Messages in this thread | | | Date | Thu, 2 Mar 2017 07:31:39 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings |
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* 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
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