[PATCH] D29711: [globalisel] Sort RuleMatchers by priority.

[Kristof Beyls via Phabricator via llvm-commits]

kristof.beyls added inline comments.


================
Comment at: utils/TableGen/GlobalISelEmitter.cpp:224-230
+    // Wider types should have priority over narrower types within the same
+    // family but the order of the families doesn't matter. So s64 is more
+    // important than s32 and f64 is more important than f32 but s64 and f64 are
+    // unordered.
+    // FIXME: Don't do a lexical comparison of the type name. Check the size
+    //        instead. This only works for the cases we emit so far and is only
+    //        done this way because we can't delve into LLT's yet.
----------------
ab wrote:
> rovka wrote:
> > dsanders wrote:
> > > ab wrote:
> > > > dsanders wrote:
> > > > > ab wrote:
> > > > > > Huh, how can this be a problem?
> > > > > It works fine for s32 and s64 since "s32" < "s64" but the overall ordering is wrong when the number of digits vary:
> > > > >   "s128" < "s16" < "s32" < "s64" < "s8"
> > > > > Once I've dealt with the LLT layering issue we'll be able to do a numeric comparison for the size such that the order is:
> > > > >   s8 < s16 < s32 < s64 < s128
> > > > Sorry; I'm asking why does the ordering matter in the first place?
> > > Ah ok. Thinking about it again, I'm not sure it does. My original thinking was partly based on a comment from D26878 that talked about the order being correct by luck but we don't have the widenable concept at the moment. The other part of the thinking was based on the order I've seen the matcher tables be emitted but I think that order stems more from additional Predicates<> than from the type.
> > Ha, I had the same comment in mind!
> Ah yeah you're right, if we want to reinstate the isWidenable logic, we'd need this ordering.  But one of the reasons why I'm more and more convinced it's a bad idea is precisely the need for an ordering:  I have a gut feeling the patterns should never require that for correctness, letting us order them as we please for performance.  In which case, the types should never be a distinguishing factor (as we're only looking at performance indicators for breaking "ties" between "equivalent" patterns).  Does that sound right?  If so, I guess the type should just be ignored entirely.
I'm trying to follow the discussion here, but find it a bit hard to understand in depth.
I'm wondering if priority of patterns/rules is written up somewhere already?
(This comment seems to suggest it's still very much an in-progress design discussion, so isn't written up anywhere?)
I'm assuming the priority of which pattern to try and match first will remain the same as under DAGISel? Or do we consider making some tweaks?
If it remains the same, what further ordering rules are being discussed here, and why does more ordering need to be defined compared to DAGISel?

Please don't let my comment slow you down in getting to a solution, but I think it'd be useful for this area to be explained a bit better in the end somewhere, e.g. in the comment at the top of this file.
Doesn't necessarily need to be as part of this patch.


https://reviews.llvm.org/D29711