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Date:   Fri, 16 Apr 2021 22:58:12 +0200
From:   Willy Tarreau <w@1wt.eu>
To:     Connor Kuehl <ckuehl@redhat.com>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Geoffrey Thomas <geofft@ldpreload.com>,
        Finn Behrens <me@kloenk.de>,
        Adam Bratschi-Kaye <ark.email@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH 04/13] Kbuild: Rust support
Message-ID: <20210416205812.GA11655@1wt.eu>
References: <20210414184604.23473-1-ojeda@kernel.org>
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 <CAHk-=wh_zb=K1B-N8mgHmSZDqTLgOm711NRXbTX_OwFAzDYg0Q@mail.gmail.com>
 <CANiq72nx7ngazsH7sZgc=HeU0cNj45F9+-rwQb7AkdYsRCmRbQ@mail.gmail.com>
 <YHnS92ZKZ4tRWTiA@zeniv-ca.linux.org.uk>
 <CANiq72=RLf0FiuLVL-ZeLFp9P2LxTymbzhXoyQGG=tvUY_J-Sg@mail.gmail.com>
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On Fri, Apr 16, 2021 at 03:34:50PM -0500, Connor Kuehl wrote:
> On 4/16/21 3:22 PM, Willy Tarreau wrote:
> > So it simply does the equivalent of:
> > 
> >   #define EINVAL -1234
> > 
> >   struct result {
> >      int status;
> >      int error;
> >   };
> 
> Result and Option types are more like a union with a tag that
> describes which variant it is.
> 
> struct foo_result {
>     /* if ok, then access foo_or_err.successful_foo
>      *        else, access foo_or_err.error
>      */
>     bool ok;
>     union {
>         struct foo successful_foo;
>         int error;
>     } foo_or_err;
> };

OK.

> > [..]
> > 
> > So it simply returns a pair of values instead of a single one, which
> 
> It will only return 1 value.

No, two:
  - ok in %rax (seems like it's "!ok" technically speaking since it
    returns 1 on !ok and 0 on ok)
  - foo_or_err in %rdx

However then I'm bothered because Miguel's example showed that regardless
of OK, EINVAL was always returned in foo_or_err, so maybe it's just
because his example was not well chosen but it wasn't very visible from
the source:

     bar:
             push    rbx
             mov     ebx, 1
             call    qword ptr [rip + black_box@GOTPCREL]
             test    al, al
             jne     .LBB2_2
             call    qword ptr [rip + kill_foo@GOTPCREL]
             xor     ebx, ebx
     .LBB2_2:
             mov     eax, ebx
             mov     edx, -1234
             pop     rbx
             ret

Willy