On Thu, Mar 02, 2017 at 02:46:29PM +0100, Ingo Molnar wrote: > > * Josh Poimboeuf <jpoim...@redhat.com> wrote: > > > On Thu, Mar 02, 2017 at 07:31:39AM +0100, Ingo Molnar wrote: > > > > > > * Josh Poimboeuf <jpoim...@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 <jpoim...@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? > > > > What does that matter if it's the wrong behavior? > > Well, both terminate the program, and it's obvious if you look at it with a > debugger what happened, right?
If it were obvious, we wouldn't be having this discussion :-) The only thing obvious to me was that gcc mysteriously removed a bunch of code and replaced it with a 'ud2' instruction in the middle of the function for no apparent reason. -- Josh
