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

Finkel, Hal J. via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 4 09:00:49 PST 2019

Aside from the fact that you're checking for i64 specifically instead of generally checking for illegal types, how much of this is really PPC specific? Would this be a reasonable enhancement to the SDAG logic in general?


On 1/4/19 8:03 AM, Nemanja Ivanovic wrote:
The changes seem fine to me. I don't think this is excessively intrusive and it accomplishes what is needed by targets whose call lowering can introduce illegal types.
Adding Justin Bogner as the owner of SDAG and Hal Finkel as the PPC back end owner for their opinions.


On Thu, Jan 3, 2019 at 4:54 PM Justin Hibbits <jrh29 at alumni.cwru.edu<mailto:jrh29 at alumni.cwru.edu>> wrote:
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
* 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

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<mailto: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<mailto: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<mailto:llvm-dev at lists.llvm.org>
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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