[Bug target/88828] New: Inefficient update of the first element of vector registers

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88828

            Bug ID: 88828
           Summary: Inefficient update of the first element of vector
                    registers
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: ubizjak at gmail dot com
  Target Milestone: ---

[hjl@gnu-cfl-1 pr88778]$ cat u4.i
typedef float __v4sf __attribute__ ((__vector_size__ (16)));
typedef float __m128 __attribute__ ((__vector_size__ (16), __may_alias__));

__m128
foo (__m128 x, float f)
{
  __m128 y =  __extension__ (__m128)(__v4sf)
  { f, ((__v4sf) x)[1], ((__v4sf) x)[2], ((__v4sf) x)[3] };
  return y;
}
[hjl@gnu-cfl-1 pr88778]$ gcc -S -O2 u4.i
[hjl@gnu-cfl-1 pr88778]$ cat u4.s
        .file   "u4.i"
        .text
        .p2align 4,,15
        .globl  foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        movaps  %xmm0, %xmm4
        movaps  %xmm0, %xmm3
        shufps  $85, %xmm0, %xmm4
        unpckhps        %xmm0, %xmm3
        unpcklps        %xmm4, %xmm1
        shufps  $255, %xmm0, %xmm0
        unpcklps        %xmm0, %xmm3
        movaps  %xmm1, %xmm0
        movlhps %xmm3, %xmm0
        ret
        .cfi_endproc
.LFE0:
        .size   foo, .-foo
        .ident  "GCC: (GNU) 8.2.1 20190109 (Red Hat 8.2.1-7)"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-cfl-1 pr88778]$ 

A simple movss will do.