Re: [go-nuts] linux/arm struct alignment seems wrong

['Dan Kortschak' via golang-nuts]

On Wed, 2020-09-09 at 13:21 +0200, Jan Mercl wrote:
> 
> 
> On Wed, Sep 9, 2020 at 1:09 PM 'Dan Kortschak' via golang-nuts <
> golang-nuts@googlegroups.com> wrote:
> 
> > What does cgo -godefs give you? On my amd64 and arm64 I get this:
> >
> > ```
> > ~/cznic $ cat main.go
> > package main
> >
> > /*
> > struct s {
> >         long long i;
> > } x;
> > */
> > import "C"
> >
> > type S struct {
> >         i int64
> > }
> >
> > type C_s C.struct_s
> >
> > ~/cznic $ go tool cgo -godefs main.go
> > // Code generated by cmd/cgo -godefs; DO NOT EDIT.
> > // cgo -godefs main.go
> >
> > package main
> >
> > type S struct {
> >         i int64
> > }
> >
> > type C_s struct {
> >         I int64
> > }
> > ```
> 
> amd64
> =====================================================================
> ====
> 
> jnml@e5-1650:~/tmp$ go version
> go version go1.15.1 linux/amd64
> jnml@e5-1650:~/tmp$ cat main.go 
> package main
> 
> /*
> struct s {
>         long long i;
> } x;
> */
> import "C"
> 
> type S struct {
>         i int64
> }
> 
> type C_s C.struct_s
> jnml@e5-1650:~/tmp$ go tool cgo -godefs main.go
> // Code generated by cmd/cgo -godefs; DO NOT EDIT.
> // cgo -godefs main.go
> 
> package main
> 
> type S struct {
>         i int64
> }
> 
> type C_s struct {
>         I int64
> }
> jnml@e5-1650:~/tmp$ 
> 
> arm
> =====================================================================
> ======
> 
> pi@raspberrypi:~/src/tmp.tmp $ go version
> go version go1.15.1 linux/arm
> pi@raspberrypi:~/src/tmp.tmp $ cat main.go
> package main
> 
> /*
> struct s {
>         long long i;
> } x;
> */
> import "C"
> 
> type S struct {
>         i int64
> }
> 
> type C_s C.struct_s
> pi@raspberrypi:~/src/tmp.tmp $ go tool cgo -godefs main.go
> // Code generated by cmd/cgo -godefs; DO NOT EDIT.
> // cgo -godefs main.go
> 
> package main
> 
> type S struct {
>         i int64
> }
> 
> type C_s struct {
>         I int64
> }
> pi@raspberrypi:~/src/tmp.tmp $ 
> 
> No difference between the outputs AFAICT. Seems good to me.
> 
> 

I think it comes down to these lines in src/cmd/internal/sys/arch.go
[1]

```
var ArchARM = &Arch{
        Name:      "arm",
        Family:    ARM,
        ByteOrder: binary.LittleEndian,
        PtrSize:   4,
        RegSize:   4,
        MinLC:     4,
}

var ArchARM64 = &Arch{
        Name:      "arm64",
        Family:    ARM64,
        ByteOrder: binary.LittleEndian,
        PtrSize:   8,
        RegSize:   8,
        MinLC:     4,
}
```
this line in src/cmd/compile/internal/gc/main.go [2]
```
Widthreg = thearch.LinkArch.RegSize
```

and these lines in src/cmd/compile/internal/gc/align.go [3]
```
        case TINT64, TUINT64, TFLOAT64:
                w = 8
                t.Align = uint8(Widthreg)
```

[1]https://golang.org/src/cmd/internal/sys/arch.go
[2]https://golang.org/src/cmd/compile/internal/gc/main.go
[3]https://golang.org/src/cmd/compile/internal/gc/align.go


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