[Bug target/71663] New: aarch64 Vector initialization can be improved slightly

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71663

            Bug ID: 71663
           Summary: aarch64 Vector initialization can be improved slightly
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*-*-*

Take:
#define vector __attribute__((vector_size(16)))

vector float combine (float a, float b, float c, float d)
{
  return (vector float) { a, b, c, d };
}

--- CUT ---
Currently we produce:
        movi    v4.4s, 0
        ins     v4.s[0], v0.s[0]
        ins     v4.s[1], v1.s[0]
        ins     v4.s[2], v2.s[0]
        orr     v0.16b, v4.16b, v4.16b
        ins     v0.s[3], v3.s[0]
        ret

The movi is not needed and if we did it correctly, the move (orr) is not needed
either.  Even the first ins is not needed either.

Right now we expand the first element as:
(insn 9 8 10 (set (reg:SF 80)
        (reg/v:SF 74 [ a ])) t8.c:5 -1
     (nil))

(insn 10 9 11 (set (reg:V4SF 79)
        (vec_merge:V4SF (vec_duplicate:V4SF (reg:SF 80))
            (reg:V4SF 79)
            (const_int 1 [0x1]))) t8.c:5 -1
     (nil))

But maybe if we do:
(set (reg:V4SF 79) (subreg:V4SF (reg:SF 74)))
We could remove the movi and ins.  The last move would remove itself too
because v0 is dead after the instruction.