conflicts between combine and pre global passes?

["Bin.Cheng" <amker dot cheng at gmail dot com>]

Hi,
I noticed that global passes before combine, like loop-invariant/cprop/cse2 some
time have conflicts with combine.
The combine pass can only operates with basic block, while these global passes
move insns across basic block and left no description info.

For example, a case I encountered.

(insn 77 75 78 8 (set (reg:SI 171)
        (const_int 9999 [0x270f])) diffmeasure/verify.c:157 176
{*thumb1_movsi_insn}
     (nil))

(insn 78 77 79 8 (set (reg:SI 172)
        (const_int 0 [0])) diffmeasure/verify.c:157 176 {*thumb1_movsi_insn}
     (nil))

(insn 79 78 80 8 (set (reg:SI 170)
        (plus:SI (plus:SI (reg:SI 172)
                (reg:SI 172))
            (geu:SI (reg:SI 171)
                (reg/v:SI 138 [ num ])))) diffmeasure/verify.c:157 226
{thumb1_addsi3_addgeu}
     (expr_list:REG_DEAD (reg:SI 172)
        (expr_list:REG_DEAD (reg:SI 171)
            (expr_list:REG_EQUAL (geu:SI (const_int 9999 [0x270f])
                    (reg/v:SI 138 [ num ]))
                (nil)))))

(insn 80 79 81 8 (set (reg:QI 169)
        (subreg:QI (reg:SI 170) 0)) diffmeasure/verify.c:157 187
{*thumb1_movqi_insn}
     (expr_list:REG_DEAD (reg:SI 170)
        (nil)))

(insn 81 80 82 8 (set (reg:SI 173)
        (zero_extend:SI (reg:QI 169))) diffmeasure/verify.c:157 158
{*thumb1_zero_extendqisi2_v6}
     (expr_list:REG_DEAD (reg:QI 169)
        (nil)))

(jump_insn 82 81 124 8 (set (pc)
        (if_then_else (eq (reg:SI 173)
                (const_int 0 [0]))
            (label_ref:SI 129)
            (pc))) diffmeasure/verify.c:157 196 {cbranchsi4_insn}
     (expr_list:REG_DEAD (reg:SI 173)
        (expr_list:REG_BR_PROB (const_int 900 [0x384])
            (nil)))
 -> 129)

can be combined into :
(insn 77 75 78 8 (set (reg:SI 171)
        (const_int 9999 [0x270f])) diffmeasure/verify.c:157 176
{*thumb1_movsi_insn}
     (nil))

(note 78 77 79 8 NOTE_INSN_DELETED)

(note 79 78 80 8 NOTE_INSN_DELETED)

(note 80 79 81 8 NOTE_INSN_DELETED)

(note 81 80 82 8 NOTE_INSN_DELETED)

(jump_insn 82 81 124 8 (set (pc)
        (if_then_else (ltu (reg:SI 171)
                (reg/v:SI 138 [ num ]))
            (label_ref:SI 129)
            (pc))) diffmeasure/verify.c:157 196 {cbranchsi4_insn}
     (expr_list:REG_DEAD (reg:SI 171)
        (expr_list:REG_BR_PROB (const_int 900 [0x384])
            (nil)))
 -> 129)

BUT, if pre-combine passes propagate register 172 in insn79 and delete insn78,
the resulting instructions will not be combined.

I am not sure how to handle this, so any advice will be very appreciated.
Thanks.

-- 
Best Regards.