1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
2 * vim: set ts=8 sts=2 et sw=2 tw=80:
3 * This Source Code Form is subject to the terms of the Mozilla Public
4 * License, v. 2.0. If a copy of the MPL was not distributed with this
5 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
6
7 #include "jit/ValueNumbering.h"
8
9 #include "jit/AliasAnalysis.h"
10 #include "jit/IonAnalysis.h"
11 #include "jit/JitSpewer.h"
12 #include "jit/MIRGenerator.h"
13
14 using namespace js;
15 using namespace js::jit;
16
17 /*
18 * [SMDOC] IonMonkey Value Numbering
19 *
20 * Some notes on the main algorithm here:
21 * - The SSA identifier id() is the value number. We do replaceAllUsesWith as
22 * we go, so there's always at most one visible value with a given number.
23 *
24 * - Consequently, the GVN algorithm is effectively pessimistic. This means it
25 * is not as powerful as an optimistic GVN would be, but it is simpler and
26 * faster.
27 *
28 * - We iterate in RPO, so that when visiting a block, we've already optimized
29 * and hashed all values in dominating blocks. With occasional exceptions,
30 * this allows us to do everything in a single pass.
31 *
32 * - When we do use multiple passes, we just re-run the algorithm on the whole
33 * graph instead of doing sparse propagation. This is a tradeoff to keep the
34 * algorithm simpler and lighter on inputs that don't have a lot of
35 * interesting unreachable blocks or degenerate loop induction variables, at
36 * the expense of being slower on inputs that do. The loop for this always
37 * terminates, because it only iterates when code is or will be removed, so
38 * eventually it must stop iterating.
39 *
40 * - Values are not immediately removed from the hash set when they go out of
41 * scope. Instead, we check for dominance after a lookup. If the dominance
42 * check fails, the value is removed.
43 */
44
hash(Lookup ins)45 HashNumber ValueNumberer::VisibleValues::ValueHasher::hash(Lookup ins) {
46 return ins->valueHash();
47 }
48
49 // Test whether two MDefinitions are congruent.
match(Key k,Lookup l)50 bool ValueNumberer::VisibleValues::ValueHasher::match(Key k, Lookup l) {
51 // If one of the instructions depends on a store, and the other instruction
52 // does not depend on the same store, the instructions are not congruent.
53 if (k->dependency() != l->dependency()) {
54 return false;
55 }
56
57 bool congruent =
58 k->congruentTo(l); // Ask the values themselves what they think.
59 #ifdef JS_JITSPEW
60 if (congruent != l->congruentTo(k)) {
61 JitSpew(
62 JitSpew_GVN,
63 " congruentTo relation is not symmetric between %s%u and %s%u!!",
64 k->opName(), k->id(), l->opName(), l->id());
65 }
66 #endif
67 return congruent;
68 }
69
rekey(Key & k,Key newKey)70 void ValueNumberer::VisibleValues::ValueHasher::rekey(Key& k, Key newKey) {
71 k = newKey;
72 }
73
VisibleValues(TempAllocator & alloc)74 ValueNumberer::VisibleValues::VisibleValues(TempAllocator& alloc)
75 : set_(alloc) {}
76
77 // Look up the first entry for |def|.
findLeader(const MDefinition * def) const78 ValueNumberer::VisibleValues::Ptr ValueNumberer::VisibleValues::findLeader(
79 const MDefinition* def) const {
80 return set_.lookup(def);
81 }
82
83 // Look up the first entry for |def|.
84 ValueNumberer::VisibleValues::AddPtr
findLeaderForAdd(MDefinition * def)85 ValueNumberer::VisibleValues::findLeaderForAdd(MDefinition* def) {
86 return set_.lookupForAdd(def);
87 }
88
89 // Insert a value into the set.
add(AddPtr p,MDefinition * def)90 bool ValueNumberer::VisibleValues::add(AddPtr p, MDefinition* def) {
91 return set_.add(p, def);
92 }
93
94 // Insert a value onto the set overwriting any existing entry.
overwrite(AddPtr p,MDefinition * def)95 void ValueNumberer::VisibleValues::overwrite(AddPtr p, MDefinition* def) {
96 set_.replaceKey(p, def);
97 }
98
99 // |def| will be discarded, so remove it from any sets.
forget(const MDefinition * def)100 void ValueNumberer::VisibleValues::forget(const MDefinition* def) {
101 Ptr p = set_.lookup(def);
102 if (p && *p == def) {
103 set_.remove(p);
104 }
105 }
106
107 // Clear all state.
clear()108 void ValueNumberer::VisibleValues::clear() { set_.clear(); }
109
110 #ifdef DEBUG
111 // Test whether |def| is in the set.
has(const MDefinition * def) const112 bool ValueNumberer::VisibleValues::has(const MDefinition* def) const {
113 Ptr p = set_.lookup(def);
114 return p && *p == def;
115 }
116 #endif
117
118 // Call MDefinition::justReplaceAllUsesWith, and add some GVN-specific asserts.
ReplaceAllUsesWith(MDefinition * from,MDefinition * to)119 static void ReplaceAllUsesWith(MDefinition* from, MDefinition* to) {
120 MOZ_ASSERT(from != to, "GVN shouldn't try to replace a value with itself");
121 MOZ_ASSERT(from->type() == to->type(), "Def replacement has different type");
122 MOZ_ASSERT(!to->isDiscarded(),
123 "GVN replaces an instruction by a removed instruction");
124
125 // We don't need the extra setting of UseRemoved flags that the regular
126 // replaceAllUsesWith does because we do it ourselves.
127 from->justReplaceAllUsesWith(to);
128 }
129
130 // Test whether |succ| is a successor of |block|.
HasSuccessor(const MControlInstruction * block,const MBasicBlock * succ)131 static bool HasSuccessor(const MControlInstruction* block,
132 const MBasicBlock* succ) {
133 for (size_t i = 0, e = block->numSuccessors(); i != e; ++i) {
134 if (block->getSuccessor(i) == succ) {
135 return true;
136 }
137 }
138 return false;
139 }
140
141 // Given a block which has had predecessors removed but is still reachable, test
142 // whether the block's new dominator will be closer than its old one and whether
143 // it will expose potential optimization opportunities.
ComputeNewDominator(MBasicBlock * block,MBasicBlock * old)144 static MBasicBlock* ComputeNewDominator(MBasicBlock* block, MBasicBlock* old) {
145 MBasicBlock* now = block->getPredecessor(0);
146 for (size_t i = 1, e = block->numPredecessors(); i < e; ++i) {
147 MBasicBlock* pred = block->getPredecessor(i);
148 // Note that dominators haven't been recomputed yet, so we have to check
149 // whether now dominates pred, not block.
150 while (!now->dominates(pred)) {
151 MBasicBlock* next = now->immediateDominator();
152 if (next == old) {
153 return old;
154 }
155 if (next == now) {
156 MOZ_ASSERT(block == old,
157 "Non-self-dominating block became self-dominating");
158 return block;
159 }
160 now = next;
161 }
162 }
163 MOZ_ASSERT(old != block || old != now,
164 "Missed self-dominating block staying self-dominating");
165 return now;
166 }
167
168 // Test for any defs which look potentially interesting to GVN.
BlockHasInterestingDefs(MBasicBlock * block)169 static bool BlockHasInterestingDefs(MBasicBlock* block) {
170 return !block->phisEmpty() || *block->begin() != block->lastIns();
171 }
172
173 // Walk up the dominator tree from |block| to the root and test for any defs
174 // which look potentially interesting to GVN.
ScanDominatorsForDefs(MBasicBlock * block)175 static bool ScanDominatorsForDefs(MBasicBlock* block) {
176 for (MBasicBlock* i = block;;) {
177 if (BlockHasInterestingDefs(block)) {
178 return true;
179 }
180
181 MBasicBlock* immediateDominator = i->immediateDominator();
182 if (immediateDominator == i) {
183 break;
184 }
185 i = immediateDominator;
186 }
187 return false;
188 }
189
190 // Walk up the dominator tree from |now| to |old| and test for any defs which
191 // look potentially interesting to GVN.
ScanDominatorsForDefs(MBasicBlock * now,MBasicBlock * old)192 static bool ScanDominatorsForDefs(MBasicBlock* now, MBasicBlock* old) {
193 MOZ_ASSERT(old->dominates(now),
194 "Refined dominator not dominated by old dominator");
195
196 for (MBasicBlock* i = now; i != old; i = i->immediateDominator()) {
197 if (BlockHasInterestingDefs(i)) {
198 return true;
199 }
200 }
201 return false;
202 }
203
204 // Given a block which has had predecessors removed but is still reachable, test
205 // whether the block's new dominator will be closer than its old one and whether
206 // it will expose potential optimization opportunities.
IsDominatorRefined(MBasicBlock * block)207 static bool IsDominatorRefined(MBasicBlock* block) {
208 MBasicBlock* old = block->immediateDominator();
209 MBasicBlock* now = ComputeNewDominator(block, old);
210
211 // If this block is just a goto and it doesn't dominate its destination,
212 // removing its predecessors won't refine the dominators of anything
213 // interesting.
214 MControlInstruction* control = block->lastIns();
215 if (*block->begin() == control && block->phisEmpty() && control->isGoto() &&
216 !block->dominates(control->toGoto()->target())) {
217 return false;
218 }
219
220 // We've computed block's new dominator. Test whether there are any
221 // newly-dominating definitions which look interesting.
222 if (block == old) {
223 return block != now && ScanDominatorsForDefs(now);
224 }
225 MOZ_ASSERT(block != now, "Non-self-dominating block became self-dominating");
226 return ScanDominatorsForDefs(now, old);
227 }
228
229 // |def| has just had one of its users release it. If it's now dead, enqueue it
230 // for discarding, otherwise just make note of it.
handleUseReleased(MDefinition * def,UseRemovedOption useRemovedOption)231 bool ValueNumberer::handleUseReleased(MDefinition* def,
232 UseRemovedOption useRemovedOption) {
233 if (IsDiscardable(def)) {
234 values_.forget(def);
235 if (!deadDefs_.append(def)) {
236 return false;
237 }
238 } else {
239 if (useRemovedOption == SetUseRemoved) {
240 def->setUseRemovedUnchecked();
241 }
242 }
243 return true;
244 }
245
246 // Discard |def| and anything in its use-def subtree which is no longer needed.
discardDefsRecursively(MDefinition * def)247 bool ValueNumberer::discardDefsRecursively(MDefinition* def) {
248 MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");
249
250 return discardDef(def) && processDeadDefs();
251 }
252
253 // Assuming |resume| is unreachable, release its operands.
254 // It might be nice to integrate this code with prepareForDiscard, however GVN
255 // needs it to call handleUseReleased so that it can observe when a definition
256 // becomes unused, so it isn't trivial to do.
releaseResumePointOperands(MResumePoint * resume)257 bool ValueNumberer::releaseResumePointOperands(MResumePoint* resume) {
258 for (size_t i = 0, e = resume->numOperands(); i < e; ++i) {
259 if (!resume->hasOperand(i)) {
260 continue;
261 }
262 MDefinition* op = resume->getOperand(i);
263 resume->releaseOperand(i);
264
265 // We set the UseRemoved flag when removing resume point operands,
266 // because even though we may think we're certain that a particular
267 // branch might not be taken, the type information might be incomplete.
268 if (!handleUseReleased(op, SetUseRemoved)) {
269 return false;
270 }
271 }
272 return true;
273 }
274
275 // Assuming |phi| is dead, release and remove its operands. If an operand
276 // becomes dead, push it to the discard worklist.
releaseAndRemovePhiOperands(MPhi * phi)277 bool ValueNumberer::releaseAndRemovePhiOperands(MPhi* phi) {
278 // MPhi saves operands in a vector so we iterate in reverse.
279 for (int o = phi->numOperands() - 1; o >= 0; --o) {
280 MDefinition* op = phi->getOperand(o);
281 phi->removeOperand(o);
282 if (!handleUseReleased(op, DontSetUseRemoved)) {
283 return false;
284 }
285 }
286 return true;
287 }
288
289 // Assuming |def| is dead, release its operands. If an operand becomes dead,
290 // push it to the discard worklist.
releaseOperands(MDefinition * def)291 bool ValueNumberer::releaseOperands(MDefinition* def) {
292 for (size_t o = 0, e = def->numOperands(); o < e; ++o) {
293 MDefinition* op = def->getOperand(o);
294 def->releaseOperand(o);
295 if (!handleUseReleased(op, DontSetUseRemoved)) {
296 return false;
297 }
298 }
299 return true;
300 }
301
302 // Discard |def| and mine its operands for any subsequently dead defs.
discardDef(MDefinition * def)303 bool ValueNumberer::discardDef(MDefinition* def) {
304 #ifdef JS_JITSPEW
305 JitSpew(JitSpew_GVN, " Discarding %s %s%u",
306 def->block()->isMarked() ? "unreachable" : "dead", def->opName(),
307 def->id());
308 #endif
309 #ifdef DEBUG
310 MOZ_ASSERT(def != nextDef_, "Invalidating the MDefinition iterator");
311 if (def->block()->isMarked()) {
312 MOZ_ASSERT(!def->hasUses(), "Discarding def that still has uses");
313 } else {
314 MOZ_ASSERT(IsDiscardable(def), "Discarding non-discardable definition");
315 MOZ_ASSERT(!values_.has(def), "Discarding a definition still in the set");
316 }
317 #endif
318
319 MBasicBlock* block = def->block();
320 if (def->isPhi()) {
321 MPhi* phi = def->toPhi();
322 if (!releaseAndRemovePhiOperands(phi)) {
323 return false;
324 }
325 block->discardPhi(phi);
326 } else {
327 MInstruction* ins = def->toInstruction();
328 if (MResumePoint* resume = ins->resumePoint()) {
329 if (!releaseResumePointOperands(resume)) {
330 return false;
331 }
332 }
333 if (!releaseOperands(ins)) {
334 return false;
335 }
336 block->discardIgnoreOperands(ins);
337 }
338
339 // If that was the last definition in the block, it can be safely removed
340 // from the graph.
341 if (block->phisEmpty() && block->begin() == block->end()) {
342 MOZ_ASSERT(block->isMarked(),
343 "Reachable block lacks at least a control instruction");
344
345 // As a special case, don't remove a block which is a dominator tree
346 // root so that we don't invalidate the iterator in visitGraph. We'll
347 // check for this and remove it later.
348 if (block->immediateDominator() != block) {
349 JitSpew(JitSpew_GVN, " Block block%u is now empty; discarding",
350 block->id());
351 graph_.removeBlock(block);
352 blocksRemoved_ = true;
353 } else {
354 JitSpew(JitSpew_GVN,
355 " Dominator root block%u is now empty; will discard later",
356 block->id());
357 }
358 }
359
360 return true;
361 }
362
363 // Recursively discard all the defs on the deadDefs_ worklist.
processDeadDefs()364 bool ValueNumberer::processDeadDefs() {
365 MDefinition* nextDef = nextDef_;
366 while (!deadDefs_.empty()) {
367 MDefinition* def = deadDefs_.popCopy();
368
369 // Don't invalidate the MDefinition iterator. This is what we're going
370 // to visit next, so we won't miss anything.
371 if (def == nextDef) {
372 continue;
373 }
374
375 if (!discardDef(def)) {
376 return false;
377 }
378 }
379 return true;
380 }
381
382 // Test whether |block|, which is a loop header, has any predecessors other than
383 // |loopPred|, the loop predecessor, which it doesn't dominate.
hasNonDominatingPredecessor(MBasicBlock * block,MBasicBlock * loopPred)384 static bool hasNonDominatingPredecessor(MBasicBlock* block,
385 MBasicBlock* loopPred) {
386 MOZ_ASSERT(block->isLoopHeader());
387 MOZ_ASSERT(block->loopPredecessor() == loopPred);
388
389 for (uint32_t i = 0, e = block->numPredecessors(); i < e; ++i) {
390 MBasicBlock* pred = block->getPredecessor(i);
391 if (pred != loopPred && !block->dominates(pred)) {
392 return true;
393 }
394 }
395 return false;
396 }
397
398 // A loop is about to be made reachable only through an OSR entry into one of
399 // its nested loops. Fix everything up.
fixupOSROnlyLoop(MBasicBlock * block,MBasicBlock * backedge)400 bool ValueNumberer::fixupOSROnlyLoop(MBasicBlock* block,
401 MBasicBlock* backedge) {
402 // Create an empty and unreachable(!) block which jumps to |block|. This
403 // allows |block| to remain marked as a loop header, so we don't have to
404 // worry about moving a different block into place as the new loop header,
405 // which is hard, especially if the OSR is into a nested loop. Doing all
406 // that would produce slightly more optimal code, but this is so
407 // extraordinarily rare that it isn't worth the complexity.
408 MBasicBlock* fake =
409 MBasicBlock::New(graph_, block->info(), nullptr, MBasicBlock::NORMAL);
410 if (fake == nullptr) {
411 return false;
412 }
413
414 graph_.insertBlockBefore(block, fake);
415 fake->setImmediateDominator(fake);
416 fake->addNumDominated(1);
417 fake->setDomIndex(fake->id());
418 fake->setUnreachable();
419
420 // Create zero-input phis to use as inputs for any phis in |block|.
421 // Again, this is a little odd, but it's the least-odd thing we can do
422 // without significant complexity.
423 for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
424 iter != end; ++iter) {
425 MPhi* phi = *iter;
426 MPhi* fakePhi = MPhi::New(graph_.alloc(), phi->type());
427 fake->addPhi(fakePhi);
428 if (!phi->addInputSlow(fakePhi)) {
429 return false;
430 }
431 }
432
433 fake->end(MGoto::New(graph_.alloc(), block));
434
435 if (!block->addPredecessorWithoutPhis(fake)) {
436 return false;
437 }
438
439 // Restore |backedge| as |block|'s loop backedge.
440 block->clearLoopHeader();
441 block->setLoopHeader(backedge);
442
443 JitSpew(JitSpew_GVN, " Created fake block%u", fake->id());
444 hasOSRFixups_ = true;
445 return true;
446 }
447
448 // Remove the CFG edge between |pred| and |block|, after releasing the phi
449 // operands on that edge and discarding any definitions consequently made dead.
removePredecessorAndDoDCE(MBasicBlock * block,MBasicBlock * pred,size_t predIndex)450 bool ValueNumberer::removePredecessorAndDoDCE(MBasicBlock* block,
451 MBasicBlock* pred,
452 size_t predIndex) {
453 MOZ_ASSERT(
454 !block->isMarked(),
455 "Block marked unreachable should have predecessors removed already");
456
457 // Before removing the predecessor edge, scan the phi operands for that edge
458 // for dead code before they get removed.
459 MOZ_ASSERT(nextDef_ == nullptr);
460 for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
461 iter != end;) {
462 MPhi* phi = *iter++;
463 MOZ_ASSERT(!values_.has(phi),
464 "Visited phi in block having predecessor removed");
465 MOZ_ASSERT(!phi->isGuard());
466
467 MDefinition* op = phi->getOperand(predIndex);
468 phi->removeOperand(predIndex);
469
470 nextDef_ = iter != end ? *iter : nullptr;
471 if (!handleUseReleased(op, DontSetUseRemoved) || !processDeadDefs()) {
472 return false;
473 }
474
475 // If |nextDef_| became dead while we had it pinned, advance the
476 // iterator and discard it now.
477 while (nextDef_ && !nextDef_->hasUses() &&
478 !nextDef_->isGuardRangeBailouts()) {
479 phi = nextDef_->toPhi();
480 iter++;
481 nextDef_ = iter != end ? *iter : nullptr;
482 if (!discardDefsRecursively(phi)) {
483 return false;
484 }
485 }
486 }
487 nextDef_ = nullptr;
488
489 block->removePredecessorWithoutPhiOperands(pred, predIndex);
490 return true;
491 }
492
493 // Remove the CFG edge between |pred| and |block|, and if this makes |block|
494 // unreachable, mark it so, and remove the rest of its incoming edges too. And
495 // discard any instructions made dead by the entailed release of any phi
496 // operands.
removePredecessorAndCleanUp(MBasicBlock * block,MBasicBlock * pred)497 bool ValueNumberer::removePredecessorAndCleanUp(MBasicBlock* block,
498 MBasicBlock* pred) {
499 MOZ_ASSERT(!block->isMarked(),
500 "Removing predecessor on block already marked unreachable");
501
502 // We'll be removing a predecessor, so anything we know about phis in this
503 // block will be wrong.
504 for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
505 iter != end; ++iter) {
506 values_.forget(*iter);
507 }
508
509 // If this is a loop header, test whether it will become an unreachable
510 // loop, or whether it needs special OSR-related fixups.
511 bool isUnreachableLoop = false;
512 if (block->isLoopHeader()) {
513 if (block->loopPredecessor() == pred) {
514 if (MOZ_UNLIKELY(hasNonDominatingPredecessor(block, pred))) {
515 JitSpew(JitSpew_GVN,
516 " "
517 "Loop with header block%u is now only reachable through an "
518 "OSR entry into the middle of the loop!!",
519 block->id());
520 } else {
521 // Deleting the entry into the loop makes the loop unreachable.
522 isUnreachableLoop = true;
523 JitSpew(JitSpew_GVN,
524 " "
525 "Loop with header block%u is no longer reachable",
526 block->id());
527 }
528 #ifdef JS_JITSPEW
529 } else if (block->hasUniqueBackedge() && block->backedge() == pred) {
530 JitSpew(JitSpew_GVN, " Loop with header block%u is no longer a loop",
531 block->id());
532 #endif
533 }
534 }
535
536 // Actually remove the CFG edge.
537 if (!removePredecessorAndDoDCE(block, pred,
538 block->getPredecessorIndex(pred))) {
539 return false;
540 }
541
542 // We've now edited the CFG; check to see if |block| became unreachable.
543 if (block->numPredecessors() == 0 || isUnreachableLoop) {
544 JitSpew(JitSpew_GVN, " Disconnecting block%u", block->id());
545
546 // Remove |block| from its dominator parent's subtree. This is the only
547 // immediately-dominated-block information we need to update, because
548 // everything dominated by this block is about to be swept away.
549 MBasicBlock* parent = block->immediateDominator();
550 if (parent != block) {
551 parent->removeImmediatelyDominatedBlock(block);
552 }
553
554 // Completely disconnect it from the CFG. We do this now rather than
555 // just doing it later when we arrive there in visitUnreachableBlock
556 // so that we don't leave a partially broken loop sitting around. This
557 // also lets visitUnreachableBlock assert that numPredecessors() == 0,
558 // which is a nice invariant.
559 if (block->isLoopHeader()) {
560 block->clearLoopHeader();
561 }
562 for (size_t i = 0, e = block->numPredecessors(); i < e; ++i) {
563 if (!removePredecessorAndDoDCE(block, block->getPredecessor(i), i)) {
564 return false;
565 }
566 }
567
568 // Clear out the resume point operands, as they can hold things that
569 // don't appear to dominate them live.
570 if (MResumePoint* resume = block->entryResumePoint()) {
571 if (!releaseResumePointOperands(resume) || !processDeadDefs()) {
572 return false;
573 }
574 if (MResumePoint* outer = block->outerResumePoint()) {
575 if (!releaseResumePointOperands(outer) || !processDeadDefs()) {
576 return false;
577 }
578 }
579 MOZ_ASSERT(nextDef_ == nullptr);
580 for (MInstructionIterator iter(block->begin()), end(block->end());
581 iter != end;) {
582 MInstruction* ins = *iter++;
583 nextDef_ = iter != end ? *iter : nullptr;
584 if (MResumePoint* resume = ins->resumePoint()) {
585 if (!releaseResumePointOperands(resume) || !processDeadDefs()) {
586 return false;
587 }
588 }
589 }
590 nextDef_ = nullptr;
591 } else {
592 #ifdef DEBUG
593 MOZ_ASSERT(block->outerResumePoint() == nullptr,
594 "Outer resume point in block without an entry resume point");
595 for (MInstructionIterator iter(block->begin()), end(block->end());
596 iter != end; ++iter) {
597 MOZ_ASSERT(iter->resumePoint() == nullptr,
598 "Instruction with resume point in block without entry "
599 "resume point");
600 }
601 #endif
602 }
603
604 // Use the mark to note that we've already removed all its predecessors,
605 // and we know it's unreachable.
606 block->mark();
607 }
608
609 return true;
610 }
611
612 // Return a simplified form of |def|, if we can.
simplified(MDefinition * def) const613 MDefinition* ValueNumberer::simplified(MDefinition* def) const {
614 return def->foldsTo(graph_.alloc());
615 }
616
617 // If an equivalent and dominating value already exists in the set, return it.
618 // Otherwise insert |def| into the set and return it.
leader(MDefinition * def)619 MDefinition* ValueNumberer::leader(MDefinition* def) {
620 // If the value isn't suitable for eliminating, don't bother hashing it. The
621 // convention is that congruentTo returns false for node kinds that wish to
622 // opt out of redundance elimination.
623 // TODO: It'd be nice to clean up that convention (bug 1031406).
624 if (!def->isEffectful() && def->congruentTo(def)) {
625 // Look for a match.
626 VisibleValues::AddPtr p = values_.findLeaderForAdd(def);
627 if (p) {
628 MDefinition* rep = *p;
629 if (!rep->isDiscarded() && rep->block()->dominates(def->block())) {
630 // We found a dominating congruent value.
631 return rep;
632 }
633
634 // The congruent value doesn't dominate. It never will again in this
635 // dominator tree, so overwrite it.
636 values_.overwrite(p, def);
637 } else {
638 // No match. Add a new entry.
639 if (!values_.add(p, def)) {
640 return nullptr;
641 }
642 }
643
644 #ifdef JS_JITSPEW
645 JitSpew(JitSpew_GVN, " Recording %s%u", def->opName(), def->id());
646 #endif
647 }
648
649 return def;
650 }
651
652 // Test whether |phi| is dominated by a congruent phi.
hasLeader(const MPhi * phi,const MBasicBlock * phiBlock) const653 bool ValueNumberer::hasLeader(const MPhi* phi,
654 const MBasicBlock* phiBlock) const {
655 if (VisibleValues::Ptr p = values_.findLeader(phi)) {
656 const MDefinition* rep = *p;
657 return rep != phi && rep->block()->dominates(phiBlock);
658 }
659 return false;
660 }
661
662 // Test whether there are any phis in |header| which are newly optimizable, as a
663 // result of optimizations done inside the loop. This is not a sparse approach,
664 // but restarting is rare enough in practice. Termination is ensured by
665 // discarding the phi triggering the iteration.
loopHasOptimizablePhi(MBasicBlock * header) const666 bool ValueNumberer::loopHasOptimizablePhi(MBasicBlock* header) const {
667 // If the header is unreachable, don't bother re-optimizing it.
668 if (header->isMarked()) {
669 return false;
670 }
671
672 // Rescan the phis for any that can be simplified, since they may be reading
673 // values from backedges.
674 for (MPhiIterator iter(header->phisBegin()), end(header->phisEnd());
675 iter != end; ++iter) {
676 MPhi* phi = *iter;
677 MOZ_ASSERT_IF(!phi->hasUses(), !DeadIfUnused(phi));
678
679 if (phi->operandIfRedundant() || hasLeader(phi, header)) {
680 return true; // Phi can be simplified.
681 }
682 }
683 return false;
684 }
685
686 // Visit |def|.
visitDefinition(MDefinition * def)687 bool ValueNumberer::visitDefinition(MDefinition* def) {
688 // Nop does not fit in any of the previous optimization, as its only purpose
689 // is to reduce the register pressure by keeping additional resume
690 // point. Still, there is no need consecutive list of MNop instructions, and
691 // this will slow down every other iteration on the Graph.
692 if (def->isNop()) {
693 MNop* nop = def->toNop();
694 MBasicBlock* block = nop->block();
695
696 // We look backward to know if we can remove the previous Nop, we do not
697 // look forward as we would not benefit from the folding made by GVN.
698 MInstructionReverseIterator iter = ++block->rbegin(nop);
699
700 // This nop is at the beginning of the basic block, just replace the
701 // resume point of the basic block by the one from the resume point.
702 if (iter == block->rend()) {
703 JitSpew(JitSpew_GVN, " Removing Nop%u", nop->id());
704 nop->moveResumePointAsEntry();
705 block->discard(nop);
706 return true;
707 }
708
709 // The previous instruction is also a Nop, no need to keep it anymore.
710 MInstruction* prev = *iter;
711 if (prev->isNop()) {
712 JitSpew(JitSpew_GVN, " Removing Nop%u", prev->id());
713 block->discard(prev);
714 return true;
715 }
716
717 // The Nop is introduced to capture the result and make sure the operands
718 // are not live anymore when there are no further uses. Though when
719 // all operands are still needed the Nop doesn't decrease the liveness
720 // and can get removed.
721 MResumePoint* rp = nop->resumePoint();
722 if (rp && rp->numOperands() > 0 &&
723 rp->getOperand(rp->numOperands() - 1) == prev &&
724 !nop->block()->lastIns()->isThrow() &&
725 !prev->isAssertRecoveredOnBailout()) {
726 size_t numOperandsLive = 0;
727 for (size_t j = 0; j < prev->numOperands(); j++) {
728 for (size_t i = 0; i < rp->numOperands(); i++) {
729 if (prev->getOperand(j) == rp->getOperand(i)) {
730 numOperandsLive++;
731 break;
732 }
733 }
734 }
735
736 if (numOperandsLive == prev->numOperands()) {
737 JitSpew(JitSpew_GVN, " Removing Nop%u", nop->id());
738 block->discard(nop);
739 }
740 }
741
742 return true;
743 }
744
745 // Skip optimizations on instructions which are recovered on bailout, to
746 // avoid mixing instructions which are recovered on bailouts with
747 // instructions which are not.
748 if (def->isRecoveredOnBailout()) {
749 return true;
750 }
751
752 // If this instruction has a dependency() into an unreachable block, we'll
753 // need to update AliasAnalysis.
754 MDefinition* dep = def->dependency();
755 if (dep != nullptr && (dep->isDiscarded() || dep->block()->isDead())) {
756 JitSpew(JitSpew_GVN, " AliasAnalysis invalidated");
757 if (updateAliasAnalysis_ && !dependenciesBroken_) {
758 // TODO: Recomputing alias-analysis could theoretically expose more
759 // GVN opportunities.
760 JitSpew(JitSpew_GVN, " Will recompute!");
761 dependenciesBroken_ = true;
762 }
763 // Temporarily clear its dependency, to protect foldsTo, which may
764 // wish to use the dependency to do store-to-load forwarding.
765 def->setDependency(def->toInstruction());
766 } else {
767 dep = nullptr;
768 }
769
770 // Look for a simplified form of |def|.
771 MDefinition* sim = simplified(def);
772 if (sim != def) {
773 if (sim == nullptr) {
774 return false;
775 }
776
777 bool isNewInstruction = sim->block() == nullptr;
778
779 // If |sim| doesn't belong to a block, insert it next to |def|.
780 if (isNewInstruction) {
781 def->block()->insertAfter(def->toInstruction(), sim->toInstruction());
782 }
783
784 #ifdef JS_JITSPEW
785 JitSpew(JitSpew_GVN, " Folded %s%u to %s%u", def->opName(), def->id(),
786 sim->opName(), sim->id());
787 #endif
788 MOZ_ASSERT(!sim->isDiscarded());
789 ReplaceAllUsesWith(def, sim);
790
791 // The node's foldsTo said |def| can be replaced by |rep|. If |def| is a
792 // guard, then either |rep| is also a guard, or a guard isn't actually
793 // needed, so we can clear |def|'s guard flag and let it be discarded.
794 def->setNotGuardUnchecked();
795
796 if (def->isGuardRangeBailouts()) {
797 sim->setGuardRangeBailoutsUnchecked();
798 }
799
800 if (DeadIfUnused(def)) {
801 if (!discardDefsRecursively(def)) {
802 return false;
803 }
804
805 // If that ended up discarding |sim|, then we're done here.
806 if (sim->isDiscarded()) {
807 return true;
808 }
809 }
810
811 if (!rerun_ && def->isPhi() && !sim->isPhi()) {
812 rerun_ = true;
813 JitSpew(JitSpew_GVN,
814 " Replacing phi%u may have enabled cascading optimisations; "
815 "will re-run",
816 def->id());
817 }
818
819 // Otherwise, procede to optimize with |sim| in place of |def|.
820 def = sim;
821
822 // If the simplified instruction was already part of the graph, then we
823 // probably already visited and optimized this instruction.
824 if (!isNewInstruction) {
825 return true;
826 }
827 }
828
829 // Now that foldsTo is done, re-enable the original dependency. Even though
830 // it may be pointing into a discarded block, it's still valid for the
831 // purposes of detecting congruent loads.
832 if (dep != nullptr) {
833 def->setDependency(dep);
834 }
835
836 // Look for a dominating def which makes |def| redundant.
837 MDefinition* rep = leader(def);
838 if (rep != def) {
839 if (rep == nullptr) {
840 return false;
841 }
842 if (rep->updateForReplacement(def)) {
843 #ifdef JS_JITSPEW
844 JitSpew(JitSpew_GVN, " Replacing %s%u with %s%u", def->opName(),
845 def->id(), rep->opName(), rep->id());
846 #endif
847 ReplaceAllUsesWith(def, rep);
848
849 // The node's congruentTo said |def| is congruent to |rep|, and it's
850 // dominated by |rep|. If |def| is a guard, it's covered by |rep|,
851 // so we can clear |def|'s guard flag and let it be discarded.
852 def->setNotGuardUnchecked();
853
854 if (DeadIfUnused(def)) {
855 // discardDef should not add anything to the deadDefs, as the
856 // redundant operation should have the same input operands.
857 mozilla::DebugOnly<bool> r = discardDef(def);
858 MOZ_ASSERT(
859 r,
860 "discardDef shouldn't have tried to add anything to the worklist, "
861 "so it shouldn't have failed");
862 MOZ_ASSERT(deadDefs_.empty(),
863 "discardDef shouldn't have added anything to the worklist");
864 }
865 def = rep;
866 }
867 }
868
869 return true;
870 }
871
872 // Visit the control instruction at the end of |block|.
visitControlInstruction(MBasicBlock * block)873 bool ValueNumberer::visitControlInstruction(MBasicBlock* block) {
874 // Look for a simplified form of the control instruction.
875 MControlInstruction* control = block->lastIns();
876 MDefinition* rep = simplified(control);
877 if (rep == control) {
878 return true;
879 }
880
881 if (rep == nullptr) {
882 return false;
883 }
884
885 MControlInstruction* newControl = rep->toControlInstruction();
886 MOZ_ASSERT(!newControl->block(),
887 "Control instruction replacement shouldn't already be in a block");
888 #ifdef JS_JITSPEW
889 JitSpew(JitSpew_GVN, " Folded control instruction %s%u to %s%u",
890 control->opName(), control->id(), newControl->opName(),
891 graph_.getNumInstructionIds());
892 #endif
893
894 // If the simplification removes any CFG edges, update the CFG and remove
895 // any blocks that become dead.
896 size_t oldNumSuccs = control->numSuccessors();
897 size_t newNumSuccs = newControl->numSuccessors();
898 if (newNumSuccs != oldNumSuccs) {
899 MOZ_ASSERT(newNumSuccs < oldNumSuccs,
900 "New control instruction has too many successors");
901 for (size_t i = 0; i != oldNumSuccs; ++i) {
902 MBasicBlock* succ = control->getSuccessor(i);
903 if (HasSuccessor(newControl, succ)) {
904 continue;
905 }
906 if (succ->isMarked()) {
907 continue;
908 }
909 if (!removePredecessorAndCleanUp(succ, block)) {
910 return false;
911 }
912 if (succ->isMarked()) {
913 continue;
914 }
915 if (!rerun_) {
916 if (!remainingBlocks_.append(succ)) {
917 return false;
918 }
919 }
920 }
921 }
922
923 if (!releaseOperands(control)) {
924 return false;
925 }
926 block->discardIgnoreOperands(control);
927 block->end(newControl);
928 if (block->entryResumePoint() && newNumSuccs != oldNumSuccs) {
929 block->flagOperandsOfPrunedBranches(newControl);
930 }
931 return processDeadDefs();
932 }
933
934 // |block| is unreachable. Mine it for opportunities to delete more dead
935 // code, and then discard it.
visitUnreachableBlock(MBasicBlock * block)936 bool ValueNumberer::visitUnreachableBlock(MBasicBlock* block) {
937 JitSpew(JitSpew_GVN, " Visiting unreachable block%u%s%s%s", block->id(),
938 block->isLoopHeader() ? " (loop header)" : "",
939 block->isSplitEdge() ? " (split edge)" : "",
940 block->immediateDominator() == block ? " (dominator root)" : "");
941
942 MOZ_ASSERT(block->isMarked(),
943 "Visiting unmarked (and therefore reachable?) block");
944 MOZ_ASSERT(block->numPredecessors() == 0,
945 "Block marked unreachable still has predecessors");
946 MOZ_ASSERT(block != graph_.entryBlock(), "Removing normal entry block");
947 MOZ_ASSERT(block != graph_.osrBlock(), "Removing OSR entry block");
948 MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");
949
950 // Disconnect all outgoing CFG edges.
951 for (size_t i = 0, e = block->numSuccessors(); i < e; ++i) {
952 MBasicBlock* succ = block->getSuccessor(i);
953 if (succ->isDead() || succ->isMarked()) {
954 continue;
955 }
956 if (!removePredecessorAndCleanUp(succ, block)) {
957 return false;
958 }
959 if (succ->isMarked()) {
960 continue;
961 }
962 // |succ| is still reachable. Make a note of it so that we can scan
963 // it for interesting dominator tree changes later.
964 if (!rerun_) {
965 if (!remainingBlocks_.append(succ)) {
966 return false;
967 }
968 }
969 }
970
971 // Discard any instructions with no uses. The remaining instructions will be
972 // discarded when their last use is discarded.
973 MOZ_ASSERT(nextDef_ == nullptr);
974 for (MDefinitionIterator iter(block); iter;) {
975 MDefinition* def = *iter++;
976 if (def->hasUses()) {
977 continue;
978 }
979 nextDef_ = iter ? *iter : nullptr;
980 if (!discardDefsRecursively(def)) {
981 return false;
982 }
983 }
984
985 nextDef_ = nullptr;
986 MControlInstruction* control = block->lastIns();
987 return discardDefsRecursively(control);
988 }
989
990 // Visit all the phis and instructions |block|.
visitBlock(MBasicBlock * block)991 bool ValueNumberer::visitBlock(MBasicBlock* block) {
992 MOZ_ASSERT(!block->isMarked(), "Blocks marked unreachable during GVN");
993 MOZ_ASSERT(!block->isDead(), "Block to visit is already dead");
994
995 JitSpew(JitSpew_GVN, " Visiting block%u", block->id());
996
997 // Visit the definitions in the block top-down.
998 MOZ_ASSERT(nextDef_ == nullptr);
999 for (MDefinitionIterator iter(block); iter;) {
1000 if (!graph_.alloc().ensureBallast()) {
1001 return false;
1002 }
1003 MDefinition* def = *iter++;
1004
1005 // Remember where our iterator is so that we don't invalidate it.
1006 nextDef_ = iter ? *iter : nullptr;
1007
1008 // If the definition is dead, discard it.
1009 if (IsDiscardable(def)) {
1010 if (!discardDefsRecursively(def)) {
1011 return false;
1012 }
1013 continue;
1014 }
1015
1016 if (!visitDefinition(def)) {
1017 return false;
1018 }
1019 }
1020 nextDef_ = nullptr;
1021
1022 if (!graph_.alloc().ensureBallast()) {
1023 return false;
1024 }
1025
1026 return visitControlInstruction(block);
1027 }
1028
1029 // Visit all the blocks dominated by dominatorRoot.
visitDominatorTree(MBasicBlock * dominatorRoot)1030 bool ValueNumberer::visitDominatorTree(MBasicBlock* dominatorRoot) {
1031 JitSpew(JitSpew_GVN,
1032 " Visiting dominator tree (with %" PRIu64
1033 " blocks) rooted at block%u%s",
1034 uint64_t(dominatorRoot->numDominated()), dominatorRoot->id(),
1035 dominatorRoot == graph_.entryBlock()
1036 ? " (normal entry block)"
1037 : dominatorRoot == graph_.osrBlock()
1038 ? " (OSR entry block)"
1039 : dominatorRoot->numPredecessors() == 0
1040 ? " (odd unreachable block)"
1041 : " (merge point from normal entry and OSR entry)");
1042 MOZ_ASSERT(dominatorRoot->immediateDominator() == dominatorRoot,
1043 "root is not a dominator tree root");
1044
1045 // Visit all blocks dominated by dominatorRoot, in RPO. This has the nice
1046 // property that we'll always visit a block before any block it dominates,
1047 // so we can make a single pass through the list and see every full
1048 // redundance.
1049 size_t numVisited = 0;
1050 size_t numDiscarded = 0;
1051 for (ReversePostorderIterator iter(graph_.rpoBegin(dominatorRoot));;) {
1052 MOZ_ASSERT(iter != graph_.rpoEnd(), "Inconsistent dominator information");
1053 MBasicBlock* block = *iter++;
1054 // We're only visiting blocks in dominatorRoot's tree right now.
1055 if (!dominatorRoot->dominates(block)) {
1056 continue;
1057 }
1058
1059 // If this is a loop backedge, remember the header, as we may not be able
1060 // to find it after we simplify the block.
1061 MBasicBlock* header =
1062 block->isLoopBackedge() ? block->loopHeaderOfBackedge() : nullptr;
1063
1064 if (block->isMarked()) {
1065 // This block has become unreachable; handle it specially.
1066 if (!visitUnreachableBlock(block)) {
1067 return false;
1068 }
1069 ++numDiscarded;
1070 } else {
1071 // Visit the block!
1072 if (!visitBlock(block)) {
1073 return false;
1074 }
1075 ++numVisited;
1076 }
1077
1078 // If the block is/was a loop backedge, check to see if the block that
1079 // is/was its header has optimizable phis, which would want a re-run.
1080 if (!rerun_ && header && loopHasOptimizablePhi(header)) {
1081 JitSpew(JitSpew_GVN,
1082 " Loop phi in block%u can now be optimized; will re-run GVN!",
1083 header->id());
1084 rerun_ = true;
1085 remainingBlocks_.clear();
1086 }
1087
1088 MOZ_ASSERT(numVisited <= dominatorRoot->numDominated() - numDiscarded,
1089 "Visited blocks too many times");
1090 if (numVisited >= dominatorRoot->numDominated() - numDiscarded) {
1091 break;
1092 }
1093 }
1094
1095 totalNumVisited_ += numVisited;
1096 values_.clear();
1097 return true;
1098 }
1099
1100 // Visit all the blocks in the graph.
visitGraph()1101 bool ValueNumberer::visitGraph() {
1102 // Due to OSR blocks, the set of blocks dominated by a blocks may not be
1103 // contiguous in the RPO. Do a separate traversal for each dominator tree
1104 // root. There's always the main entry, and sometimes there's an OSR entry,
1105 // and then there are the roots formed where the OSR paths merge with the
1106 // main entry paths.
1107 for (ReversePostorderIterator iter(graph_.rpoBegin());;) {
1108 MOZ_ASSERT(iter != graph_.rpoEnd(), "Inconsistent dominator information");
1109 MBasicBlock* block = *iter;
1110 if (block->immediateDominator() == block) {
1111 if (!visitDominatorTree(block)) {
1112 return false;
1113 }
1114
1115 // Normally unreachable blocks would be removed by now, but if this
1116 // block is a dominator tree root, it has been special-cased and left
1117 // in place in order to avoid invalidating our iterator. Now that
1118 // we've finished the tree, increment the iterator, and then if it's
1119 // marked for removal, remove it.
1120 ++iter;
1121 if (block->isMarked()) {
1122 JitSpew(JitSpew_GVN, " Discarding dominator root block%u",
1123 block->id());
1124 MOZ_ASSERT(
1125 block->begin() == block->end(),
1126 "Unreachable dominator tree root has instructions after tree walk");
1127 MOZ_ASSERT(block->phisEmpty(),
1128 "Unreachable dominator tree root has phis after tree walk");
1129 graph_.removeBlock(block);
1130 blocksRemoved_ = true;
1131 }
1132
1133 MOZ_ASSERT(totalNumVisited_ <= graph_.numBlocks(),
1134 "Visited blocks too many times");
1135 if (totalNumVisited_ >= graph_.numBlocks()) {
1136 break;
1137 }
1138 } else {
1139 // This block a dominator tree root. Proceed to the next one.
1140 ++iter;
1141 }
1142 }
1143 totalNumVisited_ = 0;
1144 return true;
1145 }
1146
insertOSRFixups()1147 bool ValueNumberer::insertOSRFixups() {
1148 ReversePostorderIterator end(graph_.end());
1149 for (ReversePostorderIterator iter(graph_.begin()); iter != end;) {
1150 MBasicBlock* block = *iter++;
1151
1152 // Only add fixup block above for loops which can be reached from OSR.
1153 if (!block->isLoopHeader()) {
1154 continue;
1155 }
1156
1157 // If the loop header is not self-dominated, then this loop does not
1158 // have to deal with a second entry point, so there is no need to add a
1159 // second entry point with a fixup block.
1160 if (block->immediateDominator() != block) {
1161 continue;
1162 }
1163
1164 if (!fixupOSROnlyLoop(block, block->backedge())) {
1165 return false;
1166 }
1167 }
1168
1169 return true;
1170 }
1171
1172 // OSR fixups serve the purpose of representing the non-OSR entry into a loop
1173 // when the only real entry is an OSR entry into the middle. However, if the
1174 // entry into the middle is subsequently folded away, the loop may actually
1175 // have become unreachable. Mark-and-sweep all blocks to remove all such code.
cleanupOSRFixups()1176 bool ValueNumberer::cleanupOSRFixups() {
1177 // Mark.
1178 Vector<MBasicBlock*, 0, JitAllocPolicy> worklist(graph_.alloc());
1179 unsigned numMarked = 2;
1180 graph_.entryBlock()->mark();
1181 graph_.osrBlock()->mark();
1182 if (!worklist.append(graph_.entryBlock()) ||
1183 !worklist.append(graph_.osrBlock())) {
1184 return false;
1185 }
1186 while (!worklist.empty()) {
1187 MBasicBlock* block = worklist.popCopy();
1188 for (size_t i = 0, e = block->numSuccessors(); i != e; ++i) {
1189 MBasicBlock* succ = block->getSuccessor(i);
1190 if (!succ->isMarked()) {
1191 ++numMarked;
1192 succ->mark();
1193 if (!worklist.append(succ)) {
1194 return false;
1195 }
1196 } else if (succ->isLoopHeader() && succ->loopPredecessor() == block &&
1197 succ->numPredecessors() == 3) {
1198 // Unmark fixup blocks if the loop predecessor is marked after
1199 // the loop header.
1200 succ->getPredecessor(1)->unmarkUnchecked();
1201 }
1202 }
1203
1204 // OSR fixup blocks are needed if and only if the loop header is
1205 // reachable from its backedge (via the OSR block) and not from its
1206 // original loop predecessor.
1207 //
1208 // Thus OSR fixup blocks are removed if the loop header is not
1209 // reachable, or if the loop header is reachable from both its backedge
1210 // and its original loop predecessor.
1211 if (block->isLoopHeader()) {
1212 MBasicBlock* maybeFixupBlock = nullptr;
1213 if (block->numPredecessors() == 2) {
1214 maybeFixupBlock = block->getPredecessor(0);
1215 } else {
1216 MOZ_ASSERT(block->numPredecessors() == 3);
1217 if (!block->loopPredecessor()->isMarked()) {
1218 maybeFixupBlock = block->getPredecessor(1);
1219 }
1220 }
1221
1222 if (maybeFixupBlock && !maybeFixupBlock->isMarked() &&
1223 maybeFixupBlock->numPredecessors() == 0) {
1224 MOZ_ASSERT(maybeFixupBlock->numSuccessors() == 1,
1225 "OSR fixup block should have exactly one successor");
1226 MOZ_ASSERT(maybeFixupBlock != graph_.entryBlock(),
1227 "OSR fixup block shouldn't be the entry block");
1228 MOZ_ASSERT(maybeFixupBlock != graph_.osrBlock(),
1229 "OSR fixup block shouldn't be the OSR entry block");
1230 maybeFixupBlock->mark();
1231 }
1232 }
1233 }
1234
1235 // And sweep.
1236 return RemoveUnmarkedBlocks(mir_, graph_, numMarked);
1237 }
1238
ValueNumberer(MIRGenerator * mir,MIRGraph & graph)1239 ValueNumberer::ValueNumberer(MIRGenerator* mir, MIRGraph& graph)
1240 : mir_(mir),
1241 graph_(graph),
1242 // Initialize the value set. It's tempting to pass in a length that is a
1243 // function of graph_.getNumInstructionIds(). But if we start out with a
1244 // large capacity, it will be far larger than the actual element count for
1245 // most of the pass, so when we remove elements, it would often think it
1246 // needs to compact itself. Empirically, just letting the HashTable grow
1247 // as needed on its own seems to work pretty well.
1248 values_(graph.alloc()),
1249 deadDefs_(graph.alloc()),
1250 remainingBlocks_(graph.alloc()),
1251 nextDef_(nullptr),
1252 totalNumVisited_(0),
1253 rerun_(false),
1254 blocksRemoved_(false),
1255 updateAliasAnalysis_(false),
1256 dependenciesBroken_(false),
1257 hasOSRFixups_(false) {}
1258
run(UpdateAliasAnalysisFlag updateAliasAnalysis)1259 bool ValueNumberer::run(UpdateAliasAnalysisFlag updateAliasAnalysis) {
1260 updateAliasAnalysis_ = updateAliasAnalysis == UpdateAliasAnalysis;
1261
1262 JitSpew(JitSpew_GVN, "Running GVN on graph (with %" PRIu64 " blocks)",
1263 uint64_t(graph_.numBlocks()));
1264
1265 // Adding fixup blocks only make sense iff we have a second entry point into
1266 // the graph which cannot be reached any more from the entry point.
1267 if (graph_.osrBlock()) {
1268 if (!insertOSRFixups()) {
1269 return false;
1270 }
1271 }
1272
1273 // Top level non-sparse iteration loop. If an iteration performs a
1274 // significant change, such as discarding a block which changes the
1275 // dominator tree and may enable more optimization, this loop takes another
1276 // iteration.
1277 int runs = 0;
1278 for (;;) {
1279 if (!visitGraph()) {
1280 return false;
1281 }
1282
1283 // Test whether any block which was not removed but which had at least
1284 // one predecessor removed will have a new dominator parent.
1285 while (!remainingBlocks_.empty()) {
1286 MBasicBlock* block = remainingBlocks_.popCopy();
1287 if (!block->isDead() && IsDominatorRefined(block)) {
1288 JitSpew(JitSpew_GVN,
1289 " Dominator for block%u can now be refined; will re-run GVN!",
1290 block->id());
1291 rerun_ = true;
1292 remainingBlocks_.clear();
1293 break;
1294 }
1295 }
1296
1297 if (blocksRemoved_) {
1298 if (!AccountForCFGChanges(mir_, graph_, dependenciesBroken_,
1299 /* underValueNumberer = */ true)) {
1300 return false;
1301 }
1302
1303 blocksRemoved_ = false;
1304 dependenciesBroken_ = false;
1305 }
1306
1307 if (mir_->shouldCancel("GVN (outer loop)")) {
1308 return false;
1309 }
1310
1311 // If no further opportunities have been discovered, we're done.
1312 if (!rerun_) {
1313 break;
1314 }
1315
1316 rerun_ = false;
1317
1318 // Enforce an arbitrary iteration limit. This is rarely reached, and
1319 // isn't even strictly necessary, as the algorithm is guaranteed to
1320 // terminate on its own in a finite amount of time (since every time we
1321 // re-run we discard the construct which triggered the re-run), but it
1322 // does help avoid slow compile times on pathological code.
1323 ++runs;
1324 if (runs == 6) {
1325 JitSpew(JitSpew_GVN, "Re-run cutoff of %d reached. Terminating GVN!",
1326 runs);
1327 break;
1328 }
1329
1330 JitSpew(JitSpew_GVN,
1331 "Re-running GVN on graph (run %d, now with %" PRIu64 " blocks)",
1332 runs, uint64_t(graph_.numBlocks()));
1333 }
1334
1335 if (MOZ_UNLIKELY(hasOSRFixups_)) {
1336 if (!cleanupOSRFixups()) {
1337 return false;
1338 }
1339 hasOSRFixups_ = false;
1340 }
1341
1342 return true;
1343 }
1344