[LLVMdev] Virtual methods (was: LLVMBuilder vs LLVMFoldingBuilder)

[Joachim Durchholz]

Am Freitag, den 04.04.2008, 11:19 -0700 schrieb Chris Lattner:
> On Fri, 4 Apr 2008, Joachim Durchholz wrote:
> >> No, please don't do this.  The idea of llvmbuilder is that it is a
> >> "free" wrapper around the other existing API calls.  Making the
> >> methods virtual would make them much more expensive.
> >
> > Wouldn't the class of the objects be known at compile time in most
> > cases? This is essentially just a case of precomputing constants, so I
> > think this should be possible.
> >
> > If yes, the compiler can predetermine the type, hence the virtual method
> > table that will be used, and can replace the virtual call with a static
> > one.
> 
> Please verify that this actually happens in practice with llvm-gcc and 
> gcc.

If that's already the case (which I'll gladly believe), where does the
performance overhead for virtual functions come from?
(Just curious.)

Regards,
Jo