2020. április 24., péntek 14:26:08 UTC+2 időpontban Yulrizka a következőt
írta:
>
> Hello,
>
>
> I was playing around with this code
>
> main_var.go
> package main
>
> func main() {
> const size = 1000000
>
> slice := make([]SomeStruct, size)
> for _, s := range slice { // line 7
> _ = s
> }
>
> }
>
>
> type_small.go
> package main
>
> type SomeStruct struct {
> ID0 int64
> ID1 int64
> ID2 int64
> ID3 int64
> ID4 int64
> ID5 int64
> ID6 int64
> ID7 int64
> ID8 int64
> }
>
>
> I noted that if i added anoteher 64 bit int64 `ID9` (total of 10 * 8 byte
> = 80 byte) integer to the struct, the for-loop becomes slower.
> And if I compare the assembly, it added instruction to copy the element
>
> // with 9 int64 (72 bytes)
> 0x001d 00029 (main_var.go:6) LEAQ type."".SomeStruct(SB), AX
> 0x0024 00036 (main_var.go:6) MOVQ AX, (SP)
> 0x0028 00040 (main_var.go:6) MOVQ $1000000, 8(SP)
> 0x0031 00049 (main_var.go:6) MOVQ $1000000, 16(SP)
> 0x003a 00058 (main_var.go:6) CALL runtime.makeslice(SB)
> 0x003f 00063 (main_var.go:6) XORL AX, AX
> 0x0041 00065 (main_var.go:7) INCQ AX
> 0x0044 00068 (main_var.go:7) CMPQ AX, $1000000
> 0x004a 00074 (main_var.go:7) JLT 65
> 0x004c 00076 (main_var.go:7) MOVQ 32(SP), BP
> 0x0051 00081 (main_var.go:7) ADDQ $40, SP
> 0x0055 00085 (main_var.go:7) RET
> 0x0056 00086 (main_var.go:7) NOP
> 0x0056 00086 (main_var.go:3) CALL runtime.morestack_noctxt(SB)
> 0x005b 00091 (main_var.go:3) JMP 0
>
> // with 10 int64 (80 bytes), it added DUFFCOPY instruction
> 0x001d 00029 (main_var.go:6) LEAQ type."".SomeStruct(SB), AX
> 0x0024 00036 (main_var.go:6) MOVQ AX, (SP)
> 0x0028 00040 (main_var.go:6) MOVQ $1000000, 8(SP)
> 0x0031 00049 (main_var.go:6) MOVQ $1000000, 16(SP)
> 0x003a 00058 (main_var.go:6) CALL runtime.makeslice(SB)
> 0x003f 00063 (main_var.go:6) MOVQ 24(SP), AX
> 0x0044 00068 (main_var.go:6) XORL CX, CX
> 0x0046 00070 (main_var.go:7) JMP 76
> 0x0048 00072 (main_var.go:7) ADDQ $80, AX
> 0x004c 00076 (main_var.go:7) LEAQ ""..autotmp_7+32(SP), DI
> 0x0051 00081 (main_var.go:7) MOVQ AX, SI
> 0x0054 00084 (main_var.go:7) DUFFCOPY $826 # <-- copy the
> element
> 0x0067 00103 (main_var.go:7) INCQ CX
> 0x006a 00106 (main_var.go:7) CMPQ CX, $1000000
> 0x0071 00113 (main_var.go:7) JLT 72
> 0x0073 00115 (main_var.go:7) MOVQ 112(SP), BP
> 0x0078 00120 (main_var.go:7) ADDQ $120, SP
> 0x007c 00124 (main_var.go:7) RET
> 0x007d 00125 (main_var.go:7) NOP
> 0x007d 00125 (main_var.go:3) CALL runtime.morestack_noctxt(SB)
> 0x0082 00130 (main_var.go:3) JMP 0
>
>
> I am just curious to why the bahavior is different on larger struct (> 80
> bytes) even though in both cases the element of the slice is not being use.
>
> Thank you
>
After grepping the sources for DUFFCOPY,
./cmd/compile/internal/ssa/rewriteAMD64.go suggests that it is a DUFFCOPY
before SSA rewrites it to something faster - and this is a size-depending
operation.
But I may be totally wrong...
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