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

[Alex Bradbury via llvm-dev]

On Wed, 2 Jan 2019 at 16:41, Alex Bradbury <asb at lowrisc.org> wrote:
>
> On Wed, 2 Jan 2019 at 15:54, 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.
>
> Hi Justin,
>
> I don't think this is a bug in ConvertNodeToLibCall - rather, it's
> just a case where you can be tripped up by the very late lowering of
> intrinsics calls.

Sorry, I was imprecise. This isn't a general problem with lowering to
intrinsics / libcalls, just where a SelectionDAG node is expanded to a
libcall.

Best,

Alex