This aims to provide Go developers with an overiew of the inner workings of gc.
The gc code: https://github.com/golang/go/tree/master/src/cmd/compile
We're going to define the function for the sake of simplicity to look at its AST and SSA
package main
import "fmt"
func myFunc() int {
a := 1
b := 2
if a < 2 {
a = b
}
return a + b
}
func main() {
fmt.Println(myFunc())
}
GOSSAFUNC=myFunc go tool compile main.go
This invocation will generate ssa.html. If you open it in your browser, you'll see something like this.
Source code is tokenized and parsed.
The corresponding code for this step; https://github.com/golang/go/tree/master/src/cmd/compile/internal/syntax
The AST is a tree-like data structure that represents the structure of a program's source code in a more abstract way. This step involves constructing the AST from the parsed source code.
AST is something like this.
Run ast.go, if you want to see the full form of AST.
go run ast.go
Output
0 *ast.GenDecl {
1 . TokPos: main.go:3:1
2 . Tok: import
3 . Lparen: -
4 . Specs: []ast.Spec (len = 1) {
5 . . 0: *ast.ImportSpec {
6 . . . Path: *ast.BasicLit {
7 . . . . ValuePos: main.go:3:8
8 . . . . Kind: STRING
9 . . . . Value: "\"fmt\""
10 . . . }
11 . . . EndPos: -
12 . . }
13 . }
14 . Rparen: -
15 }
0 *ast.FuncDecl {
1 . Name: *ast.Ident {
2 . . NamePos: main.go:5:6
3 . . Name: "myFunc"
4 . . Obj: *ast.Object {
5 . . . Kind: func
6 . . . Name: "myFunc"
7 . . . Decl: *(obj @ 0)
8 . . }
9 . }
10 . Type: *ast.FuncType {
11 . . Func: main.go:5:1
12 . . Params: *ast.FieldList {
13 . . . Opening: main.go:5:12
14 . . . Closing: main.go:5:13
15 . . }
16 . . Results: *ast.FieldList {
17 . . . Opening: -
18 . . . List: []*ast.Field (len = 1) {
19 . . . . 0: *ast.Field {
20 . . . . . Type: *ast.Ident {
21 . . . . . . NamePos: main.go:5:15
22 . . . . . . Name: "int"
23 . . . . . }
24 . . . . }
25 . . . }
26 . . . Closing: -
27 . . }
28 . }
29 . Body: *ast.BlockStmt {
30 . . Lbrace: main.go:5:19
31 . . List: []ast.Stmt (len = 4) {
32 . . . 0: *ast.AssignStmt {
33 . . . . Lhs: []ast.Expr (len = 1) {
34 . . . . . 0: *ast.Ident {
35 . . . . . . NamePos: main.go:6:2
36 . . . . . . Name: "a"
37 . . . . . . Obj: *ast.Object {
38 . . . . . . . Kind: var
39 . . . . . . . Name: "a"
40 . . . . . . . Decl: *(obj @ 32)
41 . . . . . . }
42 . . . . . }
43 . . . . }
44 . . . . TokPos: main.go:6:4
45 . . . . Tok: :=
46 . . . . Rhs: []ast.Expr (len = 1) {
47 . . . . . 0: *ast.BasicLit {
48 . . . . . . ValuePos: main.go:6:7
49 . . . . . . Kind: INT
50 . . . . . . Value: "1"
51 . . . . . }
52 . . . . }
53 . . . }
54 . . . 1: *ast.AssignStmt {
55 . . . . Lhs: []ast.Expr (len = 1) {
56 . . . . . 0: *ast.Ident {
57 . . . . . . NamePos: main.go:7:2
58 . . . . . . Name: "b"
59 . . . . . . Obj: *ast.Object {
60 . . . . . . . Kind: var
61 . . . . . . . Name: "b"
62 . . . . . . . Decl: *(obj @ 54)
63 . . . . . . }
64 . . . . . }
65 . . . . }
66 . . . . TokPos: main.go:7:4
67 . . . . Tok: :=
68 . . . . Rhs: []ast.Expr (len = 1) {
69 . . . . . 0: *ast.BasicLit {
70 . . . . . . ValuePos: main.go:7:7
71 . . . . . . Kind: INT
72 . . . . . . Value: "2"
73 . . . . . }
74 . . . . }
75 . . . }
76 . . . 2: *ast.IfStmt {
77 . . . . If: main.go:9:2
78 . . . . Cond: *ast.BinaryExpr {
79 . . . . . X: *ast.Ident {
80 . . . . . . NamePos: main.go:9:5
81 . . . . . . Name: "a"
82 . . . . . . Obj: *(obj @ 37)
83 . . . . . }
84 . . . . . OpPos: main.go:9:7
85 . . . . . Op: <
86 . . . . . Y: *ast.BasicLit {
87 . . . . . . ValuePos: main.go:9:9
88 . . . . . . Kind: INT
89 . . . . . . Value: "2"
90 . . . . . }
91 . . . . }
92 . . . . Body: *ast.BlockStmt {
93 . . . . . Lbrace: main.go:9:11
94 . . . . . List: []ast.Stmt (len = 1) {
95 . . . . . . 0: *ast.AssignStmt {
96 . . . . . . . Lhs: []ast.Expr (len = 1) {
97 . . . . . . . . 0: *ast.Ident {
98 . . . . . . . . . NamePos: main.go:10:3
99 . . . . . . . . . Name: "a"
100 . . . . . . . . . Obj: *(obj @ 37)
101 . . . . . . . . }
102 . . . . . . . }
103 . . . . . . . TokPos: main.go:10:5
104 . . . . . . . Tok: =
105 . . . . . . . Rhs: []ast.Expr (len = 1) {
106 . . . . . . . . 0: *ast.Ident {
107 . . . . . . . . . NamePos: main.go:10:7
108 . . . . . . . . . Name: "b"
109 . . . . . . . . . Obj: *(obj @ 59)
110 . . . . . . . . }
111 . . . . . . . }
112 . . . . . . }
113 . . . . . }
114 . . . . . Rbrace: main.go:11:2
115 . . . . }
116 . . . }
117 . . . 3: *ast.ReturnStmt {
118 . . . . Return: main.go:13:2
119 . . . . Results: []ast.Expr (len = 1) {
120 . . . . . 0: *ast.BinaryExpr {
121 . . . . . . X: *ast.Ident {
122 . . . . . . . NamePos: main.go:13:9
123 . . . . . . . Name: "a"
124 . . . . . . . Obj: *(obj @ 37)
125 . . . . . . }
126 . . . . . . OpPos: main.go:13:11
127 . . . . . . Op: +
128 . . . . . . Y: *ast.Ident {
129 . . . . . . . NamePos: main.go:13:13
130 . . . . . . . Name: "b"
131 . . . . . . . Obj: *(obj @ 59)
132 . . . . . . }
133 . . . . . }
134 . . . . }
135 . . . }
136 . . }
137 . . Rbrace: main.go:14:1
138 . }
139 }
0 *ast.FuncDecl {
1 . Name: *ast.Ident {
2 . . NamePos: main.go:16:6
3 . . Name: "main"
4 . . Obj: *ast.Object {
5 . . . Kind: func
6 . . . Name: "main"
7 . . . Decl: *(obj @ 0)
8 . . }
9 . }
10 . Type: *ast.FuncType {
11 . . Func: main.go:16:1
12 . . Params: *ast.FieldList {
13 . . . Opening: main.go:16:10
14 . . . Closing: main.go:16:11
15 . . }
16 . }
17 . Body: *ast.BlockStmt {
18 . . Lbrace: main.go:16:13
19 . . List: []ast.Stmt (len = 1) {
20 . . . 0: *ast.ExprStmt {
21 . . . . X: *ast.CallExpr {
22 . . . . . Fun: *ast.SelectorExpr {
23 . . . . . . X: *ast.Ident {
24 . . . . . . . NamePos: main.go:17:2
25 . . . . . . . Name: "fmt"
26 . . . . . . }
27 . . . . . . Sel: *ast.Ident {
28 . . . . . . . NamePos: main.go:17:6
29 . . . . . . . Name: "Println"
30 . . . . . . }
31 . . . . . }
32 . . . . . Lparen: main.go:17:13
33 . . . . . Args: []ast.Expr (len = 1) {
34 . . . . . . 0: *ast.CallExpr {
35 . . . . . . . Fun: *ast.Ident {
36 . . . . . . . . NamePos: main.go:17:14
37 . . . . . . . . Name: "myFunc"
38 . . . . . . . . Obj: *ast.Object {
39 . . . . . . . . . Kind: func
40 . . . . . . . . . Name: "myFunc"
41 . . . . . . . . . Decl: *ast.FuncDecl {
42 . . . . . . . . . . Name: *ast.Ident {
43 . . . . . . . . . . . NamePos: main.go:5:6
44 . . . . . . . . . . . Name: "myFunc"
45 . . . . . . . . . . . Obj: *(obj @ 38)
46 . . . . . . . . . . }
47 . . . . . . . . . . Type: *ast.FuncType {
48 . . . . . . . . . . . Func: main.go:5:1
49 . . . . . . . . . . . Params: *ast.FieldList {
50 . . . . . . . . . . . . Opening: main.go:5:12
51 . . . . . . . . . . . . Closing: main.go:5:13
52 . . . . . . . . . . . }
53 . . . . . . . . . . . Results: *ast.FieldList {
54 . . . . . . . . . . . . Opening: -
55 . . . . . . . . . . . . List: []*ast.Field (len = 1) {
56 . . . . . . . . . . . . . 0: *ast.Field {
57 . . . . . . . . . . . . . . Type: *ast.Ident {
58 . . . . . . . . . . . . . . . NamePos: main.go:5:15
59 . . . . . . . . . . . . . . . Name: "int"
60 . . . . . . . . . . . . . . }
61 . . . . . . . . . . . . . }
62 . . . . . . . . . . . . }
63 . . . . . . . . . . . . Closing: -
64 . . . . . . . . . . . }
65 . . . . . . . . . . }
66 . . . . . . . . . . Body: *ast.BlockStmt {
67 . . . . . . . . . . . Lbrace: main.go:5:19
68 . . . . . . . . . . . List: []ast.Stmt (len = 4) {
69 . . . . . . . . . . . . 0: *ast.AssignStmt {
70 . . . . . . . . . . . . . Lhs: []ast.Expr (len = 1) {
71 . . . . . . . . . . . . . . 0: *ast.Ident {
72 . . . . . . . . . . . . . . . NamePos: main.go:6:2
73 . . . . . . . . . . . . . . . Name: "a"
74 . . . . . . . . . . . . . . . Obj: *ast.Object {
75 . . . . . . . . . . . . . . . . Kind: var
76 . . . . . . . . . . . . . . . . Name: "a"
77 . . . . . . . . . . . . . . . . Decl: *(obj @ 69)
78 . . . . . . . . . . . . . . . }
79 . . . . . . . . . . . . . . }
80 . . . . . . . . . . . . . }
81 . . . . . . . . . . . . . TokPos: main.go:6:4
82 . . . . . . . . . . . . . Tok: :=
83 . . . . . . . . . . . . . Rhs: []ast.Expr (len = 1) {
84 . . . . . . . . . . . . . . 0: *ast.BasicLit {
85 . . . . . . . . . . . . . . . ValuePos: main.go:6:7
86 . . . . . . . . . . . . . . . Kind: INT
87 . . . . . . . . . . . . . . . Value: "1"
88 . . . . . . . . . . . . . . }
89 . . . . . . . . . . . . . }
90 . . . . . . . . . . . . }
91 . . . . . . . . . . . . 1: *ast.AssignStmt {
92 . . . . . . . . . . . . . Lhs: []ast.Expr (len = 1) {
93 . . . . . . . . . . . . . . 0: *ast.Ident {
94 . . . . . . . . . . . . . . . NamePos: main.go:7:2
95 . . . . . . . . . . . . . . . Name: "b"
96 . . . . . . . . . . . . . . . Obj: *ast.Object {
97 . . . . . . . . . . . . . . . . Kind: var
98 . . . . . . . . . . . . . . . . Name: "b"
99 . . . . . . . . . . . . . . . . Decl: *(obj @ 91)
100 . . . . . . . . . . . . . . . }
101 . . . . . . . . . . . . . . }
102 . . . . . . . . . . . . . }
103 . . . . . . . . . . . . . TokPos: main.go:7:4
104 . . . . . . . . . . . . . Tok: :=
105 . . . . . . . . . . . . . Rhs: []ast.Expr (len = 1) {
106 . . . . . . . . . . . . . . 0: *ast.BasicLit {
107 . . . . . . . . . . . . . . . ValuePos: main.go:7:7
108 . . . . . . . . . . . . . . . Kind: INT
109 . . . . . . . . . . . . . . . Value: "2"
110 . . . . . . . . . . . . . . }
111 . . . . . . . . . . . . . }
112 . . . . . . . . . . . . }
113 . . . . . . . . . . . . 2: *ast.IfStmt {
114 . . . . . . . . . . . . . If: main.go:9:2
115 . . . . . . . . . . . . . Cond: *ast.BinaryExpr {
116 . . . . . . . . . . . . . . X: *ast.Ident {
117 . . . . . . . . . . . . . . . NamePos: main.go:9:5
118 . . . . . . . . . . . . . . . Name: "a"
119 . . . . . . . . . . . . . . . Obj: *(obj @ 74)
120 . . . . . . . . . . . . . . }
121 . . . . . . . . . . . . . . OpPos: main.go:9:7
122 . . . . . . . . . . . . . . Op: <
123 . . . . . . . . . . . . . . Y: *ast.BasicLit {
124 . . . . . . . . . . . . . . . ValuePos: main.go:9:9
125 . . . . . . . . . . . . . . . Kind: INT
126 . . . . . . . . . . . . . . . Value: "2"
127 . . . . . . . . . . . . . . }
128 . . . . . . . . . . . . . }
129 . . . . . . . . . . . . . Body: *ast.BlockStmt {
130 . . . . . . . . . . . . . . Lbrace: main.go:9:11
131 . . . . . . . . . . . . . . List: []ast.Stmt (len = 1) {
132 . . . . . . . . . . . . . . . 0: *ast.AssignStmt {
133 . . . . . . . . . . . . . . . . Lhs: []ast.Expr (len = 1) {
134 . . . . . . . . . . . . . . . . . 0: *ast.Ident {
135 . . . . . . . . . . . . . . . . . . NamePos: main.go:10:3
136 . . . . . . . . . . . . . . . . . . Name: "a"
137 . . . . . . . . . . . . . . . . . . Obj: *(obj @ 74)
138 . . . . . . . . . . . . . . . . . }
139 . . . . . . . . . . . . . . . . }
140 . . . . . . . . . . . . . . . . TokPos: main.go:10:5
141 . . . . . . . . . . . . . . . . Tok: =
142 . . . . . . . . . . . . . . . . Rhs: []ast.Expr (len = 1) {
143 . . . . . . . . . . . . . . . . . 0: *ast.Ident {
144 . . . . . . . . . . . . . . . . . . NamePos: main.go:10:7
145 . . . . . . . . . . . . . . . . . . Name: "b"
146 . . . . . . . . . . . . . . . . . . Obj: *(obj @ 96)
147 . . . . . . . . . . . . . . . . . }
148 . . . . . . . . . . . . . . . . }
149 . . . . . . . . . . . . . . . }
150 . . . . . . . . . . . . . . }
151 . . . . . . . . . . . . . . Rbrace: main.go:11:2
152 . . . . . . . . . . . . . }
153 . . . . . . . . . . . . }
154 . . . . . . . . . . . . 3: *ast.ReturnStmt {
155 . . . . . . . . . . . . . Return: main.go:13:2
156 . . . . . . . . . . . . . Results: []ast.Expr (len = 1) {
157 . . . . . . . . . . . . . . 0: *ast.BinaryExpr {
158 . . . . . . . . . . . . . . . X: *ast.Ident {
159 . . . . . . . . . . . . . . . . NamePos: main.go:13:9
160 . . . . . . . . . . . . . . . . Name: "a"
161 . . . . . . . . . . . . . . . . Obj: *(obj @ 74)
162 . . . . . . . . . . . . . . . }
163 . . . . . . . . . . . . . . . OpPos: main.go:13:11
164 . . . . . . . . . . . . . . . Op: +
165 . . . . . . . . . . . . . . . Y: *ast.Ident {
166 . . . . . . . . . . . . . . . . NamePos: main.go:13:13
167 . . . . . . . . . . . . . . . . Name: "b"
168 . . . . . . . . . . . . . . . . Obj: *(obj @ 96)
169 . . . . . . . . . . . . . . . }
170 . . . . . . . . . . . . . . }
171 . . . . . . . . . . . . . }
172 . . . . . . . . . . . . }
173 . . . . . . . . . . . }
174 . . . . . . . . . . . Rbrace: main.go:14:1
175 . . . . . . . . . . }
176 . . . . . . . . . }
177 . . . . . . . . }
178 . . . . . . . }
179 . . . . . . . Lparen: main.go:17:20
180 . . . . . . . Ellipsis: -
181 . . . . . . . Rparen: main.go:17:21
182 . . . . . . }
183 . . . . . }
184 . . . . . Ellipsis: -
185 . . . . . Rparen: main.go:17:22
186 . . . . }
187 . . . }
188 . . }
189 . . Rbrace: main.go:18:1
190 . }
191 }The corresponding code for this step: https://github.com/golang/go/tree/master/src/cmd/compile/internal/gc
SSA form is a way of representing a program's control flow in a more structured way, making it easier to perform optimizations. This step involves converting the AST into SSA form.
Unoptimized SSA is something like this.
In b1 block, v6 and v7 is the assignment of 1, 2 to a, b. And If v8 -> b3 b2 is to decide whether to jump program execution to either b2(if true) or b3(if false).
The corresponding code for this step; https://github.com/golang/go/tree/master/src/cmd/compile/internal/gc
This step involves performing various optimizations on the code in SSA form to improve its performance. These optimizations can range from simple dead code elimination to more complex techniques like loop unrolling.
Optimized SSA is something like this.
In the optimized version, the unnecessary memory allocation, initialization, and copying operations have been removed. Instead, the local variables have been directly referenced by their names. Also, the function arguments have been directly used instead of being stored in local variables. Furthermore, the unnecessary variables v9 and v11 has been removed, and the variables v10 and v12 have been replaced with constant values.
The corresponding code for this step; https://github.com/golang/go/tree/master/src/cmd/compile/internal/ssa
The SSA that is architecture-independent is converted into assembly that is architecture-dependent.
go tool objdump -S main.o > main.objdump
TEXT "".myFunc(SB) gofile../Users/iamstgt/~/How_gc_Works/main.go
return a + b
0x13cd b804000000 MOVL $0x4, AX
0x13d2 c3 RET
TEXT "".main(SB) gofile../Users/iamstgt/~/How_gc_Works/main.go
func main() {
0x13d3 493b6610 CMPQ 0x10(R14), SP [2:2]R_USEIFACE:type.int [2:2]R_USEIFACE:type.*os.File
0x13d7 765b JBE 0x1434
0x13d9 4883ec40 SUBQ $0x40, SP
0x13dd 48896c2438 MOVQ BP, 0x38(SP)
0x13e2 488d6c2438 LEAQ 0x38(SP), BP
fmt.Println(myFunc())
0x13e7 440f117c2428 MOVUPS X15, 0x28(SP)
0x13ed b804000000 MOVL $0x4, AX
0x13f2 90 NOPL
0x13f3 e800000000 CALL 0x13f8 [1:5]R_CALL:runtime.convT64<1>
0x13f8 488d0d00000000 LEAQ 0(IP), CX [3:7]R_PCREL:type.int
0x13ff 48894c2428 MOVQ CX, 0x28(SP)
0x1404 4889442430 MOVQ AX, 0x30(SP)
return Fprintln(os.Stdout, a...)
0x1409 488b1d00000000 MOVQ 0(IP), BX [3:7]R_PCREL:os.Stdout
0x1410 488d0500000000 LEAQ 0(IP), AX [3:7]R_PCREL:go.itab.*os.File,io.Writer
0x1417 488d4c2428 LEAQ 0x28(SP), CX
0x141c bf01000000 MOVL $0x1, DI
0x1421 4889fe MOVQ DI, SI
0x1424 e800000000 CALL 0x1429 [1:5]R_CALL:fmt.Fprintln
}
0x1429 488b6c2438 MOVQ 0x38(SP), BP
0x142e 4883c440 ADDQ $0x40, SP
0x1432 90 NOPL
0x1433 c3 RET
func main() {
0x1434 e800000000 CALL 0x1439 [1:5]R_CALL:runtime.morestack_noctxt
0x1439 eb98 JMP "".main(SB)
This invovation will generate the main.objdump. Objdump command disassembles executable files and it printes a disassembly of all text symbols (code) in the binary.
Finally, the assembly is compiled into machine code that can be executed on a target CPU architecture.
The corresponding code for this step (for the x86 architecture); https://github.com/golang/go/tree/master/src/cmd/compile/internal/x86
There's one more step to generate the ready-to-use output. This is not a role of gc, but rather the linker that combines multiple object files into one and generates an executable binary that is executed by CPU.