[Bug rtl-optimization/84101] [7/8/9 Regression] -O3 and -ftree-vectorize trying too hard for function returning trivial pair-of-uint64_t-structure

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84101

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
#c4 has another thing:
struct S { float a, b; };
struct S
foo (float x)
{
  return (struct S) { x, x };
}
where we generate bad code mainly because we have DImode as DECL_MODE of the
RESULT_DECL, even when it is a structure containing two floats.  Not sure what
we'd get e.g. if we treated it like V2SFmode instead, but that is problematic
because it is MMX-ish mode, or if it would be possible to combine it into a
vec_concatv2sf somehow.  Though, that should be really tracked in a different
PR.