Question

Sometimes, mainly for optimization purposes, very simple operations are implemented as complicated and clumsy code.

One example is this integer initialization function:

void assign( int* arg )
{
    __asm__ __volatile__ ( "mov %%eax, %0" : "=m" (*arg));
}

Then:

int a;
assign ( &a );

But actually I don’t understand why is it written in this way…

Have you seen any example with real reasons to do so?

Answer 1

In the case of your example, I think it is a result of the fallacious assumption that writing code in assembly is automatically faster.

The problem is that the person who wrote this didn’t understand WHY assembly can sometimes run faster. That is, you know more than the compiler what you are trying to do and can sometimes use this knowledge to write code at a lower level that is more performant based on not having to make assumptions that the compiler will.

In the case of a simple variable assignment, I seriously doubt that holds true and the code is likely to perform slower because it has the additional overhead of managing the assign function on the stack. Mind you, it won’t be noticeably slower, the main cost here is code that is less readable and maintainable.

This is a textbook example of why you shouldn’t implement optimizations without understanding WHY it is an optimization.