嵌套循环性能
算法 刘宇帅 5年前 阅读量: 1192
下面有两个函数，猜一下哪一个执行的快
func nestFor1() {
a := 0
for i := 0; i < 10000000; i++ {
for j := 0; j < 2; j++ {
a = a + i
a = a + j
}
}
}
func nestFor2() {
a := 0
for i := 0; i < 2; i++ {
for j := 0; j < 10000000; j++ {
a = a + i
a = a + j
}
}
}从 golang 语言层面看这两个函数运行起来怎么都是一样的。
我们跑下 Benchmark
func BenchmarkNestFor1(b *testing.B) {
for i := 0; i < b.N; i++ {
nestFor1()
}
}
func BenchmarkNestFor2(b *testing.B) {
for i := 0; i < b.N; i++ {
nestFor2()
}
}跑测试
> $ go test -bench=NestFor [20:38:19]
goos: darwin
goarch: amd64
pkg: github.com/yushuailiu/go-algorithm/golang
BenchmarkNestFor1-4 100 10355409 ns/op
BenchmarkNestFor2-4 200 6776159 ns/op
PASS
ok github.com/yushuailiu/go-algorithm/golang 3.127s我们可以看到 nestFor1 比 nestFor2 慢了快一半了。
go 语言最终是要编译成汇编进而转化成机器语言，而性能就体现在指令少，而上面问题的根本原因是 nestFor1 比 nestFor2 执行的指令少，首先两者的内层循环跳转次数都是 10000000*2 而 nestFor1 的外层跳转次数是 10000000 nestFor2 的外层跳转次数是 2，同理 nestFor2 的外层循环中的比较次数和递增次数都少于 nestFor1，所以结论就是把循环次数较多的循环放在能不可以提升性能，虽然这是微乎其微的但是养成这样的习惯也挺好的。
感兴趣的同学可以看下两个函数汇编指令各个指令执行的次数：
例子源码
生成汇编源码指令
go tool compile -N -S nest_for_test.gonestFor1 汇编 ()
.nestFor1 STEXT nosplit size=135 args=0x0 locals=0x20
0x0000 00000 (nest_for_test.go:5) TEXT "".nestFor1(SB), NOSPLIT, $32-0
0x0000 00000 (nest_for_test.go:5) SUBQ $32, SP
0x0004 00004 (nest_for_test.go:5) MOVQ BP, 24(SP)
0x0009 00009 (nest_for_test.go:5) LEAQ 24(SP), BP
0x000e 00014 (nest_for_test.go:5) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:5) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:5) FUNCDATA $3, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:6) PCDATA $2, $0
0x000e 00014 (nest_for_test.go:6) PCDATA $0, $0
0x000e 00014 (nest_for_test.go:6) MOVQ $0, "".a+16(SP)
0x0017 00023 (nest_for_test.go:7) MOVQ $0, "".i+8(SP)
0x0020 00032 (nest_for_test.go:7) JMP 34
0x0022 00034 (nest_for_test.go:7) CMPQ "".i+8(SP), $10000000
0x002b 00043 (nest_for_test.go:7) JLT 47
0x002d 00045 (nest_for_test.go:7) JMP 125
0x002f 00047 (nest_for_test.go:8) MOVQ $0, "".j(SP)
0x0037 00055 (nest_for_test.go:8) JMP 57
0x0039 00057 (nest_for_test.go:8) CMPQ "".j(SP), $2
0x003e 00062 (nest_for_test.go:8) JLT 66
0x0040 00064 (nest_for_test.go:8) JMP 108
0x0042 00066 (nest_for_test.go:9) MOVQ "".a+16(SP), AX
0x0047 00071 (nest_for_test.go:9) ADDQ "".i+8(SP), AX
0x004c 00076 (nest_for_test.go:9) MOVQ AX, "".a+16(SP)
0x0051 00081 (nest_for_test.go:10) MOVQ "".j(SP), CX
0x0055 00085 (nest_for_test.go:10) ADDQ CX, AX
0x0058 00088 (nest_for_test.go:10) MOVQ AX, "".a+16(SP)
0x005d 00093 (nest_for_test.go:10) JMP 95
0x005f 00095 (nest_for_test.go:8) MOVQ "".j(SP), AX
0x0063 00099 (nest_for_test.go:8) INCQ AX
0x0066 00102 (nest_for_test.go:8) MOVQ AX, "".j(SP)
0x006a 00106 (nest_for_test.go:8) JMP 57
0x006c 00108 (nest_for_test.go:7) PCDATA $2, $-2
0x006c 00108 (nest_for_test.go:7) PCDATA $0, $-2
0x006c 00108 (nest_for_test.go:7) JMP 110
0x006e 00110 (nest_for_test.go:7) PCDATA $2, $0
0x006e 00110 (nest_for_test.go:7) PCDATA $0, $0
0x006e 00110 (nest_for_test.go:7) MOVQ "".i+8(SP), AX
0x0073 00115 (nest_for_test.go:7) INCQ AX
0x0076 00118 (nest_for_test.go:7) MOVQ AX, "".i+8(SP)
0x007b 00123 (nest_for_test.go:7) JMP 34
0x007d 00125 (<unknown line number>) PCDATA $2, $-2
0x007d 00125 (<unknown line number>) PCDATA $0, $-2
0x007d 00125 (<unknown line number>) MOVQ 24(SP), BP
0x0082 00130 (<unknown line number>) ADDQ $32, SP
0x0086 00134 (<unknown line number>) RET
0x0000 48 83 ec 20 48 89 6c 24 18 48 8d 6c 24 18 48 c7 H.. H.l$.H.l$.H.
0x0010 44 24 10 00 00 00 00 48 c7 44 24 08 00 00 00 00 D$.....H.D$.....
0x0020 eb 00 48 81 7c 24 08 80 96 98 00 7c 02 eb 4e 48 ..H.|$.....|..NH
0x0030 c7 04 24 00 00 00 00 eb 00 48 83 3c 24 02 7c 02 ..$......H.<$.|.
0x0040 eb 2a 48 8b 44 24 10 48 03 44 24 08 48 89 44 24 .*H.D$.H.D$.H.D$
0x0050 10 48 8b 0c 24 48 01 c8 48 89 44 24 10 eb 00 48 .H..$H..H.D$...H
0x0060 8b 04 24 48 ff c0 48 89 04 24 eb cd eb 00 48 8b ..$H..H..$....H.
0x0070 44 24 08 48 ff c0 48 89 44 24 08 eb a5 48 8b 6c D$.H..H.D$...H.l
0x0080 24 18 48 83 c4 20 c3 $.H.. .nestFor2 汇编
.nestFor2 STEXT nosplit size=135 args=0x0 locals=0x20
0x0000 00000 (nest_for_test.go:15) TEXT "".nestFor2(SB), NOSPLIT, $32-0
0x0000 00000 (nest_for_test.go:15) SUBQ $32, SP
0x0004 00004 (nest_for_test.go:15) MOVQ BP, 24(SP)
0x0009 00009 (nest_for_test.go:15) LEAQ 24(SP), BP
0x000e 00014 (nest_for_test.go:15) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:15) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:15) FUNCDATA $3, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x000e 00014 (nest_for_test.go:16) PCDATA $2, $0
0x000e 00014 (nest_for_test.go:16) PCDATA $0, $0
0x000e 00014 (nest_for_test.go:16) MOVQ $0, "".a+16(SP)
0x0017 00023 (nest_for_test.go:17) MOVQ $0, "".i+8(SP)
0x0020 00032 (nest_for_test.go:17) JMP 34
0x0022 00034 (nest_for_test.go:17) CMPQ "".i+8(SP), $2
0x0028 00040 (nest_for_test.go:17) JLT 44
0x002a 00042 (nest_for_test.go:17) JMP 125
0x002c 00044 (nest_for_test.go:18) MOVQ $0, "".j(SP)
0x0034 00052 (nest_for_test.go:18) JMP 54
0x0036 00054 (nest_for_test.go:18) CMPQ "".j(SP), $10000000
0x003e 00062 (nest_for_test.go:18) JLT 66
0x0040 00064 (nest_for_test.go:18) JMP 108
0x0042 00066 (nest_for_test.go:19) MOVQ "".a+16(SP), AX
0x0047 00071 (nest_for_test.go:19) ADDQ "".i+8(SP), AX
0x004c 00076 (nest_for_test.go:19) MOVQ AX, "".a+16(SP)
0x0051 00081 (nest_for_test.go:20) MOVQ "".j(SP), CX
0x0055 00085 (nest_for_test.go:20) ADDQ CX, AX
0x0058 00088 (nest_for_test.go:20) MOVQ AX, "".a+16(SP)
0x005d 00093 (nest_for_test.go:20) JMP 95
0x005f 00095 (nest_for_test.go:18) MOVQ "".j(SP), AX
0x0063 00099 (nest_for_test.go:18) INCQ AX
0x0066 00102 (nest_for_test.go:18) MOVQ AX, "".j(SP)
0x006a 00106 (nest_for_test.go:18) JMP 54
0x006c 00108 (nest_for_test.go:17) PCDATA $2, $-2
0x006c 00108 (nest_for_test.go:17) PCDATA $0, $-2
0x006c 00108 (nest_for_test.go:17) JMP 110
0x006e 00110 (nest_for_test.go:17) PCDATA $2, $0
0x006e 00110 (nest_for_test.go:17) PCDATA $0, $0
0x006e 00110 (nest_for_test.go:17) MOVQ "".i+8(SP), AX
0x0073 00115 (nest_for_test.go:17) INCQ AX
0x0076 00118 (nest_for_test.go:17) MOVQ AX, "".i+8(SP)
0x007b 00123 (nest_for_test.go:17) JMP 34
0x007d 00125 (<unknown line number>) PCDATA $2, $-2
0x007d 00125 (<unknown line number>) PCDATA $0, $-2
0x007d 00125 (<unknown line number>) MOVQ 24(SP), BP
0x0082 00130 (<unknown line number>) ADDQ $32, SP
0x0086 00134 (<unknown line number>) RET
0x0000 48 83 ec 20 48 89 6c 24 18 48 8d 6c 24 18 48 c7 H.. H.l$.H.l$.H.
0x0010 44 24 10 00 00 00 00 48 c7 44 24 08 00 00 00 00 D$.....H.D$.....
0x0020 eb 00 48 83 7c 24 08 02 7c 02 eb 51 48 c7 04 24 ..H.|$..|..QH..$
0x0030 00 00 00 00 eb 00 48 81 3c 24 80 96 98 00 7c 02 ......H.<$....|.
0x0040 eb 2a 48 8b 44 24 10 48 03 44 24 08 48 89 44 24 .*H.D$.H.D$.H.D$
0x0050 10 48 8b 0c 24 48 01 c8 48 89 44 24 10 eb 00 48 .H..$H..H.D$...H
0x0060 8b 04 24 48 ff c0 48 89 04 24 eb ca eb 00 48 8b ..$H..H..$....H.
0x0070 44 24 08 48 ff c0 48 89 44 24 08 eb a5 48 8b 6c D$.H..H.D$...H.l
0x0080 24 18 48 83 c4 20 c3 $.H.. .