String compare in words?

[Márcio Martins via Digitalmars-d-learn]

On Monday, 30 May 2016 at 09:28:29 UTC, ag0aep6g wrote:
> On 05/29/2016 10:40 PM, qznc wrote:
>> bool string_cmp_opt(immutable(ubyte)[] x, immutable(ubyte)[] 
>> y) {
>
> Having "string" in the function name may be a bit misleading. 
> This doesn't have any special functionality for 
> text/characters/Unicode, does it?
>
> Should have const parameters, not immutable.
>
>>      pragma(inline, false);
>
> I think you have to put this pragma on the function signature, 
> not in the body. Also, why prevent inlining of the function?
>
>>      if (x.length != y.length) return false;
>>      int i=0;
>
> int isn't large enough for array lengths.
>
>>      // 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];
>
> Typo: Should be `&y[i]` here.
>
>>              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?
>
> Did you benchmark this against the built-in `==`, with ldc or 
> gdc?
>
> If this is correct and faster than the built-in `==`, why isn't 
> it the built-in `==`?

I too expected it to compile to a memcmp call, but according to 
asm.dlang.org DMD with -O and -release, DMD compiles a == b to a 
byte-wise compare.

I suppose for very tiny strings this is the fastest, but for 
slightly larger strings, calling memcmp() would be faster. I 
think inlining a string comparison is also not great for code 
size. In the general case, for element types with trivial 
equality, a call to memcmp() will always be preferable, right?