[Bug tree-optimization/99101] optimization bug with -ffinite-loops

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99101

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|og10 (devel/omp/gcc-10)     |11.0
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
           Keywords|                            |wrong-code
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-02-15
          Component|c++                         |tree-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  But note that C++ requires forward progress and thus technically
the infinite loop invokes undefined behavior.

CDDCE2 does (with -fno-tree-sra to show at_eof_)

Assume loop 1 to be finite: it has an exit and -ffinite-loops is on.
Removing basic block 14

   <bb 2> [local count: 1073741824]:
-  comp_md_.at_eof_ = 0;

   <bb 3> [local count: 108447916235]:
-  # prephitmp_47 = PHI <1(2), 0(15)>
-  # prephitmp_22 = PHI <0(2), prephitmp_27(15)>
-  if (prephitmp_47 > 0)
-    goto <bb 16>; [33.00%]
-  else
-    goto <bb 15>; [67.00%]
-
-  <bb 16> [local count: 35787812164]:
-
-  <bb 4> [local count: 53412352861]:
-  # i_9 = PHI <i_8(14), 0(16)>
-  # prephitmp_11 = PHI <pretmp_2(14), prephitmp_22(16)>
-  if (prephitmp_11 != 0)
-    goto <bb 12>; [34.00%]
-  else
-    goto <bb 5>; [66.00%]
-
-  <bb 5> [local count: 11633210387]:
-  comp_md_.at_eof_ = 1;
   _14 = std::basic_ostream<char>::operator<< (&cout, 1);
   _15 = _14->_vptr.basic_ostream;
   _16 = MEM[(long int *)_15 + -24B];
@@ -99,28 +88,12 @@
   _43 = OBJ_TYPE_REF(_37;(const struct ctype)_23->6) (_23, 10);
   _10 = (int) _43;

-  <bb 11> [local count: 11628557050]:
+  <bb 11> [local count: 53407699675]:
   # prephitmp_24 = PHI <_13(8), 10(9), _10(10)>
   _21 = std::basic_ostream<char>::put (_14, prephitmp_24);
   std::basic_ostream<char>::flush (_21);

-  <bb 12> [local count: 53407699675]:
-  i_8 = i_9 + 1;
-  if (i_8 < prephitmp_47)
-    goto <bb 14>; [33.00%]
-  else
-    goto <bb 13>; [67.00%]
-
-  <bb 13> [local count: 35783158878]:
-  pretmp_25 = comp_md_.at_eof_;
-  goto <bb 15>; [100.00%]
-
-  <bb 14> [local count: 17624540797]:
-  pretmp_2 = comp_md_.at_eof_;
-  goto <bb 4>; [100.00%]
-
   <bb 15> [local count: 108443262898]:
-  # prephitmp_27 = PHI <pretmp_25(13), prephitmp_22(3)>
   goto <bb 3>; [100.00%]

 }

so the question would be why the std::cout << 1 call doesn't add its
control dependence as necessary here.  Hmm, and the reason is we have
BB 5 only control dependent on itself.

And I have a hunch that this boils down to post dominators being
"wrong" here because of the missing edges to exit.  Indeed if
connecting infinite loops to exit in CD-DCE the bug is fixed.