[Bug tree-optimization/103721] [12 regression] wrong code generated for loop with conditional since r12-4790-g4b3a325f07acebf4

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103721

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #4 from Jeffrey A. Law <law at gcc dot gnu.org> ---
But what doesn't make any sense here is the folding in this block:

<bb 9> [local count: 1073741824]:
# searchVolume_5 = PHI <searchVolume_11(8), world_7(D)(2)>
# currentVolume_6 = PHI <currentVolume_8(8), 0(2)>
_2 = searchVolume_5 != currentVolume_6;
_3 = searchVolume_5 != 0;
_4 = _2 & _3;
if (_4 != 0)
  goto <bb 3>; [89.00%]
else
  goto <bb 7>; [11.00%]


In fold_using_range::range_of_range_op we have:

(gdb) p debug_tree (op1)
 <ssa_name 0x7ffff77fef30
    type <integer_type 0x7ffff780e5e8 int sizes-gimplified public SI
        size <integer_cst 0x7ffff77f3f90 constant 32>
        unit-size <integer_cst 0x7ffff77f3fa8 constant 4>
        align:32 warn_if_not_align:0 symtab:0 alias-set 1 canonical-type
0x7ffff780e5e8 precision:32 min <integer_cst 0x7ffff77f3f48 -2147483648> max
<integer_cst 0x7ffff77f3f60 2147483647>
        pointer_to_this <pointer_type 0x7ffff7816a80>>
    visited var <var_decl 0x7ffff7a12cf0 searchVolume>
    def_stmt searchVolume_5 = PHI <searchVolume_11(8), world_7(D)(2)>
    version:5>
$113 = void
(gdb) p debug_tree (op2)
 <ssa_name 0x7ffff77fef78
    type <integer_type 0x7ffff780e5e8 int sizes-gimplified public SI
        size <integer_cst 0x7ffff77f3f90 constant 32>
        unit-size <integer_cst 0x7ffff77f3fa8 constant 4>
        align:32 warn_if_not_align:0 symtab:0 alias-set 1 canonical-type
0x7ffff780e5e8 precision:32 min <integer_cst 0x7ffff77f3f48 -2147483648> max
<integer_cst 0x7ffff77f3f60 2147483647>
        pointer_to_this <pointer_type 0x7ffff7816a80>>
    visited var <var_decl 0x7ffff7a12d80 currentVolume>
    def_stmt currentVolume_6 = PHI <currentVolume_8(8), 0(2)>
    version:6>
$114 = void
(gdb) p rel
$115 = EQ_EXPR

If I'm reading the code correctly I think that means that the ranger has
determined that _5 and _6 are equal.  But I don't see how it can possibly make
that determination with this CFG:

int f (int world)
{
  int currentVolume;
  int searchVolume;
  int ipos.0_1;
  _Bool _2;
  _Bool _3;
  _Bool _4;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  goto <bb 9>; [100.00%]
;;    succ:       9

;;   basic block 3, loop depth 1
;;    pred:       9
  ipos.0_1 = ipos;
  if (ipos.0_1 != 0)
    goto <bb 8>; [50.00%]
  else
    goto <bb 4>; [50.00%]
;;    succ:       8
;;                4

;;   basic block 4, loop depth 1
;;    pred:       3
;;    succ:       8

;;   basic block 8, loop depth 1
;;    pred:       4
;;                3
  # searchVolume_11 = PHI <1(4), 0(3)>
  # currentVolume_8 = PHI <searchVolume_5(4), searchVolume_5(3)>
;;    succ:       9

;;   basic block 9, loop depth 1
;;    pred:       8
;;                2
  # searchVolume_5 = PHI <searchVolume_11(8), world_7(D)(2)>
  # currentVolume_6 = PHI <currentVolume_8(8), 0(2)>
  _2 = searchVolume_5 != currentVolume_6;
  _3 = searchVolume_5 != 0;
  _4 = _2 & _3;
  if (_4 != 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 7>; [11.00%]
;;    succ:       3
;;                7

;;   basic block 7, loop depth 0
;;    pred:       9
  return currentVolume_6;
;;    succ:       EXIT

}

This feels like it's got to be a problem in the equivalence handling -- it's
largely outside the threader.

My recollection of equivalences in loops is that they're exceedingly hard to
get correct once you follow the backedge -- particularly since you have to
invalidate some equivalences once you traverse that backedge.  Finding the set
that needed to be invalidated was expensive and the book keeping turned out to
be too hard to do reliably so I ripped it all out.

How does equivalence handling in the Ranger world work once you traverse the
backedge of a loop?