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.
