The document discusses how Go implements maps efficiently at runtime without generics. It explains that Go uses a single hashmap implementation where the compiler generates a maptype struct at compile time for each unique map declaration. This maptype struct contains type information for the key, value, and buckets, as well as functions for hashing and comparing keys. At runtime, functions like mapaccess1 and mapinsert use the maptype to access or modify the map without relying on interfaces. This approach provides better performance than Java by avoiding boxing, pointer chasing, and predetermining memory sizes, while keeping compile times and binary sizes lower than C++.

1. How the Go runtimeHow the Go runtime
implement mapsimplement maps
efficientlyefficiently
David Chou

2. We are UmboWe are Umbo
Computer VisionComputer Vision
We buildWe build
autonomous videoautonomous video
security systemsecurity system

3. Golang TaipeiGolang Taipei
Streaming MeetupStreaming Meetup
david74.chou @ facebook
david74.chou @ medium
david7482 @ github

4. How the Go runtimeHow the Go runtime
implements maps efficientlyimplements maps efficiently
(without generics)(without generics)
Dave Cheney, GoCon Spring 2018
https://dave.cheney.net/2018/05/29/how-the-go-runtime-
implements-maps-efficiently-without-generics

5. Go2 Generic Design Proposal

6. C++C++ JAVAJAVA
template<
class Key,
class T,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator< std::pai
> class unordered_map;
Class HashMap<K,V>
java.lang.Object
java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values

7. GoGo
var m map[string]

8. map(key)map(key) →→ valuevalue
The map functionThe map function

9. Go uses HashMap

10. hash(key)hash(key) →→ integerinteger
The hash functionThe hash function

11. 0
1
2
3
4
5
6
7
key value
Hashmap Bucket: 3
Hashmap Data StructureHashmap Data Structure

12. 0
1
2
3
4
5
6
7
key value
pkg/errors 2903
spf13/cobra 7136
golang/go 40260
Hashmap Bucket: 3
insert(star, "golang/go", 40260)insert(star, "golang/go", 40260)
"golang/go"
HashFunction
78356113
Mask

13. Four properties of a hash mapFour properties of a hash map
1. A hash function for the key
2. An equality function to compare keys
3. Need to know the size of the key type
4. Need to know the size of the value type

14. C++C++
template<
class Key,
class T,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator< std::pai
> class unordered_map;
class Key
class T
std::hash<Key>
std::equal_to<Key>

15. 0
1
2
3
4
5
6
7
key value
pkg/errors 2903
spf13/cobra 7136
golang/go 40260
Hashmap Bucket: 3
insert(star, "golang/go", 40260)insert(star, "golang/go", 40260)
"golang/go"
std::hash<key>
78356113
Mask
std::equal_to<key>

16. JAVAJAVA
Class HashMap<K,V>
java.lang.Object
java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
K and V are Object
Object.equals()
Object.hashCode()
Need boxing for primitive types

17. 0
1
2
3
4
5
6
7
key value next
pkg/errors 2903
Hashmap Bucket: 3
insert(star, "golang/go", 40260)insert(star, "golang/go", 40260)
"golang/go"
key.hashCode()
78356113
Mask
spf13/cobra 7136
golang/go 40260 null
key.equals()

18. C++C++
Pros
The size of key and value are always known
Array implementation
No need for boxing or pointer chasing
Cons
Larger binary size. Different types means different maps.
Slower compile time.
Larger memory footprint for predetermined size for each
array element.

19. JAVAJAVA
Pros
Single implementation for any subclass of Object
Faster compile time and smaller binary size
Linked list implementation. No predetermined size for
each array element.
Cons
Boxing would increase gc preasure
Slower for boxing and linked list pointer chasing

20. Go's hashmapGo's hashmap
implementaionimplementaion

21. Use interface{} ?Use interface{} ?
Code generation ?Code generation ?
NoNo
NoNo

22. Compiler + RuntimeCompiler + Runtime

23. v := m["key"] → runtime.mapaccess1(m, ”key", &v)
v, ok := m["key"] → runtime.mapaccess2(m, ”key”, &v, &ok)
m["key"] = 9001 → runtime.mapinsert(m, ”key", 9001)
delete(m, "key") → runtime.mapdelete(m, “key”)
Compile time rewritingCompile time rewriting

24. func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer)
unsafe.Pointer
mapaccess1mapaccess1

25. Different maptype values forDifferent maptype values for
each unique map declarationeach unique map declaration
map[string]int → var mt1 maptype{...}
map[string]http.Header → var mt2 maptype{...}
map[structA]structB → var mt3 maptype{...}

26. type maptype struct {
typ _type
key *_type
elem *_type
bucket *_type // internal type representing a hash
hmap *_type // internal type representing a hmap
keysize uint8 // size of key slot
indirectkey bool // store ptr to key instead of key i
valuesize uint8 // size of value slot
indirectvalue bool // store ptr to value instead of val
bucketsize uint16 // size of bucket
reflexivekey bool // true if k==k for all keys
needkeyupdate bool // true if we need to update key on
}
type _type struct {
size uintp
alg *typeA
...
}
type typeAlg struct {
// function for hashing objects
// (ptr to object, seed) -> hash
hash func(unsafe.Pointer, uintpt
// function for comparing object
// (ptr to object A, ptr to obje
equal func(unsafe.Pointer, unsafe
}

27. C++C++
map<K0,V0>map<K0,V0>map<K0,V0>map<K0,V0>
Compile Time
JAVAJAVA
map<K,V>
Run Time
Object0Object0Object0Object0
GoGo
map<K,V>
Compile Time
maptype0maptype0maptype0maptype0

28. ConclusionConclusion
A good compromise between C++ and JAVA
Single hashmap implementation to reduce binary size
Already known the the size of key and value.
Array implementation for better performance.
Could use primitive types without boxing.
No extra gc preasure

29. Any Question?Any Question?