Performance improvement to strnlen().

[Lee Thomas]

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