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From: Peter Zijlstra <peterz@infradead.org>
To: Christophe de Dinechin <christophe@dinechin.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: Thu, 19 May 2022 13:16:08 +0200	[thread overview]
Message-ID: <20220519111608.GF2578@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <5AEAD35F-10E2-41A3-8269-E8358160D33B@dinechin.org>

On Mon, Apr 25, 2022 at 04:07:43PM +0200, Christophe de Dinechin wrote:

> >> 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? 

Yeah, because: x + y - x - 1 == y - 1, so wth would you want to write it
with the x on. That's just silly.

> It actually relies on a
> precise layout that is not enforced in this code, not even documented,
> but actually enforced by the linker script.

It has a comment pointing at the linker script, and we have:

	/* Make sure the linker didn't screw up */
	BUG_ON(&idle_sched_class + 1 != &fair_sched_class ||
	       &fair_sched_class + 1 != &rt_sched_class ||
	       &rt_sched_class + 1   != &dl_sched_class);
#ifdef CONFIG_SMP
	BUG_ON(&dl_sched_class + 1 != &stop_sched_class);
#endif

On boot to verify the layout is as we expect.

> > 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)

We build with -fno-strict-aliasing, it must not assume anything like
that, unless restrict is used.

In this case, they're not two objects but the same one. Just because
linker script can't really get us a sensible array definition.

> > 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.

We're usually fairly quick to call a compiler broken if doesn't do what
we want it to. Dodgy optimizations go out the window real fast.

  reply	other threads:[~2022-05-19 11:16 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
2022-05-19 11:16       ` Peter Zijlstra [this message]
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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