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From: Peter Zijlstra <peterz@infradead.org>
To: kbuild-all@lists.01.org
Subject: Re: [peterz-queue:perf/core 26/39] arch/x86/include/asm/perf_event.h:290:19: error: flexible array member in a struct with no named members
Date: Tue, 07 Jul 2020 16:44:56 +0200
Message-ID: <20200707144456.GP4800@hirez.programming.kicks-ass.net>
In-Reply-To: <20200707135110.GA11125@embeddedor>
List-Id: <oe-kbuild-all.lists.linux.dev>

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On Tue, Jul 07, 2020 at 08:51:10AM -0500, Gustavo A. R. Silva wrote:
> On Tue, Jul 07, 2020 at 12:24:43PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 06, 2020 at 09:08:41PM -0400, Liang, Kan wrote:
> > =

> > > >     288	=

> > > >     289	struct pebs_lbr {
> > > >   > 290		struct lbr_entry lbr[]; /* Variable length */
> > > >     291	};
> > > >     292	=

> > > =

> > > The struct pebs_lbr only has one member. To fix the warning, it seems=
 we
> > > have to define the struct as below.
> > > =

> > > =

> > > diff --git a/arch/x86/include/asm/perf_event.h
> > > b/arch/x86/include/asm/perf_event.h
> > > index 2338200..a387e14 100644
> > > --- a/arch/x86/include/asm/perf_event.h
> > > +++ b/arch/x86/include/asm/perf_event.h
> > > @@ -283,7 +283,7 @@ struct pebs_xmm {
> > >  };
> > > =

> > >  struct pebs_lbr {
> > > -	struct lbr_entry lbr[]; /* Variable length */
> > > +	struct lbr_entry lbr[0]; /* Variable length */
> > >  };
> > > =

> > =

> > Right, but then we get trouble like the below again. There's basically
> =

> Yep, zero-length and one-element arrays are being deprecated[1].
> =

> > no good solution here :-(
> > =

> > Gustavo, any clues?
> > =

> =

> Yep; the issue is that "Flexible array members may only appear as the
> last member of a struct that is otherwise non-empty"[2].
> =

> The solution is to declare something like this:
> =

> struct pebs_lbr {
> 	size_t num_lbr; /* count for number of lbr entries */
> 	struct lbr_entry lbr[];
> };
> =


Can't.. this is hardware provided layout. The thing is that the array
size is variable, so [0] or [] would be correct.

That said, Kan, I suppose we can simply remove struct pebs_lbr and use
struct lbr_entry * directly. There is very little actual point in having
struct pebs_lbr.

I'll frob the patches and push out again.
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