[llvm-dev] Get llvm to optimize through thunk method

[Reid Kleckner via llvm-dev]

If you add varargs to the thunk prototype and to the musttail call inside
of it, the inliner should be able to inline through it. Check out the LLVM
tests at llvm/test/Transforms/Inline/inline-varargs.ll for examples of what
currently works.

On Thu, Oct 4, 2018 at 12:11 PM Carlo Kok <ck at remobjects.com> wrote:

>
>
> On Thu, Oct 4, 2018, at 20:53, Friedman, Eli wrote:
> > On 10/4/2018 11:14 AM, Carlo Kok wrote:
> > >
> > > On Thu, Oct 4, 2018, at 20:05, Friedman, Eli wrote:
> > >> On 10/4/2018 1:58 AM, Carlo Kok via llvm-dev wrote:
> > >>> Hi,
> > >>>
> > >>> is there a way I can tell llvm to optimize through a call to a thunk
> method?
> > >>>
> > >>> https://gist.github.com/carlokok/3ba9cb1a36a145a3071a7b901867d442
> > >>>
> > >>> On line 37 I cast & call
> _intf__rtti_t27_issue81139__static__dispatchtest_d_Test_vmthelper0
> > >>>
> > >>> which is a thunk method that calls
> > >>> _intfstub_mi_t27_issue81139__static__dispatchtest_d_Test4_Test
> > >>>
> > >>> Is there a way to tell the optimizer to get rid of the thunk call?
> Since llvm can prove it will end up in
> _intfstub_mi_t27_issue81139__static__dispatchtest_d_Test4_Test
> > >>> given the parameters passed?
> > >> LLVM is conservative if the callee of a call is bitcast in a
> non-trivial
> > >> way; it treats the call as opaque.
> > >>
> > >> What are you trying to accomplish by passing the wrong number of
> > >> arguments to the call?
> > >>
> > > The vmt helper does a tail call that acts like a thunk. The final
> method has the right number of parameters and I adjust the sig to match the
> calling convention. It's essentially a trick to do interface dispatch
> without requiring an explicit vmt per interface.
> >
> > In addition to the inlining problem you've noted, defining the thunk
> > with too few arguments is likely to corrupt the remaining arguments.
> >
> > I think you can avoid the issue with corrupted arguments by defining the
> > thunk using a varargs type.  But I don't think there's any existing
> > solution for your inlining issue.
> >
>
> Thnx. I'm not getting corruption so I'm good there. Was hoping for a
> solution to the in lining but that can't be helped.
>
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