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/hydrogen-instructions.h"
6#include "src/hydrogen-store-elimination.h"
7
8namespace v8 {
9namespace internal {
10
11#define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
12
13// Performs a block-by-block local analysis for removable stores.
14void HStoreEliminationPhase::Run() {
15  GVNFlagSet flags;  // Use GVN flags as an approximation for some instructions.
16  flags.RemoveAll();
17
18  flags.Add(kArrayElements);
19  flags.Add(kArrayLengths);
20  flags.Add(kStringLengths);
21  flags.Add(kBackingStoreFields);
22  flags.Add(kDoubleArrayElements);
23  flags.Add(kDoubleFields);
24  flags.Add(kElementsPointer);
25  flags.Add(kInobjectFields);
26  flags.Add(kExternalMemory);
27  flags.Add(kStringChars);
28  flags.Add(kTypedArrayElements);
29
30  for (int i = 0; i < graph()->blocks()->length(); i++) {
31    unobserved_.Rewind(0);
32    HBasicBlock* block = graph()->blocks()->at(i);
33    if (!block->IsReachable()) continue;
34    for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
35      HInstruction* instr = it.Current();
36      if (instr->CheckFlag(HValue::kIsDead)) continue;
37
38      // TODO(titzer): eliminate unobserved HStoreKeyed instructions too.
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
57void 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      // TODO(titzer): remove map word clearing from folded allocations.
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 v8::internal
122