On Tue, Sep 26, 2023 at 11:06 PM j2gg0s <feys1...@gmail.com> wrote:
>
> Related go code:
> 22 //go:noinline
> 23 func add(a, b int) int {
> 24 defer func() {
> 25 fmt.Println(3)
> 26 }()
> 27 return a + b
> 28 }
>
> Build by: GOOS=linux GOARCH=amd64 go21 build main.go
>
> Disassembler by: objdump -D -S main
> Assembly code:
> 179228 000000000047ae20 <main.add>:
> 179229 ; func add(a, b int) int {
> 179230 47ae20: 49 3b 66 10 cmpq 16(%r14), %rsp
> 179231 47ae24: 76 5b jbe 0x47ae81
> <main.add+0x61>
> 179232 47ae26: 55 pushq %rbp
> 179233 47ae27: 48 89 e5 movq %rsp, %rbp
> 179234 47ae2a: 48 83 ec 18 subq $24, %rsp
> 179235 47ae2e: 66 44 0f d6 7c 24 10 movq %xmm15, 16(%rsp)
> 179236 47ae35: c6 44 24 07 00 movb $0, 7(%rsp)
> 179237 47ae3a: 48 c7 44 24 08 00 00 00 00 movq $0, 8(%rsp)
> 179238 ; return a + b
> 179239 47ae43: 48 01 d8 addq %rbx, %rax
> 179240 ; defer func() {
> 179241 47ae46: 48 8d 0d 9b 06 02 00 leaq 132763(%rip), %rcx
> # 0x49b4e8 <go:func.*+0x220>
> 179242 47ae4d: 48 89 4c 24 10 movq %rcx, 16(%rsp)
> 179243 47ae52: c6 44 24 07 01 movb $1, 7(%rsp)
> 179244 ; return a + b
> 179245 47ae57: 48 89 44 24 08 movq %rax, 8(%rsp)
> 179246 47ae5c: c6 44 24 07 00 movb $0, 7(%rsp)
> 179247 47ae61: e8 7a 00 00 00 callq 0x47aee0
> <main.add.func1>
> 179248 47ae66: 48 8b 44 24 08 movq 8(%rsp), %rax
> 179249 47ae6b: 48 83 c4 18 addq $24, %rsp
> 179250 47ae6f: 5d popq %rbp
> 179251 47ae70: c3 retq
>
>
> According to my understanding:
> 1. `179241 47ae46: 48 8d 0d 9b 06 02 00 leaq 132763(%rip),
> %rcx # 0x49b4e8 <go:func.*+0x220>` want to load address of
> main.add.func1 to register rcx. Is this understanding correct?
Sort of. 0x49b4e8 is the address of the function descriptor for
main.add.func1. If you look at address 0x49b4e8 you will see that it
contains the value 0x47aee0, which is the address of the code for
main.add.func1. For function descriptors see
https://go.dev/s/go11func.
> 2.1 rip 0x47ae4d, 0x47ae4d + 132763 = 0x49b4e8,
Yes, as the disassembler comment says.
> 2.2 main.add.func1's address is 0x47b0c0
> objdump -D -S main | grep "main.add.func1>:" -A 31
> 000000000047aee0 <main.add.func1>:
Well, 0x47aee0, not 0x47b0c0
> 2.3 i just found 0x49b4e8 in rodata
> 2.4 So, What is stored at 0x49b4e8?
The code for main.add.func1.
Ian
> 在2023年9月27日星期三 UTC+8 12:44:10<Ian Lance Taylor> 写道：
>>
>> On Tue, Sep 26, 2023 at 9:10 PM j2gg0s <feys...@gmail.com> wrote:
>> >
>> > Already read it, I actually started here.
>> >
>> > What I can't understand is x64 rip-relative address.
>> >
>> > leaq 132795(%rip), %rcx # 0x49b4e8 <go:func.*+0x220>
>> >
>> > why 132795, and what is loaded into rcx
>>
>> It's a reference to another address in the program. The comment from
>> the disassembler tells you that it refers to the address go:func.* +
>> 0x220. The linker constructs the offset 132795.
>>
>> Ian
>>
>>
>>
>> > 在2023年9月27日星期三 UTC+8 09:38:17<Ian Lance Taylor> 写道：
>> >>
>> >> On Tue, Sep 26, 2023 at 7:43 AM j2gg0s <feys...@gmail.com> wrote:
>> >> >
>> >> > How to understand assmbly code ` 47ae26: 48 8d 0d bb 06 02 00 leaq
>> >> > 132795(%rip), %rcx # 0x49b4e8 <go:func.*+0x220>`
>> >>
>> >> The best place to start is
>> >>
>> >> https://go.googlesource.com/proposal/+/refs/heads/master/design/34481-opencoded-defers.md
>> >>
>> >> Ian
>> >
>> > --
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>
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