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* Copyright (c) 2006, 2023, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation.
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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#include "precompiled.hpp"
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#include "memory/allocation.inline.hpp"
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#include "opto/castnode.hpp"
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#include "opto/cfgnode.hpp"
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#include "opto/connode.hpp"
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#include "opto/convertnode.hpp"
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#include "opto/loopnode.hpp"
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#include "opto/opaquenode.hpp"
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#include "opto/predicates.hpp"
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#include "opto/rootnode.hpp"
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// Loop Unswitching is a loop optimization to move an invariant, non-loop-exiting test in the loop body before the loop.
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// Such a test is either always true or always false in all loop iterations and could therefore only be executed once.
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// To achieve that, we duplicate the loop and change the original and cloned loop as follows:
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// - Original loop -> true-path-loop:
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// The true-path of the invariant, non-loop-exiting test in the original loop
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// is kept while the false-path is killed. We call this unswitched loop version
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// - Cloned loop -> false-path-loop:
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// The false-path of the invariant, non-loop-exiting test in the cloned loop
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// is kept while the true-path is killed. We call this unswitched loop version
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// the false-path loop.
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// The invariant, non-loop-exiting test can now be moved before both loops (to only execute it once) and turned into a
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// loop selector If node to select at runtime which unswitched loop version should be executed.
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// - Loop selector true? Execute the true-path-loop.
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// - Loop selector false? Execute the false-path-loop.
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// Note that even though an invariant test that exits the loop could also be optimized with Loop Unswitching, it is more
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// efficient to simply peel the loop which achieves the same result in a simpler manner (also see policy_peeling()).
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// The following graphs summarizes the Loop Unswitching optimization.
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// We start with the original loop:
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// which is unswitched into a true-path-loop and a false-path-loop together with a loop selector:
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// [Initialized Assertion Predicates]
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// loop selector If (invariant-test)
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// [Cloned Parse Predicates] [Cloned Parse Predicates]
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// [Cloned Template [Cloned Template
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// Assertion Predicates] Assertion Predicates]
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// True-Path-Loop False-Path-Loop
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// cloned stmt1 cloned stmt1
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// cloned if-path cloned else-path
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// cloned stmt2 cloned stmt2
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// Return true if the loop should be unswitched or false otherwise.
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bool IdealLoopTree::policy_unswitching(PhaseIdealLoop* phase) const {
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if (!LoopUnswitching) {
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if (!_head->is_Loop()) {
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// If nodes are depleted, some transform has miscalculated its needs.
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assert(!phase->exceeding_node_budget(), "sanity");
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// check for vectorized loops, any unswitching was already applied
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if (_head->is_CountedLoop() && _head->as_CountedLoop()->is_unroll_only()) {
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LoopNode* head = _head->as_Loop();
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if (head->unswitch_count() + 1 > head->unswitch_max()) {
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if (phase->find_unswitch_candidate(this) == nullptr) {
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// Too speculative if running low on nodes.
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return phase->may_require_nodes(est_loop_clone_sz(2));
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// Find an invariant test in the loop body that does not exit the loop. If multiple tests are found, we pick the first
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// one in the loop body. Return the "unswitch candidate" If to apply Loop Unswitching on.
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IfNode* PhaseIdealLoop::find_unswitch_candidate(const IdealLoopTree* loop) const {
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LoopNode* head = loop->_head->as_Loop();
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IfNode* unswitch_candidate = nullptr;
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Node* n = head->in(LoopNode::LoopBackControl);
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Node* n_dom = idom(n);
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if (n->is_Region()) {
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if (n_dom->is_If()) {
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IfNode* iff = n_dom->as_If();
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if (iff->in(1)->is_Bool()) {
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BoolNode* bol = iff->in(1)->as_Bool();
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if (bol->in(1)->is_Cmp()) {
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// If condition is invariant and not a loop exit,
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// then found reason to unswitch.
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if (loop->is_invariant(bol) && !loop->is_loop_exit(iff)) {
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assert(iff->Opcode() == Op_If || iff->is_RangeCheck() || iff->is_BaseCountedLoopEnd(), "valid ifs");
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unswitch_candidate = iff;
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return unswitch_candidate;
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// This class creates an If node (i.e. loop selector) that selects if the true-path-loop or the false-path-loop should be
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// executed at runtime. This is done by finding an invariant and non-loop-exiting unswitch candidate If node (guaranteed
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// to exist at this point) to perform Loop Unswitching on.
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class UnswitchedLoopSelector : public StackObj {
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PhaseIdealLoop* const _phase;
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IdealLoopTree* const _outer_loop;
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Node* const _original_loop_entry;
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IfNode* const _unswitch_candidate;
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IfNode* const _selector;
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IfTrueNode* const _true_path_loop_proj;
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IfFalseNode* const _false_path_loop_proj;
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enum PathToLoop { TRUE_PATH, FALSE_PATH };
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UnswitchedLoopSelector(IdealLoopTree* loop)
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: _phase(loop->_phase),
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_outer_loop(loop->skip_strip_mined()->_parent),
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_original_loop_entry(loop->_head->as_Loop()->skip_strip_mined()->in(LoopNode::EntryControl)),
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_unswitch_candidate(find_unswitch_candidate(loop)),
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_selector(create_selector_if()),
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_true_path_loop_proj(create_proj_to_loop(TRUE_PATH)->as_IfTrue()),
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_false_path_loop_proj(create_proj_to_loop(FALSE_PATH)->as_IfFalse()) {
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NONCOPYABLE(UnswitchedLoopSelector);
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IfNode* find_unswitch_candidate(IdealLoopTree* loop) {
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IfNode* unswitch_candidate = _phase->find_unswitch_candidate(loop);
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assert(unswitch_candidate != nullptr, "guaranteed to exist by policy_unswitching");
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assert(_phase->is_member(loop, unswitch_candidate), "must be inside original loop");
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return unswitch_candidate;
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IfNode* create_selector_if() const {
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const uint dom_depth = _phase->dom_depth(_original_loop_entry);
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_phase->igvn().rehash_node_delayed(_original_loop_entry);
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BoolNode* unswitch_candidate_bool = _unswitch_candidate->in(1)->as_Bool();
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IfNode* selector_if = IfNode::make_with_same_profile(_unswitch_candidate, _original_loop_entry,
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unswitch_candidate_bool);
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_phase->register_node(selector_if, _outer_loop, _original_loop_entry, dom_depth);
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IfProjNode* create_proj_to_loop(const PathToLoop path_to_loop) {
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const uint dom_depth = _phase->dom_depth(_original_loop_entry);
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IfProjNode* proj_to_loop;
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if (path_to_loop == TRUE_PATH) {
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proj_to_loop = new IfTrueNode(_selector);
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proj_to_loop = new IfFalseNode(_selector);
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_phase->register_node(proj_to_loop, _outer_loop, _selector, dom_depth);
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IfNode* unswitch_candidate() const {
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return _unswitch_candidate;
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IfNode* selector() const {
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IfTrueNode* true_path_loop_proj() const {
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return _true_path_loop_proj;
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IfFalseNode* false_path_loop_proj() const {
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return _false_path_loop_proj;
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// Class to unswitch the original loop and create Predicates at the new unswitched loop versions. The newly cloned loop
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// becomes the false-path-loop while original loop becomes the true-path-loop.
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class OriginalLoop : public StackObj {
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LoopNode* const _loop_head; // OuterStripMinedLoopNode if loop strip mined, else just the loop head.
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IdealLoopTree* const _loop;
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PhaseIdealLoop* const _phase;
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OriginalLoop(IdealLoopTree* loop, Node_List& old_new)
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: _loop_head(loop->_head->as_Loop()->skip_strip_mined()),
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_phase(loop->_phase) {}
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NONCOPYABLE(OriginalLoop);
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void fix_loop_entries(IfProjNode* true_path_loop_entry, IfProjNode* false_path_loop_entry) {
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_phase->replace_loop_entry(_loop_head, true_path_loop_entry);
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LoopNode* false_path_loop_strip_mined_head = old_to_new(_loop_head)->as_Loop();
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_phase->replace_loop_entry(false_path_loop_strip_mined_head, false_path_loop_entry);
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Node* old_to_new(const Node* old) const {
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return _old_new[old->_idx];
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void verify_unswitched_loop_versions(LoopNode* true_path_loop_head,
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const UnswitchedLoopSelector& unswitched_loop_selector) const {
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verify_unswitched_loop_version(true_path_loop_head, unswitched_loop_selector.true_path_loop_proj());
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verify_unswitched_loop_version(old_to_new(true_path_loop_head)->as_Loop(),
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unswitched_loop_selector.false_path_loop_proj());
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static void verify_unswitched_loop_version(LoopNode* loop_head, IfProjNode* loop_selector_if_proj) {
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Node* entry = loop_head->skip_strip_mined()->in(LoopNode::EntryControl);
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const Predicates predicates(entry);
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// When skipping all predicates, we should end up at 'loop_selector_if_proj'.
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assert(loop_selector_if_proj == predicates.entry(), "should end up at loop selector If");
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// Remove the unswitch candidate If nodes in both unswitched loop versions which are now dominated by the loop selector
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// If node. Keep the true-path-path in the true-path-loop and the false-path-path in the false-path-loop by setting
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// the bool input accordingly. The unswitch candidate If nodes are folded in the next IGVN round.
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void remove_unswitch_candidate_from_loops(const UnswitchedLoopSelector& unswitched_loop_selector) {
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IfNode* unswitching_candidate = unswitched_loop_selector.unswitch_candidate();
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_phase->igvn().rehash_node_delayed(unswitching_candidate);
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_phase->dominated_by(unswitched_loop_selector.true_path_loop_proj(), unswitching_candidate);
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IfNode* unswitching_candidate_clone = _old_new[unswitching_candidate->_idx]->as_If();
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_phase->igvn().rehash_node_delayed(unswitching_candidate_clone);
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_phase->dominated_by(unswitched_loop_selector.false_path_loop_proj(), unswitching_candidate_clone);
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// Unswitch the original loop on the invariant loop selector by creating a true-path-loop and a false-path-loop.
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// Remove the unswitch candidate If from both unswitched loop versions which are now covered by the loop selector If.
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void unswitch(const UnswitchedLoopSelector& unswitched_loop_selector) {
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_phase->clone_loop(_loop, _old_new, _phase->dom_depth(_loop_head),
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PhaseIdealLoop::CloneIncludesStripMined, unswitched_loop_selector.selector());
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// At this point, the selector If projections are the corresponding loop entries.
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// clone_parse_and_assertion_predicates_to_unswitched_loop() could clone additional predicates after the selector
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// If projections. The loop entries are updated accordingly.
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IfProjNode* true_path_loop_entry = unswitched_loop_selector.true_path_loop_proj();
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IfProjNode* false_path_loop_entry = unswitched_loop_selector.false_path_loop_proj();
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_phase->clone_parse_and_assertion_predicates_to_unswitched_loop(_loop, _old_new,
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true_path_loop_entry, false_path_loop_entry);
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fix_loop_entries(true_path_loop_entry, false_path_loop_entry);
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DEBUG_ONLY(verify_unswitched_loop_versions(_loop->_head->as_Loop(), unswitched_loop_selector);)
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_phase->recompute_dom_depth();
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remove_unswitch_candidate_from_loops(unswitched_loop_selector);
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// See comments below file header for more information about Loop Unswitching.
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void PhaseIdealLoop::do_unswitching(IdealLoopTree* loop, Node_List& old_new) {
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assert(LoopUnswitching, "LoopUnswitching must be enabled");
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LoopNode* original_head = loop->_head->as_Loop();
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if (has_control_dependencies_from_predicates(original_head)) {
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NOT_PRODUCT(trace_loop_unswitching_impossible(original_head);)
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NOT_PRODUCT(trace_loop_unswitching_count(loop, original_head);)
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C->print_method(PHASE_BEFORE_LOOP_UNSWITCHING, 4, original_head);
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revert_to_normal_loop(original_head);
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const UnswitchedLoopSelector unswitched_loop_selector(loop);
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OriginalLoop original_loop(loop, old_new);
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original_loop.unswitch(unswitched_loop_selector);
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hoist_invariant_check_casts(loop, old_new, unswitched_loop_selector);
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add_unswitched_loop_version_bodies_to_igvn(loop, old_new);
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LoopNode* new_head = old_new[original_head->_idx]->as_Loop();
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increment_unswitch_counts(original_head, new_head);
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NOT_PRODUCT(trace_loop_unswitching_result(unswitched_loop_selector, original_head, new_head);)
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C->print_method(PHASE_AFTER_LOOP_UNSWITCHING, 4, new_head);
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C->set_major_progress();
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bool PhaseIdealLoop::has_control_dependencies_from_predicates(LoopNode* head) {
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Node* entry = head->skip_strip_mined()->in(LoopNode::EntryControl);
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const Predicates predicates(entry);
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if (predicates.has_any()) {
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assert(entry->is_IfProj(), "sanity - must be ifProj since there is at least one predicate");
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if (entry->outcnt() > 1) {
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// Bailout if there are predicates from which there are additional control dependencies (i.e. from loop
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// entry 'entry') to previously partially peeled statements since this case is not handled and can lead
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// to a wrong execution. Remove this bailout, once this is fixed.
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void PhaseIdealLoop::trace_loop_unswitching_impossible(const LoopNode* original_head) {
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if (TraceLoopUnswitching) {
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tty->print_cr("Loop Unswitching \"%d %s\" not possible due to control dependencies",
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original_head->_idx, original_head->Name());
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void PhaseIdealLoop::trace_loop_unswitching_count(IdealLoopTree* loop, LoopNode* original_head) {
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tty->print("Unswitch %d ", original_head->unswitch_count() + 1);
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void PhaseIdealLoop::trace_loop_unswitching_result(const UnswitchedLoopSelector& unswitched_loop_selector,
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const LoopNode* original_head, const LoopNode* new_head) {
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if (TraceLoopUnswitching) {
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IfNode* unswitch_candidate = unswitched_loop_selector.unswitch_candidate();
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IfNode* loop_selector = unswitched_loop_selector.selector();
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tty->print_cr("Loop Unswitching:");
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tty->print_cr("- Unswitch-Candidate-If: %d %s", unswitch_candidate->_idx, unswitch_candidate->Name());
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tty->print_cr("- Loop-Selector-If: %d %s", loop_selector->_idx, loop_selector->Name());
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tty->print_cr("- True-Path-Loop (=Orig): %d %s", original_head->_idx, original_head->Name());
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tty->print_cr("- False-Path-Loop (=Clone): %d %s", new_head->_idx, new_head->Name());
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// When unswitching a counted loop, we need to convert it back to a normal loop since it's not a proper pre, main or,
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// post loop anymore after loop unswitching.
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void PhaseIdealLoop::revert_to_normal_loop(const LoopNode* loop_head) {
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CountedLoopNode* cl = loop_head->isa_CountedLoop();
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if (cl != nullptr && !cl->is_normal_loop()) {
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cl->set_normal_loop();
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// Hoist invariant CheckCastPPNodes out of each unswitched loop version to the appropriate loop selector If projection.
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void PhaseIdealLoop::hoist_invariant_check_casts(const IdealLoopTree* loop, const Node_List& old_new,
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const UnswitchedLoopSelector& unswitched_loop_selector) {
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IfNode* unswitch_candidate = unswitched_loop_selector.unswitch_candidate();
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IfNode* loop_selector = unswitched_loop_selector.selector();
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GrowableArray<CheckCastPPNode*> loop_invariant_check_casts;
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for (DUIterator_Fast imax, i = unswitch_candidate->fast_outs(imax); i < imax; i++) {
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IfProjNode* proj = unswitch_candidate->fast_out(i)->as_IfProj();
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// Copy to a worklist for easier manipulation
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for (DUIterator_Fast jmax, j = proj->fast_outs(jmax); j < jmax; j++) {
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CheckCastPPNode* check_cast = proj->fast_out(j)->isa_CheckCastPP();
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if (check_cast != nullptr && loop->is_invariant(check_cast->in(1))) {
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loop_invariant_check_casts.push(check_cast);
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IfProjNode* loop_selector_if_proj = loop_selector->proj_out(proj->_con)->as_IfProj();
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while (loop_invariant_check_casts.length() > 0) {
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CheckCastPPNode* cast = loop_invariant_check_casts.pop();
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Node* cast_clone = cast->clone();
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cast_clone->set_req(0, loop_selector_if_proj);
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_igvn.replace_input_of(cast, 1, cast_clone);
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register_new_node(cast_clone, loop_selector_if_proj);
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// Same for the clone
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Node* use_clone = old_new[cast->_idx];
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_igvn.replace_input_of(use_clone, 1, cast_clone);
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// Enable more optimizations possibilities in the next IGVN round.
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void PhaseIdealLoop::add_unswitched_loop_version_bodies_to_igvn(IdealLoopTree* loop, const Node_List& old_new) {
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loop->record_for_igvn();
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for(int i = loop->_body.size() - 1; i >= 0 ; i--) {
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Node* n = loop->_body[i];
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Node* n_clone = old_new[n->_idx];
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_igvn._worklist.push(n_clone);
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void PhaseIdealLoop::increment_unswitch_counts(LoopNode* original_head, LoopNode* new_head) {
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const int unswitch_count = original_head->unswitch_count() + 1;
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original_head->set_unswitch_count(unswitch_count);
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new_head->set_unswitch_count(unswitch_count);