Re: [PATCH] Convert strlen pass from evrp to ranger.

[Aldy Hernandez <aldyh@redhat.com>]

On Thu, Oct 21, 2021 at 12:20 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Wed, Oct 20, 2021 at 10:58 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
> >
> >
> >
> > On 10/18/2021 2:17 AM, Aldy Hernandez wrote:
> > >
> > >
> > > On 10/18/21 12:52 AM, Jeff Law wrote:
> > >>
> > >>
> > >> On 10/8/2021 9:12 AM, Aldy Hernandez via Gcc-patches wrote:
> > >>> The following patch converts the strlen pass from evrp to ranger,
> > >>> leaving DOM as the last remaining user.
> > >> So is there any reason why we can't convert DOM as well?   DOM's use
> > >> of EVRP is pretty limited.  You've mentioned FP bits before, but my
> > >> recollection is those are not part of the EVRP analysis DOM uses.
> > >> Hell, give me a little guidance and I'll do the work...
> > >
> > > Not only will I take you up on that offer, but I can provide 90% of
> > > the work.  Here be dragons, though (well, for me, maybe not for you ;-)).
> > >
> > > DOM is actually an evrp pass at -O1 in disguise.  The reason it really
> > > is a covert evrp pass is because:
> > >
> > > a) It calls extract_range_from_stmt on each statement.
> > >
> > > b) It folds conditionals with simplify_using_ranges.
> > >
> > > c) But most importantly, it exports discovered ranges when it's done
> > > (evrp_range_analyzer(true)).
> > >
> > > If you look at the evrp pass, you'll notice that that's basically what
> > > it does, albeit with the substitute and fold engine, which also calls
> > > gimple fold plus other goodies.
> > >
> > > But I could argue that we've made DOM into an evrp pass without
> > > noticing.  The last item (c) is particularly invasive because these
> > > exported ranges show up in other passes unexpectedly.  For instance, I
> > > saw an RTL pass at -O1 miss an optimization because it was dependent
> > > on some global range being set.  IMO, DOM should not export global
> > > ranges it discovered during its walk (do one thing and do it well),
> > > but I leave it to you experts to pontificate.
> > All true.  But I don't think we've got many, if any, hard dependencies
> > on those behaviors.
> >
> > >
> > > The attached patch is rather trivial.  It's mostly deleting state.  It
> > > seems DOM spends a lot of time massaging the IL so that it can fold
> > > conditionals or thread paths.  None of this is needed, because the
> > > ranger can do all of this.  Well, except floats, but...
> > Massaging the IL should only take two forms IIRC.
> >
> > First, if we have a simplification we can do.  That could be const/copy
> > propagation, replacing an expression with an SSA_NAME or constant and
> > the like.  It doesn't massage the IL just to massage the IL.
> >
> > Second, we do temporarily copy propagate the current known values of an
> > SSA name into use points and then see if that allows us to determine if
> > a statement is already in the hash tables.  But we undo that so that
> > nobody should see that temporary change in state.
>
> Are you sure we still do that?  I can't find it at least.

I couldn't either, but perhaps what Jeff is referring to is the ad-hoc
copy propagation that happens in the DOM's use of the threader:

      /* Make a copy of the uses & vuses into USES_COPY, then cprop into
         the operands.  */
      FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
        {
          tree tmp = NULL;
          tree use = USE_FROM_PTR (use_p);

          copy[i++] = use;
          if (TREE_CODE (use) == SSA_NAME)
        tmp = SSA_NAME_VALUE (use);
          if (tmp)
        SET_USE (use_p, tmp);
        }

      cached_lhs = simplifier->simplify (stmt, stmt, bb, this);

      /* Restore the statement's original uses/defs.  */
      i = 0;
      FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
        SET_USE (use_p, copy[i++]);

Aldy