[Issue 9666] Invalid struct passing + redundant struct copy on struct assignment

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9666



--- Comment #1 from Kenji Hara <k.hara.pg at gmail.com> 2013-03-08 05:00:03 PST ---
(In reply to comment #0)
> dmd generates two copies, first - when passing argument "d" to setDate(), and
> second - when assigning parameter "d" to x. The latter is redundant.
> 
> #dmd main.d -v -release
> ....
> function  e.setDate
> Date x = _D1e4Date6__initZ;
> x.opAssign((Date __cpcttmp6 = __cpcttmp6.__cpctor(d); , __cpcttmp6))
> function  D main
> Date d = _D1e4Date6__initZ;
> setDate((Date __cpcttmp7 = __cpcttmp7.__cpctor(d);, __cpcttmp7))
> ....
> 
> This frontend pseudo-code survives CT and is present in object file as a call
> to __cpctor inside setDate() asm output.

This is correct behavior. Date has user defined postblit, so its assignment
operator is implicitly generated as opAssign. And, it is implemented by
copy-and-swap method.

struct Date {
  this(this) { ... }
  ref Date opAssign(Date rhs) {
     swap(this, rhs);
  }
}

void setDate(Date d)
{
  Date x;
  //x = d;  // is lowered to:
  x.opAssign(d);  // d is copied.
            // and then it is swapped with x in Date.opAssign.
}

The copy in setDate is essentially necessary. It may be removed with _smart
optimizer_, but today's dmd doesn't do it enough.

So I think this is an enhancement that requests more optimization.

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