String compare in words?

[chmike via Digitalmars-d-learn]

On Sunday, 29 May 2016 at 20:40:52 UTC, qznc wrote:
> On Sunday, 29 May 2016 at 18:15:16 UTC, qznc wrote:
>> On Sunday, 29 May 2016 at 17:38:17 UTC, Jonathan M Davis wrote:
>>> And if you're not simply comparing for equality, what are you 
>>> looking to figure out? Without more information about what 
>>> you're trying to do, it's kind of hard to help you.
>>
>> If I write the comparison naively, the assembly clearly shows 
>> a "movzbl" [0]. It loads a single byte! The other single byte 
>> load is encoded in the address mode of "cmp". Implementation:
>>
>> bool stringcmp(string x, string y) {
>>   foreach(i; 0..x.length) {
>>     if (x[i] != y[i]) // byte compare
>>       return false;
>>   }
>>   return true;
>> }
>>
>> It makes no sense to load single bytes here. Since we only 
>> want to check for equality, we could load two full words and 
>> compare four or eight bytes in one go.
>
> Ok, to answer my own question, this looks good:
>
> bool string_cmp_opt(immutable(ubyte)[] x, immutable(ubyte)[] y) 
> {
>     pragma(inline, false);
>     if (x.length != y.length) return false;
>     int i=0;
>     // word-wise compare is faster than byte-wise
>     if (x.length > size_t.sizeof)
>         for (; i < x.length - size_t.sizeof; i+=size_t.sizeof) {
>             size_t* xw = cast(size_t*) &x[i];
>             size_t* yw = cast(size_t*) &x[i];
>             if (*xw != *yw) return false;
>         }
>     // last sub-word part
>     for (; i < x.length; i+=1) {
>         if (x[i] != y[i]) // byte compare
>             return false;
>     }
>     return true;
> }
>
> Any comments or recommendations?

I don't know if this would be faster, but here is my attempt.

It assumes the arrays start at an address multiple of 8.

if (x is y)
     return true;
if (x.length != y.length)
     return false;
size_t l = x.length;
ubyte* a = x.ptr, b = y.ptr;
for (size_t n = l>>3; n != 0; --n, a+=8, b+=8)
     if (*cast(long*)a ^ *cast(long*)b)
         return false;
if (l & 4)
{
     if (*cast(int*)a ^ *cast(int*)b)
         return false;
     a+= 4;
     b+= 4;
}
if (l & 2)
{
     if (*cast(short*)a ^ *cast(short*)b)
         return false;
     a+=2;
     b+=2;
}
return (l & 1) && (*a ^ *b) ? false : true;

If the pointers are not on an address multiple of 8, one has to 
inverse the trailing tests to consume the bytes in front of the 
array until the address becomes a multiple of 8.

The trailing tests could eventually be replaced by a simple 
sequential byte compare. I don't know which is faster.