C provenance semantics proposal

[Ralf Jung]

Hi all,

>>>  if (a == b)
>>> ...
>>>
>>>  # c = PHI <a, b>
>>>
>>> where we'd lose eliding such a conditional.  IMHO that's bad
>>> and very undesirable.
>> But if we only suppress this optimization for pointers is it that terrible?
> 
> I've at least seen a lot of cases with c = PHI <a, 0> for null pointer
> checks.  It's just we're going to chase a lot of cases down even
> knowing RTL will fuck up later big times.

Even if pointer substitution based on "==" is not possible in general, it can
still be possible (depending on other choices in the IR semantics) to do
substitution for "== NULL" pointer tests.  This is the conclusion we came to
when studying similar questions for LLVM IR in
<https://people.mpi-sws.org/~jung/twinsem/twinsem.pdf>.

Kind regards,
Ralf