[Bug tree-optimization/51879] Missed tail merging with non-const/pure calls

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51879

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-01-18 
10:36:39 UTC ---
I'm afraid a lot would break.  It really depends on what you use VN for and on
what code.
If you have:
  D.12345_1 = bar (7);
  D.12346_2 = bar (7);
and bar isn't const/pure call, then if VN equivalences D.12345_1 and D.12346_2,
it is wrong.  Of course if you have:
<bb7>:
  D.12345_1 = bar (7);
  goto bb9;
<bb8>:
  D.12346_2 = bar (7);
<bb9>:
  D.12347_3 = PHI <D.12345_1(7), D.12346_2(8)>
(this case), you could VN them the same.