[Bug target/104039] AArch64 Redundant instruction moving general to vector register

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104039

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
   Last reconfirmed|                            |2022-01-15
     Ever confirmed|0                           |1
             Target|                            |aarch64
           Keywords|                            |ra
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.
The problem is the register allocator.
It goes from:
(insn 18 10 19 2 (set (reg:DI 99)
        (reg:DI 0 x0 [ a ])) "/app/example.cpp":4:49 53 {*movdi_aarch64}
     (expr_list:REG_DEAD (reg:DI 0 x0 [ a ])
        (nil)))
(insn 19 18 8 2 (set (reg:DI 100)
        (reg:DI 1 x1 [ b ])) "/app/example.cpp":4:49 53 {*movdi_aarch64}
     (expr_list:REG_DEAD (reg:DI 1 x1 [ b ])
        (nil)))
(insn 8 19 15 2 (set (reg:V2DI 95 [ a ])
        (subreg:V2DI (reg:DI 99) 0)) "/app/example.cpp":5:16 1206
{*aarch64_simd_movv2di}
     (expr_list:REG_DEAD (reg:DI 99)
        (nil)))
To:
(insn 8 10 15 2 (set (reg:V2DI 32 v0 [orig:95 a ] [95])
        (reg:V2DI 0 x0 [99])) "/app/example.cpp":5:16 1206
{*aarch64_simd_movv2di}
     (nil))

And then that gets split into:
(insn 22 10 23 2 (set (reg:V2DI 32 v0 [orig:95 a ] [95])
        (vec_concat:V2DI (reg:DI 0 x0 [99])
            (const_int 0 [0]))) "/app/example.cpp":5:16 1919
{move_lo_quad_internal_v2di}
     (nil))
(insn 23 22 15 2 (set (reg:V2DI 32 v0 [orig:95 a ] [95])
        (vec_concat:V2DI (vec_select:DI (reg:V2DI 32 v0 [orig:95 a ] [95])
                (parallel:V2DI [
                        (const_int 0 [0])
                    ]))
            (reg:DI 1 x1 [+8 ]))) "/app/example.cpp":5:16 1932
{aarch64_simd_move_hi_quad_v2di}
     (nil))

Maybe we should have done the expansion to use an fmov in the first place via a
special pattern which then gets expanded correctly might be better, I don't
know.