On 10/24/2014 03:09 PM, Andreas Dilger wrote: > On Oct 24, 2014, at 9:10 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >> On Fri, Oct 24, 2014 at 7:04 PM, Sasha Levin <sasha.levin@xxxxxxxxxx> wrote: >>> On 10/24/2014 09:42 AM, Peter Zijlstra wrote: >>>> On Fri, Oct 24, 2014 at 09:23:35AM -0400, Sasha Levin wrote: >>>>> >>>>> i >> 32 may happen to be "i", but is there anything that prevents the compiler from returning, let's say, 42? >>>> >>>> Not really, although gcc seems to opt for the 'sane' option and emit >>>> the instruction and let the arch figure out how to deal with it. >>>> Hence the 'fun' difference between x86 and ARM. >>> >>> It's interesting how many different views on undefined behaviour there are between kernel folks. >>> >>> Everything between Ted Ts'o saying that GCC can launch nethack on oversized shifts, to DaveM saying he will file a GCC bug if the >>> behaviour isn't sane w.r.t to memcpy(). >> >> One of the benefits of fixing such issues (or not letting them into >> code in the first place) is just saving numerous hours of top-notch >> engineers spent on disputes like this. > > By the principle of least surprise, I would expect "__u32 >> N", where > N >= 32 to return zero instead of random garbage. For N < 32 it will > return progressively smaller numbers, until it has shifted away all of > the set bits, at which turn it will return 0. For it suddenly to jump > up once N = 32 is used, is counter-intuitive. > That's why it is undefined. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
