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

["Martin Liška" <mliska@suse.cz>]

PING^2

On 3/4/22 14:47, Martin Liška wrote:
> 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.  */
>