linux-kernel - Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

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Message-ID: <Ylg73c83AJGwz9UN@hirez.programming.kicks-ass.net>
Date:   Thu, 14 Apr 2022 17:21:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe de Dinechin <dinechin@...hat.com>
Cc:     trivial@...nel.org, Ben Segall <bsegall@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

On Thu, Apr 14, 2022 at 05:08:53PM +0200, Christophe de Dinechin wrote:
> With gcc version 12.0.1 20220401 (Red Hat 12.0.1-0) (GCC), the following
> errors are reported in sched.h when building after `make defconfig`:

<snip tons of noise>

> Rewrite the definitions of sched_class_highest and for_class_range to
> avoid this error as follows:
> 
> 1/ The sched_class_highest is rewritten to be relative to
>   __begin_sched_classes, so that GCC sees it as being part of the
>   __begin_sched_classes array and not a distinct __end_sched_classes
>   array.
> 
> 2/ The for_class_range macro is modified to replace the comparison with
>   an out-of-bound pointer __begin_sched_classes - 1 with an equivalent,
>   but in-bounds comparison.
> 
> In that specific case, I believe that the GCC analysis is correct and
> potentially valuable for other arrays, so it makes sense to keep it
> enabled.
> 
> Signed-off-by: Christophe de Dinechin <christophe@...echin.org>
> Signed-off-by: Christophe de Dinechin <dinechin@...hat.com>
> ---
>  kernel/sched/sched.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8dccb34eb190..6350fbc7418d 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2193,11 +2193,18 @@ const struct sched_class name##_sched_class \
>  extern struct sched_class __begin_sched_classes[];
>  extern struct sched_class __end_sched_classes[];
>  
> -#define sched_class_highest (__end_sched_classes - 1)
> +/*
> + * sched_class_highests is really __end_sched_classes - 1, but written in a way
> + * that makes it clear that it is within __begin_sched_classes[] and not outside
> + * of __end_sched_classes[].
> + */
> +#define sched_class_highest (__begin_sched_classes + \
> +			     (__end_sched_classes - __begin_sched_classes - 1))
>  #define sched_class_lowest  (__begin_sched_classes - 1)
>  
> +/* The + 1 below places the pointers within the range of their array */
>  #define for_class_range(class, _from, _to) \
> -	for (class = (_from); class != (_to); class--)
> +	for (class = (_from); class + 1 != (_to) + 1; class--)

Urgh, so now we get less readable code, just because GCC is being
stupid?

What's wrong with negative array indexes? memory is memory, stuff works.