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
10namespace v8 {
11namespace 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.
16void 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
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      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
82void 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
98void 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