[Bug c++/37922] [4.4 Regression] code generation error

["jakub at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #11 from jakub at gcc dot gnu dot org  2008-12-16 13:09 -------
The problem is that CSE2 extends the live range of CC register, so before DSE1
we have in _ZN11rot_mx_infoC1ERK6rot_mx function:
(insn 15 14 16 2 pr37922.C:469 (set (reg:CC 17 flags)
        (compare:CC (reg/v:SI 75 [ proper_order ])
            (const_int 0 [0x0]))) 5 {*cmpsi_1_insn} (nil))

(jump_insn 16 15 17 2 pr37922.C:469 (set (pc)
        (if_then_else (eq (reg:CC 17 flags)
                (const_int 0 [0x0]))
            (label_ref:DI 128)
            (pc))) 618 {*jcc_1} (expr_list:REG_DEAD (reg:CC 17 flags)
        (expr_list:REG_BR_PROB (const_int 3900 [0xf3c])
            (nil))))
...
(insn 28 27 30 3 pr37922.C:472 (set (reg:SI 71 [ proper_r$den_ ])
        (mem/s/c:SI (plus:DI (reg/f:DI 20 frame)
                (const_int -60 [0xffffffffffffffc4])) [7 proper_r.den_+0 S4
A32])) 47 {*movsi_1} (nil))

(jump_insn 30 28 31 3 pr37922.C:475 (set (pc)
        (if_then_else (lt (reg:CC 17 flags)
                (const_int 0 [0x0]))
            (label_ref 35)
            (pc))) 618 {*jcc_1} (expr_list:REG_DEAD (reg:CC 17 flags)
        (expr_list:REG_BR_PROB (const_int 2700 [0xa8c])
            (nil))))

and no insn in between jump_insn 16 and 30 touch the flags register (insn 29
which was comparing (reg:SI 75) and (const_int 0) was eliminated during CSE2).

DSE in 4.3/4.4 then attempts to optimize the store of a 64-bit value in insn 27
followed by 32-bit read from the upper 32-bits of that value in insn 28, but
the
sequence it inserts for it (shift down by 32 bytes) unfortunately clobbers CC
register, but replace_read doesn't check for that.
Also, the REG_DEAD note which wasn't removed during CSE2 looks weird.
I guess in replace_read before calling validate_change we should check if insns
don't clobber or modify any hard registers that are live at that point, but am
not sure if DF is initialized for such kind of query at this point.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zadeck at gcc dot gnu dot
                   |                            |org
             Status|WAITING                     |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-12-16 13:09:52
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37922