[Bug target/44883] Combine separate shift and add instructions into a single one

[carrot at google dot com]

------- Comment #2 from carrot at google dot com  2010-07-09 00:04 -------
(In reply to comment #1)
> >So in function fwprop_addr before deciding propagate an expression should we
> also check if it is the only use of the corresponding def?
> 
> It does somewhat.  Though address cost might be lower for r2+r3 than just r8. 
> Please make sure that fwprop_addr has the correct address cost.
> 

It occurs before register allocation, it is hard to say (plus (reg 144) (reg
143)) is cheaper than (reg 137).

But the address cost looks really strange. The arm/thumb2 address cost function
is

arm_arm_address_cost (rtx x)
{
  enum rtx_code c  = GET_CODE (x);
  if (c == PRE_INC || c == PRE_DEC || c == POST_INC || c == POST_DEC)
    return 0;
  if (c == MEM || c == LABEL_REF || c == SYMBOL_REF)
    return 10;
  if (c == PLUS)
    {
      if (GET_CODE (XEXP (x, 1)) == CONST_INT)
        return 2;
      if (ARITHMETIC_P (XEXP (x, 0)) || ARITHMETIC_P (XEXP (x, 1)))
        return 3;
      return 4;
    }
  return 6;
}

Give it a single register, the cost is 6. Give it the rtx (plus (reg 144) (reg
143)), the cost is 4. So a single register is more expensive than a plus
expression, not reasonable.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44883