Re: Identical basic blocks live long in RTL flow.

[Jeff Law]

On 01/16/2013 02:28 AM, Bin.Cheng wrote:

Hi,
For below simple function from newlib:

static int
is_option (char *argv_element, int only)
{
   return ((argv_element == 0)
       || (argv_element[0] == '-') || (only && argv_element[0] == '+'));
}

The expanded rtl is like:

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

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.



jeff