On Mon, Nov 29, 2021 at 10:16 AM Oren Ish-Shalom <tuna.is.good.for....@gmail.com> wrote: > > I have a question in SO regarding parameter passing in ELF binaries: > > https://stackoverflow.com/questions/70150497/go-binaries-parameter-passing > > One of the comments referenced me to this mailing list, so I'm re-posting > here: > > I’m trying to understand parameter passing in go ELF binaries > > mov %rdi,0x8(%rsp) <----- first parameter (?) > mov %r8,0x10(%rsp) <----- second > mov %r9,0x18(%rsp) <----- third > mov %r10,0x20(%rsp) <----- fourth > mov %rcx,0x28(%rsp) <----- fifth > sub %edx,%ebx > movslq %ebx,%rcx > mov %rcx,0x30(%rsp) <----- sixth > callq 5aaba0 <math/big.nat.shl> <----- call to native golang func > > It seems that parameters are passed on stack, but when I look at the function > here (line 681): > > // z = x << s > func (z nat) shl(x nat, s uint) nat { ... } > > The number of parameter is just 2, and in the ELF it looks like 6 (?)
First, let me say that you asked about ELF binaries but the parameter passing used by the gc compiler doesn't have anything to do with whether the output is ELF. It only depends on the GOARCH and GOOS values. Second, in 1.17 for amd64/linux, amd64/windows, and amd64/darwin, the compiler changed the way that parameter passing works. Before that release it passed all arguments on the stack for those platforms. In 1.17 it started passing the first several parameters in registers. For an introduction to that see https://go.googlesource.com/proposal/+/refs/heads/master/design/40724-register-calling.md. Third, I think you are assuming that every parameter is passed in a single register or stack location. That is not the case. Slices are a struct of three values, where each value requires a register/stack location. Fourth, when calling a method, the receiver value is passed as an ordinary argument. So the method func (z nat) shl(x nat, s uint) nat { ... } has a receiver and two arguments. The nat values are slices (nat is defined as []Word), which are three values each. So calling this method requires passing seven values in registers or the stack. Hope this helps. Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcWv_veYae47jY4%3DUbuq%3D_HDrs6vm2%2BnBbLYaSAStTTX8w%40mail.gmail.com.
