On 10/24/2014 02:05 PM, One Thousand Gnomes wrote: > On Fri, 24 Oct 2014 19:10:49 +0400 > 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. > > Also it means when someone quietly changes the default behaviour next > year in the compiler they won't spend months trying to work out why it > broke. > > gcc has one behaviour but people also try and build the kernel with icc > and with llvm. In addition in some cases you risk the compiler simply > generating an undefined in hardware operation and the hardware behaviour > changing. If x >> 32 is undefined then generating "load Y with the > shift, shift X left by Y" is fine. What happens in future silicon - who > knows. > > Most of the kernel is already very careful about the >> 32 problem. > The real question is if we can rely on the gcc-ism: (x >> (S-y)) | (x << y) ... where S is the number of bits to indicate a rotate. This is technically a gcc extension to the C language. -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
