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From: Ingo Molnar <mingo@kernel.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings
Date: Thu, 2 Mar 2017 07:31:39 +0100	[thread overview]
Message-ID: <20170302063139.GA594@gmail.com> (raw)
In-Reply-To: <20170302010342.fshczxfiiz3txgac@treble>


* Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Wed, Mar 01, 2017 at 11:42:54PM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 1, 2017 at 5:53 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > On Wed, Mar 01, 2017 at 04:27:29PM +0100, Arnd Bergmann wrote:
> > 
> > > I see no apparent reason for the ud2.
> > 
> > It's the possible division by zero. This change would avoid the ud2:
> > 
> > diff --git a/drivers/i2c/busses/i2c-img-scb.c b/drivers/i2c/busses/i2c-img-scb.c
> > index db8e8b40569d..a2b09c518225 100644
> > --- a/drivers/i2c/busses/i2c-img-scb.c
> > +++ b/drivers/i2c/busses/i2c-img-scb.c
> > @@ -1196,6 +1196,8 @@ static int img_i2c_init(struct img_i2c *i2c)
> >         clk_khz /= prescale;
> > 
> >         /* Setup the clock increment value */
> > +       if (clk_khz < 1)
> > +               clk_khz = 1;
> >         inc = (256 * 16 * bitrate_khz) / clk_khz;
> > 
> >         /*
> 
> Ok, I see what gcc is doing.
> 
> 	clk_khz = clk_get_rate(i2c->scb_clk) / 1000;
> 	...
> 	inc = (256 * 16 * bitrate_khz) / clk_khz;
> 
> Because CONFIG_HAVE_CLK isn't set, clk_get_rate() returns 0, which means
> clk_khz is always zero, so the last statement *always* results in a
> divide-by-zero.  So that looks like a bug in the code.
> 
> However, I'm baffled by how gcc handles it.  Instead of:
> 
>   a) reporting a compile-time warning/error; or
> 
>   b) letting the #DE (divide error) exception happen;
> 
> it inserts a 'ud2', resulting in a #UD (invalid opcode).  Why?!?

Well, technically an invalid opcode is shorter code than generating an (integer) 
division by zero exception, right?

Thanks,

	Ingo

  reply	other threads:[~2017-03-02  6:31 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-10 12:56 [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings Arnd Bergmann
2016-10-10 20:23 ` Josh Poimboeuf
2016-10-11  8:08   ` Arnd Bergmann
2016-10-11 12:20     ` Josh Poimboeuf
2016-10-11 13:30       ` Arnd Bergmann
2016-10-11 15:05         ` Josh Poimboeuf
2016-10-11 15:51           ` Josh Poimboeuf
2016-10-11 20:38             ` Arnd Bergmann
2016-10-12 13:01               ` Josh Poimboeuf
2016-10-13 12:46               ` Another gcc corruption bug (was Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings) Josh Poimboeuf
2016-10-13 17:57                 ` Denys Vlasenko
2016-10-13 20:15                   ` Josh Poimboeuf
2017-03-01  9:34               ` [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings Arnd Bergmann
2017-03-01  9:45                 ` Arnd Bergmann
2017-03-01 14:40                   ` Josh Poimboeuf
2017-03-01 15:27                     ` Arnd Bergmann
2017-03-01 16:53                       ` Josh Poimboeuf
2017-03-01 22:05                         ` Arnd Bergmann
2017-03-01 22:42                         ` Arnd Bergmann
2017-03-02  1:03                           ` Josh Poimboeuf
2017-03-02  6:31                             ` Ingo Molnar [this message]
2017-03-02 12:49                               ` Josh Poimboeuf
2017-03-02 13:46                                 ` Ingo Molnar
2017-03-02 14:08                                   ` Josh Poimboeuf
2017-03-02 14:46                                     ` Ingo Molnar
2017-03-02 22:49                             ` Arnd Bergmann
2017-03-02 23:05                               ` Josh Poimboeuf
2017-03-03  8:58                                 ` Arnd Bergmann
2017-03-03 11:27                                   ` Arnd Bergmann
2017-03-01 14:31                 ` Josh Poimboeuf
2017-03-01 15:21                   ` Arnd Bergmann
2017-03-02 18:25                     ` Josh Poimboeuf
2017-03-02 22:43                       ` Arnd Bergmann
2017-03-02 22:57                         ` [PATCH] objtool: fix another gcc jump table detection issue Josh Poimboeuf
2017-03-02 23:01                           ` Arnd Bergmann
2016-10-11  1:53 ` [PATCH] objtool: support '-mtune=atom' stack frame setup instruction Josh Poimboeuf

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