Re: [PATCH] Preserve loops from CFG build until after RTL loop opts

[Richard Biener <rguenther at suse dot de>]

On Sun, 28 Apr 2013, Tom de Vries wrote:

> On 26/04/13 16:27, Tom de Vries wrote:
> > On 25/04/13 16:19, Richard Biener wrote:
> > <SNIP>
> >> and compared to the previous patch changed the tree-ssa-tailmerge.c
> >> part to deal with merging of loop latch and loop preheader (even
> >> if that's a really bad idea) to not regress gcc.dg/pr50763.c.
> >> Any suggestion on how to improve that part welcome.
> <SNIP>
> > So I think this is really a cornercase, and we should disregard it if that makes
> > things simpler.
> > 
> > Rather than fixing up the loop structure, we could prevent tail-merge in these
> > cases.
> > 
> > The current fix tests for current_loops == NULL, and I'm not sure that can still
> > happen there, given that we have PROP_loops.
> 
> Richard,
> 
> I've found that it happens in these g++ test-cases:
>   g++.dg/ext/mv1.C
>   g++.dg/ext/mv12.C
>   g++.dg/ext/mv2.C
>   g++.dg/ext/mv5.C
>   g++.dg/torture/covariant-1.C
>   g++.dg/torture/pr43068.C
>   g++.dg/torture/pr47714.C
> This seems rare enough to just bail out of tail-merge in those cases.
> 
> > It's not evident to me that the test bb2->loop_father->latch == bb2 is
> > sufficient. Before calling tail_merge_optimize, we call loop_optimizer_finalize
> > in which we assert that LOOPS_MAY_HAVE_MULTIPLE_LATCHES from there on, so in
> > theory we might miss some latches.
> > 
> > But I guess that pre (having started out with simple latches) maintains simple
> > latches throughout, and that tail-merge does the same.
> 
> I've added a comment related to this in the patch.
> 
> Bootstrapped and reg-tested (ada inclusive) on x86_64.
> 
> OK for trunk?

+  if (bb == NULL
+      /* Be conservative with loop structure.  It's not evident that this
test
+        is sufficient.  Before tail-merge, we've just called
+        loop_optimizer_finalize, and LOOPS_MAY_HAVE_MULTIPLE_LATCHES is 
now
+        set, so there's no guarantee that the loop->latch value is still
valid.
+        But we assume that, since we've forced LOOPS_HAVE_SIMPLE_LATCHES 
at
the
+        start of pre, we've kept that property intact throughout pre, and
are
+        keeping it throughout tail-merge using this test.  */
+      || bb->loop_father->latch == bb)
     return;

A more complete test would be to use what the bb_loop_header_p predicate
does - skip latch _edges_.  Not sure if that's easily possible in
the loop looking at succs

  FOR_EACH_EDGE (e, ei, bb->succs)
    {
      int index = e->dest->index;
      bitmap_set_bit (same->succs, index);
      same_succ_edge_flags[index] = e->flags;
    }

but we'd skip all edges for which

    dominated_by_p (CDI_DOMINATORS, e->src, e->dest)

of course that's equal to skipping the whole basic-block if the
above is true.

I suppose the patch is ok as-is for now, but let's keep the above
in mind (I want to audit the whole bootstrap process for loops
that vanish and eventually re-appear, I just didn't get around
thinking about a proper way to efficiently instrument for that).

Thanks,
Richard.