On Fri, Apr 24, 2020 at 1:32 PM Yulrizka <yulri...@gmail.com> wrote:
>
> 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.
In the first case, there was a move, but it was discarded in the SSA
pass because it was dead.
I would guess that the SSA pass is not smart enough to see that the
call to the builtin function is dead, so it doesn't remove it.
If I'm right, it's basically a missing optimization in the compiler.
But it doesn't seem like a very important one; not many people write
range loops that name the value but don't use it.
Ian
> 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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