1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
|
/*
* Copyright (C) 2015 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "boolean_simplifier.h"
namespace art {
void HBooleanSimplifier::TryRemovingNegatedCondition(HBasicBlock* block) {
DCHECK(block->EndsWithIf());
// Check if the condition is a Boolean negation.
HIf* if_instruction = block->GetLastInstruction()->AsIf();
HInstruction* boolean_not = if_instruction->InputAt(0);
if (!boolean_not->IsBooleanNot()) {
return;
}
// Make BooleanNot's input the condition of the If and swap branches.
if_instruction->ReplaceInput(boolean_not->InputAt(0), 0);
block->SwapSuccessors();
// Remove the BooleanNot if it is now unused.
if (!boolean_not->HasUses()) {
boolean_not->GetBlock()->RemoveInstruction(boolean_not);
}
}
// Returns true if 'block1' and 'block2' are empty, merge into the same single
// successor and the successor can only be reached from them.
static bool BlocksDoMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
if (!block1->IsSingleGoto() || !block2->IsSingleGoto()) return false;
HBasicBlock* succ1 = block1->GetSuccessors().Get(0);
HBasicBlock* succ2 = block2->GetSuccessors().Get(0);
return succ1 == succ2 && succ1->GetPredecessors().Size() == 2u;
}
// Returns true if the outcome of the branching matches the boolean value of
// the branching condition.
static bool PreservesCondition(HInstruction* input_true, HInstruction* input_false) {
return input_true->IsIntConstant() && input_true->AsIntConstant()->IsOne()
&& input_false->IsIntConstant() && input_false->AsIntConstant()->IsZero();
}
// Returns true if the outcome of the branching is exactly opposite of the
// boolean value of the branching condition.
static bool NegatesCondition(HInstruction* input_true, HInstruction* input_false) {
return input_true->IsIntConstant() && input_true->AsIntConstant()->IsZero()
&& input_false->IsIntConstant() && input_false->AsIntConstant()->IsOne();
}
// Returns an instruction with the opposite boolean value from 'cond'.
static HInstruction* GetOppositeCondition(HInstruction* cond) {
HGraph* graph = cond->GetBlock()->GetGraph();
ArenaAllocator* allocator = graph->GetArena();
if (cond->IsCondition()) {
HInstruction* lhs = cond->InputAt(0);
HInstruction* rhs = cond->InputAt(1);
if (cond->IsEqual()) {
return new (allocator) HNotEqual(lhs, rhs);
} else if (cond->IsNotEqual()) {
return new (allocator) HEqual(lhs, rhs);
} else if (cond->IsLessThan()) {
return new (allocator) HGreaterThanOrEqual(lhs, rhs);
} else if (cond->IsLessThanOrEqual()) {
return new (allocator) HGreaterThan(lhs, rhs);
} else if (cond->IsGreaterThan()) {
return new (allocator) HLessThanOrEqual(lhs, rhs);
} else {
DCHECK(cond->IsGreaterThanOrEqual());
return new (allocator) HLessThan(lhs, rhs);
}
} else if (cond->IsIntConstant()) {
HIntConstant* int_const = cond->AsIntConstant();
if (int_const->IsZero()) {
return graph->GetIntConstant(1);
} else {
DCHECK(int_const->IsOne());
return graph->GetIntConstant(0);
}
} else {
// General case when 'cond' is another instruction of type boolean,
// as verified by SSAChecker.
return new (allocator) HBooleanNot(cond);
}
}
void HBooleanSimplifier::TryRemovingBooleanSelection(HBasicBlock* block) {
DCHECK(block->EndsWithIf());
// Find elements of the pattern.
HIf* if_instruction = block->GetLastInstruction()->AsIf();
HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
if (!BlocksDoMergeTogether(true_block, false_block)) {
return;
}
HBasicBlock* merge_block = true_block->GetSuccessors().Get(0);
if (!merge_block->HasSinglePhi()) {
return;
}
HPhi* phi = merge_block->GetFirstPhi()->AsPhi();
HInstruction* true_value = phi->InputAt(merge_block->GetPredecessorIndexOf(true_block));
HInstruction* false_value = phi->InputAt(merge_block->GetPredecessorIndexOf(false_block));
// Check if the selection negates/preserves the value of the condition and
// if so, generate a suitable replacement instruction.
HInstruction* if_condition = if_instruction->InputAt(0);
HInstruction* replacement;
if (NegatesCondition(true_value, false_value)) {
replacement = GetOppositeCondition(if_condition);
if (replacement->GetBlock() == nullptr) {
block->InsertInstructionBefore(replacement, if_instruction);
}
} else if (PreservesCondition(true_value, false_value)) {
replacement = if_condition;
} else {
return;
}
// Replace the selection outcome with the new instruction.
phi->ReplaceWith(replacement);
merge_block->RemovePhi(phi);
// Delete the true branch and merge the resulting chain of blocks
// 'block->false_block->merge_block' into one.
true_block->DisconnectAndDelete();
block->MergeWith(false_block);
block->MergeWith(merge_block);
// No need to update any dominance information, as we are simplifying
// a simple diamond shape, where the join block is merged with the
// entry block. Any following blocks would have had the join block
// as a dominator, and `MergeWith` handles changing that to the
// entry block.
}
void HBooleanSimplifier::Run() {
// Iterate in post order in the unlikely case that removing one occurrence of
// the selection pattern empties a branch block of another occurrence.
// Otherwise the order does not matter.
for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
if (!block->EndsWithIf()) continue;
// If condition is negated, remove the negation and swap the branches.
TryRemovingNegatedCondition(block);
// If this is a boolean-selection diamond pattern, replace its result with
// the condition value (or its negation) and simplify the graph.
TryRemovingBooleanSelection(block);
}
}
} // namespace art
|
