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