pass a struct by value/ref and size of the struct

[kinke via Digitalmars-d-learn]

On Tuesday, 22 March 2016 at 07:35:49 UTC, ZombineDev wrote:
> If the object is larger than the size of a register on the 
> target machine, it is implicitly passed by ref

That's incorrect. As Johan pointed out, this is somewhat true for 
the Win64 ABI (but it firstly copies the argument before passing 
a pointer to it!), but it's not for the 32-bit x86 and x86_64 
System V (used on all non-Windows platforms) ABIs. System V is 
especially elaborate and may pass structs up to twice the size of 
a register in 2 registers. Bigger structs passed by value are 
blitted into the function arguments stack in memory. They are 
then accessed by the callee via a stack offset, that's correct, 
but I wouldn't call that implicit-by-ref-passing, as copying does 
take place, unless the optimizer decides it's unnecessary.

So passing structs > 64-bit by value on Win64 never pays off 
(there's always an indirection); using `const ref(T)` where 
possible makes sure you at least elide the copy. But then again, 
you'll very soon find out that it's not really an option as 
rvalues cannot be passed byref in D, something a lot of people 
[including myself if not already obvious :)] hate about D.