Comment 6 for bug 1263576

I think the actual issue is with the code in aarch64_build_constant:

      /* zcount contains the number of additional MOVK instructions
  required if the constant is built up with an initial MOVZ instruction,
  while ncount is the number of MOVK instructions required if starting
  with a MOVN instruction. Choose the sequence that yields the fewest
  number of instructions, preferring MOVZ instructions when they are both
  the same. */
      if (ncount < zcount)
 {
   emit_move_insn (gen_rtx_REG (Pmode, regnum),
     GEN_INT ((~val) & 0xffff));
   tval = 0xffff;
 }
      else
 {
   emit_move_insn (gen_rtx_REG (Pmode, regnum),
     GEN_INT (val & 0xffff));
   tval = 0;
 }

The GEN_INT in the first if branch is incorrect as it truncates the immediate at 16-bits, and so we will never generate the "MOVN" form. What it should be instead is: GEN_INT (~((~val) & 0xffff)) or equivalent.