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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       // TODO(titzer): eliminate unobserved HStoreKeyed instructions too.
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       // TODO(titzer): remove map word clearing from folded allocations.
72       i++;
73     }
74   }
75   // Only non-transitioning stores are removable.
76   if (!store->has_transition()) {
77     TRACE(("-- Might remove store S%d\n", store->id()));
78     unobserved_.Add(store, zone());
79   }
80 }
81 
82 
ProcessLoad(HLoadNamedField * load)83 void HStoreEliminationPhase::ProcessLoad(HLoadNamedField* load) {
84   HValue* object = load->object()->ActualValue();
85   int i = 0;
86   while (i < unobserved_.length()) {
87     HStoreNamedField* prev = unobserved_.at(i);
88     if (aliasing_->MayAlias(object, prev->object()->ActualValue()) &&
89         load->access().Equals(prev->access())) {
90       TRACE(("-- Observed store S%d by load L%d\n", prev->id(), load->id()));
91       unobserved_.Remove(i);
92     } else {
93       i++;
94     }
95   }
96 }
97 
98 
ProcessInstr(HInstruction * instr,GVNFlagSet flags)99 void HStoreEliminationPhase::ProcessInstr(HInstruction* instr,
100     GVNFlagSet flags) {
101   if (unobserved_.length() == 0) return;  // Nothing to do.
102   if (instr->CanDeoptimize()) {
103     TRACE(("-- Observed stores at I%d (%s might deoptimize)\n",
104            instr->id(), instr->Mnemonic()));
105     unobserved_.Rewind(0);
106     return;
107   }
108   if (instr->CheckChangesFlag(kNewSpacePromotion)) {
109     TRACE(("-- Observed stores at I%d (%s might GC)\n",
110            instr->id(), instr->Mnemonic()));
111     unobserved_.Rewind(0);
112     return;
113   }
114   if (instr->DependsOnFlags().ContainsAnyOf(flags)) {
115     TRACE(("-- Observed stores at I%d (GVN flags of %s)\n",
116            instr->id(), instr->Mnemonic()));
117     unobserved_.Rewind(0);
118     return;
119   }
120 }
121 
122 }  // namespace internal
123 }  // namespace v8
124