Re: [PATCH] Improve profile handling in switch lowering.

["Martin Liška" <mliska@suse.cz>]

PING^1

On 1/26/22 12:11, Martin Liška wrote:
> Hello.
> 
> Right now, switch lowering does not update basic_block::count values
> so that they are uninitiliazed. Moreover, I've updated probability scaling
> when a more complex expansion happens. There are still some situations where
> the profile is a bit imprecise, but the patch improves rapidly the current situation.
> 
> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> 
> Ready to be installed?
> Thanks,
> Martin
> 
>      PR tree-optimization/101301
>      PR tree-optimization/103680
> 
> gcc/ChangeLog:
> 
>      * tree-switch-conversion.cc (bit_test_cluster::emit):
>      Handle correctly remaining probability.
>      (switch_decision_tree::try_switch_expansion): Fix BB's count
>      where a cluster expansion happens.
>      (switch_decision_tree::emit_cmp_and_jump_insns): Fill up also
>      BB count.
>      (switch_decision_tree::do_jump_if_equal): Likewise.
>      (switch_decision_tree::emit_case_nodes): Handle special case
>      for BT expansion which can also fallback to a default BB.
>      * tree-switch-conversion.h (cluster::cluster): Add
>      m_default_prob probability.
> ---
>   gcc/tree-switch-conversion.cc | 51 ++++++++++++++++++++++++-----------
>   gcc/tree-switch-conversion.h  |  8 +++++-
>   2 files changed, 43 insertions(+), 16 deletions(-)
> 
> diff --git a/gcc/tree-switch-conversion.cc b/gcc/tree-switch-conversion.cc
> index 670397c87e4..d6679e8dee3 100644
> --- a/gcc/tree-switch-conversion.cc
> +++ b/gcc/tree-switch-conversion.cc
> @@ -1538,10 +1538,12 @@ bit_test_cluster::emit (tree index_expr, tree index_type,
>         test[k].target_bb = n->m_case_bb;
>         test[k].label = n->m_case_label_expr;
>         test[k].bits = 0;
> +      test[k].prob = profile_probability::never ();
>         count++;
>       }
> 
>         test[k].bits += n->get_range (n->get_low (), n->get_high ());
> +      test[k].prob += n->m_prob;
> 
>         lo = tree_to_uhwi (int_const_binop (MINUS_EXPR, n->get_low (), minval));
>         if (n->get_high () == NULL_TREE)
> @@ -1629,6 +1631,11 @@ bit_test_cluster::emit (tree index_expr, tree index_type,
>                     /*simple=*/true, NULL_TREE,
>                     /*before=*/true, GSI_SAME_STMT);
> 
> +  profile_probability subtree_prob = m_subtree_prob;
> +  profile_probability default_prob = m_default_prob;
> +  if (!default_prob.initialized_p ())
> +    default_prob = m_subtree_prob.invert ();
> +
>     if (m_handles_entire_switch && entry_test_needed)
>       {
>         tree range = int_const_binop (MINUS_EXPR, maxval, minval);
> @@ -1639,9 +1646,10 @@ bit_test_cluster::emit (tree index_expr, tree index_type,
>                       /*simple=*/true, NULL_TREE,
>                       /*before=*/true, GSI_SAME_STMT);
>         tmp = fold_build2 (GT_EXPR, boolean_type_node, idx, range);
> +      default_prob = default_prob.apply_scale (1, 2);
>         basic_block new_bb
>       = hoist_edge_and_branch_if_true (&gsi, tmp, default_bb,
> -                     profile_probability::unlikely ());
> +                     default_prob);
>         gsi = gsi_last_bb (new_bb);
>       }
> 
> @@ -1662,14 +1670,12 @@ bit_test_cluster::emit (tree index_expr, tree index_type,
>     else
>       csui = tmp;
> 
> -  profile_probability prob = profile_probability::always ();
> -
>     /* for each unique set of cases:
>          if (const & csui) goto target  */
>     for (k = 0; k < count; k++)
>       {
> -      prob = profile_probability::always ().apply_scale (test[k].bits,
> -                             bt_range);
> +      profile_probability prob = test[k].prob / (subtree_prob + default_prob);
> +      subtree_prob -= test[k].prob;
>         bt_range -= test[k].bits;
>         tmp = wide_int_to_tree (word_type_node, test[k].mask);
>         tmp = fold_build2 (BIT_AND_EXPR, word_type_node, csui, tmp);
> @@ -1908,9 +1914,13 @@ switch_decision_tree::try_switch_expansion (vec<cluster *> &clusters)
>         /* Emit cluster-specific switch handling.  */
>         for (unsigned i = 0; i < clusters.length (); i++)
>       if (clusters[i]->get_type () != SIMPLE_CASE)
> -      clusters[i]->emit (index_expr, index_type,
> -                 gimple_switch_default_label (m_switch),
> -                 m_default_bb, gimple_location (m_switch));
> +      {
> +        edge e = single_pred_edge (clusters[i]->m_case_bb);
> +        e->dest->count = e->src->count.apply_probability (e->probability);
> +        clusters[i]->emit (index_expr, index_type,
> +                   gimple_switch_default_label (m_switch),
> +                   m_default_bb, gimple_location (m_switch));
> +      }
>       }
> 
>     fix_phi_operands_for_edges ();
> @@ -2162,6 +2172,7 @@ switch_decision_tree::emit_cmp_and_jump_insns (basic_block bb, tree op0,
>     edge false_edge = split_block (bb, cond);
>     false_edge->flags = EDGE_FALSE_VALUE;
>     false_edge->probability = prob.invert ();
> +  false_edge->dest->count = bb->count.apply_probability (prob.invert ());
> 
>     edge true_edge = make_edge (bb, label_bb, EDGE_TRUE_VALUE);
>     true_edge->probability = prob;
> @@ -2192,6 +2203,7 @@ switch_decision_tree::do_jump_if_equal (basic_block bb, tree op0, tree op1,
>     edge false_edge = split_block (bb, cond);
>     false_edge->flags = EDGE_FALSE_VALUE;
>     false_edge->probability = prob.invert ();
> +  false_edge->dest->count = bb->count.apply_probability (prob.invert ());
> 
>     edge true_edge = make_edge (bb, label_bb, EDGE_TRUE_VALUE);
>     true_edge->probability = prob;
> @@ -2227,7 +2239,7 @@ switch_decision_tree::emit_case_nodes (basic_block bb, tree index,
>                    node->m_c->m_case_bb, p, loc);
>         /* Since this case is taken at this point, reduce its weight from
>        subtree_weight.  */
> -      node->m_c->m_subtree_prob -= p;
> +      node->m_c->m_subtree_prob -= node->m_c->m_prob;
> 
>         if (node->m_left != NULL && node->m_right != NULL)
>       {
> @@ -2246,6 +2258,7 @@ switch_decision_tree::emit_case_nodes (basic_block bb, tree index,
>              / (node->m_c->m_subtree_prob + default_prob));
>             bb = do_jump_if_equal (bb, index, node->m_right->m_c->get_low (),
>                        node->m_right->m_c->m_case_bb, p, loc);
> +          node->m_c->m_subtree_prob -= node->m_right->m_c->m_prob;
> 
>             p = (node->m_left->m_c->m_prob
>              / (node->m_c->m_subtree_prob + default_prob));
> @@ -2262,6 +2275,7 @@ switch_decision_tree::emit_case_nodes (basic_block bb, tree index,
>             p = ((node->m_right->m_c->m_subtree_prob
>               + default_prob.apply_scale (1, 2))
>              / (node->m_c->m_subtree_prob + default_prob));
> +          test_bb->count = bb->count.apply_probability (p);
>             bb = emit_cmp_and_jump_insns (bb, index, node->m_c->get_high (),
>                           GT_EXPR, test_bb, p, loc);
>             default_prob = default_prob.apply_scale (1, 2);
> @@ -2348,21 +2362,28 @@ switch_decision_tree::emit_case_nodes (basic_block bb, tree index,
>        is the one to branch to.  */
>         if (node->has_child () || node->m_c->get_type () != SIMPLE_CASE)
>       {
> +      bool is_bt = node->m_c->get_type () == BIT_TEST;
> +      int parts = is_bt ? 3 : 2;
> +
>         /* Branch to a label where we will handle it later.  */
>         basic_block test_bb = split_edge (single_succ_edge (bb));
>         redirect_edge_succ (single_pred_edge (test_bb),
>                     single_succ_edge (bb)->dest);
> 
> -
> -       profile_probability right_prob = profile_probability::never ();
> -       if (node->m_right)
> -         right_prob = node->m_right->m_c->m_subtree_prob;
> -      p = ((right_prob + default_prob.apply_scale (1, 2))
> +      profile_probability right_prob = profile_probability::never ();
> +      if (node->m_right)
> +        right_prob = node->m_right->m_c->m_subtree_prob;
> +      p = ((right_prob + default_prob.apply_scale (1, parts))
>              / (node->m_c->m_subtree_prob + default_prob));
> +      test_bb->count = bb->count.apply_probability (p);
> 
>         bb = emit_cmp_and_jump_insns (bb, index, node->m_c->get_high (),
>                       GT_EXPR, test_bb, p, loc);
> -      default_prob = default_prob.apply_scale (1, 2);
> +
> +      default_prob = default_prob.apply_scale (1, parts);
> +      node->m_c->m_subtree_prob -= right_prob;
> +      if (is_bt)
> +        node->m_c->m_default_prob = default_prob;
> 
>         /* Value belongs to this node or to the left-hand subtree.  */
>         p = node->m_c->m_prob / (node->m_c->m_subtree_prob + default_prob);
> diff --git a/gcc/tree-switch-conversion.h b/gcc/tree-switch-conversion.h
> index e969c051a05..5f3f99353ba 100644
> --- a/gcc/tree-switch-conversion.h
> +++ b/gcc/tree-switch-conversion.h
> @@ -102,6 +102,10 @@ public:
>     /* Probability of reaching subtree rooted at this node.  */
>     profile_probability m_subtree_prob;
> 
> +  /* Probability of default case when reaching the node.
> +     It is used by bit-test right now.  */
> +  profile_probability m_default_prob;
> +
>   protected:
>     /* Default constructor.  */
>     cluster () {}
> @@ -110,7 +114,8 @@ protected:
>   cluster::cluster (tree case_label_expr, basic_block case_bb,
>             profile_probability prob, profile_probability subtree_prob):
>     m_case_label_expr (case_label_expr), m_case_bb (case_bb), m_prob (prob),
> -  m_subtree_prob (subtree_prob)
> +  m_subtree_prob (subtree_prob),
> +  m_default_prob (profile_probability::uninitialized ())
>   {
>   }
> 
> @@ -542,6 +547,7 @@ public:
>     basic_block target_bb;
>     tree label;
>     int bits;
> +  profile_probability prob;
> 
>     /* Comparison function for qsort to order bit tests by decreasing
>        probability of execution.  */