[Bug tree-optimization/62217] [4.9/5 Regression] DOM confuses complete unrolling which in turn causes VRP to warn

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.