stuff like this at the cost of clarity. A good enough C compiler will already handle this nicely, for example, multiplication by constants is optimized by some compilers, here's one possible optimization for reg*3 on x86:

8D0440          LEA EAX,DWORD PTR DS:[EAX+EAX*2]
Which is faster than your add by 3, especially if the variable is not in a separate register.

tl;dr: Stop microoptimizing, a good compiler will take care of it.