Re: Identical basic blocks live long in RTL flow.

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

On Thu, Jan 17, 2013 at 1:29 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >
>> >Basic blocks 8/9/10 are identical and live until pass jump2, which is
>> >after register allocation.
>> >I think these duplicated BBs do not contain additional information and
>> >should be better to be removed ASAP, because they might interfere with
>> >other passes like ifcvt.
>> >
>> >So should this issue be handled like in jump pass?
>> In this case, I'd expect DCE to optimize away the redundant assignments.
>
> Yeah, we also used to be able to optimize these out in crossjumping before
> regalloc but with SSA we now produce different pseudos in different copies so I
> think it was path by Steven that disabled crossjumping proir jump2 (both passes
> also ought to be renamed)
>

Unfortunately, this case can't be handled by DCE because just before
pass pass_into_cfg_layout_mode, the rtl is like:


    1: NOTE_INSN_DELETED
    9: NOTE_INSN_BASIC_BLOCK 2
    2: r113:SI=r0:SI
    3: r114:SI=r1:SI
    4: NOTE_INSN_FUNCTION_BEG
   11: pc={(r113:SI==0)?L39:pc}
      REG_BR_PROB 0x3f6
   12: NOTE_INSN_BASIC_BLOCK 4
   13: r111:SI=zero_extend([r113:SI])
   14: pc={(r111:SI==0x2d)?L43:pc}
      REG_BR_PROB 0x7c7
   15: NOTE_INSN_BASIC_BLOCK 5
   16: pc={(r114:SI!=0)?L20:pc}
      REG_BR_PROB 0x1388
   22: L22:
   17: NOTE_INSN_BASIC_BLOCK 6
    6: r110:SI=0
   18: pc=L24
   19: barrier
   20: L20:
   21: NOTE_INSN_BASIC_BLOCK 7
   23: pc={(r111:SI!=0x2b)?L22:pc}
      REG_BR_PROB 0x1f49
   35: NOTE_INSN_BASIC_BLOCK 8
    8: r110:SI=0x1
   36: pc=L24
   37: barrier
   39: L39:
   38: NOTE_INSN_BASIC_BLOCK 9
    7: r110:SI=0x1
   40: pc=L24
   41: barrier
   43: L43:
   42: NOTE_INSN_BASIC_BLOCK 10
    5: r110:SI=0x1
   24: L24:
   25: NOTE_INSN_BASIC_BLOCK 11
   26: r112:SI=r110:SI
   30: r0:SI=r112:SI
   33: use r0:SI

Insns 36/40 is deleted in pass pass_into_cfg_layout_mode to make each
edge possibly fall-through. The edges <8, 11> and <9, 11> still exist
in control flow and insns 7/8 are not dead code. It's just coincident
to have three identical basic blocks here. I don't think any passes
other than cross-jumping can handle this, right?

Thanks
--
Best Regards.