[Bug target/113027] New: aarch64 is missing vec_set and vec_extract for structure modes

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113027

            Bug ID: 113027
           Summary: aarch64 is missing vec_set and vec_extract for
                    structure modes
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
  Target Milestone: ---

The lack of vec_set and vec_extract optabs for structure modes means that the
following testcase spills to the stack when compiled at -O2:

#include <arm_neon.h>

float64x2x2_t f1 (float64x2x2_t x) { x.val[0][1] += 1.0; return x; }
float64x2x3_t f2 (float64x2x3_t x) { x.val[0][0] = x.val[1][1] + x.val[2][0];
return x; }
float64x2x4_t f3 (float64x2x4_t x) { x.val[0][0] = x.val[1][1] + x.val[2][0] -
x.val[3][1]; return x; }

For example:

f1:
        sub     sp, sp, #32
        fmov    d31, 1.0e+0
        st1     {v0.2d - v1.2d}, [sp]
        ldr     d30, [sp, 8]
        fadd    d31, d31, d30
        str     d31, [sp, 8]
        ld1     {v0.2d - v1.2d}, [sp]
        add     sp, sp, 32
        ret

With the extra patterns, we instead get:

f1:
        dup     d31, v0.d[1]
        fmov    d30, 1.0e+0
        fadd    d30, d31, d30
        ins     v0.d[1], v30.d[0]
        ret
f2:
        dup     d31, v1.d[1]
        fadd    d31, d31, d2
        ins     v0.d[0], v31.d[0]
        ret
f3:
        dup     d31, v1.d[1]
        dup     d30, v3.d[1]
        fadd    d31, d31, d2
        fsub    d30, d31, d30
        ins     v0.d[0], v30.d[0]
        ret

Fixing this might also make it possible to use structure modes for arrays (c.f.
PR109543).