[PATCH] D123537: [VPlan] Model first exit values using VPLiveOut.

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:8717
+  // Only handle single-exit loops for now.
+  if (!ExitBB->getSinglePredecessor())
+    return;
----------------
fhahn wrote:
> Ayal wrote:
> > fhahn wrote:
> > > Ayal wrote:
> > > > Can fold both early exits into one.
> > > > 
> > > > Also early exit if requiresScalarEpilogue()?
> > > > Can fold both early exits into one.
> > >  
> > > Done, thanks!
> > > 
> > > > Also early exit if requiresScalarEpilogue()?
> > > 
> > > I think we should still create live-outs if `requiresScalarEpilogue`, otherwise some recipes may be considered dead if they only have uses outside the loop. The latest version of the patch uses that property to remove a workaround in VPlanTransforms that used IR def use chains to detect users outside the loop.
> > >> Also early exit if requiresScalarEpilogue()?
> > > I think we should still create live-outs if requiresScalarEpilogue, otherwise some recipes may be considered dead if they only have uses outside the loop. The latest version of the patch uses that property to remove a workaround in VPlanTransforms that used IR def use chains to detect users outside the loop.
> > 
> > This perhaps deserves a comment somewhere, explaining that VPLiveOuts are used to represent loop live out values that originally fed LCSSA phis in Exit Block, where these phis may or may not require fixing - if their edge is removed due to required scalar epilogue - in which case they are still live out but feed only preheader of scalar epilogue?
> I'll adjust the comment here.
> I'll adjust the comment here.

Thanks!

Perhaps instead of deleting LiveOuts whose phis are fixed or need not be fixed due to required scalar epilog, such LiveOuts should be marked as "fixed" - so that queries of outside users still hold while preventing their future fixes(?). This may become clearer when external users of inductions/reductions/recurrences are introduced.


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  https://reviews.llvm.org/D123537/new/

https://reviews.llvm.org/D123537