https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62217
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Jeffrey A. Law from comment #5) > Kirill, you are correct WRT propagation of "b" for "i". Prior to DOM1 we > have: > > ;; basic block 3, loop depth 1, count 0, freq 9100, maybe hot > ;; prev block 2, next block 4, flags: (NEW, REACHABLE) > ;; pred: 7 [91.0%] (TRUE_VALUE,EXECUTABLE) > if (i_1 == b_6(D)) > goto <bb 4>; > else > goto <bb 5>; > ;; succ: 4 [0.0%] (TRUE_VALUE,EXECUTABLE) > ;; 5 [100.0%] (FALSE_VALUE,EXECUTABLE) > > ;; basic block 4, loop depth 1, count 0, freq 2, maybe hot > ;; prev block 3, next block 5, flags: (NEW, REACHABLE) > ;; pred: 3 [0.0%] (TRUE_VALUE,EXECUTABLE) > g_x[i_1] = *x1_7(D); > goto <bb 6>; > ;; succ: 6 [100.0%] (FALLTHRU,EXECUTABLE) > > ;; basic block 5, loop depth 1, count 0, freq 9098, maybe hot > ;; prev block 4, next block 6, flags: (NEW, REACHABLE) > ;; pred: 3 [100.0%] (FALSE_VALUE,EXECUTABLE) > g_x[i_1] = *x2_9(D); > ;; succ: 6 [100.0%] (FALLTHRU,EXECUTABLE) > > > DOM records that i_1 and b_6 are equivalent on the edge bb3->bb4. So in bb4 > it replaces i_1 with b_6. Presumably that's causing problems downstream in > the optimization pipeline. The easiest way to think about this is we record > that i_1 can be legitimately replaced with b_6 in that context. We could > just have easily recorded that b_6 can be replaced with i_1. > > I don't think we have any heuristics for which of those two equivalences to > record, it's strictly based on the order of appearance (which is likely > determined elsewhere by canonicalization rules). > > If you want to propose some heuristics, I'm all ears. One might be to put > the object with the least number of references on the lhs. THe idea would > be to try and ultimately get that use count to 0/1 which may allow that > object to die at the comparison. There may be some reasonable heuristic > around loop depth of the names as well. ie, do we want to replace uses of > a non-loop object with a loop object or vice versa? > > Anyway, open to suggestions here... The rule is simple - we should always replace with the more dominating definition because that's what value-numbering would do to be able to make the other defs unused.
