Date: Fri, 19 Jan 2018 10:30:20 -0800 From: Conrad Meyer <cem@freebsd.org> To: "Rodney W. Grimes" <rgrimes@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r328159 - head/sys/modules Message-ID: <CAG6CVpUj3SfiuHAaPMB1zGXpXPw=U-CsHgk%2BivEPyrzhvrrPKw@mail.gmail.com> In-Reply-To: <201801191737.w0JHbM90073097@pdx.rh.CN85.dnsmgr.net> References: <CAG6CVpV6Suft3v-=08f5UH6BTH2NEJgU_4kYd-UphLZ6yoJB4Q@mail.gmail.com> <201801191737.w0JHbM90073097@pdx.rh.CN85.dnsmgr.net>
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On Fri, Jan 19, 2018 at 9:37 AM, Rodney W. Grimes <freebsd@pdx.rh.cn85.dnsmgr.net> wrote: > If you think in assembler it is easy to understand why this is UB, > most (all) architectures Right Logic or Arithmetic Shift only accept an > operand that is a size that can hold log2(wordsize). This is a logical right shift by a constant larger than the width of the left operand. As a result, it would a constant zero in any emitted machine code. It is a bug in the C standard and a concession to naive, non-optimizing compilers that this is considered UB.
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