[Bug tree-optimization/104515] [11/12/13 Regression] trivially-destructible destructors interfere with loop optimization - maybe related to lifetime-dse.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104515

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed.  The issue is that store motion of v_7(D)->end cannot be
> performed on
> 
>   <bb 3> [local count: 955630225]:
>   # i_13 = PHI <i_10(6), 0(5)>
>   _1 = v_7(D)->end;
>   _2 = _1 + 18446744073709551612;
>   v_7(D)->end = _2;
>   MEM[(T *)_1 + -4B] ={v} {CLOBBER};   // <- inserted by -flifetime-dse
>   i_10 = i_13 + 1;
>   if (n_6(D) > i_10)
>     goto <bb 6>; [89.00%]
>   else
>     goto <bb 4>; [11.00%]
> 
>   <bb 6> [local count: 850510901]:
>   goto <bb 3>; [100.00%]

It's possible to perform store-motion of the above as the following,
preserving the CLOBBER in the loop but necessarily re-issueing the
last iteration CLOBBER on the exit edge.  I _think_ that's fine to do.
The oacc-kernels pipeline does LIM quite early before IPA.  For the
later LIM passes we might as well go with eliminating the non-decl
CLOBBERs earlier - but the issue also exists for decl CLOBBERs of course.

Note duplicating "lifetime" CLOBBERs might prove bad (we only have EOL
clobbers right now).

  <bb 3> [local count: 955630225]:
  # i_13 = PHI <i_10(6), 0(5)>
  # v__end_lsm.3_4 = PHI <v__end_lsm.3_12(6), v__end_lsm.3_3(5)>
  _1 = v__end_lsm.3_4;
  _2 = _1 + 18446744073709551612;
  v__end_lsm.3_12 = _2;
  MEM[(int *)_1 + -4B] ={v} {CLOBBER};
  i_10 = i_13 + 1;
  if (n_6(D) > i_10)
    goto <bb 6>; [89.00%]
  else
    goto <bb 7>; [11.00%]

  <bb 6> [local count: 850510901]:
  goto <bb 3>; [100.00%]

  <bb 7> [local count: 105119324]:
  # _17 = PHI <_1(3)>
  # v__end_lsm.3_19 = PHI <v__end_lsm.3_12(3)>
  v_7(D)->end = v__end_lsm.3_19;
  MEM[(int *)_17 + -4B] ={v} {CLOBBER};

  <bb 4> [local count: 118111600]:
  return;