[Issue 12308] Request pragma for very thin struct wrappers

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12308



--- Comment #2 from Adam D. Ruppe <destructionator at gmail.com> 2014-03-07 07:18:14 PST ---
I don't think it has been explained fully yet. A struct with just an int is not
fully interchangeable for an int because they are returned differently by
functions, including in C. So this isn't really a matter of fixing anything,
since it isn't broken, it is a different spec.

indeed, since it changes the calling convention, how would this mangle? I'd say
the same as the type it wraps. The struct just disappears as far as the backend
is concerned.


A few other points in the thread: couldn't the hidden this be changed to pass
by value? Maybe, though taking the address of this wouldn't work, I think.

   A* ptr() { return &this; }

works now, just returning EAX (which is the hidden this pointer in the first
place). Trying that with a regular int is (awesomely!) an error:

int* foo(int a) { return &a; }
test56.d(17): Error: escaping reference to local a

* * *

Could we just change the D calling convention to pass small structs by value*
while keeping extern(C) the same for compatibility? Yes, I think we can, and I
think it would be a win. But I'd still like it to be available in extern(C) too
for cases like library typedef, where we want it to look different on the D
side, but remain ABI identical to the naked type.


* Note: this is already what happens for most function arguments:

void test(A a);

calling that looks like:
   mov EAX, [a];
   call test;

Which is identical to test(int a).


The difference is returning a value and the hidden this pointer. Returning a
value is done by hidden pointer too.

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