Date: Mon, 27 May 2013 10:37:40 +0200 From: =?UTF-8?Q?V=C3=A1clav_Zeman?= <vhaisman@gmail.com> To: Lee Thomas <lee_thomas@aslantools.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Performance improvement to strnlen(). Message-ID: <CAKw7uVi2oofoy=yJQiHR927oGfy74UuYFq68r=C=gUVXBXZuKw@mail.gmail.com> In-Reply-To: <2adc4d8e7c55e92d935f61efb4c9d723@lthomas.net> References: <2adc4d8e7c55e92d935f61efb4c9d723@lthomas.net>
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On 26 May 2013 21:01, Lee Thomas wrote: > On 2013-05-26 08:00, V=C3=A1clav Zeman wrote: >> >> On 05/25/2013 10:27 PM, Lee Thomas wrote: >>> >>> + lp =3D (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK); >>> + va =3D (*lp - mask01); >>> + vb =3D ((~*lp) & mask80); >> >> I do not think that this correct C. This is type punning violating the >> rules of the language. > > > Hello V=C3=A1clav, > > The aliasing here is safe, because there are no writes through either of = the > pointers, and the reads are correctly aligned. I disagree. IANALL but AFAIK, this is simply not allowed by the language =3D> UB =3D> even though it seems to work in this instance, you are just lucky the UB is actually doing what you expect. -- VZ
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