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. Cheers, Andreas
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