Re: Merging identical functions in GCC

["Daniel Berlin" <dberlin at dberlin dot org>]

On 15 Sep 2006 09:27:32 -0700, Ian Lance Taylor <iant@google.com> wrote:
> "Steven Bosscher" <stevenb.gcc@gmail.com> writes:
>
> > Isn't this what you describe here the same as COMDAT?
>
> Not exactly.  COMDAT means that the compiler can generate multiple
> instances of, say, the list<int> functions, and only one version will
> be included in the final executable.  However, if you use both
> list<int> and list<long>, they will both wind up in the executable.
>
> The proposal here is that in the case where list<int> and list<long>
> generate identical code, only one version winds up in the executable,
> with two symbols associated with it.
>
> I think Danny has a 75% implementation based on hashing the RTL for a
> section and using that to select the COMDAT section signature.  Or the
> hashing can be done in the linker, but it still needs compiler help to
> know when it is permissible.

Yup, that's what i've got.
It works when they generate value wise identical code (effectively it
allows as different of code your value numbering can determine is the
same).
There are of course, collisions possible in the hash, and in the
absolute worst case, it is possible to end up with no warning but
wrong results.  In particular, if you get a collision and the length
of the functions happens to be the same, i don't believe the linker
will complain about the symbol size changing.