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
|
---
title: "Garbage collection in the VM"
date: 2018-12-05T17:45:53+03:00
draft: false
outputs: ["Reveal"]
---
# Garbage Collection in the VM
---
## Contents
1. Allocation
2. Mark and sweep
3. Embedability
4. Incremental
5. Optimizations
---
## Theory reminders
1. [Theory](https://www.slideshare.net/eelcovisser/garbage-collection-69688448)
0. [Barriers](https://www.cs.kent.ac.uk/pubs/2010/3011/content.pdf)
0. [Lua GC](http://www.inf.puc-rio.br/~roberto/talks/gc-lua.pdf)
0. [V8 GC](http://jayconrod.com/posts/55/a-tour-of-v8-garbage-collection)
0. [Generational GC](https://www.ps.uni-saarland.de/courses/gc-ws01/slides/generational_gc.pdf)
0. [Java Z GC](http://cr.openjdk.java.net/~pliden/slides/ZGC-FOSDEM-2018.pdf)
0. [LuaJIT GC](http://wiki.luajit.org/New-Garbage-Collector)
---
## Tri-colour marking
In the core of concurrent and incremental garbage collectors
- _white_ - not used, _dead_ object
- _black_ - used, _alive_ object, whose references have been marked
- _gray_ - used, _alive_ object, whose references have yet to be marked
> references are the objects that are referenced by the current object
---
### Invariants
- Weak:
> All white objects pointed to by a black object are reachable from some grey
> object through a chain of white objects.
- Strong:
> There are no pointers from black objects to white objects.
---
### Escapes
In the incremental or concurrent GC, while the marking phase is running, the
mutator can change the references of a black object and break the above
invariants.
---
### Barriers
Barriers are code that gets executed by the mutator every time it mutates the
heap. They will enforce that the invariants are being kept.
- read
- write
- deletion
---
#### Read Barrier
Ensure the invariants by not allowing getting a _white_ object out of a _gray_
one
def read(object, field):
if is_grey(object):
shade(object)
return object[field]
Not used, since _read_ operations are more than _write_ operations in a program
and having a slower _read_ is worse than having a slower _write_
---
#### Write Barrier
Ensure the invariants by not allowing to set a _white_ object as a reference
inside a _black_ object
def write(object, field, value):
object[field] = value
if is_black(object):
shade(value)
---
## Disclaimer
> This is how GC can be implemented. Everything is subject to change.
---
## Allocation
Having a custom allocator is a *must* have for most VM
- decent performance
- optimizations
---
### Allocators and C++
[Allocators in
C++](https://sofiacpp.github.io/advanced-cpp/slides/17_allocators.html)
---
### Simplicity matters
For simplicity, we are going to not use a special allocator.
---
## Mark and sweep
- mark reacheable objects starting from variables (roots)
- sweep
- unmarked objects are returned to the free heap
- marked objects are unmarked (for the next cycle)
---
### Mark
- every object starts as dead
- reacheable objects are marked as alive
---
#### Reachable objects
---
#### Roots
- registers
- stack
- global object / environment
Where are these stored?
---
#### Roots
- The `data_stack`
- everything upto `m_SP`
- The global object
---
#### Marking
void Spasm::Mark() {
for (auto cell = &data_stack[0]; cell != m_SP; ++cell) {
Mark(cell); // ?
}
Mark(m_Global);
}
---
#### Visit every object in the heap?
- Visitor pattern again
---
struct MarkVisitor
{
typedef void ResultType;
void Visit(double d) const {}
void VisitUndefined() const {}
void VisitNull() const {}
void Visit(StringValue& value) const;
void Visit(ArrayValue& value) const;
void Visit(ObjectValue& value) const;
void Visit(FunctionValue& value) const;
};
---
void Visit(StringValue& value) const {
if (Dead(value)) {
SetAlive(value);
}
}
---
void Visit(ArrayValue& value) const {
if (Dead(value)) {
SetAlive(value);
for (auto i = 0u; i < value.length(); ++i) {
value.item(i).Visit(*this);
}
}
}
---
void Visit(ObjectValue& value) const {
if (Dead(value)) {
SetAlive(value);
for (auto* p : value.m_Properties) {
p->first.Visit(*this);
p->second.Visit(*this);
}
value.m_Prototype->Visit(*this);
}
}
---
#### State
Where to store whether an object is *dead* or *alive*
- `bool` per object, string, array, function
- `bitmap` that holds one bit for a range of objects
- `bit` per pointer
---
# ?
|
