https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120409
--- Comment #14 from H.J. Lu <hjl.tools at gmail dot com> --- (In reply to Iain Sandoe from comment #13) > (In reply to H.J. Lu from comment #12) > > (In reply to Iain Sandoe from comment #5) > > > well .. the visible difference in gimple, but this is code outside of the > > > coroutines changes (i.e. just a bit of regular C++). > > > > > > However, it does seem we create the return object correctly .. and then > > > return an empty one. We seem to be failing to do copy elision. > > > > > > > On x32, OptionalPromiseReturn is 16 bytes, which is returned in 2 registers. > > You can't assume that copy elision will happen. You need to prepare for it. > > Hmm .. but I think copy elision is mandated by c++17 - so we need to figure > out a solution< i guess. There is no guarantee that all ABIs will pass and return struct in the same location, stack or register. I don't see how a high level language can mandate copy elision for the incoming argument on stack and the return value in registers. Copy elision won't happen on x64-64 if the return struct size is 16 bytes or less.
