Re: GCC's ICF vs. gold's ICF

[Frank Tetzel <s1445051 at mail dot zih dot tu-dresden dot de>]

> > why is the ICF pass in gcc not folding member functions which
> > depend on a template parameter but happen to generate identical
> > code? Is it because it is not identical on the IR level in the
> > compiler? Can I somehow dump the IR in text form?  
> 
> You can look at the ICF dump generated when you pass
> -fdump-ipa-icf-details
> 
> And yes, ICF has to consider IL differences that may result in
> different allowed followup optimizations while when the IL is final
> (such as link-time) no such considerations have to be made.  A very
> simple example would be signed vs. unsigned integer multiplication
> where from the former IL overflow would be undefined and
> optimizations can exploit that while not for the latter.

Thanks for the information. If I read the dump correctly, it also
considers the return type and that seems to be the problem in my tiny
test program.

snippet from dump:

  group: with 1 classes:
    class with id: 1, hash: 2170673536, items: 2
      MyArray<float, 1024>::operator[](unsigned int)/4 MyArray<int, 1024>::operator[](unsigned int)/3 
  false returned: 'alias sets are different' (compatible_types_p:244)
  false returned: 'result types are different' (equals_wpa:676)

The body of the functions look identical, but the return type differs.
So in C++, ICF is "disabled" for templated functions with a template
parameter as return type.

But why is the return type preventing code folding? Because we do not
know the calling convention at this point in time?