DIP 1014:Hooking D's struct move semantics--Community Review Round 1

[Shachar Shemesh]

On 17/05/18 22:29, Manu wrote:
> 
> This is great!
> I've wanted this on numerous occasions when interacting with C++ code.
> This will make interaction more complete.
> 
> Within self-contained D code, I have avoided self-pointers by using
> self-offsets instead in the past (a bit hack-ey). But this nicely
> tidies up one more little rough-edge in the language.
> 
> :+1:
> 

Following Andrei's advice, I've actually started writing a couple of 
examples to illustrate why this is needed.

The first was this:

struct S {
   static int global;
   int local;

   Something selector; // Decides whether we want local or global.
}

Let's further assume that we have an array of S instances with random 
uniform distribution between local and global.

Obviously, Something can be an enum or a boolean. If it is, however, 
then we have to perform a condition to select the correct value. The 
problem with conditionals is that if the CPU misses a guess about what 
they are (and in our case, the CPU is going to miss about 50% of the 
time), they are extremely expensive to evaluate.

Performance wise, a much saner approach is:
alias Something = int*;

Of course, this means our struct now has a self referencing pointer.

What I'm getting at is that even if there are alternatives to structs 
pointing at themselves, they may not be performance wise comparable to 
pointers.

Of course, the second example was a struct that has no internal 
pointers, but rather maintains global pointers pointing to it. This 
problem is quite a bit harder to solve.

Shachar