ssa_phi_elimination.cc revision 102cbed1e52b7c5f09458b44903fe97bb3e14d5f 
1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#include "ssa_phi_elimination.h"
18
19namespace art {
20
21void SsaDeadPhiElimination::Run() {
22  // Add to the worklist phis referenced by non-phi instructions.
23  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
24    HBasicBlock* block = it.Current();
25    for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
26      HPhi* phi = it.Current()->AsPhi();
27      for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
28        HUseListNode<HInstruction>* current = it.Current();
29        HInstruction* user = current->GetUser();
30        if (!user->IsPhi()) {
31          worklist_.Add(phi);
32          phi->SetLive();
33          break;
34        } else {
35          phi->SetDead();
36        }
37      }
38    }
39  }
40
41  // Process the worklist by propagating liveness to phi inputs.
42  while (!worklist_.IsEmpty()) {
43    HPhi* phi = worklist_.Pop();
44    for (HInputIterator it(phi); !it.Done(); it.Advance()) {
45      HInstruction* input = it.Current();
46      if (input->IsPhi() && input->AsPhi()->IsDead()) {
47        worklist_.Add(input->AsPhi());
48        input->AsPhi()->SetLive();
49      }
50    }
51  }
52
53  // Remove phis that are not live. Visit in post order so that phis
54  // that are not inputs of loop phis can be removed when they have
55  // no users left (dead phis might use dead phis).
56  for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
57    HBasicBlock* block = it.Current();
58    HInstruction* current = block->GetFirstPhi();
59    HInstruction* next = nullptr;
60    while (current != nullptr) {
61      next = current->GetNext();
62      if (current->AsPhi()->IsDead()) {
63        if (current->HasUses()) {
64          for (HUseIterator<HInstruction> it(current->GetUses()); !it.Done(); it.Advance()) {
65            HUseListNode<HInstruction>* user_node = it.Current();
66            HInstruction* user = user_node->GetUser();
67            DCHECK(user->IsLoopHeaderPhi());
68            DCHECK(user->AsPhi()->IsDead());
69            // Just put itself as an input. The phi will be removed in this loop anyway.
70            user->SetRawInputAt(user_node->GetIndex(), user);
71            current->RemoveUser(user, user_node->GetIndex());
72          }
73        }
74        if (current->HasEnvironmentUses()) {
75          for (HUseIterator<HEnvironment> it(current->GetEnvUses()); !it.Done(); it.Advance()) {
76            HUseListNode<HEnvironment>* user_node = it.Current();
77            HEnvironment* user = user_node->GetUser();
78            user->SetRawEnvAt(user_node->GetIndex(), nullptr);
79            current->RemoveEnvironmentUser(user, user_node->GetIndex());
80          }
81        }
82        block->RemovePhi(current->AsPhi());
83      }
84      current = next;
85    }
86  }
87}
88
89void SsaRedundantPhiElimination::Run() {
90  // Add all phis in the worklist.
91  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
92    HBasicBlock* block = it.Current();
93    for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
94      worklist_.Add(it.Current()->AsPhi());
95    }
96  }
97
98  while (!worklist_.IsEmpty()) {
99    HPhi* phi = worklist_.Pop();
100
101    // If the phi has already been processed, continue.
102    if (!phi->IsInBlock()) {
103      continue;
104    }
105
106    // Find if the inputs of the phi are the same instruction.
107    HInstruction* candidate = phi->InputAt(0);
108    // A loop phi cannot have itself as the first phi. Note that this
109    // check relies on our simplification pass ensuring the pre-header
110    // block is first in the list of predecessors of the loop header.
111    DCHECK(!phi->IsLoopHeaderPhi() || phi->GetBlock()->IsLoopPreHeaderFirstPredecessor());
112    DCHECK_NE(phi, candidate);
113
114    for (size_t i = 1; i < phi->InputCount(); ++i) {
115      HInstruction* input = phi->InputAt(i);
116      // For a loop phi, if the input is the phi, the phi is still candidate for
117      // elimination.
118      if (input != candidate && input != phi) {
119        candidate = nullptr;
120        break;
121      }
122    }
123
124    // If the inputs are not the same, continue.
125    if (candidate == nullptr) {
126      continue;
127    }
128
129    if (phi->IsInLoop()) {
130      // Because we're updating the users of this phi, we may have new
131      // phis candidate for elimination if this phi is in a loop. Add phis that
132      // used this phi to the worklist.
133      for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
134        HUseListNode<HInstruction>* current = it.Current();
135        HInstruction* user = current->GetUser();
136        if (user->IsPhi()) {
137          worklist_.Add(user->AsPhi());
138        }
139      }
140    }
141    phi->ReplaceWith(candidate);
142    phi->GetBlock()->RemovePhi(phi);
143  }
144}
145
146}  // namespace art
147