[Bug target/69493] New: Poor code generation for return of struct containing vectors on PPC64LE

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69493

            Bug ID: 69493
           Summary: Poor code generation for return of struct containing
                    vectors on PPC64LE
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wschmidt at gcc dot gnu.org
                CC: dje at gcc dot gnu.org, munroesj at us dot ibm.com
  Target Milestone: ---
            Target: powerpc64le-*-linux*

For this simple test using the ELFv2 ABI, we generate code that correctly
returns the structures in v2 and v3 (vs34 and vs35).  However, there is an
unnecessary store and reload of the vectors prior to the return, which will
perform extremely poorly on POWER processors.

 typedef struct
 {
   __vector double vx0;
   __vector double vx1;
 } vdoublex4_t;

  vdoublex4_t
  test_big_double (long double a, long double b)
  {
    vdoublex4_t result;
    /* return a 0.0 */
    result.vx0[0] = __builtin_unpack_longdouble (a, 0);
    result.vx0[1] = __builtin_unpack_longdouble (a, 1);
    result.vx1[0] = __builtin_unpack_longdouble (b, 0);
    result.vx1[1] = __builtin_unpack_longdouble (b, 1);

    return (result);
  }

$ gcc -S -O2 poor.c

poor.s:
        .file   "poor.c"
        .abiversion 2
        .section        ".toc","aw"
        .section        ".text"
        .align 2
        .p2align 4,,15
        .globl test_big_double
        .type   test_big_double, @function
test_big_double:
        fmr 10,1
        fmr 11,3
        addi 8,1,-96
        li 10,32
        xxlxor 12,12,12
        li 9,48
        xxlor 0,12,12
        xxpermdi 12,12,10,0
        xxpermdi 0,0,11,0
        xxpermdi 12,2,12,1
        xxpermdi 0,4,0,1
        xxpermdi 12,12,12,2
        xxpermdi 0,0,0,2
        stxvd2x 12,8,10
        stxvd2x 0,8,9
        lxvd2x 34,8,10
        lxvd2x 35,8,9
        xxpermdi 34,34,34,2
        xxpermdi 35,35,35,2
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
        .size   test_big_double,.-test_big_double
        .ident  "GCC: (GNU) 6.0.0 20160125 (experimental) [trunk revision
232783]"
        .section        .note.GNU-stack,"",@progbits