linux-kernel - Re: [PATCH] sparc: mdesc: Fix compile error seen with gcc 11.x

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Message-ID: <20210914151747.GC3457579@roeck-us.net>
Date:   Tue, 14 Sep 2021 08:17:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Arnd Bergmann' <arnd@...nel.org>, Sam Ravnborg <sam@...nborg.org>,
        "David S . Miller" <davem@...emloft.net>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sparc: mdesc: Fix compile error seen with gcc 11.x

On Tue, Sep 14, 2021 at 03:03:51PM +0000, David Laight wrote:
> From: Arnd Bergmann
> > Sent: 14 September 2021 15:54
> > 
> > On Tue, Sep 14, 2021 at 4:24 PM Guenter Roeck <linux@...ck-us.net> wrote:
> > > On 9/14/21 7:17 AM, David Laight wrote:
> > > >> Sorry, I didn't realize that a field of size 0 increases the structure size
> > > >> on sparc. I had checked the size of the old and the new structure with gcc
> > > >> on x86_64 and didn't see a field size increase.
> > > >
> > > > clang output doesn't change:
> > > >
> > > > https://godbolt.org/z/bTeeq19j1
> > > >
> > > > gcc ought to generate the same size.
> > > >
> > > > It ought to be 'char data[];' though.
> > > >
> > >
> > > I am never sure if [] or [0] is "correct". Anyway, is there agreement that this
> > > is an acceptable solution ? I'll be happy to resend if that is the case.
> > 
> > Yes, looks good to me, in the [] version. I think the [0] version can be
> > interpreted as a zero-length array that may not be accessed, while the
> > [] flexible array syntax clearly means that extra data follows, and it's
> > part of the C standard now, while [0] is a gcc extension.
> 
> More problematic is where is the correct place for the 'char data[]'.
> It follows the header rather than being part of it.

I personally always prefer the simple solution, and I don't really care
about such nuances. I take it as granted that a header is followed by data,
and I think that a zero-length field at the end of a header is a perfectly
valid means to express that, but that is just my personal opinion.

Anyway, I take that as non-agreement and won't resend at this time.

Thanks,
Guenter