[PATCH] D65607: [DAGCombiner] try to convert opposing shifts to casts

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D65607#1613601 <https://reviews.llvm.org/D65607#1613601>, @mstorsjo wrote:

> In D65607#1613461 <https://reviews.llvm.org/D65607#1613461>, @spatel wrote:
>
> > In D65607#1613432 <https://reviews.llvm.org/D65607#1613432>, @mstorsjo wrote:
> >
> > > In D65607#1613403 <https://reviews.llvm.org/D65607#1613403>, @lebedev.ri wrote:
> > >
> > > > In D65607#1613401 <https://reviews.llvm.org/D65607#1613401>, @mstorsjo wrote:
> > > >
> > > > > This caused failed asserts ("Unexpected illegal type!" in LegalizeDAG) when building Qt,  with https://martin.st/temp/qpainterpath-preproc.cpp, built with `clang++ -target i686-w64-mingw32 -c -O3 -std=c++17 qpainterpath-preproc.cpp`. Will file a full proper bug later if necessary.
> > > >
> > > >
> > > > Would be good to have a reproducer, even just `N->dumprFull()` may shed some light.
> > >
> > >
> > > `Node->dumprFull();` for the failed assert prints
> > >
> > >   t51: i32 = sign_extend t50
> > >     t50: i29 = add t48, Constant:i29<-3>
> > >       t48: i29 = truncate t46
> > >         t46: i32 = <<Unknown Node #438>> t8, Constant:i32<3>
> > >           t8: i32,ch = CopyFromReg t0, Register:i32 %13
> > >             t7: i32 = Register %13
> > >           t45: i32 = Constant<3>
> > >       t49: i29 = Constant<-3>
> > >
> >
> >
> > I can guess how we should limit this transform, but my translation of that DAG to IR does not crash for me:
> >
> >   define i32 @trunc29(i32 %x) nounwind {
> >     %u2 = trunc i32 %x to i29
> >     %u3 = add i29 %u2, -3
> >     %r = sext i29 %u3 to i32
> >     ret i32 %r
> >   }
> >  
> >
> >
> > ...so I need more info.
>
>
> The original standalone preprocessed C++ code I linked before was reproducible on its own already, but I went ahead and made a reduced IR reproducer out of it, and filed a bug for it at https://bugs.llvm.org/show_bug.cgi?id=42880.


Thank you for reducing that! I submitted:
rL367766 <https://reviews.llvm.org/rL367766>
...as an immediate fix, but I'm still not sure what happens later to cause the assert.


Repository:
  rL LLVM

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