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
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
|
/*
* Copyright (C) 2014 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 "constant_folding.h"
namespace art {
// This visitor tries to simplify operations that yield a constant. For example
// `input * 0` is replaced by a null constant.
class InstructionWithAbsorbingInputSimplifier : public HGraphVisitor {
public:
explicit InstructionWithAbsorbingInputSimplifier(HGraph* graph) : HGraphVisitor(graph) {}
private:
void VisitShift(HBinaryOperation* shift);
void VisitAnd(HAnd* instruction) OVERRIDE;
void VisitMul(HMul* instruction) OVERRIDE;
void VisitOr(HOr* instruction) OVERRIDE;
void VisitRem(HRem* instruction) OVERRIDE;
void VisitShl(HShl* instruction) OVERRIDE;
void VisitShr(HShr* instruction) OVERRIDE;
void VisitSub(HSub* instruction) OVERRIDE;
void VisitUShr(HUShr* instruction) OVERRIDE;
void VisitXor(HXor* instruction) OVERRIDE;
};
void HConstantFolding::Run() {
InstructionWithAbsorbingInputSimplifier simplifier(graph_);
// Process basic blocks in reverse post-order in the dominator tree,
// so that an instruction turned into a constant, used as input of
// another instruction, may possibly be used to turn that second
// instruction into a constant as well.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
// Traverse this block's instructions in (forward) order and
// replace the ones that can be statically evaluated by a
// compile-time counterpart.
for (HInstructionIterator inst_it(block->GetInstructions());
!inst_it.Done(); inst_it.Advance()) {
HInstruction* inst = inst_it.Current();
if (inst->IsBinaryOperation()) {
// Constant folding: replace `op(a, b)' with a constant at
// compile time if `a' and `b' are both constants.
HConstant* constant =
inst->AsBinaryOperation()->TryStaticEvaluation();
if (constant != nullptr) {
inst->GetBlock()->ReplaceAndRemoveInstructionWith(inst, constant);
} else {
inst->Accept(&simplifier);
}
} else if (inst->IsUnaryOperation()) {
// Constant folding: replace `op(a)' with a constant at compile
// time if `a' is a constant.
HConstant* constant =
inst->AsUnaryOperation()->TryStaticEvaluation();
if (constant != nullptr) {
inst->GetBlock()->ReplaceAndRemoveInstructionWith(inst, constant);
}
} else if (inst->IsDivZeroCheck()) {
// We can safely remove the check if the input is a non-null constant.
HDivZeroCheck* check = inst->AsDivZeroCheck();
HInstruction* check_input = check->InputAt(0);
if (check_input->IsConstant() && !check_input->AsConstant()->IsZero()) {
check->ReplaceWith(check_input);
check->GetBlock()->RemoveInstruction(check);
}
}
}
}
}
void InstructionWithAbsorbingInputSimplifier::VisitShift(HBinaryOperation* instruction) {
DCHECK(instruction->IsShl() || instruction->IsShr() || instruction->IsUShr());
HInstruction* left = instruction->GetLeft();
if (left->IsConstant() && left->AsConstant()->IsZero()) {
// Replace code looking like
// SHL dst, 0, shift_amount
// with
// CONSTANT 0
instruction->ReplaceWith(left);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitAnd(HAnd* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
if ((input_cst != nullptr) && input_cst->IsZero()) {
// Replace code looking like
// AND dst, src, 0
// with
// CONSTANT 0
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitMul(HMul* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
Primitive::Type type = instruction->GetType();
if (Primitive::IsIntOrLongType(type) &&
(input_cst != nullptr) && input_cst->IsZero()) {
// Replace code looking like
// MUL dst, src, 0
// with
// CONSTANT 0
// Integral multiplication by zero always yields zero, but floating-point
// multiplication by zero does not always do. For example `Infinity * 0.0`
// should yield a NaN.
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitOr(HOr* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
if (input_cst == nullptr) {
return;
}
if (Int64FromConstant(input_cst) == -1) {
// Replace code looking like
// OR dst, src, 0xFFF...FF
// with
// CONSTANT 0xFFF...FF
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitRem(HRem* instruction) {
Primitive::Type type = instruction->GetType();
if (!Primitive::IsIntegralType(type)) {
return;
}
HBasicBlock* block = instruction->GetBlock();
if (instruction->GetLeft()->IsConstant() &&
instruction->GetLeft()->AsConstant()->IsZero()) {
// Replace code looking like
// REM dst, 0, src
// with
// CONSTANT 0
instruction->ReplaceWith(instruction->GetLeft());
block->RemoveInstruction(instruction);
}
HConstant* cst_right = instruction->GetRight()->AsConstant();
if (((cst_right != nullptr) &&
(cst_right->IsOne() || cst_right->IsMinusOne())) ||
(instruction->GetLeft() == instruction->GetRight())) {
// Replace code looking like
// REM dst, src, 1
// or
// REM dst, src, -1
// or
// REM dst, src, src
// with
// CONSTANT 0
ArenaAllocator* allocator = GetGraph()->GetArena();
block->ReplaceAndRemoveInstructionWith(instruction,
HConstant::NewConstant(allocator, type, 0));
}
}
void InstructionWithAbsorbingInputSimplifier::VisitShl(HShl* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitShr(HShr* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitSub(HSub* instruction) {
Primitive::Type type = instruction->GetType();
if (!Primitive::IsIntegralType(type)) {
return;
}
HBasicBlock* block = instruction->GetBlock();
ArenaAllocator* allocator = GetGraph()->GetArena();
// We assume that GVN has run before, so we only perform a pointer
// comparison. If for some reason the values are equal but the pointers are
// different, we are still correct and only miss an optimisation
// opportunity.
if (instruction->GetLeft() == instruction->GetRight()) {
// Replace code looking like
// SUB dst, src, src
// with
// CONSTANT 0
// Note that we cannot optimise `x - x` to `0` for floating-point. It does
// not work when `x` is an infinity.
block->ReplaceAndRemoveInstructionWith(instruction,
HConstant::NewConstant(allocator, type, 0));
}
}
void InstructionWithAbsorbingInputSimplifier::VisitUShr(HUShr* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitXor(HXor* instruction) {
if (instruction->GetLeft() == instruction->GetRight()) {
// Replace code looking like
// XOR dst, src, src
// with
// CONSTANT 0
Primitive::Type type = instruction->GetType();
HBasicBlock* block = instruction->GetBlock();
ArenaAllocator* allocator = GetGraph()->GetArena();
block->ReplaceAndRemoveInstructionWith(instruction,
HConstant::NewConstant(allocator, type, 0));
}
}
} // namespace art
|
