[Bug tree-optimization/102880] [12 Regression] Dead Code Elimination Regression at -O3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102880

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|missed-optimization         |
             Status|NEW                         |ASSIGNED
   Target Milestone|---                         |12.0
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
The difference after DSE1 is (for main):

@@ -235,20 +251,9 @@

   <bb 13> :
   b.11_31 = b;
-  if (b.11_31 != 0)
-    goto <bb 15>; [INV]
-  else
-    goto <bb 14>; [INV]

   <bb 14> :
-
-  <bb 15> :
-  # iftmp.9_36 = PHI <1(13), 0(14)>
-  d.12_32 = d;
-  _33 = d.12_32 & iftmp.9_36;
-
-  <bb 17> :
-  # iftmp.7_35 = PHI <1(10), 0(15)>
+  # iftmp.7_35 = PHI <1(10), 0(11), 0(13), 0(12)>
   _34 = (short int) iftmp.7_35;
   ai = _34;
   return 0;

where this then causes less CDDCE:

-marking necessary through iftmp.7_35 stmt iftmp.7_35 = PHI <1(10), 0(15)>
-processing: iftmp.7_35 = PHI <1(10), 0(15)>
+marking necessary through iftmp.7_35 stmt iftmp.7_35 = PHI <1(10), 0(11),
0(13), 0(12)>
+processing: iftmp.7_35 = PHI <1(10), 0(11), 0(13), 0(12)>

 Marking useful stmt: if (e.8_26 != 0)

+Marking useful stmt: if (iftmp.14_83 != 0)
+
+Marking useful stmt: if (_28 != 0)
+
+processing: if (_28 != 0)
+
+marking necessary through _28 stmt _28 = (char) ah.10_27;
+processing: _28 = (char) ah.10_27;
+
+marking necessary through ah.10_27 stmt ah.10_27 = ah;
+processing: ah.10_27 = ah;
+
+processing: if (iftmp.14_83 != 0)
+
+marking necessary through iftmp.14_83 stmt iftmp.14_83 = a.part.0 (1, _29);
+processing: iftmp.14_83 = a.part.0 (1, _29);
+
+marking necessary through _29 stmt _29 = (int) _28;
+processing: _29 = (int) _28;
+

the key here is I guess that if we'd split the CFG so that PHIs only
have _different_ arguments we would be able to reduce the number of
control dependences.

That seems like a general issue of CD-DCE, in this case exposed by the
extra DCE from DSE which in turn triggers block merging from CFG cleanup
(we trigger that once we removed a PHI which we did).

Now the question is whether there's a good way to alter the control
dependence query in a way to make this work without factoring the PHIs
first.  I suppose querying control dependence of the common dominator
of the same-argument PHI edges might work.