[Bug tree-optimization/94846] Failure to optimize jnc+inc into adc

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94846

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-11-25
             Status|UNCONFIRMED                 |NEW
                 CC|                            |rguenth at gcc dot gnu.org
          Component|rtl-optimization            |tree-optimization

--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
This looks like a tree-optimization problem. A store to *p_5(D) could sink all
the way to bb5. RTL gets expanded from:

  ...
  if (_10 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870912]:
  *p_5(D) = _1;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 536870913]:
  _2 = _1 + 1;
  *p_5(D) = _2;

  <bb 5> [local count: 1073741824]:
  # prephitmp_11 = PHI <_1(3), _2(4)>
  return prephitmp_11;

ifcvt RTL pass (_.ce1) is unable to convert:

   ...
   17: r86:SI=r89:SI
      REG_DEAD r89:SI
   18: flags:CCZ=cmp(r85:SI,0)
      REG_DEAD r85:SI
   19: pc={(flags:CCZ!=0)?L24:pc}
      REG_DEAD flags:CCZ
      REG_BR_PROB 536870916
   20: NOTE_INSN_BASIC_BLOCK 5
   21: [r87:DI]=r89:SI
      REG_DEAD r87:DI
      ; pc falls through to BB 7
   24: L24:
   25: NOTE_INSN_BASIC_BLOCK 6
   26: {r86:SI=r89:SI+0x1;clobber flags:CC;}
      REG_UNUSED flags:CC
   27: [r87:DI]=r86:SI
      REG_DEAD r87:DI
   32: L32:
   35: NOTE_INSN_BASIC_BLOCK 7
   ...


IF-THEN-ELSE-JOIN block found, pass 2, test 2, then 5, else 6, join 7

===

In case p is not a pointer, RTL optimizers start with:

  ...
  if (_8 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  p_5 = p_4 + 1;

  <bb 4> [local count: 1073741824]:
  # p_1 = PHI <p_4(2), p_5(3)>
  return p_1;

and RTL ifcvt pass is able to convert this form to addcc:

IF-THEN-JOIN block found, pass 2, test 2, then 5, join 6

if-conversion succeeded through noce_try_addcc