对处理器执行除法和模的最佳方法是什么？

Question

How to perform a division AND a modulo at the same time. Is it possible for the processor ?

Like :

int a, b = 8 / 3; //a = 2, b = 2

Or is there an operation which is better than :

int a = 8 / 3;
int b = 8 % 3;

Maybe this is better ?

int a = 8 / 3;
int b = 8 - a * 3;

Thanks.

Answer 1

Consider the following function:

std::pair<int, int> divmod(int x, int y)
{
    return { x / y, x % y };
}

Compiling this with g++ -std=c++11 -O1 -S spits out the following assembly code:

movl    %edi, %eax
cltd
idivl   %esi
salq    $32, %rdx
movl    %eax, %eax
orq     %rdx, %rax
ret

As you can see, it only contains a single division on line 3. Optimizers are very good at this stuff.

Answer 2

Maybe this is better?

Why would it be? It's obscure, both to the programmer and the compiler/optimiser. Without having looked at the compiler output, I'd imagine that any decent optimiser sees your first code and says “ah, a div and a mod being performed, I'd better emit the divmod ¹ opcode”. Whereas in the second case the optimiser is well within its right to shrug and leave it at that.

As a general rule (with many exceptions though), the cleanest code which has its semantics stated directly is also the code that is easiest to optimise.

---

¹ For a given processor, the opcode name may differ.

Answer 3

Perhaps you are creating a solution to a problem that <cstdlib> 's std::div is provided to solve?

namespace std
{

struct div_t
{
    int quot;
    int rem;
};
struct ldiv_t
{
    long int quot;
    long int rem;
};
struct lldiv_t
{
    long long int quot;
    long long int rem;
};

  div_t div ( int numer, int denom );
 ldiv_t div ( long int numer, long int denom );
lldiv_t div ( long long int numer, long long int denom );

}; // namespace std

http://www.cplusplus.com/reference/cstdlib/div/