RVO not applied to object with large array

[arnetheduck]

type X = object
  v: array[1024*1024*8, int]

proc f(): X =
  result.v[0] = 1

var x = (ref X)()
x[] = f()
echo x.v[0]

note how huge object is placed on stack, crashing the application on linux (which has an 8mb default stack size)

N_LIB_PRIVATE N_NIMCALL(tyObject_X__KqwtR1mZdQ5eNccTtsN3pA, f__KQQBzps5MRoTwkbIifdBMQ)(void) {
        tyObject_X__KqwtR1mZdQ5eNccTtsN3pA result;
...
        return result;

[arnetheduck@tempus tmp]$ nim --version
Nim Compiler Version 1.2.0 [Linux: amd64]
Compiled at 2020-04-03
Copyright (c) 2006-2020 by Andreas Rumpf

active boot switches: -d:release

[mratsim]

var x = f() should be changed to

var x: ref X
new x
x[] = f()

Otherwise the caller would overflow as well.

Second, I think Nim is actually the C-array semantics there (so there is a conflict between RVO and C-arrays)

[arnetheduck]

fixed example - C array sematics are not relevant - in fact, if you change to f(): array[...] it'll pass return value by pointer like it should

[Araq]

Yeah, I noticed this too after I wrote the RFC... ;-)