1 /*
2  * Copyright (C) 2015 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 #ifndef ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_
18 #define ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_
19 
20 #include "nodes.h"
21 
22 namespace art {
23 
24 // Helper class for finding common dominators of two or more blocks in a graph.
25 // The domination information of a graph must not be modified while there is
26 // a CommonDominator object as it's internal state could become invalid.
27 class CommonDominator {
28  public:
29   // Convenience function to find the common dominator of 2 blocks.
ForPair(HBasicBlock * block1,HBasicBlock * block2)30   static HBasicBlock* ForPair(HBasicBlock* block1, HBasicBlock* block2) {
31     CommonDominator finder(block1);
32     finder.Update(block2);
33     return finder.Get();
34   }
35 
36   // Create a finder starting with a given block.
CommonDominator(HBasicBlock * block)37   explicit CommonDominator(HBasicBlock* block)
38       : dominator_(block), chain_length_(ChainLength(block)) {
39   }
40 
41   // Update the common dominator with another block.
Update(HBasicBlock * block)42   void Update(HBasicBlock* block) {
43     DCHECK(block != nullptr);
44     if (dominator_ == nullptr) {
45       dominator_ = block;
46       chain_length_ = ChainLength(block);
47       return;
48     }
49     HBasicBlock* block2 = dominator_;
50     DCHECK(block2 != nullptr);
51     if (block == block2) {
52       return;
53     }
54     size_t chain_length = ChainLength(block);
55     size_t chain_length2 = chain_length_;
56     // Equalize the chain lengths
57     for ( ; chain_length > chain_length2; --chain_length) {
58       block = block->GetDominator();
59       DCHECK(block != nullptr);
60     }
61     for ( ; chain_length2 > chain_length; --chain_length2) {
62       block2 = block2->GetDominator();
63       DCHECK(block2 != nullptr);
64     }
65     // Now run up the chain until we hit the common dominator.
66     while (block != block2) {
67       --chain_length;
68       block = block->GetDominator();
69       DCHECK(block != nullptr);
70       block2 = block2->GetDominator();
71       DCHECK(block2 != nullptr);
72     }
73     dominator_ = block;
74     chain_length_ = chain_length;
75   }
76 
Get()77   HBasicBlock* Get() const {
78     return dominator_;
79   }
80 
81  private:
ChainLength(HBasicBlock * block)82   static size_t ChainLength(HBasicBlock* block) {
83     size_t result = 0;
84     while (block != nullptr) {
85       ++result;
86       block = block->GetDominator();
87     }
88     return result;
89   }
90 
91   HBasicBlock* dominator_;
92   size_t chain_length_;
93 };
94 
95 }  // namespace art
96 
97 #endif  // ART_COMPILER_OPTIMIZING_COMMON_DOMINATOR_H_
98