http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51879
--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-18 10:45:01 UTC --- (In reply to comment #2) > 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. Yes, but not for D.12345_1 = bar (7); D.12346_2 = bar (7); so you can't really value-number the calls the same. As we are working on SSA SCCs and not on a CFG (and thus do not do predicated value-numbering) that ability is useless.
