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
9namespace v8 {
10namespace 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.
15void 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
58void 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
83void 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
99void 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