Re: [PATCH] tree-optimization/106593 - fix ICE with backward threading

[Richard Biener <rguenther@suse.de>]

On Fri, 12 Aug 2022, Aldy Hernandez wrote:

> On Fri, Aug 12, 2022 at 12:59 PM Richard Biener <rguenther@suse.de> wrote:
> >
> > With the last re-org I failed to make sure to not add SSA names
> > nor supported by ranger into m_imports which then triggers an
> > ICE in range_on_path_entry because range_of_expr returns false.  I've
> > noticed that range_on_path_entry does mightly complicated things
> > that don't make sense to me and the commentary might just be
> > out of date.  For the sake of it I replaced it with range_on_entry
> > and statistics show we thread _more_ jumps with that, so better
> > not do magic there.
> 
> Hang on, hang on.  range_on_path_entry was written that way for a
> reason.  Andrew and I had numerous discussions about this.  For that
> matter, my first implementation did exactly what you're proposing, but
> he had reservations about using range_on_entry, which IIRC he thought
> should be removed from the (public) API because it had a tendency to
> blow up lookups.
> 
> Let's wait for Andrew to chime in on this.  If indeed the commentary
> is out of date, I would much rather use range_on_entry like you
> propose, but he and I have fought many times about this... over
> various versions of the path solver :).
> 
> For now I would return VARYING in range_on_path_entry if range_of_expr
> returns false.  We shouldn't be ICEing when we can gracefully handle
> things.  This gcc_unreachable was there to catch implementation issues
> during development.
> 
> I would keep your gimple_range_ssa_p check regardless.  No sense doing
> extra work if we're absolutely sure we won't handle it.

OK, I'll push just the gimple_range_ssa_p then since that resolves
the PR on its own.  I was first misled about the gcc_unreachable
and my brain hurt understanding this function ... (also as to
why using range_of_expr on a _random_ stmt would be OK).

That said, nothing seems to be (publicly) using range_on_entry,
so if it shouldn't be used (but it's used privately!) then
make it private.

Thanks,
Richard.

> Aldy
> 
> >
> > Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
> >
> > Will push if that succeeds.
> >
> >         PR tree-optimization/106593
> >         * tree-ssa-threadbackward.cc (back_threader::find_paths):
> >         If the imports from the conditional do not satisfy
> >         gimple_range_ssa_p don't try to thread anything.
> >         * gimple-range-path.cc (range_on_path_entry): Just
> >         call range_on_entry.
> > ---
> >  gcc/gimple-range-path.cc       | 33 +--------------------------------
> >  gcc/tree-ssa-threadbackward.cc |  6 +++++-
> >  2 files changed, 6 insertions(+), 33 deletions(-)
> >
> > diff --git a/gcc/gimple-range-path.cc b/gcc/gimple-range-path.cc
> > index b6148eb5bd7..a7d277c31b8 100644
> > --- a/gcc/gimple-range-path.cc
> > +++ b/gcc/gimple-range-path.cc
> > @@ -153,38 +153,7 @@ path_range_query::range_on_path_entry (vrange &r, tree name)
> >  {
> >    gcc_checking_assert (defined_outside_path (name));
> >    basic_block entry = entry_bb ();
> > -
> > -  // Prefer to use range_of_expr if we have a statement to look at,
> > -  // since it has better caching than range_on_edge.
> > -  gimple *last = last_stmt (entry);
> > -  if (last)
> > -    {
> > -      if (m_ranger->range_of_expr (r, name, last))
> > -       return;
> > -      gcc_unreachable ();
> > -    }
> 
> I
> > -
> > -  // If we have no statement, look at all the incoming ranges to the
> > -  // block.  This can happen when we're querying a block with only an
> > -  // outgoing edge (no statement but the fall through edge), but for
> > -  // which we can determine a range on entry to the block.
> > -  Value_Range tmp (TREE_TYPE (name));
> > -  bool changed = false;
> > -  r.set_undefined ();
> > -  for (unsigned i = 0; i < EDGE_COUNT (entry->preds); ++i)
> > -    {
> > -      edge e = EDGE_PRED (entry, i);
> > -      if (e->src != ENTRY_BLOCK_PTR_FOR_FN (cfun)
> > -         && m_ranger->range_on_edge (tmp, e, name))
> > -       {
> > -         r.union_ (tmp);
> > -         changed = true;
> > -       }
> > -    }
> > -
> > -  // Make sure we don't return UNDEFINED by mistake.
> > -  if (!changed)
> > -    r.set_varying (TREE_TYPE (name));
> > +  m_ranger->range_on_entry (r, entry, name);
> >  }
> >
> >  // Return the range of NAME at the end of the path being analyzed.
> > diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
> > index 0a992213dad..669098e4ec3 100644
> > --- a/gcc/tree-ssa-threadbackward.cc
> > +++ b/gcc/tree-ssa-threadbackward.cc
> > @@ -525,7 +525,11 @@ back_threader::find_paths (basic_block bb, tree name)
> >        bitmap_clear (m_imports);
> >        ssa_op_iter iter;
> >        FOR_EACH_SSA_TREE_OPERAND (name, stmt, iter, SSA_OP_USE)
> > -       bitmap_set_bit (m_imports, SSA_NAME_VERSION (name));
> > +       {
> > +         if (!gimple_range_ssa_p (name))
> > +           return;
> > +         bitmap_set_bit (m_imports, SSA_NAME_VERSION (name));
> > +       }
> >
> >        // Interesting is the set of imports we still not have see
> >        // the definition of.  So while imports only grow, the
> > --
> > 2.35.3
> >
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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