Re: Relocation avoidance for function pointers

[Martin Sebor <msebor at gmail dot com>]

On 10/07/2016 01:28 PM, Florian Weimer wrote:
> On 10/07/2016 08:20 PM, Martin Sebor wrote:
> > On 10/07/2016 09:51 AM, Florian Weimer wrote:
> > > It is possible to compute the difference between two labels and store
> > > the result in a variable.  For PIC code, this can be used to avoid
> > > relocations in jump tables.
> > >
> > > I try to do something similar for function pointers.  (Function pointer
> > > arithmetic is a GCC extension.)  However, GCC rejects that with “error:
> > > initializer element is not constant”.  Is there any way around this? The
> > > required relocation should be the same (for static functions anyway).
> >
> > I can't think of a workaround.  There have been a number of bugs
> > and enhancement requests to have GCC treat more expressions as
> > integer or address constant.  Clang users especially expect some
> > of the basic cases accepted there to work in GCC as well.
> > A couple of the recent requests I remember are 69960 and 71713.
> > I don't think they cover your case so opening a new request for
> > it might be helpful.
>
> Thanks.  I found <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38295>
> and reopened it.

I forgot about this one.  Since labels are local to function they're
not subject to the same reordering that functions can be subject to.
Do you not see a potential problem with that or with the relaxation
Andrew mentions?

Martin