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

[Aldy Hernandez <aldyh@redhat.com>]

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 out of date.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is out of date, I would much rather use range_on_entry like you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.

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
>