[PATCH] D28490: [LV] Don't panic when encountering the IV of an outer loop.

[Michael Kuperstein via Phabricator via llvm-commits]

mkuper added inline comments.


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Comment at: test/Transforms/LoopVectorize/pr31190.ll:8
+
+; CHECK: LV: PHI is a recurrence with respect to an outer loop.
+; CHECK: LV: Not vectorizing: Cannot prove legality.
----------------
mssimpso wrote:
> mkuper wrote:
> > delena wrote:
> > > Can you add the source code to the comments? You try to vectorize the "for.body3" loop, right? What PHI belongs to the outer loop?
> > Yes, it's trying to vectorize the inner loop (for.body3).
> > The problematic PHI is: 
> > %0 = phi i32 [ undef, %for.cond1.preheader ], [ %inc54, %for.body3 ]
> > 
> > This phi is in the inner loop, but %inc54 is an IV phi in the outer loop:
> > %inc54 = phi i32 [ %inc5, %for.cond1.for.inc4_crit_edge ], [ %c.promoted, %entry ]
> > So SCEV recognizes %0 as an AddRec, but with respect to the outer loop.
> > 
> > I'll add the above explanation in a comment.
> > Not sure about the source, though - the IR was, I think, hand-reduced by @davide from the source in PR31190, so I'm not sure having the "original" source he makes a lot of sense.
> > 
> > So SCEV recognizes %0 as an AddRec, but with respect to the outer loop.
> 
> I haven't tried this so I'm not really sure, but would SE->getSCEVAtScope(Phi, TheLoop) be useful here?
I think it will work, but that doesn't really help. We'll just end up with an unidentified phi, so we won't vectorize regardless.
I'd prefer to leave this explicit, but I don't mind trying getSCEVatScope if you think that's better.


https://reviews.llvm.org/D28490