From: Jason Gerecke <killertofu@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
LKML <linux-kernel@vger.kernel.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Jason Baron <jbaron@akamai.com>,
Jason Gerecke <jason.gerecke@wacom.com>,
Ard Biesheuvel <ardb@kernel.org>,
"x86@kernel.org" <x86@kernel.org>
Subject: Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints
Date: Fri, 12 Feb 2021 07:55:15 -0800 [thread overview]
Message-ID: <CANRwn3SZpdNqp__RH8cUtVLA1-Cz3c+uOG+eyisd9vrZVW1EKQ@mail.gmail.com> (raw)
In-Reply-To: <YCaebvpeLjY5oRAs@hirez.programming.kicks-ass.net>
On Fri, Feb 12, 2021 at 7:27 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Feb 12, 2021 at 09:40:59AM -0500, Steven Rostedt wrote:
> > On Thu, 11 Feb 2021 13:48:48 -0800
> > Jason Gerecke <killertofu@gmail.com> wrote:
> >
> > > When compiling an external kernel module with `-O0` or `-O1`, the following
> > > compile error may be reported:
> > >
> > > ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
> > > 25 | asm_volatile_goto("1:"
> > > | ^~~~~~~~~~~~~~~~~
> > >
> > > It appears that these lower optimization levels prevent GCC from detecting
> > > that the key/branch arguments can be treated as constants and used as
> > > immediate operands. To work around this, explicitly add the `const` label.
> >
> > Yes this makes sense. The "i" constraint needs to be a constant.
>
> Right, using -O[01] seems a little daft though. But yeah, that patch is
> correct and won't cause harm.
>
> I've queued it for after the next merge window.
Thanks. Only reason I even tried compiling at those levels was to play
around with GCC's new static analyzer options. They seem to be
ineffective at more normal optimization levels.
next prev parent reply other threads:[~2021-02-12 15:56 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-11 21:48 [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints Jason Gerecke
2021-02-12 14:40 ` Steven Rostedt
2021-02-12 15:27 ` Peter Zijlstra
2021-02-12 15:55 ` Jason Gerecke [this message]
2021-02-12 16:14 ` Josh Poimboeuf
2021-03-03 8:15 ` [tip: locking/core] " tip-bot2 for Jason Gerecke
2021-03-06 11:54 ` tip-bot2 for Jason Gerecke
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