Re: [PATCH] Avoid further recomputations in path_range_query once path is finalized.

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

On Tue, Aug 16, 2022 at 3:59 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> [Richi, I'm trying to make things more obvious for others working for the
> cose base.  What do you think?]
>
> This makes a few things explicit to avoid misuse.  First, we add a
> flag to differentiate between a path whose depdenency ranges are being
> computed, and a path that is finalized and no further calculations
> should be done.  I've also enhanced the comments to document what
> queries are supported.  Finally, I added some asserts to make sure
> range_of_expr is only used on the final conditional once the path has
> been finalized.
>
> Unfortunately, the forward threader builds dummy statements it passes
> to the path solver.  Most of these don't have a BB associated with
> them.  I've cleared the BB field for the one that still had one, to
> make the asserts above work.
>
> No difference in thread counts over my cc1 .ii files.
>
> gcc/ChangeLog:
>
>         * gimple-range-path.cc (path_range_query::path_range_query):
>         Initialized m_path_finalized.
>         (path_range_query::internal_range_of_expr): Avoid further
>         recomputations once path is finalized.
>         Take basic_block instead of stmt as argument.
>         (path_range_query::range_of_expr): Document.  Add asserts.
>         (path_range_query::compute_ranges): Set m_path_finalized.
>         * gimple-range-path.h (path_range_query::internal_range_of_expr):
>         Replace statement argument with basic block.
>         (class path_range_query): Add m_path_finalized.
>         * tree-ssa-threadedge.cc
>         (jump_threader::simplify_control_stmt_condition): Clear BB field
>         in dummy_switch.
> ---
>  gcc/gimple-range-path.cc   | 50 ++++++++++++++++++++++++++++++++++----
>  gcc/gimple-range-path.h    |  7 +++++-
>  gcc/tree-ssa-threadedge.cc |  1 +
>  3 files changed, 52 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/gimple-range-path.cc b/gcc/gimple-range-path.cc
> index c99d77dd340..a8412dd090b 100644
> --- a/gcc/gimple-range-path.cc
> +++ b/gcc/gimple-range-path.cc
> @@ -40,7 +40,8 @@ path_range_query::path_range_query (bool resolve, gimple_ranger *ranger)
>    : m_cache (new ssa_global_cache),
>      m_has_cache_entry (BITMAP_ALLOC (NULL)),
>      m_resolve (resolve),
> -    m_alloced_ranger (!ranger)
> +    m_alloced_ranger (!ranger),
> +    m_path_finalized (false)
>  {
>    if (m_alloced_ranger)
>      m_ranger = new gimple_ranger;
> @@ -159,7 +160,7 @@ path_range_query::range_on_path_entry (vrange &r, tree name)
>  // Return the range of NAME at the end of the path being analyzed.
>
>  bool
> -path_range_query::internal_range_of_expr (vrange &r, tree name, gimple *stmt)
> +path_range_query::internal_range_of_expr (vrange &r, tree name, basic_block bb)
>  {
>    if (!r.supports_type_p (TREE_TYPE (name)))
>      return false;
> @@ -174,8 +175,16 @@ path_range_query::internal_range_of_expr (vrange &r, tree name, gimple *stmt)
>        return true;
>      }
>
> -  if (stmt
> -      && range_defined_in_block (r, name, gimple_bb (stmt)))
> +  // Avoid further recomputations once the path has been finalized.
> +  if (m_path_finalized)
> +    {
> +      gimple_range_global (r, name);

I suppose we can't assert here instead?

> +      return true;
> +    }
> +
> +  // We're being called as part of the calculation of ranges for exit
> +  // dependencies.  Set the cache as we traverse the path top-down.
> +  if (bb && range_defined_in_block (r, name, bb))
>      {
>        if (TREE_CODE (name) == SSA_NAME)
>         {
> @@ -192,10 +201,37 @@ path_range_query::internal_range_of_expr (vrange &r, tree name, gimple *stmt)
>    return true;
>  }
>
> +// This, as well as range_of_stmt, are the main entry points for
> +// making queries about a path.
> +//
> +// Once the ranges for the exit dependencies have been pre-calculated,
> +// the path is considered finalized, and the only valid query is
> +// asking the range of a NAME at the end of the path.
> +//
> +// Note that this method can also be called internally before the path
> +// is finalized, as part of the path traversal pre-calculating the
> +// ranges for exit dependencies.  In this case, it may be called on
> +// statements that are not the final conditional as described above.
> +
>  bool
>  path_range_query::range_of_expr (vrange &r, tree name, gimple *stmt)
>  {
> -  if (internal_range_of_expr (r, name, stmt))
> +  basic_block bb = stmt ? gimple_bb (stmt) : NULL;
> +
> +  // Once a path is finalized, the only valid queries are of the final
> +  // statement in the exit block.

why?  shouldn't we support queries for all dependent ranges (what
the user specified as imports argument to compute_ranges)?

If we want to restrict things this way shouldn't we simply not expose
this function but instead just path_range_query::range_of_exit,
or even what the threader uses as find_taken_edge?

The loop header copying case would also work with this,
it's interested in the known outgoing edge of the path.

> +  if (flag_checking && m_path_finalized && stmt)
> +    {
> +      // The forward threader may query dummy statements without a
> +      // basic block.
> +      if (bb)
> +       {
> +         gcc_assert (stmt == last_stmt (bb));
> +         gcc_assert (bb == exit_bb ());
> +       }
> +    }
> +
> +  if (internal_range_of_expr (r, name, bb))
>      {
>        if (r.undefined_p ())
>         m_undefined_path = true;
> @@ -608,6 +644,7 @@ path_range_query::compute_ranges (const vec<basic_block> &path,
>
>    set_path (path);
>    m_undefined_path = false;
> +  m_path_finalized = false;
>
>    if (dependencies)
>      bitmap_copy (m_exit_dependencies, dependencies);
> @@ -642,6 +679,7 @@ path_range_query::compute_ranges (const vec<basic_block> &path,
>
>        move_next ();
>      }
> +  m_path_finalized = true;
>
>    if (DEBUG_SOLVER)
>      {
> @@ -730,6 +768,8 @@ jt_fur_source::query_relation (tree op1, tree op2)
>  }
>
>  // Return the range of STMT at the end of the path being analyzed.
> +//
> +// See notes in range_of_expr as to what are considered valid queries.
>
>  bool
>  path_range_query::range_of_stmt (vrange &r, gimple *stmt, tree)
> diff --git a/gcc/gimple-range-path.h b/gcc/gimple-range-path.h
> index 3cb794e34a9..61a7d0f656e 100644
> --- a/gcc/gimple-range-path.h
> +++ b/gcc/gimple-range-path.h
> @@ -46,7 +46,7 @@ public:
>    void debug ();
>
>  private:
> -  bool internal_range_of_expr (vrange &r, tree name, gimple *);
> +  bool internal_range_of_expr (vrange &r, tree name, basic_block);
>    bool defined_outside_path (tree name);
>    void range_on_path_entry (vrange &r, tree name);
>    path_oracle *get_path_oracle () { return (path_oracle *)m_oracle; }
> @@ -113,6 +113,11 @@ private:
>    // True if m_ranger was allocated in this class and must be freed at
>    // destruction.
>    bool m_alloced_ranger;
> +
> +// Once the ranges for the exit dependencies have been pre-calculated,
> +// the path is considered finalized and this is set to TRUE.
> +// Otherwise, the path is being analyzed.
> +  bool m_path_finalized;
>  };
>
>  #endif // GCC_TREE_SSA_THREADSOLVER_H
> diff --git a/gcc/tree-ssa-threadedge.cc b/gcc/tree-ssa-threadedge.cc
> index e64e4f209f7..213796c10cc 100644
> --- a/gcc/tree-ssa-threadedge.cc
> +++ b/gcc/tree-ssa-threadedge.cc
> @@ -451,6 +451,7 @@ jump_threader::simplify_control_stmt_condition (edge e, gimple *stmt)
>                  possible, the simplified value will be a CASE_LABEL_EXPR of
>                  the label that is proven to be taken.  */
>               gswitch *dummy_switch = as_a<gswitch *> (gimple_copy (stmt));
> +             gimple_set_bb (dummy_switch, NULL);

I think it's not wrong to say the stmt is in the block though

>               gimple_switch_set_index (dummy_switch, cached_lhs);
>               cached_lhs = m_simplifier->simplify (dummy_switch, stmt, e->src,
>                                                    m_state);
> --
> 2.37.1
>