1 /*
2 * Copyright (c) 2006, 2023, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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23 */
24
25 #include "precompiled.hpp"
26 #include "memory/allocation.inline.hpp"
27 #include "opto/castnode.hpp"
28 #include "opto/cfgnode.hpp"
29 #include "opto/connode.hpp"
30 #include "opto/convertnode.hpp"
31 #include "opto/loopnode.hpp"
32 #include "opto/opaquenode.hpp"
33 #include "opto/predicates.hpp"
34 #include "opto/rootnode.hpp"
35
36 // Loop Unswitching is a loop optimization to move an invariant, non-loop-exiting test in the loop body before the loop.
37 // Such a test is either always true or always false in all loop iterations and could therefore only be executed once.
38 // To achieve that, we duplicate the loop and change the original and cloned loop as follows:
39 // - Original loop -> true-path-loop:
40 // The true-path of the invariant, non-loop-exiting test in the original loop
41 // is kept while the false-path is killed. We call this unswitched loop version
42 // the true-path-loop.
43 // - Cloned loop -> false-path-loop:
44 // The false-path of the invariant, non-loop-exiting test in the cloned loop
45 // is kept while the true-path is killed. We call this unswitched loop version
46 // the false-path loop.
47 //
48 // The invariant, non-loop-exiting test can now be moved before both loops (to only execute it once) and turned into a
49 // loop selector If node to select at runtime which unswitched loop version should be executed.
50 // - Loop selector true? Execute the true-path-loop.
51 // - Loop selector false? Execute the false-path-loop.
52 //
53 // Note that even though an invariant test that exits the loop could also be optimized with Loop Unswitching, it is more
54 // efficient to simply peel the loop which achieves the same result in a simpler manner (also see policy_peeling()).
55 //
56 // The following graphs summarizes the Loop Unswitching optimization.
57 // We start with the original loop:
58 //
59 // [Predicates]
60 // |
61 // Original Loop
62 // stmt1
63 // if (invariant-test)
64 // if-path
65 // else
66 // else-path
67 // stmt2
68 // Endloop
69 //
70 //
71 // which is unswitched into a true-path-loop and a false-path-loop together with a loop selector:
72 //
73 //
74 // [Initialized Assertion Predicates]
75 // |
76 // loop selector If (invariant-test)
77 // / \
78 // true? false?
79 // / \
80 // [Cloned Parse Predicates] [Cloned Parse Predicates]
81 // [Cloned Template [Cloned Template
82 // Assertion Predicates] Assertion Predicates]
83 // | |
84 // True-Path-Loop False-Path-Loop
85 // cloned stmt1 cloned stmt1
86 // cloned if-path cloned else-path
87 // cloned stmt2 cloned stmt2
88 // Endloop Endloop
89
90
91 // Return true if the loop should be unswitched or false otherwise.
92 bool IdealLoopTree::policy_unswitching(PhaseIdealLoop* phase) const {
93 if (!LoopUnswitching) {
94 return false;
95 }
96 if (!_head->is_Loop()) {
97 return false;
98 }
99
100 // If nodes are depleted, some transform has miscalculated its needs.
101 assert(!phase->exceeding_node_budget(), "sanity");
102
103 // check for vectorized loops, any unswitching was already applied
104 if (_head->is_CountedLoop() && _head->as_CountedLoop()->is_unroll_only()) {
105 return false;
106 }
107
108 LoopNode* head = _head->as_Loop();
109 if (head->unswitch_count() + 1 > head->unswitch_max()) {
110 return false;
111 }
112 if (phase->find_unswitch_candidate(this) == nullptr) {
113 return false;
114 }
115
116 // Too speculative if running low on nodes.
117 return phase->may_require_nodes(est_loop_clone_sz(2));
118 }
119
120 // Find an invariant test in the loop body that does not exit the loop. If multiple tests are found, we pick the first
121 // one in the loop body. Return the "unswitch candidate" If to apply Loop Unswitching on.
122 IfNode* PhaseIdealLoop::find_unswitch_candidate(const IdealLoopTree* loop) const {
123 LoopNode* head = loop->_head->as_Loop();
124 IfNode* unswitch_candidate = nullptr;
125 Node* n = head->in(LoopNode::LoopBackControl);
126 while (n != head) {
127 Node* n_dom = idom(n);
128 if (n->is_Region()) {
129 if (n_dom->is_If()) {
130 IfNode* iff = n_dom->as_If();
131 if (iff->in(1)->is_Bool()) {
132 BoolNode* bol = iff->in(1)->as_Bool();
133 if (bol->in(1)->is_Cmp()) {
134 // If condition is invariant and not a loop exit,
135 // then found reason to unswitch.
136 if (loop->is_invariant(bol) && !loop->is_loop_exit(iff)) {
137 assert(iff->Opcode() == Op_If || iff->is_RangeCheck() || iff->is_BaseCountedLoopEnd(), "valid ifs");
138 unswitch_candidate = iff;
139 }
140 }
141 }
142 }
143 }
144 n = n_dom;
145 }
146 return unswitch_candidate;
147 }
148
149 // This class creates an If node (i.e. loop selector) that selects if the true-path-loop or the false-path-loop should be
150 // executed at runtime. This is done by finding an invariant and non-loop-exiting unswitch candidate If node (guaranteed
151 // to exist at this point) to perform Loop Unswitching on.
152 class UnswitchedLoopSelector : public StackObj {
153 PhaseIdealLoop* const _phase;
154 IdealLoopTree* const _outer_loop;
155 Node* const _original_loop_entry;
156 IfNode* const _unswitch_candidate;
157 IfNode* const _selector;
158 IfTrueNode* const _true_path_loop_proj;
159 IfFalseNode* const _false_path_loop_proj;
160
161 enum PathToLoop { TRUE_PATH, FALSE_PATH };
162
163 public:
164 UnswitchedLoopSelector(IdealLoopTree* loop)
165 : _phase(loop->_phase),
166 _outer_loop(loop->skip_strip_mined()->_parent),
167 _original_loop_entry(loop->_head->as_Loop()->skip_strip_mined()->in(LoopNode::EntryControl)),
168 _unswitch_candidate(find_unswitch_candidate(loop)),
169 _selector(create_selector_if()),
170 _true_path_loop_proj(create_proj_to_loop(TRUE_PATH)->as_IfTrue()),
171 _false_path_loop_proj(create_proj_to_loop(FALSE_PATH)->as_IfFalse()) {
172 }
173 NONCOPYABLE(UnswitchedLoopSelector);
174
175 private:
176 IfNode* find_unswitch_candidate(IdealLoopTree* loop) {
177 IfNode* unswitch_candidate = _phase->find_unswitch_candidate(loop);
178 assert(unswitch_candidate != nullptr, "guaranteed to exist by policy_unswitching");
179 assert(_phase->is_member(loop, unswitch_candidate), "must be inside original loop");
180 return unswitch_candidate;
181 }
182
183 IfNode* create_selector_if() const {
184 const uint dom_depth = _phase->dom_depth(_original_loop_entry);
185 _phase->igvn().rehash_node_delayed(_original_loop_entry);
186 BoolNode* unswitch_candidate_bool = _unswitch_candidate->in(1)->as_Bool();
187 IfNode* selector_if = IfNode::make_with_same_profile(_unswitch_candidate, _original_loop_entry,
188 unswitch_candidate_bool);
189 _phase->register_node(selector_if, _outer_loop, _original_loop_entry, dom_depth);
190 return selector_if;
191 }
192
193 IfProjNode* create_proj_to_loop(const PathToLoop path_to_loop) {
194 const uint dom_depth = _phase->dom_depth(_original_loop_entry);
195 IfProjNode* proj_to_loop;
196 if (path_to_loop == TRUE_PATH) {
197 proj_to_loop = new IfTrueNode(_selector);
198 } else {
199 proj_to_loop = new IfFalseNode(_selector);
200 }
201 _phase->register_node(proj_to_loop, _outer_loop, _selector, dom_depth);
202 return proj_to_loop;
203 }
204
205 public:
206 IfNode* unswitch_candidate() const {
207 return _unswitch_candidate;
208 }
209
210 IfNode* selector() const {
211 return _selector;
212 }
213
214 IfTrueNode* true_path_loop_proj() const {
215 return _true_path_loop_proj;
216 }
217
218 IfFalseNode* false_path_loop_proj() const {
219 return _false_path_loop_proj;
220 }
221 };
222
223 // Class to unswitch the original loop and create Predicates at the new unswitched loop versions. The newly cloned loop
224 // becomes the false-path-loop while original loop becomes the true-path-loop.
225 class OriginalLoop : public StackObj {
226 LoopNode* const _loop_head; // OuterStripMinedLoopNode if loop strip mined, else just the loop head.
227 IdealLoopTree* const _loop;
228 Node_List& _old_new;
229 PhaseIdealLoop* const _phase;
230
231 public:
232 OriginalLoop(IdealLoopTree* loop, Node_List& old_new)
233 : _loop_head(loop->_head->as_Loop()->skip_strip_mined()),
234 _loop(loop),
235 _old_new(old_new),
236 _phase(loop->_phase) {}
237 NONCOPYABLE(OriginalLoop);
238
239 private:
240 void fix_loop_entries(IfProjNode* true_path_loop_entry, IfProjNode* false_path_loop_entry) {
241 _phase->replace_loop_entry(_loop_head, true_path_loop_entry);
242 LoopNode* false_path_loop_strip_mined_head = old_to_new(_loop_head)->as_Loop();
243 _phase->replace_loop_entry(false_path_loop_strip_mined_head, false_path_loop_entry);
244 }
245
246 Node* old_to_new(const Node* old) const {
247 return _old_new[old->_idx];
248 }
249
250 #ifdef ASSERT
251 void verify_unswitched_loop_versions(LoopNode* true_path_loop_head,
252 const UnswitchedLoopSelector& unswitched_loop_selector) const {
253 verify_unswitched_loop_version(true_path_loop_head, unswitched_loop_selector.true_path_loop_proj());
254 verify_unswitched_loop_version(old_to_new(true_path_loop_head)->as_Loop(),
255 unswitched_loop_selector.false_path_loop_proj());
256 }
257
258 static void verify_unswitched_loop_version(LoopNode* loop_head, IfProjNode* loop_selector_if_proj) {
259 Node* entry = loop_head->skip_strip_mined()->in(LoopNode::EntryControl);
260 const Predicates predicates(entry);
261 // When skipping all predicates, we should end up at 'loop_selector_if_proj'.
262 assert(loop_selector_if_proj == predicates.entry(), "should end up at loop selector If");
263 }
264 #endif // ASSERT
265
266 // Remove the unswitch candidate If nodes in both unswitched loop versions which are now dominated by the loop selector
267 // If node. Keep the true-path-path in the true-path-loop and the false-path-path in the false-path-loop by setting
268 // the bool input accordingly. The unswitch candidate If nodes are folded in the next IGVN round.
269 void remove_unswitch_candidate_from_loops(const UnswitchedLoopSelector& unswitched_loop_selector) {
270 IfNode* unswitching_candidate = unswitched_loop_selector.unswitch_candidate();
271 _phase->igvn().rehash_node_delayed(unswitching_candidate);
272 _phase->dominated_by(unswitched_loop_selector.true_path_loop_proj(), unswitching_candidate);
273
274 IfNode* unswitching_candidate_clone = _old_new[unswitching_candidate->_idx]->as_If();
275 _phase->igvn().rehash_node_delayed(unswitching_candidate_clone);
276 _phase->dominated_by(unswitched_loop_selector.false_path_loop_proj(), unswitching_candidate_clone);
277 }
278
279 public:
280 // Unswitch the original loop on the invariant loop selector by creating a true-path-loop and a false-path-loop.
281 // Remove the unswitch candidate If from both unswitched loop versions which are now covered by the loop selector If.
282 void unswitch(const UnswitchedLoopSelector& unswitched_loop_selector) {
283 _phase->clone_loop(_loop, _old_new, _phase->dom_depth(_loop_head),
284 PhaseIdealLoop::CloneIncludesStripMined, unswitched_loop_selector.selector());
285
286 // At this point, the selector If projections are the corresponding loop entries.
287 // clone_parse_and_assertion_predicates_to_unswitched_loop() could clone additional predicates after the selector
288 // If projections. The loop entries are updated accordingly.
289 IfProjNode* true_path_loop_entry = unswitched_loop_selector.true_path_loop_proj();
290 IfProjNode* false_path_loop_entry = unswitched_loop_selector.false_path_loop_proj();
291 _phase->clone_parse_and_assertion_predicates_to_unswitched_loop(_loop, _old_new,
292 true_path_loop_entry, false_path_loop_entry);
293
294 fix_loop_entries(true_path_loop_entry, false_path_loop_entry);
295
296 DEBUG_ONLY(verify_unswitched_loop_versions(_loop->_head->as_Loop(), unswitched_loop_selector);)
297
298 _phase->recompute_dom_depth();
299 remove_unswitch_candidate_from_loops(unswitched_loop_selector);
300 }
301 };
302
303 // See comments below file header for more information about Loop Unswitching.
304 void PhaseIdealLoop::do_unswitching(IdealLoopTree* loop, Node_List& old_new) {
305 assert(LoopUnswitching, "LoopUnswitching must be enabled");
306
307 LoopNode* original_head = loop->_head->as_Loop();
308 if (has_control_dependencies_from_predicates(original_head)) {
309 NOT_PRODUCT(trace_loop_unswitching_impossible(original_head);)
310 return;
311 }
312
313 NOT_PRODUCT(trace_loop_unswitching_count(loop, original_head);)
314 C->print_method(PHASE_BEFORE_LOOP_UNSWITCHING, 4, original_head);
315
316 revert_to_normal_loop(original_head);
317
318 const UnswitchedLoopSelector unswitched_loop_selector(loop);
319 OriginalLoop original_loop(loop, old_new);
320 original_loop.unswitch(unswitched_loop_selector);
321
322 hoist_invariant_check_casts(loop, old_new, unswitched_loop_selector);
323 add_unswitched_loop_version_bodies_to_igvn(loop, old_new);
324
325 LoopNode* new_head = old_new[original_head->_idx]->as_Loop();
326 increment_unswitch_counts(original_head, new_head);
327
328 NOT_PRODUCT(trace_loop_unswitching_result(unswitched_loop_selector, original_head, new_head);)
329 C->print_method(PHASE_AFTER_LOOP_UNSWITCHING, 4, new_head);
330 C->set_major_progress();
331 }
332
333 bool PhaseIdealLoop::has_control_dependencies_from_predicates(LoopNode* head) {
334 Node* entry = head->skip_strip_mined()->in(LoopNode::EntryControl);
335 const Predicates predicates(entry);
336 if (predicates.has_any()) {
337 assert(entry->is_IfProj(), "sanity - must be ifProj since there is at least one predicate");
338 if (entry->outcnt() > 1) {
339 // Bailout if there are predicates from which there are additional control dependencies (i.e. from loop
340 // entry 'entry') to previously partially peeled statements since this case is not handled and can lead
341 // to a wrong execution. Remove this bailout, once this is fixed.
342 return true;
343 }
344 }
345 return false;
346 }
347
348 #ifndef PRODUCT
349 void PhaseIdealLoop::trace_loop_unswitching_impossible(const LoopNode* original_head) {
350 if (TraceLoopUnswitching) {
351 tty->print_cr("Loop Unswitching \"%d %s\" not possible due to control dependencies",
352 original_head->_idx, original_head->Name());
353 }
354 }
355
356 void PhaseIdealLoop::trace_loop_unswitching_count(IdealLoopTree* loop, LoopNode* original_head) {
357 if (TraceLoopOpts) {
358 tty->print("Unswitch %d ", original_head->unswitch_count() + 1);
359 loop->dump_head();
360 }
361 }
362
363 void PhaseIdealLoop::trace_loop_unswitching_result(const UnswitchedLoopSelector& unswitched_loop_selector,
364 const LoopNode* original_head, const LoopNode* new_head) {
365 if (TraceLoopUnswitching) {
366 IfNode* unswitch_candidate = unswitched_loop_selector.unswitch_candidate();
367 IfNode* loop_selector = unswitched_loop_selector.selector();
368 tty->print_cr("Loop Unswitching:");
369 tty->print_cr("- Unswitch-Candidate-If: %d %s", unswitch_candidate->_idx, unswitch_candidate->Name());
370 tty->print_cr("- Loop-Selector-If: %d %s", loop_selector->_idx, loop_selector->Name());
371 tty->print_cr("- True-Path-Loop (=Orig): %d %s", original_head->_idx, original_head->Name());
372 tty->print_cr("- False-Path-Loop (=Clone): %d %s", new_head->_idx, new_head->Name());
373 }
374 }
375 #endif
376
377 // When unswitching a counted loop, we need to convert it back to a normal loop since it's not a proper pre, main or,
378 // post loop anymore after loop unswitching.
379 void PhaseIdealLoop::revert_to_normal_loop(const LoopNode* loop_head) {
380 CountedLoopNode* cl = loop_head->isa_CountedLoop();
381 if (cl != nullptr && !cl->is_normal_loop()) {
382 cl->set_normal_loop();
383 }
384 }
385
386 // Hoist invariant CheckCastPPNodes out of each unswitched loop version to the appropriate loop selector If projection.
387 void PhaseIdealLoop::hoist_invariant_check_casts(const IdealLoopTree* loop, const Node_List& old_new,
388 const UnswitchedLoopSelector& unswitched_loop_selector) {
389 IfNode* unswitch_candidate = unswitched_loop_selector.unswitch_candidate();
390 IfNode* loop_selector = unswitched_loop_selector.selector();
391 ResourceMark rm;
392 GrowableArray<CheckCastPPNode*> loop_invariant_check_casts;
393 for (DUIterator_Fast imax, i = unswitch_candidate->fast_outs(imax); i < imax; i++) {
394 IfProjNode* proj = unswitch_candidate->fast_out(i)->as_IfProj();
395 // Copy to a worklist for easier manipulation
396 for (DUIterator_Fast jmax, j = proj->fast_outs(jmax); j < jmax; j++) {
397 CheckCastPPNode* check_cast = proj->fast_out(j)->isa_CheckCastPP();
398 if (check_cast != nullptr && loop->is_invariant(check_cast->in(1))) {
399 loop_invariant_check_casts.push(check_cast);
400 }
401 }
402 IfProjNode* loop_selector_if_proj = loop_selector->proj_out(proj->_con)->as_IfProj();
403 while (loop_invariant_check_casts.length() > 0) {
404 CheckCastPPNode* cast = loop_invariant_check_casts.pop();
405 Node* cast_clone = cast->clone();
406 cast_clone->set_req(0, loop_selector_if_proj);
407 _igvn.replace_input_of(cast, 1, cast_clone);
408 register_new_node(cast_clone, loop_selector_if_proj);
409 // Same for the clone
410 Node* use_clone = old_new[cast->_idx];
411 _igvn.replace_input_of(use_clone, 1, cast_clone);
412 }
413 }
414 }
415
416 // Enable more optimizations possibilities in the next IGVN round.
417 void PhaseIdealLoop::add_unswitched_loop_version_bodies_to_igvn(IdealLoopTree* loop, const Node_List& old_new) {
418 loop->record_for_igvn();
419 for(int i = loop->_body.size() - 1; i >= 0 ; i--) {
420 Node* n = loop->_body[i];
421 Node* n_clone = old_new[n->_idx];
422 _igvn._worklist.push(n_clone);
423 }
424 }
425
426 void PhaseIdealLoop::increment_unswitch_counts(LoopNode* original_head, LoopNode* new_head) {
427 const int unswitch_count = original_head->unswitch_count() + 1;
428 original_head->set_unswitch_count(unswitch_count);
429 new_head->set_unswitch_count(unswitch_count);
430 }
431