[llvm-dev] Potential bug in SelectionDAGLegalize::ConvertNodeToLibcall()?

[Justin Hibbits via llvm-dev]

Hi Nemanja,

I'm attaching a patch that builds on D54583 and implements what we
discussed on IRC earlier today.  Particularly:

* Make LowerCallTo() a virtual function, so it can be wrapped by a
  subclass.
* Implement LowerCallTo() in PPCTargetLowering to wrap
  TargetLowering::LowerCallTo() and legalize the return node when
  targeting SPE.
* Augment PPCTargetLowering::LowerCall_32SVR4() to legalize MVT::f64
  arguments that have been pre-processed into
  EXTRACT_ELEMENT(i64 BITCAST f64, 0/1)

The purpose of this being to legalize intermediate illegal types
post-type legalization.

Is there a better approach?  Comments from anyone else?

- Justin

On Wed, 2 Jan 2019 11:39:59 -0500
Nemanja Ivanovic <nemanja.i.ibm at gmail.com> wrote:

> It sounds like the legalizer is lowering `fmaxnum` to a libcall
> because it is not a legal node for `f64` and in doing so, it is
> producing the `build_pair` to reassemble the results of the libcall.
> And presumably, it is assuming that the new nodes do not need
> legalization or something along those lines.
> 
> Justin, it would probably be good if you could provide the DAG before
> and after legalization both with and without your patch. Then we can
> see how it was handled before your patch and how it is handled now
> and the difference should allude to the problem.
> 
> N
> 
> On Wed, Jan 2, 2019 at 10:54 AM Justin Hibbits via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> 
> > Hi,
> >
> > I have a custom lowering operation on ISD::BITCAST for the
> > PowerPC/SPE target, to convert 'f64 bitcast (i64 build_pair i32,
> > i32)' into a 'f64 BUILD_SPE64 i32, i32' node, which can be seen at
> > https://reviews.llvm.org/D54583. However, when building
> > compiler-rt's lib/builtins/divdc3.c an assertion is triggered that
> > BUILD_PAIR is not legal on line 24.  There should be no
> > bitcast(buildpair) anywhere in the generation, as it should all be
> > lowered.  However, this is not the case when lowering to a libcall
> > it seems.  The relevant debug output, is here:
> >
> > Creating new node: t118: i64 = build_pair t117,t116,
> > /home/chmeee/freebsd/contrib/compiler-rt/lib/builtins/divdc3.c:24:22
> > Creating new node: t119: f64 = bitcast t118,
> > /home/chmeee/freebsd/contrib/compiler-rt/lib/builtins/divdc3.c:24:22
> > Created libcall: t119: f64 = bitcast t118,
> > /home/chmeee/freebsd/contrib/compiler-rt/lib/builtins/divdc3.c:24:22
> > Successfully converted node to libcall
> >  ... replacing: t38: f64 = fmaxnum t36, t37,
> > /home/chmeee/freebsd/contrib/compiler-rt/lib/builtins/divdc3.c:24:22
> >      with:      t119: f64 = bitcast t118,
> > /home/chmeee/freebsd/contrib/compiler-rt/lib/builtins/divdc3.c:24:22
> >
> > Is this a real bug, or am I missing something in my patch?  After
> > spending quite a while on it I'm at a loss.
> >
> > Thanks,
> > Justin
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >  

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