Date: Wed, 19 Sep 2012 19:14:17 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Attilio Rao <attilio@freebsd.org> Cc: Davide Italiano <davide@freebsd.org>, src-committers@freebsd.org, John Baldwin <jhb@freebsd.org>, Jeff Roberson <jeff@freebsd.org>, Dimitry Andric <dim@freebsd.org>, svn-src-projects@freebsd.org, Konstantin Belousov <kostikbel@gmail.com> Subject: Re: svn commit: r238907 - projects/calloutng/sys/kern Message-ID: <20120919190309.R1306@besplex.bde.org> In-Reply-To: <CAJ-FndD90M2BAEcrjWG0YFCUrNu5Yad-5sRv=n8QXFbtYmo%2BjQ@mail.gmail.com> References: <201207301350.q6UDobCI099069@svn.freebsd.org> <CAJ-FndBj8tpC_BJXs_RH8sG2TBG8yA=Lxu3-GTVT9Ap_zOCuVQ@mail.gmail.com> <CAJ-FndDnO7wjnWPV0tTu%2BUGHjsxa3YDarMxmyei3ZmjLAFvRkQ@mail.gmail.com> <201207301732.33474.jhb@freebsd.org> <CAJ-FndD5EO12xsWOAe6u0EvX00q33wxO4OivnGjzj0=T2Oe8uA@mail.gmail.com> <CAJ-FndCRg0UCThFkatp=tw7rUWWCvhsApLE=iztLpxpGBC1F9w@mail.gmail.com> <20120918083324.GX37286@deviant.kiev.zoral.com.ua> <CAJ-FndAXw6zjXr=zB3gAVQDKUV_K4=SF39iYQQOV23NkfJ=MPw@mail.gmail.com> <CAJ-FndCmg-GdTf9FWQGZPYB-iGuEt-JH8HrCL%2BwaOqJ%2B8ZUbHQ@mail.gmail.com> <20120919041811.GM37286@deviant.kiev.zoral.com.ua> <CAJ-FndD90M2BAEcrjWG0YFCUrNu5Yad-5sRv=n8QXFbtYmo%2BjQ@mail.gmail.com>
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On Wed, 19 Sep 2012, Attilio Rao wrote:
> On Wed, Sep 19, 2012 at 5:18 AM, Konstantin Belousov
> <kostikbel@gmail.com> wrote:
>> On Tue, Sep 18, 2012 at 08:00:48PM +0100, Attilio Rao wrote:
>>> ...
>>> Here is the patch. I didn't use a real extern function body for it,
>>> but just went with an empty macro.
>>> ...
>>> +/*
>>> + * Compiler memory barriers, specific to gcc and clang.
>>> + */
>>> +#if defined(__GNUC__)
>>> +#define __compiler_membar() __asm __volatile(" " : : : "memory")
>>> +#else
>>> +#define __compiler_membar() struct __hack
>>> +#endif
>>
>> I would not call this an empty macro. If this works at all, it requires
>> c99 compiler. Why not just do
>> extern void __compiler_membar(void);
>> for !GNUC. Note that we never supplied actual implementation for the
>> placeholders, as evidenced e.g. by cpufunc.h or fpu.c.
>
> So the main reason for this is to keep compliancy with c'89? Are you
> sure it is so important nowadays?
> I'm always under the impression that we are already using c'99
> specific features, like structs initializers, etc.
'struct __hack' requires a c83 compiler. It is used extensively and worked
in 1993.
One point of using an extern function is to break properly when a feature
is missing. For a random compiler, you wouldn't know if it needs the
feature. OTOH, your original version with no support breaks even better,
by giving a spew of error messages (at least with -Wmumble-prototypes).
Many of the features in sys/cdefs.h are handled in this way. E.g.,
using __packed is a syntax error unless it is known to work (for
__GNUC_PREREQ(2, 7) || __INTEL_COMPILER), except for lint it is bogusly
stubbed out so that it is not properly broken for line.
Bruce
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