[Bug target/43597] Move and compare with 0 can be combined

["steven at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from steven at gcc dot gnu dot org  2010-03-31 14:59 -------
In .final, the insns look like this:
@(insn:TI 9 7 8 t.c:8 (set (reg:CC 24 cc)
@        (compare:CC (reg:SI 0 r0 [orig:134 f ] [134])
@            (const_int 0 [0x0]))) 220 {*arm_cmpsi_insn} (expr_list:REG_DEAD
(reg:SI 0 r0 [orig:134 f ] [134])
@        (nil)))
        cmp     r0, #0  @ 9     *arm_cmpsi_insn/1       [length = 4]
@(insn 8 9 10 t.c:7 (set (reg/v:SI 5 r5 [orig:134 f ] [134])
@        (reg:SI 0 r0)) 658 {*thumb2_movsi_insn} (nil))
        mov     r5, r0  @ 8     *thumb2_movsi_insn/1    [length = 4]



This, AFAICT, would match with the pattern and constraints of movsi_compare0:

;; If copying one reg to another we can set the condition codes according to
;; its value.  Such a move is common after a return from subroutine and the
;; result is being tested against zero.

(define_insn "*movsi_compare0"
  [(set (reg:CC CC_REGNUM)
        (compare:CC (match_operand:SI 1 "s_register_operand" "0,r")
                    (const_int 0)))
   (set (match_operand:SI 0 "s_register_operand" "=r,r")
        (match_dup 1))]
  "TARGET_32BIT"
  "@
   cmp%?\\t%0, #0
   sub%.\\t%0, %1, #0"
  [(set_attr "conds" "set")]
)


But this insns can probably only be formed in the .combine pass, since (to the
best of my knowledge) we don't construct multiple-set insns in any other pass.

In the .combine pass, the two insns look like this (obviously reversed, as you
could already suspect from the insns numbers 9 and 8 above):

(insn 8 7 9 2 t.c:7 (set (reg/v:SI 134 [ f ])
        (reg:SI 0 r0)) 658 {*thumb2_movsi_insn} (expr_list:REG_DEAD (reg:SI 0
r0)
        (nil)))

(insn 9 8 10 2 t.c:8 (set (reg:CC 24 cc)
        (compare:CC (reg/v:SI 134 [ f ])
            (const_int 0 [0x0]))) 220 {*arm_cmpsi_insn} (nil))

But combine does not even try to combine these two insns:
------------
...
insn_cost 24: 0

Trying 9 -> 10:
Failed to match this instruction:
(set (pc)
    (if_then_else (lt (reg/v:SI 134 [ f ])
            (const_int 0 [0x0]))
        (label_ref:SI 29)
        (pc)))


foo4
...
------------


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43597