On Sun, May 05, 2019 at 12:04:00PM +0800, Bin.Cheng wrote: > On Sun, May 5, 2019 at 11:23 AM Kewen.Lin <li...@linux.ibm.com> wrote: > > I can't really follow this. If it's predicted to be transformed to doloop, > > I think it should not be undoed any more, since it's useless to consider > > this cmp iv use. Whatever IVOPTS does, the comp at loop closing should not > > be changed (although possible to use other iv), right? Do I miss something? > As mentioned, the previous comment wasn't made on top of implementing > doloop in ivopts. That would be nice but a different story. > Before we can do that, it'd better be conservative and only makes > (doloop) decision in ivopts when you are sure. As you mentioned, it's > hard to do the same checks at gimple as RTL, right? In this case, > making it a (conservative) heuristic capturing certain beneficial > cases sounds better than capturing all cases but fail in later RTL > passes.
But not *overly* conservative. If some situation almost never happens, it's better to have ivopts guess wrong some of the time than it is to just optimise less aggressively. If ivopts makes a non-optimal decision you still end up with valid code :-) Segher
