Re: [PATCH v2] Store_bit_field_1: Use SUBREG instead of REG if possible

[YunQiang Su <wzssyqa@gmail.com>]

>
> Note I think Andrews comment#7 in the PR is spot-on then, the issue
> isn't the bitfield inserts but the compare where combine elides
> the sign_extend in favor of a subreg.  That's likely some wrongdoing
> in simplify-rtx in the context of WORD_REGISTER_OPERATIONS.
>

Yes. There are 2 problems here. Any one of them can make this problem.
1) jump_insn eats sign_extend (and truncate) in
      /* Simplify X, an IF_THEN_ELSE expression.  Return the new expression.  */
      static rtx simplify_if_then_else (rtx x)

2) MIPS claims sign_extend to be deleted.
(define_insn_and_split "extendsidi2"
  [(set (match_operand:DI 0 "register_operand" "=d,l,d")
        (sign_extend:DI (match_operand:SI 1 "nonimmediate_operand" "0,0,m")))]
  "TARGET_64BIT"
  "@
   #
   #
   lw\t%0,%1"
  "&& reload_completed && register_operand (operands[1], VOIDmode)"
  [(const_int 0)]
{
  emit_note (NOTE_INSN_DELETED);
  DONE;
}
  [(set_attr "move_type" "move,move,load")
   (set_attr "mode" "DI")])

-- 
YunQiang Su