[Bug rtl-optimization/115506] Possible but missed "cmp" instruction merging (x86 & ARM, optimization)

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115506

--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
For the original testcase tree optimizers optimize to:

  <bb 4> [local count: 114863530]:
  _30 = _2 & 240;
  if (_30 == 224)
    goto <bb 7>; [34.00%]
  else
    goto <bb 5>; [66.00%]

  <bb 5> [local count: 75809929]:
  if (_30 <= 223)
    goto <bb 6>; [50.00%]
  else
    goto <bb 7>; [50.00%]


and for /* NOTE 1 */ workaround:

  <bb 4> [local count: 114863530]:
  _30 = _2 & 240;
  if (_30 == 224)
    goto <bb 7>; [34.00%]
  else
    goto <bb 5>; [66.00%]

  <bb 5> [local count: 75809929]:
  if (_30 <= 224)
    goto <bb 6>; [50.00%]
  else
    goto <bb 7>; [50.00%]


If the tree optimizer didn't over-optimize the original case and left:

  <bb 5> [local count: 75809929]:
  if (_30 < 224)
    goto <bb 6>; [50.00%]
  else
    goto <bb 7>; [50.00%]

then RTL CSE2 pass would be able to merge:

(insn 31 30 32 4 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:QI 111 [ _30 ])
            (const_int -32 [0xffffffffffffffe0]))) "pr115506.c":11:8 9
{*cmpqi_1}
     (nil))

and

(insn 36 33 37 5 (set (reg:CC 17 flags)
        (compare:CC (reg:QI 111 [ _30 ])
            (const_int -33 [0xffffffffffffffdf]))) "pr115506.c":14:15 9
{*cmpqi_1}
     (expr_list:REG_DEAD (reg:QI 111 [ _30 ])
        (nil)))

Is there a way to avoid the over-optimization with tree optimizers? RTL part
has no way to update the flags user during CSE2 pass:

(jump_insn 37 36 38 5 (set (pc)
        (if_then_else (gtu (reg:CC 17 flags)
                (const_int 0 [0]))
            (label_ref:DI 90)
            (pc))) "pr115506.c":14:15 1130 {*jcc}
     (expr_list:REG_DEAD (reg:CC 17 flags)
        (int_list:REG_BR_PROB 536870916 (nil)))