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From: Christophe de Dinechin <christophe@dinechin.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Christophe de Dinechin <dinechin@redhat.com>,
	trivial@kernel.org, Ben Segall <bsegall@google.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	virtualization@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org,
	Zhen Lei <thunder.leizhen@huawei.com>,
	Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12
Date: Mon, 25 Apr 2022 16:07:43 +0200	[thread overview]
Message-ID: <5AEAD35F-10E2-41A3-8269-E8358160D33B@dinechin.org> (raw)
In-Reply-To: <Ylg73c83AJGwz9UN@hirez.programming.kicks-ass.net>



> On 14 Apr 2022, at 17:21, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> 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>

I don’t mind removing the detailed error message.
What do others think?

> 
>> 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@dinechin.org>
>> Signed-off-by: Christophe de Dinechin <dinechin@redhat.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,

You consider the original code readable? It actually relies on a
precise layout that is not enforced in this code, not even documented,
but actually enforced by the linker script.

> just because GCC is being
> stupid?

I think that GCC is actually remarkably smart there. It tells you
that you are building pointers to A[] from B[], when there is a legit
way to say that the pointer is in A[] (which is what my patch does)

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

What’s wrong is that the compiler cannot prove theorems anymore.
These theorems are used to optimise code. When you write -1[B], the
compiler cannot optimise based on knowing this refers to A[B-A-1].

While at first, you might think that disabling a warning is a win, what comes next
is the compiler optimizing in a way you did not anticipate, mysterious bugs showing up,
and/or having to turn off some potentially useful optimisation.



  parent reply	other threads:[~2022-04-25 14:15 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-14 15:08 [PATCH 0/3] trivial: Fix several compilation errors/warnings with GCC12 Christophe de Dinechin
2022-04-14 15:08 ` [PATCH 1/3] sched/headers: Fix compilation error with GCC 12 Christophe de Dinechin
2022-04-14 15:21   ` Peter Zijlstra
2022-04-14 20:30     ` Andrew Morton
2022-04-17 15:52       ` Peter Zijlstra
2022-04-20 18:45         ` Kees Cook
2022-04-21  7:32           ` Peter Zijlstra
2022-04-25 14:23       ` Christophe de Dinechin
2022-04-17 13:27     ` David Laight
2022-04-25 14:07     ` Christophe de Dinechin [this message]
2022-05-19 11:16       ` Peter Zijlstra
2022-04-14 15:08 ` [PATCH 2/3] nodemask.h: Fix compilation error with GCC12 Christophe de Dinechin
2022-04-14 15:23   ` Peter Zijlstra
2022-04-14 15:08 ` [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation error Christophe de Dinechin
2022-04-15  8:48   ` Michael S. Tsirkin
2022-04-28  9:48     ` Christophe Marie Francois Dupont de Dinechin
2022-04-28 11:06       ` Michael S. Tsirkin
2022-05-15 21:24 ` [PATCH 0/3] trivial: Fix several compilation errors/warnings with GCC12 Davidlohr Bueso

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