Re: [PATCH] Loop unswitching: support gswitch statements.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Nov 8, 2021 at 8:45 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>
> On 11/8/21 10:05 AM, Martin Liška wrote:
> > On 9/28/21 22:39, Andrew MacLeod wrote:
> >> In Theory, modifying the IL should be fine, it happens already in
> >> places, but its not extensively tested under those conditions yet.
> >
> > Hello Andrew.
> >
> > I've just tried using a global gimple_ranger and it crashes when loop
> > unswitching duplicates
> > some BBs.
> >
> > Please try the attached patch for:
>
> hey Martin,
>
> try using this in your tree.  Since nothing else is using a growing BB
> right now, I'll let you work with it and see if everything works as
> expected before checking it in, just in case we need more tweaking.
> With this,
>
> make RUNTESTFLAGS=dg.exp=loop-unswitch*.c check-gcc
>
> runs clean.
>
>
> basically, I tried to grow it by either a factor of 10% for the current
> BB size when the grow is requested, or some double the needed extra
> size, or 128... whichever value is "maximum"    That means it shoudnt be
> asking for tooo much each time, but also not a minimum amount.
>
> Im certainly open to suggestion on how much to grow it each time.
> Note the vector being grown is ONLY fo the SSA_NAme being asked for.. so
> it really an on-demand thing just for specific names, in your case,
> mostly just the switch index.
>
> Let me know how this works for you, and if you have any other issues.

So I think in the end we shouldn't need the growing.  Ideally we'd do all
the analysis before the first transform, but for that we'd need ranger to
be able to "simplify" conditions based on a known true/false predicate
that's not yet in the IL.  Consider

 for (;;)
   {
        if (invariant < 3) // A
          {
...
          }
        if (invariant < 5) // B
          {
...
          }
   }

unswitch analysis will run into the condition 'A' and determine the loop
can be unswitched with the condition 'invariant < 3'.  To be able to
perform cost assessment and to avoid redundant unswitching we
want to determine that if we unswitch with 'invariant < 3' being
true then the condition at 'B' is true as well before actually inserting
the if (invariant < 3) outside of the loop.

So I'm thinking of assigning a gimple_uid to each condition we want to
unswitch on and have an array indexed by the uid with meta-data on
the unswitch opportunity, the "related" conditions could be marked with
the same uid (or some other), and the folding result recorded so that
at transform time we can just do the appropriate replacement without
invoking ranger again.

Now, but how do we arrange for the ranger analysis here?

We might also somehow want to remember that on the
'invariant < 3' == false copy of the loop there's still the
unswitching opportunity on 'invariant < 5', but not on the
'invariant < 5' == true copy.

Currently unswitching uses a custom simplify_using_entry_checks
which tries to do simplification only after the fact (and so costing
also is far from costing the true cost and ordering of the opportunities
to do the best first is not implemented either).

Richard.

> Andrew
>