Why is this not optimized?

[Bingfeng Mei]

Hi, 
I am looking at some code of our target, which is not optimized as expected. 
For the following RTX, I expect source of insn 17 should be propagated into 
insn 20, and insn 17 is eliminated as a result. On our target, it will become a 
predicated xor instruction instead of two. Initially, I thought fwprop pass 
should do this. 

(insn 17 16 18 3 (set (reg/v:HI 102 [ crc ])
        (xor:HI (reg/v:HI 108 [ crc ])
            (const_int 16386 [0x4002]))) coremark.c:1632 725 {xorhi3}
     (nil))
(insn 18 17 19 3 (set (reg:BI 113)
        (ne:BI (reg:QI 101 [ D.4446 ])
            (const_int 1 [0x1]))) 1397 {cmp_qimode}
     (nil))
(jump_insn 19 18 55 3 (set (pc)
        (if_then_else (ne (reg:BI 113)
                (const_int 0 [0]))
            (label_ref 23)
            (pc))) 1477 {cbranchbi4}
     (expr_list:REG_DEAD (reg:BI 113)
        (expr_list:REG_BR_PROB (const_int 7100 [0x1bbc])
            (expr_list:REG_PRED_WIDTH (const_int 1 [0x1])
                (nil))))
 -> 23)
(note 55 19 20 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 20 55 23 4 (set (reg:HI 112 [ crc ])
        (reg/v:HI 102 [ crc ])) 502 {fp_movhi}
     (expr_list:REG_DEAD (reg/v:HI 102 [ crc ])
        (nil)))
(code_label 23 20 56 5 2 "" [1 uses])


But it can't. First propagate_rtx_1 will return false because PR_CAN_APPEAR is 
false and following code is executed. 

  if (x == old_rtx)
    {
      *px = new_rtx;
      return can_appear;
    }

Even I forces PR_CAN_APPEAR to be set in flags, fwprop still won't go ahead in 
try_fwprpp_subst because old_cost is 0 (REG only rtx), and set_src_cost 
(SET_SRC (set), speed) is bigger than 0. So the change is deemed as not 
profitable, which is not correct IMO. 

If fwprop is not the place to do this optimization, where should it be done? I 
am working on up-to-date GCC 4.8. 

Thanks,
Bingfeng Mei