Most definitely interesting.
https://golang.org/src/cmd/compile/internal/ssa/rewriteAMD64.go#L54447
Am I correct to assume that DUFFCOPY is and optimization for MOVE?
If so, i wonder why the first example did not generate any MOV command at
all.
On Friday, April 24, 2020 at 4:25:51 PM UTC+2, Tamás Gulácsi wrote:
>
> 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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