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

[rguenth at gcc dot gnu.org]

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

--- Comment #14 from Richard Biener <rguenth at gcc dot gnu.org> ---
Just looking at what we feed combine:

(insn 9 8 15 2 (set (reg:V2DI 89)
        (vec_concat:V2DI (reg:DI 90 [ num ])
            (reg:DI 92))) "t.c":9:12 4182 {vec_concatv2di}
     (expr_list:REG_DEAD (reg:DI 92)
        (expr_list:REG_DEAD (reg:DI 90 [ num ])
            (nil))))
(insn 15 9 16 2 (set (reg/i:TI 0 ax)
        (subreg:TI (reg:V2DI 89) 0)) "t.c":10:1 65 {*movti_internal}
     (expr_list:REG_DEAD (reg:V2DI 89)
        (nil)))

I wonder why we can't "simplify" this into individual sets of the
hardreg pair?  fwprop sees the same thing so that's another possible
fixing point.  Not sure if the backend in the end would like to
see the above TImode set decomposed though...