[llvm-dev] Question about the order of predecessors in LoopVectorizer with VPlanNatviePath

[jingu kang via llvm-dev]

Hi Florian

Thanks for reply.

I have created a bug https://bugs.llvm.org/show_bug.cgi?id=45958 and a
review https://reviews.llvm.org/D80086
Please review them.

Thanks
JinGu Kang

2020년 5월 17일 (일) 오후 1:51, Florian Hahn <florian_hahn at apple.com>님이 작성:
>
> Hi,
>
> > On May 17, 2020, at 13:04, jingu kang via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi All,
> >
> > I have got one domination error after running LoopVectorizer with
> > VPlanNatviePath.
> >
> > Let's see simple IR snippet after loop vectorization with VPlanNatviePath.
> >
> > vector.body:
> > ...
> >  br label %for.body10.preheader67
> >
> > for.body10.preheader67:                     ; preds =
> > %for.cond.cleanup972, %vector.body
> >  %vec.phi = phi <4 x i64> [ zeroinitializer, %for.cond.cleanup972 ],
> > [ %8, %vector.body ]
> > ...
> >
> > for.cond.cleanup972:                              ; preds = %for.body1068
> >  %8 = add nuw nsw <4 x i64> %vec.phi, <i64 1, i64 1, i64 1, i64 1>
> > ...
> >  br i1 %10, label %for.cond.cleanup373, label %for.body10.preheader67
> >
> > As you can see, %vec.phi has wrong incoming basic blocks. It could be as below.
> >
> > %vec.phi = phi <4 x i64> [ %8, %for.cond.cleanup972 ], [
> > zeroinitializer, %vector.body ]
> >
> > The problem comes from "InnerLoopVectorizer::fixNonInductionPHIs()".
> > This function has assumption about the order of predecessors as below.
> >
> >    // The predecessor order is preserved and we can rely on mapping between
> >    // scalar and vector block predecessors.
> >    for (unsigned i = 0; i < NumIncomingValues; ++i) {
> >
> > It seems it assumes loop latch as first predecessor and loop preheader
> > as second one or something like that.
> >
> > I am not sure there is rule to keep the order of predecessors but I
> > can see Jump Threading pass generates different order of predecessors
> > on basic blocks.
> >
> > If llvm does not have the rule about the order of predecessors, it
> > seems VPlanNatviePath need to consider the different order of
> > predecessors. How do you think about it?
>
>
> Yes that seems like a bug. Could you create a bug repot at https://bugs.llvm.org or share the reproducer here, if you don’t have an account?
>
> Cheers,
> Florian