https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109632
--- Comment #10 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> --- After prototyping this further, I no longer think that lowering at the gimple level is the best answer. (I should have listened to Richi.) Although it works, its major drawback is that it's one-sided: it allows the current function's PARM_DECLs and returns to be lowered to individual scalars, but it does nothing for calls to other functions. Being one-sided means (a) that lowering only solves half the problem and (b) that tail calls cannot be handled easily after lowering. One thing that does seem to work is to force the structure to have V2SF (and fix the inevitable ABI fallout). That could only be done conditionally, based on a target hook. But it seems to fix both test cases: the pass-by-reference one and the pass-by-value one.
