[Bug tree-optimization/83126] [8 Regression] ICE in transform_to_exit_first_loop_alt, at tree-parloops.c:1713

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83126

Tom de Vries <vries at gcc dot gnu.org> changed:

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

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
I.

If we disable transform_to_exit_first_loop_alt, we run into this assert instead
in transform_to_exit_first_loop:
...
              gcc_assert (control_name == NULL_TREE
                          && SSA_NAME_VAR (res) == SSA_NAME_VAR (control));
...

So the problem is not specific to transform_to_exit_first_loop_alt.


II.

This is the loop we're trying to parallelize, at dce5 (so before parloops):
...
  <bb 10> [local count: 118111600]:

  <bb 5> [local count: 236258638]:
  # _22 = PHI <0(10), _5(8)>
  # c9_23 = PHI <c9_1(10), c9_14(8)>
  # e1_27 = PHI <0(10), e1_17(8)>
  # ivtmp_26 = PHI <2(10), ivtmp_24(8)>
  _4 = (short unsigned int) e1_27;
  c9_14 = _4 * c9_23;
  _5 = _22 + 1;
  e1_17 = (int) _5;
  ivtmp_24 = ivtmp_26 - 1;
  if (ivtmp_24 != 0)
    goto <bb 8>; [66.67%]
  else
    goto <bb 7>; [33.33%]

  <bb 8> [local count: 157513634]:
  goto <bb 5>; [100.00%]

  <bb 7> [local count: 78745004]:
  # c9_21 = PHI <c9_14(5)>
...

The loop has one exit phi, and 4 loop header phis. The phis are classified in
try_create_reduction_list.

The loop exit phi c9_21 together with loop header phi c9_23 is classified as a
reduction. The remaining 3 loop header phis are classified as 'simple_iv'.

The idea is that the 3 ivs will be unified by canonicalize_loop_ivs before
doing transform_to_exit_first_loop_alt or transform_to_exit_first_loop.

But, after canonicalize_loop_ivs we still have 3 loop header phis (instead of
the two we are expecting: one for the reduction and one for the iv):
...
  <bb 5> [local count: 189006911]:
  # c9_23 = PHI <c9_14(8), c9_1(15)>
  # e1_27 = PHI <e1_17(8), 0(15)>
  # ivtmp_8 = PHI <ivtmp_28(8), 0(15)>
  _22 = ivtmp_8;
  ivtmp_26 = 2 - ivtmp_8;
  _4 = (short unsigned int) e1_27;
  c9_14 = _4 * c9_23;
  _5 = _22 + 1;
  e1_17 = (int) _5;
  ivtmp_24 = ivtmp_26 - 1;
  if (ivtmp_8 < 1)
    goto <bb 8>; [66.67%]
  else
    goto <bb 7>; [33.33%]
...
The reason for this is that the e1_27 phi is not classified as simple_iv by
canonicalize_loop_ivs.

This again can be tracked down to the loop versioning transformation at the
beginning of gen_parallel_loop. Before the transformation, we have 3 simple_iv
header phis, after the transformation we have 2. This is caused by scalar
evolution failing for e1_27.


III. 

The loop versioning transformation creates a condition, and a conditional jump
to either the original loop (which is to be transformed into a parallel loop),
or a copy of the loop. As it happens the condition for this example is
hardcoded to 0 != 0, meaning we always jump to the copy. This makes the
original loop dead code, and it's possible that the scalar evolution is somehow
influenced by that.


IV.

FWIW, moving canonicalize_loop_ivs to before the loop versioning fixes the ICE.
But that needlessly transforms the copy of the loop, while this is only
necessary for the original loop (which is to be transformed into a parallel
loop).