[Bug rtl-optimization/48037] Missed optimization: unnecessary register moves

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48037

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-14 15:45:53 UTC ---
(In reply to comment #4)
> > --- Comment #3 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-03-09 12:01:10 UTC ---
> >> As of the register moves you are seeing, we have the long-time known problem
> >> that we fail to allocate registers in a way to have the function return
> >> value in-place.  Maybe we are just confusing IRA with the explicit move
> >> to that register?
> >>
> >> (insn 13 11 18 2 (set (reg:V2DF 72)
> >>         (vec_concat:V2DF (reg:DF 67)
> >>             (reg:DF 69))) t.c:8 1557 {*vec_concatv2df}
> >>      (expr_list:REG_DEAD (reg:DF 69)
> >>         (expr_list:REG_DEAD (reg:DF 67)
> >>             (nil))))
> >>
> >> (insn 18 13 21 2 (set (reg/i:V2DF 21 xmm0)
> >>         (reg:V2DF 72)) t.c:10 1127 {*movv2df_internal}
> >>      (expr_list:REG_DEAD (reg:V2DF 72)
> >>         (nil)))
> >>
> >> why is combine not able to change this to
> >>
> >> (insn 13 11 18 2 (set (reg/i:V2DF 21 xmm0)
> >>         (vec_concat:V2DF (reg:DF 67)
> >>             (reg:DF 69))) t.c:8 1557 {*vec_concatv2df}
> >>      (expr_list:REG_DEAD (reg:DF 69)
> >>         (expr_list:REG_DEAD (reg:DF 67)
> >>             (nil))))
> >>
> >> ? (it doesn't even try this combination)
> > 
> > Probably because of the check in cant_combine_insn_p (SSE_FIRST_REG is likely
> > spilled on x86).
> Right.  I recently looked into a similar problem; ISTM we ought to be
> able to come up with better rules than never combine if one of the regs
> is a likely spilled reg.

True, though relying on the combiner is also somewhat odd.  Why does
the RA not consider coalescing the return register with a pseudo?