1 // Copyright 2013 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 "src/crankshaft/hydrogen-store-elimination.h"
6
7 #include "src/crankshaft/hydrogen-instructions.h"
8 #include "src/objects-inl.h"
9
10 namespace v8 {
11 namespace internal {
12
13 #define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
14
15 // Performs a block-by-block local analysis for removable stores.
Run()16 void HStoreEliminationPhase::Run() {
17 GVNFlagSet flags; // Use GVN flags as an approximation for some instructions.
18 flags.RemoveAll();
19
20 flags.Add(kArrayElements);
21 flags.Add(kArrayLengths);
22 flags.Add(kStringLengths);
23 flags.Add(kBackingStoreFields);
24 flags.Add(kDoubleArrayElements);
25 flags.Add(kDoubleFields);
26 flags.Add(kElementsPointer);
27 flags.Add(kInobjectFields);
28 flags.Add(kExternalMemory);
29 flags.Add(kStringChars);
30 flags.Add(kTypedArrayElements);
31
32 for (int i = 0; i < graph()->blocks()->length(); i++) {
33 unobserved_.Rewind(0);
34 HBasicBlock* block = graph()->blocks()->at(i);
35 if (!block->IsReachable()) continue;
36 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
37 HInstruction* instr = it.Current();
38 if (instr->CheckFlag(HValue::kIsDead)) continue;
39
40 switch (instr->opcode()) {
41 case HValue::kStoreNamedField:
42 // Remove any unobserved stores overwritten by this store.
43 ProcessStore(HStoreNamedField::cast(instr));
44 break;
45 case HValue::kLoadNamedField:
46 // Observe any unobserved stores on this object + field.
47 ProcessLoad(HLoadNamedField::cast(instr));
48 break;
49 default:
50 ProcessInstr(instr, flags);
51 break;
52 }
53 }
54 }
55 }
56
57
ProcessStore(HStoreNamedField * store)58 void HStoreEliminationPhase::ProcessStore(HStoreNamedField* store) {
59 HValue* object = store->object()->ActualValue();
60 int i = 0;
61 while (i < unobserved_.length()) {
62 HStoreNamedField* prev = unobserved_.at(i);
63 if (aliasing_->MustAlias(object, prev->object()->ActualValue()) &&
64 prev->CanBeReplacedWith(store)) {
65 // This store is guaranteed to overwrite the previous store.
66 prev->DeleteAndReplaceWith(NULL);
67 TRACE(("++ Unobserved store S%d overwritten by S%d\n",
68 prev->id(), store->id()));
69 unobserved_.Remove(i);
70 } else {
71 i++;
72 }
73 }
74 // Only non-transitioning stores are removable.
75 if (!store->has_transition()) {
76 TRACE(("-- Might remove store S%d\n", store->id()));
77 unobserved_.Add(store, zone());
78 }
79 }
80
81
ProcessLoad(HLoadNamedField * load)82 void HStoreEliminationPhase::ProcessLoad(HLoadNamedField* load) {
83 HValue* object = load->object()->ActualValue();
84 int i = 0;
85 while (i < unobserved_.length()) {
86 HStoreNamedField* prev = unobserved_.at(i);
87 if (aliasing_->MayAlias(object, prev->object()->ActualValue()) &&
88 load->access().Equals(prev->access())) {
89 TRACE(("-- Observed store S%d by load L%d\n", prev->id(), load->id()));
90 unobserved_.Remove(i);
91 } else {
92 i++;
93 }
94 }
95 }
96
97
ProcessInstr(HInstruction * instr,GVNFlagSet flags)98 void HStoreEliminationPhase::ProcessInstr(HInstruction* instr,
99 GVNFlagSet flags) {
100 if (unobserved_.length() == 0) return; // Nothing to do.
101 if (instr->CanDeoptimize()) {
102 TRACE(("-- Observed stores at I%d (%s might deoptimize)\n",
103 instr->id(), instr->Mnemonic()));
104 unobserved_.Rewind(0);
105 return;
106 }
107 if (instr->CheckChangesFlag(kNewSpacePromotion)) {
108 TRACE(("-- Observed stores at I%d (%s might GC)\n",
109 instr->id(), instr->Mnemonic()));
110 unobserved_.Rewind(0);
111 return;
112 }
113 if (instr->DependsOnFlags().ContainsAnyOf(flags)) {
114 TRACE(("-- Observed stores at I%d (GVN flags of %s)\n",
115 instr->id(), instr->Mnemonic()));
116 unobserved_.Rewind(0);
117 return;
118 }
119 }
120
121 } // namespace internal
122 } // namespace v8
123