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From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     libc-alpha <libc-alpha@sourceware.org>,
        Rich Felker <dalias@libc.org>,
        linux-api <linux-api@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ben Maurer <bmaurer@fb.com>, Dave Watson <davejwatson@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul <paulmck@linux.vnet.ibm.com>, Paul Turner <pjt@google.com>,
        Joseph Myers <joseph@codesourcery.com>
Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)
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Date:   Tue, 26 May 2020 16:38:45 +0200
In-Reply-To: <1701081361.34159.1590503556923.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Tue, 26 May 2020 10:32:36 -0400
        (EDT)")
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* Mathieu Desnoyers:

> AFAIU, the only gain here would be to make sure we don't emit useless
> ";" in the "/* nothing */" case. But does it matter ?

I don't think C allows empty constructs like this at the top level.

>>>> And something similar for _Alignas/attribute aligned,
>>>
>>> I don't see where _Alignas is needed here ?
>>>
>>> For attribute aligned, what would be the oldest supported C and C++
>>> standards ?
>>=20
>> There are no standardized attributes for C, there is only _Alignas.
>> C++11 has an alignas specifier; it's not an attribute either.  I think
>> these are syntactically similar.
>
> There appears to be an interesting difference between attribute aligned
> and alignas. It seems like alignas cannot be used on a structure declarat=
ion,
> only on fields, e.g.:
>
> struct blah {
>         int a;
> } _Alignas (16);
>
> o.c:3:1: warning: useless =E2=80=98_Alignas=E2=80=99 in empty declaration
>  } _Alignas (16);
>
> But
>
> struct blah {
>         int _Alignas (16) a;
> };

Like the attribute, it needs to come right after the struct keyword, I
think.  (Trailing attributes can be ambiguous, but not in this case.)

> is OK. So if I change e.g. struct rseq_cs to align
> the first field:
>
> struct rseq_cs
>   {
>     /* Version of this structure.  */
>     uint32_t rseq_align (32) version;
>     /* enum rseq_cs_flags.  */
>     uint32_t flags;
>     uint64_t start_ip;
>     /* Offset from start_ip.  */
>     uint64_t post_commit_offset;
>     uint64_t abort_ip;
>   };
>
> It should work.

Indeed.

> /* Rely on GNU extensions for older standards and tls model.  */
> #ifdef __GNUC__
> # ifndef rseq_alignof
> #  define rseq_alignof(x) __alignof__ (x)
> # endif
> # ifndef rseq_alignas
> #  define rseq_alignas(x) __attribute__ ((aligned (x)))
> # endif
> # define rseq_tls_model_ie __attribute__ ((__tls_model__ ("initial-exec")=
))
> #else
> /* Specifying the TLS model on the declaration is optional.  */
> # define rseq_tls_model_ie /* Nothing.  */
> #endif
>
> /* Fall back to __thread for TLS storage class.  */
> #ifndef rseq_tls_storage_class
> # define rseq_tls_storage_class __thread
> #endif

If they are only used in the glibc headers, they should have __rseq
prefixes, so that application code doesn't start using them (in case we
have to change/fix them, or move the into <sys/cdefs.h> later).

Rest looks fine.

Thanks,
Florian