NRVO doesn't happen when return type contains an array

[timotheecour]

NRVO doesn't happen when return type contains an array

Example

when defined case7: # D20200427T030626
  type X1 = object
    a1: array[10,int] # no NRVO
  type X2 = object
    a2: seq[int] # NRVO
  proc fun1(): X1 = discard
  proc fun2(): X2 = discard
  proc main =
    var x1 = fun1()
    var x2 = fun2()
  main()

Current Output

cgen'd file shows:

N_LIB_PRIVATE N_NIMCALL(void, main__Txk9aMBuTQfWSxCVsnz9cpLw)(void) {
	tyObject_X1__8Rt52S9caw66JoaF6ps17vQ x1;
	tyObject_X2__HENPt9b0VcoiqRd762A4dqg x2;
	x1 = fun1__Mn2tcNiI7WmB3YthDb2xNA();
	nimZeroMem((void*)(&x2), sizeof(tyObject_X2__HENPt9b0VcoiqRd762A4dqg));
	fun2__opw8oBRugVywaf9af9cJzLnA((&x2));
}

Expected Output

x1 = fun1__Mn2tcNiI7WmB3YthDb2xNA();
should be:
fun1__Mn2tcNiI7WmB3YthDb2xNA(&1);

Possible Solution

• nim should apply a code transformation that avoids returning by copy and instead passes the return value by reference, eg like in RVO not exception safe nim-lang/Nim#14126 (comment)

Additional Information

• see does using result guarantee NRVO? - Nim forum