Golang：使切片性能
为什么这些基准测试结果如此不同？
func Benchmark1(b *testing.B) {
for n := 0; n < b.N; n++ {
_ = make([]byte, 8)
}
}
func Benchmark2(b *testing.B) {
length := 1
for n := 0; n < b.N; n++ {
_ = make([]byte, 7+length)
}
}
基准测试结果：
Benchmark1-8 500000000 3.37 ns/op
Benchmark2-8 30000000 50.6 ns/op
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常量表达式8在编译时计算。在make分配上够程堆栈（便宜）。变量表达式7 + length在运行时计算。在make被分配于程序堆中（昂贵的）。如果make堆栈分配的大小太大（例如， constant(64*1024)和 variable (64*1024-1)+length），则两种分配都在堆上进行，并且基准测试时间相同。$ go tool compile -m a_test.goa_test.go:5: Benchmark1 b does not escapea_test.go:7: Benchmark1 make([]byte, 8) does not escapea_test.go:14: make([]byte, 7 + length) escapes to heapa_test.go:11: Benchmark2 b does not escape$ a_test.go:package aimport "testing"func Benchmark1(b *testing.B) { for n := 0; n < b.N; n++ { _ = make([]byte, 8) }}func Benchmark2(b *testing.B) { length := 1 for n := 0; n < b.N; n++ { _ = make([]byte, 7+length) }}Go 伪汇编程序：$ go tool compile -S a_test.go基准 1："".Benchmark1 t=1 size=112 value=0 args=0x8 locals=0x20 0x0000 00000 (a_test.go:5) TEXT "".Benchmark1(SB), $32-8 0x0000 00000 (a_test.go:5) SUBQ $32, SP 0x0004 00004 (a_test.go:5) MOVQ "".b+40(FP), CX 0x0009 00009 (a_test.go:5) FUNCDATA $0, gclocals·87d20ce1b58390b294df80b886db78bf(SB) 0x0009 00009 (a_test.go:5) FUNCDATA $1, gclocals·790e5cc5051fc0affc980ade09e929ec(SB) 0x0009 00009 (a_test.go:6) MOVQ $0, AX 0x000b 00011 (a_test.go:6) NOP 0x000b 00011 (a_test.go:6) MOVQ 112(CX), BX 0x000f 00015 (a_test.go:6) CMPQ BX, AX 0x0012 00018 (a_test.go:6) JLE $0, 98 0x0014 00020 (a_test.go:7) MOVQ $0, BX 0x0016 00022 (a_test.go:7) MOVB BL, "".autotmp_0001(SP) 0x0019 00025 (a_test.go:7) MOVB BL, "".autotmp_0001+1(SP) 0x001d 00029 (a_test.go:7) MOVB BL, "".autotmp_0001+2(SP) 0x0021 00033 (a_test.go:7) MOVB BL, "".autotmp_0001+3(SP) 0x0025 00037 (a_test.go:7) MOVB BL, "".autotmp_0001+4(SP) 0x0029 00041 (a_test.go:7) MOVB BL, "".autotmp_0001+5(SP) 0x002d 00045 (a_test.go:7) MOVB BL, "".autotmp_0001+6(SP) 0x0031 00049 (a_test.go:7) MOVB BL, "".autotmp_0001+7(SP) 0x0035 00053 (a_test.go:7) LEAQ "".autotmp_0001(SP), BX 0x0039 00057 (a_test.go:7) CMPQ BX, $0 0x003d 00061 (a_test.go:7) JEQ $1, 103 0x003f 00063 (a_test.go:7) MOVQ $8, "".autotmp_0002+16(SP) 0x0048 00072 (a_test.go:7) MOVQ $8, "".autotmp_0002+24(SP) 0x0051 00081 (a_test.go:7) MOVQ BX, "".autotmp_0002+8(SP) 0x0056 00086 (a_test.go:6) INCQ AX 0x0059 00089 (a_test.go:6) NOP 0x0059 00089 (a_test.go:6) MOVQ 112(CX), BX 0x005d 00093 (a_test.go:6) CMPQ BX, AX 0x0060 00096 (a_test.go:6) JGT $0, 20 0x0062 00098 (a_test.go:9) ADDQ $32, SP 0x0066 00102 (a_test.go:9) RET 0x0067 00103 (a_test.go:7) MOVL AX, (BX) 0x0069 00105 (a_test.go:7) JMP 63Benchmark2:"".Benchmark2 t=1 size=144 value=0 args=0x8 locals=0x58 0x0000 00000 (a_test.go:11) TEXT "".Benchmark2(SB), $88-8 0x0000 00000 (a_test.go:11) MOVQ (TLS), CX 0x0009 00009 (a_test.go:11) CMPQ SP, 16(CX) 0x000d 00013 (a_test.go:11) JLS 129 0x000f 00015 (a_test.go:11) SUBQ $88, SP 0x0013 00019 (a_test.go:11) FUNCDATA $0, gclocals·87d20ce1b58390b294df80b886db78bf(SB) 0x0013 00019 (a_test.go:11) FUNCDATA $1, gclocals·790e5cc5051fc0affc980ade09e929ec(SB) 0x0013 00019 (a_test.go:12) MOVQ $1, "".length+56(SP) 0x001c 00028 (a_test.go:13) MOVQ $0, AX 0x001e 00030 (a_test.go:13) MOVQ "".b+96(FP), BP 0x0023 00035 (a_test.go:13) NOP 0x0023 00035 (a_test.go:13) MOVQ 112(BP), BX 0x0027 00039 (a_test.go:13) MOVQ AX, "".n+48(SP) 0x002c 00044 (a_test.go:13) CMPQ BX, AX 0x002f 00047 (a_test.go:13) JLE $0, 124 0x0031 00049 (a_test.go:14) MOVQ "".length+56(SP), AX 0x0036 00054 (a_test.go:14) ADDQ $7, AX 0x003a 00058 (a_test.go:14) LEAQ type.[]uint8(SB), BX 0x0041 00065 (a_test.go:14) MOVQ BX, (SP) 0x0045 00069 (a_test.go:14) MOVQ AX, 8(SP) 0x004a 00074 (a_test.go:14) MOVQ AX, 16(SP) 0x004f 00079 (a_test.go:14) PCDATA $0, $0 0x004f 00079 (a_test.go:14) CALL runtime.makeslice(SB) 0x0054 00084 (a_test.go:14) MOVQ 24(SP), BX 0x0059 00089 (a_test.go:14) MOVQ BX, "".autotmp_0005+64(SP) 0x005e 00094 (a_test.go:14) MOVQ 32(SP), BX 0x0063 00099 (a_test.go:14) MOVQ BX, "".autotmp_0005+72(SP) 0x0068 00104 (a_test.go:14) MOVQ 40(SP), BX 0x006d 00109 (a_test.go:14) MOVQ BX, "".autotmp_0005+80(SP) 0x0072 00114 (a_test.go:13) MOVQ "".n+48(SP), AX 0x0077 00119 (a_test.go:13) INCQ AX 0x007a 00122 (a_test.go:13) NOP 0x007a 00122 (a_test.go:13) JMP 30 0x007c 00124 (a_test.go:16) ADDQ $88, SP 0x0080 00128 (a_test.go:16) RET 0x0081 00129 (a_test.go:11) CALL runtime.morestack_noctxt(SB) 0x0086 00134 (a_test.go:11) JMP 0
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