Reimplementing the bulk of std.meta iteratively

[Stefan Koch]

On Tuesday, 29 September 2020 at 12:38:42 UTC, Stefan Koch wrote:
> [continuing]

So we had this:

> int mul(
>   int x,
>   int y
> ) {
>   int result = 0;
>   for(int i = 0; i < x; i++)
>   {
>     result += y;
>   }
>   return result;
> }


And the code generated was

	imull	%esi, %edi // y *= x
	xorl	%eax, %eax // result = 0
	testl	%esi, %esi // if (y)
	cmovgl	%edi, %eax // if (x > 0) result = x;
         retq               // return result;

On first glance these instructions seem superfluous
But we have to understand that the compiler had no way of knowing 
that we actually wanted to generate a mere multilply.

For example there is the loop entrance check.
Why is it there?

We can proof it's necessity quite easily.
- let's assume we did not have it.
In that case we are allowd to enter loop body even if (x == 0)
so we would execute result += y; once.
Which would cause mul(0, 32) to result in 32. And that would be a 
violation of the semantics.

So there's no way to get rid of the check.
And this code will always perform worse than an intrinsic 'mul' 
which the compiler knows about.