[Bug tree-optimization/100162] missed optimization for dead code elimination at -O3 (vs. -O2)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100162

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |12.0
                 CC|                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-04-21
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Optimized by DOM3 which sees the following difference:

-  <bb 2> [local count: 118111601]:
+  <bb 2> [local count: 955630225]:
   b.1_1 = b;
-  c[0][b.1_1] = 2;
-  c[1][b.1_1] = 2;
-  c[2][b.1_1] = 2;
-  c[3][b.1_1] = 2;
+  _27 = (sizetype) b.1_1;
+  _28 = _27 * 4;
+  vectp_c.13_26 = &c + _28;
+  MEM <vector(4) int> [(int *)vectp_c.13_26] = { 2, 2, 2, 2 };
+  vectp_c.12_30 = vectp_c.13_26 + 16;
   c[4][b.1_1] = 2;
   a = 5;
   _5 = b.1_1 != 0;
   _6 = (int) _5;
-  _8 = _6 / 2;
+  _7 = c[0][0];
+  _8 = _6 / _7;
   if (_8 != 0)

here c[0][b.1_1] takes advantage of get_ref_base_and_extent honoring the known
array size of [1] while the pointer based access is not constrained this way
which makes matching c[0][0] to *(&c + _28) = { 2, 2, 2, 2 } difficult.

The realistic chance is to catch this by improving value-numbering done
on the not unrolled loop earlier:

  <bb 3> [local count: 955630225]:
  # a.3_19 = PHI <_2(3), 0(2)>
  c[a.3_19][b.1_1] = 2;
  _2 = a.3_19 + 1;
  if (_2 <= 4)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 4> [local count: 118111600]:
  a = _2;
  _5 = b.1_1 != 0;
  _6 = (int) _5;
  _7 = c[0][0];

where we could use SCEV & friends to lookup c[0][0] at the c[a.3_19][b.1_1]
definition in vn_reference_lookup_3.

That might also help to look through loop abstraction earlier.