[Bug target/16800] PowerPC - Unnecessary rldicl

["amodra at bigpond dot net dot au" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From amodra at bigpond dot net dot au  2004-11-24 07:00 -------
I believe this problem is caused by rs6000_rtx_costs.  The two following insns
are successfully combined:

(insn 20 19 21 0 (set (reg:DI 126)
        (eq:DI (reg:CCFP 125)
            (const_int 0 [0x0]))) 410 {*rs6000.md:11408} (insn_list:REG_DEP_TRUE
19 (nil))
    (expr_list:REG_DEAD (reg:CCFP 125)
        (nil)))

(insn 21 20 22 0 (set (reg:SI 124)
        (subreg:SI (reg:DI 126) 4)) 280 {*movsi_internal1}
(insn_list:REG_DEP_TRUE 20 (nil))
    (expr_list:REG_DEAD (reg:DI 126)
        (nil)))

to

(insn 21 20 22 0 (set (reg:SI 124)
        (eq:SI (reg:CCFP 125)
            (const_int 0 [0x0]))) 409 {*rs6000.md:11376} (insn_list:REG_DEP_TRUE
20 (nil))
    (expr_list:REG_DEAD (reg:DI 126)
        (nil)))

Following that, the new insn 21 and

(insn 22 21 23 0 (set (reg:DI 121)
        (zero_extend:DI (reg:SI 124))) 24 {*rs6000.md:379}
(insn_list:REG_DEP_TRUE 21 (nil))
    (expr_list:REG_DEAD (reg:SI 124)
        (nil)))

is a combine candidate, with a good result pattern of

(set (reg:DI 121)
    (eq:DI (reg:CCFP 125)
        (const_int 0 [0x0])))

However, rs6000_rtx_costs decides this patten is more expensive than the
previous two instructions.

rejecting combination of insns 21 and 22
original costs 12 + 4 = 16
replacement cost 20

Code in question is as follows.  I don't understand why these particular
comparisons have been singled out, or why the mode == Pmode test.
This contributes 12

    case EQ:
    case GTU:
    case LTU:
      if (mode == Pmode)
	{
	  switch (outer_code)
	    {
	    case SET:
	      *total = COSTS_N_INSNS (3);
	      break;

The (const_int 0) contributes 8!  This is because outer_code is EQ, which
doesn't match anything in the rather long set of outer_code tests for CONST_INT.

An obvious fix is to handle EQ and other comparisons here.



-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dje at gcc dot gnu dot org
   Last reconfirmed|2004-08-02 01:02:22         |2004-11-24 07:00:16
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16800