Line data Source code
1 : // Copyright 2016 the V8 project authors. All rights reserved.
2 : // Use of this source code is governed by a BSD-style license that can be
3 : // found in the LICENSE file.
4 :
5 : #include <iterator>
6 :
7 : #include "src/compiler/store-store-elimination.h"
8 :
9 : #include "src/compiler/all-nodes.h"
10 : #include "src/compiler/js-graph.h"
11 : #include "src/compiler/node-properties.h"
12 : #include "src/compiler/simplified-operator.h"
13 :
14 : namespace v8 {
15 : namespace internal {
16 : namespace compiler {
17 :
18 : #define TRACE(fmt, ...) \
19 : do { \
20 : if (FLAG_trace_store_elimination) { \
21 : PrintF("RedundantStoreFinder: " fmt "\n", ##__VA_ARGS__); \
22 : } \
23 : } while (false)
24 :
25 : // CHECK_EXTRA is like CHECK, but has two or more arguments: a boolean
26 : // expression, a format string, and any number of extra arguments. The boolean
27 : // expression will be evaluated at runtime. If it evaluates to false, then an
28 : // error message will be shown containing the condition, as well as the extra
29 : // info formatted like with printf.
30 : #define CHECK_EXTRA(condition, fmt, ...) \
31 : do { \
32 : if (V8_UNLIKELY(!(condition))) { \
33 : FATAL("Check failed: %s. Extra info: " fmt, #condition, ##__VA_ARGS__); \
34 : } \
35 : } while (false)
36 :
37 : #ifdef DEBUG
38 : #define DCHECK_EXTRA(condition, fmt, ...) \
39 : CHECK_EXTRA(condition, fmt, ##__VA_ARGS__)
40 : #else
41 : #define DCHECK_EXTRA(condition, fmt, ...) ((void)0)
42 : #endif
43 :
44 : // Store-store elimination.
45 : //
46 : // The aim of this optimization is to detect the following pattern in the
47 : // effect graph:
48 : //
49 : // - StoreField[+24, kRepTagged](263, ...)
50 : //
51 : // ... lots of nodes from which the field at offset 24 of the object
52 : // returned by node #263 cannot be observed ...
53 : //
54 : // - StoreField[+24, kRepTagged](263, ...)
55 : //
56 : // In such situations, the earlier StoreField cannot be observed, and can be
57 : // eliminated. This optimization should work for any offset and input node, of
58 : // course.
59 : //
60 : // The optimization also works across splits. It currently does not work for
61 : // loops, because we tend to put a stack check in loops, and like deopts,
62 : // stack checks can observe anything.
63 :
64 : // Assumption: every byte of a JS object is only ever accessed through one
65 : // offset. For instance, byte 15 of a given object may be accessed using a
66 : // two-byte read at offset 14, or a four-byte read at offset 12, but never
67 : // both in the same program.
68 : //
69 : // This implementation needs all dead nodes removed from the graph, and the
70 : // graph should be trimmed.
71 :
72 : namespace {
73 :
74 : using StoreOffset = uint32_t;
75 :
76 : struct UnobservableStore {
77 : NodeId id_;
78 : StoreOffset offset_;
79 :
80 : bool operator==(const UnobservableStore) const;
81 : bool operator<(const UnobservableStore) const;
82 : };
83 :
84 : } // namespace
85 :
86 : namespace {
87 :
88 : // Instances of UnobservablesSet are immutable. They represent either a set of
89 : // UnobservableStores, or the "unvisited empty set".
90 : //
91 : // We apply some sharing to save memory. The class UnobservablesSet is only a
92 : // pointer wide, and a copy does not use any heap (or temp_zone) memory. Most
93 : // changes to an UnobservablesSet might allocate in the temp_zone.
94 : //
95 : // The size of an instance should be the size of a pointer, plus additional
96 : // space in the zone in the case of non-unvisited UnobservablesSets. Copying
97 : // an UnobservablesSet allocates no memory.
98 : class UnobservablesSet final {
99 : public:
100 : static UnobservablesSet Unvisited();
101 : static UnobservablesSet VisitedEmpty(Zone* zone);
102 : UnobservablesSet(); // unvisited
103 : UnobservablesSet(const UnobservablesSet& other) V8_NOEXCEPT = default;
104 :
105 : UnobservablesSet Intersect(const UnobservablesSet& other, Zone* zone) const;
106 : UnobservablesSet Add(UnobservableStore obs, Zone* zone) const;
107 : UnobservablesSet RemoveSameOffset(StoreOffset off, Zone* zone) const;
108 :
109 7870041 : const ZoneSet<UnobservableStore>* set() const { return set_; }
110 :
111 100632803 : bool IsUnvisited() const { return set_ == nullptr; }
112 3769791 : bool IsEmpty() const { return set_ == nullptr || set_->empty(); }
113 : bool Contains(UnobservableStore obs) const {
114 4554824 : return set_ != nullptr && (set_->find(obs) != set_->end());
115 : }
116 :
117 : bool operator==(const UnobservablesSet&) const;
118 : bool operator!=(const UnobservablesSet&) const;
119 :
120 : private:
121 : explicit UnobservablesSet(const ZoneSet<UnobservableStore>* set)
122 : : set_(set) {}
123 : const ZoneSet<UnobservableStore>* set_;
124 : };
125 :
126 : } // namespace
127 :
128 : namespace {
129 :
130 928808 : class RedundantStoreFinder final {
131 : public:
132 : RedundantStoreFinder(JSGraph* js_graph, Zone* temp_zone);
133 :
134 : void Find();
135 :
136 : const ZoneSet<Node*>& to_remove_const() { return to_remove_; }
137 :
138 : void Visit(Node* node);
139 :
140 : private:
141 : void VisitEffectfulNode(Node* node);
142 : UnobservablesSet RecomputeUseIntersection(Node* node);
143 : UnobservablesSet RecomputeSet(Node* node, const UnobservablesSet& uses);
144 : static bool CannotObserveStoreField(Node* node);
145 :
146 : void MarkForRevisit(Node* node);
147 : bool HasBeenVisited(Node* node);
148 :
149 : JSGraph* jsgraph() const { return jsgraph_; }
150 : Isolate* isolate() { return jsgraph()->isolate(); }
151 : Zone* temp_zone() const { return temp_zone_; }
152 : ZoneVector<UnobservablesSet>& unobservable() { return unobservable_; }
153 : UnobservablesSet& unobservable_for_id(NodeId id) {
154 : DCHECK_LT(id, unobservable().size());
155 112997446 : return unobservable()[id];
156 : }
157 : ZoneSet<Node*>& to_remove() { return to_remove_; }
158 :
159 : JSGraph* const jsgraph_;
160 : Zone* const temp_zone_;
161 :
162 : ZoneStack<Node*> revisit_;
163 : ZoneVector<bool> in_revisit_;
164 : // Maps node IDs to UnobservableNodeSets.
165 : ZoneVector<UnobservablesSet> unobservable_;
166 : ZoneSet<Node*> to_remove_;
167 : const UnobservablesSet unobservables_visited_empty_;
168 : };
169 :
170 : // To safely cast an offset from a FieldAccess, which has a potentially wider
171 : // range (namely int).
172 : StoreOffset ToOffset(int offset) {
173 4459861 : CHECK_LE(0, offset);
174 4459861 : return static_cast<StoreOffset>(offset);
175 : }
176 :
177 : StoreOffset ToOffset(const FieldAccess& access) {
178 : return ToOffset(access.offset);
179 : }
180 :
181 : unsigned int RepSizeOf(MachineRepresentation rep) {
182 2292551 : return 1u << ElementSizeLog2Of(rep);
183 : }
184 : unsigned int RepSizeOf(FieldAccess access) {
185 : return RepSizeOf(access.machine_type.representation());
186 : }
187 :
188 42683 : bool AtMostTagged(FieldAccess access) {
189 42683 : return RepSizeOf(access) <= RepSizeOf(MachineRepresentation::kTagged);
190 : }
191 :
192 2249868 : bool AtLeastTagged(FieldAccess access) {
193 2249870 : return RepSizeOf(access) >= RepSizeOf(MachineRepresentation::kTagged);
194 : }
195 :
196 : } // namespace
197 :
198 464398 : void RedundantStoreFinder::Find() {
199 464398 : Visit(jsgraph()->graph()->end());
200 :
201 41198184 : while (!revisit_.empty()) {
202 20366888 : Node* next = revisit_.top();
203 : revisit_.pop();
204 : DCHECK_LT(next->id(), in_revisit_.size());
205 20366890 : in_revisit_[next->id()] = false;
206 20366890 : Visit(next);
207 : }
208 :
209 : #ifdef DEBUG
210 : // Check that we visited all the StoreFields
211 : AllNodes all(temp_zone(), jsgraph()->graph());
212 : for (Node* node : all.reachable) {
213 : if (node->op()->opcode() == IrOpcode::kStoreField) {
214 : DCHECK_EXTRA(HasBeenVisited(node), "#%d:%s", node->id(),
215 : node->op()->mnemonic());
216 : }
217 : }
218 : #endif
219 464407 : }
220 :
221 27810649 : void RedundantStoreFinder::MarkForRevisit(Node* node) {
222 : DCHECK_LT(node->id(), in_revisit_.size());
223 83431947 : if (!in_revisit_[node->id()]) {
224 : revisit_.push(node);
225 40733886 : in_revisit_[node->id()] = true;
226 : }
227 27810650 : }
228 :
229 : bool RedundantStoreFinder::HasBeenVisited(Node* node) {
230 : return !unobservable_for_id(node->id()).IsUnvisited();
231 : }
232 :
233 464403 : void StoreStoreElimination::Run(JSGraph* js_graph, Zone* temp_zone) {
234 : // Find superfluous nodes
235 928810 : RedundantStoreFinder finder(js_graph, temp_zone);
236 464398 : finder.Find();
237 :
238 : // Remove superfluous nodes
239 :
240 507090 : for (Node* node : finder.to_remove_const()) {
241 42683 : if (FLAG_trace_store_elimination) {
242 : PrintF("StoreStoreElimination::Run: Eliminating node #%d:%s\n",
243 0 : node->id(), node->op()->mnemonic());
244 : }
245 42683 : Node* previous_effect = NodeProperties::GetEffectInput(node);
246 : NodeProperties::ReplaceUses(node, nullptr, previous_effect, nullptr,
247 42683 : nullptr);
248 42683 : node->Kill();
249 : }
250 464402 : }
251 :
252 : // Recompute unobservables-set for a node. Will also mark superfluous nodes
253 : // as to be removed.
254 :
255 12339636 : UnobservablesSet RedundantStoreFinder::RecomputeSet(
256 : Node* node, const UnobservablesSet& uses) {
257 12339636 : switch (node->op()->opcode()) {
258 : case IrOpcode::kStoreField: {
259 : Node* stored_to = node->InputAt(0);
260 2277412 : const FieldAccess& access = FieldAccessOf(node->op());
261 : StoreOffset offset = ToOffset(access);
262 :
263 : UnobservableStore observation = {stored_to->id(), offset};
264 : bool isNotObservable = uses.Contains(observation);
265 :
266 2277412 : if (isNotObservable && AtMostTagged(access)) {
267 42683 : TRACE(" #%d is StoreField[+%d,%s](#%d), unobservable", node->id(),
268 : offset, MachineReprToString(access.machine_type.representation()),
269 : stored_to->id());
270 : to_remove().insert(node);
271 42683 : return uses;
272 2234729 : } else if (isNotObservable && !AtMostTagged(access)) {
273 0 : TRACE(
274 : " #%d is StoreField[+%d,%s](#%d), repeated in future but too "
275 : "big to optimize away",
276 : node->id(), offset,
277 : MachineReprToString(access.machine_type.representation()),
278 : stored_to->id());
279 0 : return uses;
280 2234729 : } else if (!isNotObservable && AtLeastTagged(access)) {
281 2219590 : TRACE(" #%d is StoreField[+%d,%s](#%d), observable, recording in set",
282 : node->id(), offset,
283 : MachineReprToString(access.machine_type.representation()),
284 : stored_to->id());
285 2219590 : return uses.Add(observation, temp_zone());
286 15140 : } else if (!isNotObservable && !AtLeastTagged(access)) {
287 15140 : TRACE(
288 : " #%d is StoreField[+%d,%s](#%d), observable but too small to "
289 : "record",
290 : node->id(), offset,
291 : MachineReprToString(access.machine_type.representation()),
292 : stored_to->id());
293 15140 : return uses;
294 : } else {
295 0 : UNREACHABLE();
296 : }
297 : break;
298 : }
299 : case IrOpcode::kLoadField: {
300 : Node* loaded_from = node->InputAt(0);
301 2182448 : const FieldAccess& access = FieldAccessOf(node->op());
302 : StoreOffset offset = ToOffset(access);
303 :
304 2182448 : TRACE(
305 : " #%d is LoadField[+%d,%s](#%d), removing all offsets [+%d] from "
306 : "set",
307 : node->id(), offset,
308 : MachineReprToString(access.machine_type.representation()),
309 : loaded_from->id(), offset);
310 :
311 2182448 : return uses.RemoveSameOffset(offset, temp_zone());
312 : break;
313 : }
314 : default:
315 7879776 : if (CannotObserveStoreField(node)) {
316 2013810 : TRACE(" #%d:%s can observe nothing, set stays unchanged", node->id(),
317 : node->op()->mnemonic());
318 2013816 : return uses;
319 : } else {
320 5865966 : TRACE(" #%d:%s might observe anything, recording empty set",
321 : node->id(), node->op()->mnemonic());
322 5865969 : return unobservables_visited_empty_;
323 : }
324 : }
325 : UNREACHABLE();
326 : }
327 :
328 : bool RedundantStoreFinder::CannotObserveStoreField(Node* node) {
329 7847654 : return node->opcode() == IrOpcode::kLoadElement ||
330 7242162 : node->opcode() == IrOpcode::kLoad ||
331 7239422 : node->opcode() == IrOpcode::kStore ||
332 5927680 : node->opcode() == IrOpcode::kEffectPhi ||
333 5882121 : node->opcode() == IrOpcode::kStoreElement ||
334 13755101 : node->opcode() == IrOpcode::kUnsafePointerAdd ||
335 : node->opcode() == IrOpcode::kRetain;
336 : }
337 :
338 : // Initialize unobservable_ with js_graph->graph->NodeCount() empty sets.
339 464404 : RedundantStoreFinder::RedundantStoreFinder(JSGraph* js_graph, Zone* temp_zone)
340 : : jsgraph_(js_graph),
341 : temp_zone_(temp_zone),
342 : revisit_(temp_zone),
343 : in_revisit_(js_graph->graph()->NodeCount(), temp_zone),
344 : unobservable_(js_graph->graph()->NodeCount(),
345 : UnobservablesSet::Unvisited(), temp_zone),
346 : to_remove_(temp_zone),
347 928807 : unobservables_visited_empty_(UnobservablesSet::VisitedEmpty(temp_zone)) {}
348 :
349 20831295 : void RedundantStoreFinder::Visit(Node* node) {
350 : // All effectful nodes should be reachable from End via a sequence of
351 : // control, then a sequence of effect edges. In VisitEffectfulNode we mark
352 : // all effect inputs for revisiting (if they might have stale state); here
353 : // we mark all control inputs at least once.
354 :
355 20831295 : if (!HasBeenVisited(node)) {
356 59704151 : for (int i = 0; i < node->op()->ControlInputCount(); i++) {
357 20255702 : Node* control_input = NodeProperties::GetControlInput(node, i);
358 20255706 : if (!HasBeenVisited(control_input)) {
359 15084396 : MarkForRevisit(control_input);
360 : }
361 : }
362 : }
363 :
364 : bool isEffectful = (node->op()->EffectInputCount() >= 1);
365 20831247 : if (isEffectful) {
366 12339628 : VisitEffectfulNode(node);
367 : DCHECK(HasBeenVisited(node));
368 : }
369 :
370 20831392 : if (!HasBeenVisited(node)) {
371 : // Mark as visited.
372 8094944 : unobservable_for_id(node->id()) = unobservables_visited_empty_;
373 : }
374 20831392 : }
375 :
376 12339628 : void RedundantStoreFinder::VisitEffectfulNode(Node* node) {
377 12339628 : if (HasBeenVisited(node)) {
378 1241666 : TRACE("- Revisiting: #%d:%s", node->id(), node->op()->mnemonic());
379 : }
380 12339628 : UnobservablesSet after_set = RecomputeUseIntersection(node);
381 12339616 : UnobservablesSet before_set = RecomputeSet(node, after_set);
382 : DCHECK(!before_set.IsUnvisited());
383 :
384 12339635 : UnobservablesSet stored_for_node = unobservable_for_id(node->id());
385 : bool cur_set_changed =
386 13581300 : (stored_for_node.IsUnvisited() || stored_for_node != before_set);
387 12339634 : if (!cur_set_changed) {
388 : // We will not be able to update the part of this chain above any more.
389 : // Exit.
390 1241120 : TRACE("+ No change: stabilized. Not visiting effect inputs.");
391 : } else {
392 11098514 : unobservable_for_id(node->id()) = before_set;
393 :
394 : // Mark effect inputs for visiting.
395 36551318 : for (int i = 0; i < node->op()->EffectInputCount(); i++) {
396 12726397 : Node* input = NodeProperties::GetEffectInput(node, i);
397 12726399 : TRACE(" marking #%d:%s for revisit", input->id(),
398 : input->op()->mnemonic());
399 12726399 : MarkForRevisit(input);
400 : }
401 : }
402 12339639 : }
403 :
404 : // Compute the intersection of the UnobservablesSets of all effect uses and
405 : // return it. This function only works if {node} has an effect use.
406 : //
407 : // The result UnobservablesSet will always be visited.
408 12339651 : UnobservablesSet RedundantStoreFinder::RecomputeUseIntersection(Node* node) {
409 : // {first} == true indicates that we haven't looked at any elements yet.
410 : // {first} == false indicates that cur_set is the intersection of at least one
411 : // thing.
412 :
413 : bool first = true;
414 12339651 : UnobservablesSet cur_set = UnobservablesSet::Unvisited(); // irrelevant
415 :
416 96346500 : for (Edge edge : node->use_edges()) {
417 : // Skip non-effect edges
418 42003441 : if (!NodeProperties::IsEffectEdge(edge)) {
419 26702160 : continue;
420 : }
421 :
422 : Node* use = edge.from();
423 15301276 : UnobservablesSet new_set = unobservable_for_id(use->id());
424 : // Include new_set in the intersection.
425 15301276 : if (first) {
426 : // Intersection of a one-element set is that one element
427 : first = false;
428 11551829 : cur_set = new_set;
429 : } else {
430 : // Take the intersection of cur_set and new_set.
431 3749447 : cur_set = cur_set.Intersect(new_set, temp_zone());
432 : }
433 : }
434 :
435 12339618 : if (first) {
436 : // There were no effect uses.
437 : auto opcode = node->op()->opcode();
438 : // List of opcodes that may end this effect chain. The opcodes are not
439 : // important to the soundness of this optimization; this serves as a
440 : // general sanity check. Add opcodes to this list as it suits you.
441 : //
442 : // Everything is observable after these opcodes; return the empty set.
443 : DCHECK_EXTRA(
444 : opcode == IrOpcode::kReturn || opcode == IrOpcode::kTerminate ||
445 : opcode == IrOpcode::kDeoptimize || opcode == IrOpcode::kThrow,
446 : "for #%d:%s", node->id(), node->op()->mnemonic());
447 : USE(opcode); // silence warning about unused variable in release mode
448 :
449 787803 : return unobservables_visited_empty_;
450 : } else {
451 11551815 : if (cur_set.IsUnvisited()) {
452 2821361 : cur_set = unobservables_visited_empty_;
453 : }
454 :
455 11551815 : return cur_set;
456 : }
457 : }
458 :
459 : UnobservablesSet UnobservablesSet::Unvisited() { return UnobservablesSet(); }
460 :
461 : UnobservablesSet::UnobservablesSet() : set_(nullptr) {}
462 :
463 464397 : UnobservablesSet UnobservablesSet::VisitedEmpty(Zone* zone) {
464 : // Create a new empty UnobservablesSet. This allocates in the zone, and
465 : // can probably be optimized to use a global singleton.
466 : ZoneSet<UnobservableStore>* empty_set =
467 : new (zone->New(sizeof(ZoneSet<UnobservableStore>)))
468 464397 : ZoneSet<UnobservableStore>(zone);
469 464397 : return UnobservablesSet(empty_set);
470 : }
471 :
472 : // Computes the intersection of two UnobservablesSets. May return
473 : // UnobservablesSet::Unvisited() instead of an empty UnobservablesSet for
474 : // speed.
475 3749446 : UnobservablesSet UnobservablesSet::Intersect(const UnobservablesSet& other,
476 : Zone* zone) const {
477 3769791 : if (IsEmpty() || other.IsEmpty()) {
478 : return Unvisited();
479 : } else {
480 : ZoneSet<UnobservableStore>* intersection =
481 : new (zone->New(sizeof(ZoneSet<UnobservableStore>)))
482 3374 : ZoneSet<UnobservableStore>(zone);
483 : // Put the intersection of set() and other.set() in intersection.
484 : set_intersection(set()->begin(), set()->end(), other.set()->begin(),
485 : other.set()->end(),
486 : std::inserter(*intersection, intersection->end()));
487 :
488 3374 : return UnobservablesSet(intersection);
489 : }
490 : }
491 :
492 2219590 : UnobservablesSet UnobservablesSet::Add(UnobservableStore obs,
493 : Zone* zone) const {
494 : bool present = (set()->find(obs) != set()->end());
495 2219588 : if (present) {
496 0 : return *this;
497 : } else {
498 : // Make a new empty set.
499 : ZoneSet<UnobservableStore>* new_set =
500 : new (zone->New(sizeof(ZoneSet<UnobservableStore>)))
501 2219589 : ZoneSet<UnobservableStore>(zone);
502 : // Copy the old elements over.
503 : *new_set = *set();
504 : // Add the new element.
505 : bool inserted = new_set->insert(obs).second;
506 : DCHECK(inserted);
507 : USE(inserted); // silence warning about unused variable
508 :
509 2219591 : return UnobservablesSet(new_set);
510 : }
511 : }
512 :
513 2182448 : UnobservablesSet UnobservablesSet::RemoveSameOffset(StoreOffset offset,
514 : Zone* zone) const {
515 : // Make a new empty set.
516 : ZoneSet<UnobservableStore>* new_set =
517 : new (zone->New(sizeof(ZoneSet<UnobservableStore>)))
518 2182448 : ZoneSet<UnobservableStore>(zone);
519 : // Copy all elements over that have a different offset.
520 8310877 : for (auto obs : *set()) {
521 6128429 : if (obs.offset_ != offset) {
522 : new_set->insert(obs);
523 : }
524 : }
525 :
526 2182448 : return UnobservablesSet(new_set);
527 : }
528 :
529 : // Used for debugging.
530 1241666 : bool UnobservablesSet::operator==(const UnobservablesSet& other) const {
531 2483332 : if (IsUnvisited() || other.IsUnvisited()) {
532 0 : return IsEmpty() && other.IsEmpty();
533 : } else {
534 : // Both pointers guaranteed not to be nullptrs.
535 1241664 : return *set() == *other.set();
536 : }
537 : }
538 :
539 : bool UnobservablesSet::operator!=(const UnobservablesSet& other) const {
540 1241666 : return !(*this == other);
541 : }
542 :
543 : bool UnobservableStore::operator==(const UnobservableStore other) const {
544 14406 : return (id_ == other.id_) && (offset_ == other.offset_);
545 : }
546 :
547 :
548 : bool UnobservableStore::operator<(const UnobservableStore other) const {
549 151814113 : return (id_ < other.id_) || (id_ == other.id_ && offset_ < other.offset_);
550 : }
551 :
552 : #undef TRACE
553 : #undef CHECK_EXTRA
554 : #undef DCHECK_EXTRA
555 :
556 : } // namespace compiler
557 : } // namespace internal
558 122036 : } // namespace v8
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