Re: The Extension to ELF

[Ian Lance Taylor <iant@google.com>]

On Wed, Aug 8, 2012 at 4:23 AM, Frank Ch. Eigler <fche@redhat.com> wrote:
> Ian Lance Taylor <iant@google.com> writes:
>
>> [...]
>> ELF is designed to permit fast program loading at runtime, and to
>> permit fast linking.  Changing symbol and relocation values to take
>> general expressions works against that goal.
>> [...]
>
> It may interest you to know that, for an older Cygnus project (mep),
> we implemented a facility called computed/complex relocations, as an
> ELF extension.  This is a way of encoding general symbol/arithmetic
> expressions to be evaluated at link time and substituted into the
> binary output.  (It may be similar to the vms-alpha ETIR facility.)
>
> This has been merged into gnu binutils some time ago, though is not
> widely known, and only used by a single cgen-based gas port.  See the
> OBJ_COMPLEX_RELC conditionals in gas/*, the BSF_*RELC/STT_RELC logic
> in bfd/*.

Thanks, I was trying to remember that.

It is perhaps also worth noting that the IEEE-695 object file format
supports arbitrary expressions in a number of different ways,
including for relocation calculations.  The format is not widely used
today, for more or less that reason.

Ian