[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
Tue Dec 18 17:43:00 GMT 2018
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #6)
> I think subreg should deal with this. What do you mean "there aren't
> any half of TImode subregs"? insn 10 has it split at the start, and
> insn 18 at the end wants it split, too.
By that I mean that there aren't any (subreg:DI (reg:TI) ) destinations or
similar. Admittedly lower-subreg also handles shifts, and a few others, but
not vec_concat etc.
V2DI move: original cost 4, split cost 4 * 2
so it decides not to do anything about V2DImode (generally the right thing).
So, we'd need to pattern recognize a vec_concat from scalar modes (integral
only) and the result used (single use) in a subreg to twice that big integral
handle it as if it was setting the two halves of the wider reg.
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