[Development] QVariant comparison in Qt6

[Thiago Macieira]

On Monday, 21 September 2020 22:55:16 PDT Lars Knoll wrote:
> I do not want to simply return an int, as the risk that people ignore the 
> Unordered state it too big with that. So the other choice would be to add
> an enum in the Qt namespace (as I need this for QMetaType as well, which
> currently has a std::optional<int> compare()).

For internal APIs, please return just a plain int.

For public ones, either call the internal or please use 
std::optional<qsizetype>. Code generation is slightly better. Compare the 
Clang panes at

qsizetype - https://gcc.godbolt.org/z/5bTbq6
int - https://gcc.godbolt.org/z/b6jn8s

The other three compilers just show how using std::optional is worse than an 
int: both GCC and ICC currently unnecessarily spill to the stack (you can 
avoid the spillage with std::optional<signed char>). MSVC always returns non-
primitives by implicit reference, so a memory access is always present.

All the memory accesses go away if you call an internal function: 
https://gcc.godbolt.org/z/rYde7c
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel DPG Cloud Engineering