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27
28 #include "hydrogen-removable-simulates.h"
29
30 namespace v8 {
31 namespace internal {
32
Run()33 void HMergeRemovableSimulatesPhase::Run() {
34 ZoneList<HSimulate*> mergelist(2, zone());
35 for (int i = 0; i < graph()->blocks()->length(); ++i) {
36 HBasicBlock* block = graph()->blocks()->at(i);
37 // Make sure the merge list is empty at the start of a block.
38 ASSERT(mergelist.is_empty());
39 // Nasty heuristic: Never remove the first simulate in a block. This
40 // just so happens to have a beneficial effect on register allocation.
41 bool first = true;
42 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
43 HInstruction* current = it.Current();
44 if (current->IsLeaveInlined()) {
45 // Never fold simulates from inlined environments into simulates
46 // in the outer environment.
47 // (Before each HEnterInlined, there is a non-foldable HSimulate
48 // anyway, so we get the barrier in the other direction for free.)
49 // Simply remove all accumulated simulates without merging. This
50 // is safe because simulates after instructions with side effects
51 // are never added to the merge list.
52 while (!mergelist.is_empty()) {
53 mergelist.RemoveLast()->DeleteAndReplaceWith(NULL);
54 }
55 continue;
56 }
57 if (current->IsReturn()) {
58 // Drop mergeable simulates in the list. This is safe because
59 // simulates after instructions with side effects are never added
60 // to the merge list.
61 while (!mergelist.is_empty()) {
62 mergelist.RemoveLast()->DeleteAndReplaceWith(NULL);
63 }
64 continue;
65 }
66 // Skip the non-simulates and the first simulate.
67 if (!current->IsSimulate()) continue;
68 if (first) {
69 first = false;
70 continue;
71 }
72 HSimulate* current_simulate = HSimulate::cast(current);
73 if ((current_simulate->previous()->HasObservableSideEffects() &&
74 !current_simulate->next()->IsSimulate()) ||
75 !current_simulate->is_candidate_for_removal()) {
76 // This simulate is not suitable for folding.
77 // Fold the ones accumulated so far.
78 current_simulate->MergeWith(&mergelist);
79 continue;
80 } else {
81 // Accumulate this simulate for folding later on.
82 mergelist.Add(current_simulate, zone());
83 }
84 }
85
86 if (!mergelist.is_empty()) {
87 // Merge the accumulated simulates at the end of the block.
88 HSimulate* last = mergelist.RemoveLast();
89 last->MergeWith(&mergelist);
90 }
91 }
92 }
93
94 } } // namespace v8::internal
95