[Bug tree-optimization/64319] add alias runtime check to remove load after load redundancy

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64319

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias, missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-12-16
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The interesting problem to solve is of course which disabiguation checks to
insert
(and where).  And whether there is really a net positive effect in runtime
(surley not code-size).  Probably making this a value-profiling would make
sense?

As for the missed optimization with the conditonal code here the issue is that
points-to information is not flow-sensitive (or rather only in sofar as SSA
defs are flow-sensitive).

  <bb 2>:
  if (a_3(D) == b_4(D))
    goto <bb 3>;
  else
    goto <bb 4>;
...
  <bb 4>:
  *a_3(D) = 1;
  _12 = *b_4(D);
  _13 = _12 + 1;
  *b_4(D) = _13;
  _15 = *a_3(D);

And of course our points-to code doesn't look at conditionals at all to
derive alias information (it couldn't associate it with SSA names later
anyway).  It would need to add explicit IL similar to VRP (but that does
it only temporarily), and it also would require to teach the solver of
a constraint that makes two pointers non-equal (not sure if doable at all).

For the a == b case we manage to copy-propagate
both to equal values in DOM and thus optimize the loads and later DSE the
redundant store.

We could use the recently added facility to record explicit non-dependences
which is also flow-sensitive (recorded on actual memory references) when
adding the alias check or from code (DOM?) that figures out non-equivalence.
[see MR_DEPENDENCE_CLIQE / MR_DEPENDENCE_BASE]

I see this as two bugs - one to generate runtime disambiguation code, possibly
directed by profiling, and one to actually make use of present runtime alias
checks.

Confirmed.