How to understand runtime.gogo?

[j2gg0s]

As a newbie of golang's assembly, i cant understand why we MOVQ DX CX twice in runtime.gogo. WHY?

Source code:

```

   255  // func gogo(buf *gobuf)
   256  // restore state from Gobuf; longjmp
   257  TEXT runtime·gogo(SB), NOSPLIT, $0-8
   258          MOVQ    buf+0(FP), BX           // gobuf
   259          MOVQ    gobuf_g(BX), DX
   260          MOVQ    0(DX), CX               // make sure g != nil
   261          JMP     gogo<>(SB)
   262
   263  TEXT gogo<>(SB), NOSPLIT, $0
   264          get_tls(CX)
   265          MOVQ    DX, g(CX)
   266          MOVQ    DX, R14         // set the g register
   267          MOVQ    gobuf_sp(BX), SP        // restore SP
   268          MOVQ    gobuf_ret(BX), AX
   269          MOVQ    gobuf_ctxt(BX), DX
   270          MOVQ    gobuf_bp(BX), BP
   271          MOVQ    $0, gobuf_sp(BX)        // clear to help garbage collector
   272          MOVQ    $0, gobuf_ret(BX)
   273          MOVQ    $0, gobuf_ctxt(BX)
   274          MOVQ    $0, gobuf_bp(BX)
   275          MOVQ    gobuf_pc(BX), BX
   276          JMP     BX

```

Link: https://github.com/golang/go/blob/go1.17.13/src/runtime/asm_amd64.s#L255

[Ian Lance Taylor]

On Mon, Jan 16, 2023 at 8:43 AM j2gg0s <feys...@gmail.com> wrote:
>
> As a newbie of golang's assembly, i cant understand why we MOVQ DX CX twice in runtime.gogo. WHY?

I don't see any MOVQ DX, CX instructions here. Can you clarify by
saying exactly which instructions you are asking about?

Note that MOVQ 0(DX), CX treats DX as a memory address, and moves the
8-byte value at that address into CX. And MOVQ DX, g(CX) treats CX as
a memory address, and moves the value in DX into the address as "g"
bytes offset from the address in CX.

Ian

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[j2gg0s]

Thanks, solved my problem