[Bug target/102758] New: [x86] Failure to use registers optimally when swapping between (identically represented) vector types

["gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102758

            Bug ID: 102758
           Summary: [x86] Failure to use registers optimally when swapping
                    between (identically represented) vector types
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

#include <stdint.h>

typedef int64_t v2i64 __attribute__((vector_size(16)));
typedef uint16_t v8u16 __attribute__((vector_size(16)));

v2i64 f(v8u16 make_b_xxm1, v2i64 b)
{
    return (v2i64)((v8u16)b + (v8u16){1});
}

With -O3, GCC outputs this:

f(unsigned short __vector(8), long __vector(2)):
        movdqa  xmm2, XMMWORD PTR .LC0[rip]
        paddw   xmm2, xmm1
        movdqa  xmm0, xmm2
        ret

LLVM outputs this:

f(unsigned short __vector(8), long __vector(2)):
        movdqa  xmm0, xmm1
        paddw   xmm0, xmmword ptr [rip + .LCPI0_0]
        ret

It should be possible to optimize out the last `movdqa`. This seems to be
directly related to the usage of differing types here (even though the
conversion is cost-free) as replacing all usage of `v2i64` with `v8u16` makes
this be better optimized.