String compare performance

[Robert Jacques]

On Sun, 28 Nov 2010 06:44:23 -0500, bearophile <bearophileHUGS at lycos.com>  
wrote:
> Robert Jacques:
>
>> I've spent some time having fun this afternoon optimizing array-equals
>> using vectorization techniques. I found that vectorizing using ulongs
>> worked best on my system except with the shortest strings, where a  
>> simple
>> Duff's device edged it out. If you'd like to try it out on your data  
>> set:
>
> Thank you for your work :-)
> A version with your function, D version #8:

[snip]

> Your function can't be inlined because it's big, so this code isn't  
> faster than inlined code like this generated by the compiler:
> (codon.length == 3 && codon[0] == 'T' && codon[1] == 'A' && codon[2] ==  
> 'G')
>
> Bye,
> bearophile

Still, part of the point was that string comparisons in general were way  
too slow. Anyways, I've applied the same technique in a partially unrolled  
version if you want to check it out:

bool arrayComp(T, size_t N)(const T[] a, ref const T[N] b) pure nothrow {
     static if(T.sizeof*N <= uint.sizeof) {
         return a.length == N && !( (*(cast(uint*)a.ptr)  ^  
*(cast(uint*)b.ptr))  & (uint.max >> 8*(uint.sizeof  - T.sizeof*N) ));
     } else static if(T.sizeof*N <= ulong.sizeof) {
         return a.length == N && !( (*(cast(ulong*)a.ptr) ^  
*(cast(ulong*)b.ptr)) & (ulong.max>> 8*(ulong.sizeof - T.sizeof*N) ));
     } else { // Fall back to a loop
         if(a.length != N || (*(cast(ulong*)a.ptr) != *(cast(ulong*)b.ptr))  
) return false;
         enum alignment = T.sizeof*N % ulong.sizeof > 0 ? T.sizeof*N %  
ulong.sizeof : ulong.sizeof;
         auto pa      = cast(ulong*)(cast(ubyte*)a.ptr + alignment);
         auto pb      = cast(ulong*)(cast(ubyte*)b.ptr + alignment);
         auto pa_end  = cast(ulong*)(cast(ubyte*)a.ptr + T.sizeof*N);
         while (pa < pa_end) {
             if(*pa++ != *pb++ ) return false;
         }
         return true;
     }
}

Be warned that you'll have to use strings explicitly typed as immutable  
char[N], since both array literals and string literals won't match a this  
template.