Should we use R_ARM_MOVT_PREL for ARM PIC?

[Rafael Espíndola via llvm-commits]

While refactoring some code I noticed that

@a = hidden global i32 42
define i32* @f()  {
  ret i32* @a
}

produces

        movw    r0, :lower16:(_a-(LPC0_0+8))
        movt    r0, :upper16:(_a-(LPC0_0+8))
LPC0_0:
        add     r0, pc, r0

When targeting MachO but


        ldr     r0, .LCPI0_0
.LPC0_0:
        add     r0, pc, r0
...

.LCPI0_0:
        .long   a-(.LPC0_0+8)

When targeting ELF.

We could use R_ARM_MOVT_PREL on ELF to use a movw/movt pair and an add
instead of a load and an add. Would that be an improvement? We use
movw/movt for non PIC code, so I assume so.  Should I open a bug?

On a similar note, why there is no movw/movt pc relative relocation
that points to a got entry? That would be handy for when the variable
can be preempted.

Cheers,
Rafael