Date: Mon, 27 May 2013 06:20:32 -0400 From: Lee Thomas <lee_thomas@aslantools.com> To: <freebsd-hackers@freebsd.org> Subject: Re: Performance improvement to strnlen(). Message-ID: <a88fc3c3c7c4da882af20eed5a61aee1@lthomas.net> In-Reply-To: <CAKw7uVi2oofoy=yJQiHR927oGfy74UuYFq68r=C=gUVXBXZuKw@mail.gmail.com> References: <2adc4d8e7c55e92d935f61efb4c9d723@lthomas.net> <CAKw7uVi2oofoy=yJQiHR927oGfy74UuYFq68r=C=gUVXBXZuKw@mail.gmail.com>
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On 2013-05-27 04:37, Václav Zeman wrote: > On 26 May 2013 21:01, Lee Thomas wrote: >> On 2013-05-26 08:00, Václav Zeman wrote: >>> >>> On 05/25/2013 10:27 PM, Lee Thomas wrote: >>>> >>>> + lp = (const unsigned long *)((uintptr_t)str & >>>> ~LONGPTR_MASK); >>>> + va = (*lp - mask01); >>>> + vb = ((~*lp) & mask80); >>> >>> I do not think that this correct C. This is type punning violating >>> the >>> rules of the language. >> >> >> Hello Václav, >> >> 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 => UB => even though it seems to work in this instance, you > are just lucky the UB is actually doing what you expect. > > -- > VZ Hello Václav, I am not an expert in C either, so you may be right that this is technically illegal. However, I copied this code from strlen.c, which has had it, and still has it, for 4.5 years, and I can't see any way any alias analysis done by the compiler could invalidate this code. In addition, there are many places in the kernel, and in other codebases I've worked on, where this kind of type conversion is done. See for instance /sys/amd64/amd64/vm_macdep.c:200, where we compute the base of a thread's stackframe from a pointer to an unrelated type of 'struct pcb', and then write to it. I am willing to uglify the code in the way you suggest if that is the general concensus, but I think the code as it stands is both safe and more legible. Thanks, Lee
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