[Info-vax] Viable versus ideal programming languages

[Arne Vajhøj]

On 3/25/2022 9:16 AM, Dan Cross wrote:
> In article <623cb7a5$0$694$14726298 at news.sunsite.dk>,
> Arne Vajhøj  <arne at vajhoej.dk> wrote:
>> On 3/24/2022 9:08 AM, Simon Clubley wrote:
>>> On 2022-03-23, Dan Cross <cross at spitfire.i.gajendra.net> wrote:
>>>> In article <t1fpab$v69$1 at dont-email.me>,
>>>> Simon Clubley  <clubley at remove_me.eisner.decus.org-Earth.UFP> wrote:
>>>>> It seems that everyone falls back to C interface mode when trying
>>>>> to export functions for use by another language. A quick look around
>>>>> seems to show that Rust does the same.
>>>>
>>>> I suppose that if one's definition of a well-defined ABI is what
>>>> you are calling a "C interface mode" that's true, but most ABIs
>>>> are language-independent.
>>>
>>> It's at this point that I mention many languages directly call this a
>>> C interface mode, including in the syntax that they provide to achieve
>>> this... :-)
>>
>> True.
>>
>> But it really isn't a C thing - it is a calling convention thing.
>>
>> If we take Rust as an example then you can use:
> 
> Here's a small example in Rust of calling into assembler:

> Here's an example of assembler calling into Rust, including
> calling via a mangled name:

> .text
> .globl trampoline, you
> trampoline:
>          JMP     you
>          UD2

> #[no_mangle]
> pub extern fn you(a: i32, b: i32) -> i32 {
>      a + b
> }
> 
> pub extern fn mangled(a: i32, b: i32) -> i32 {
>      a + b
> }

> fn main() {
>      println!("trampoline(1, 2) = {}", unsafe { trampoline(1, 2) });
>      let s: i32;
>      unsafe {
>          std::arch::asm!(
>          r#"movl {a:e}, %edi;
>             movl {b:e}, %esi;
>             call {mangled};
>             movl %eax, {s:e}"#,
>          a = in(reg) 1, b = in(reg) 2, s = out(reg) s,
>          mangled = sym mangled, options(att_syntax));
>      }
>      println!("a = {}", s);
> }

> Note the inline assembler snippet showing that I can, in fact,
> work with mangled symbols.  Inspection of the generated object
> file shows this to be true:


> If I dump that same snippet with name demangling, one sees
> something intelligible:

>    487046:       e8 35 ff ff ff          call   486f80 <demo2::mangled>

> There is not a single line of C in this example.
> 
> Of course, all of this works because the name mangling algorithm
> is well-specified.

If I read that code correctly then it works because the
std::arch::asm ... mangled = sym mangled knows what to do.

Which is not general language interop.

Arne