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

[Carlo Kok via llvm-dev]

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.