[Bug tree-optimization/77839] [6/7 Regression] Memory- and compile time hog at -O1 and above

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77839

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
SCC consists of: .MEM_16 .MEM_15 .MEM_35 .MEM_26 ez_12 .MEM_14 wo_37 _5 _6
hy.3_7 _8 .MEM_32 hy.0_1 ez_28 _3 wo_40

Value numbering wo_37 stmt = wo_37 = PHI <0(4), wo_40(5)>
Setting value number of wo_37 to ez_12 (changed)
Value numbering _3 stmt = _3 = ez_28 ^ wo_37;
Applying pattern match.pd:580, gimple-match.c:33849
Match-and-simplified ez_28 ^ wo_37 to 0
RHS ez_28 ^ wo_37 simplified to 0
Setting value number of _3 to 0 (changed)
Value numbering wo_40 stmt = wo_40 = _3;
Setting value number of wo_40 to 0 (changed)

Value numbering wo_37 stmt = wo_37 = PHI <0(4), wo_40(5)>
Setting value number of wo_37 to 0 (changed)
Value numbering _3 stmt = _3 = ez_28 ^ wo_37;
Applying pattern match.pd:83, gimple-match.c:33640
Match-and-simplified ez_28 ^ wo_37 to ez_12
RHS ez_28 ^ wo_37 simplified to ez_12
Setting value number of _3 to ez_12 (changed)
Value numbering wo_40 stmt = wo_40 = _3;
Setting value number of wo_40 to ez_12 (changed)
...


  <bb 6>:
  # tc_11 = PHI <0(4), tc_31(5)>
  # ez_12 = PHI <0(4), ez_28(5)>
  # b8_13 = PHI <0(4), b8_30(5)>
  # wo_37 = PHI <0(4), wo_40(5)>

so we're iterating between "same PHI" and same value.  This is because
only when wo_40 == ez_12 we detect that it is actually zero ... we
fail to apply the wo_40 == 0 -> wo_37 == 0 -> ez_12 == 0 step when we
in the first iteration numbered wo_37 to ez_12.  As the comment says:

  /* First see if it is equivalent to a phi node in this block.  We prefer
     this as it allows IV elimination - see PRs 66502 and 67167.  */

we have the issue that the algorithm doesn't handle all cases, this now
shows we even can oscillate between them :/