[Bug middle-end/99293] Built-in vec_splat generates sub-optimal code for -mcpu=power10

[segher at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99293

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-02-27
     Ever confirmed|0                           |1

--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
It generates non-optimal code for older CPUs as well (it does two splats
instead of one:

        xxpermdi 0,35,35,3       # 7    [c=4 l=4]  vsx_extract_v2di/1
        xxpermdi 35,0,0,0        # 9    [c=4 l=4]  vsx_splat_v2di_reg/0
        vrlq 2,2,3

This is because we get things like

Trying 7 -> 9:
    7: r117:DI=vec_select(r127:V1TI#0,parallel)
      REG_DEAD r127:V1TI
    9: r124:V2DI=vec_duplicate(r117:DI)
      REG_DEAD r117:DI
Failed to match this instruction:
(set (reg:V2DI 124)
    (vec_duplicate:V2DI (vec_select:DI (subreg:V2DI (reg:V1TI 127) 0)
            (parallel [
                    (const_int 0 [0])
                ]))))

(the patterns we do have use vec_concat instead).

Confirmed.