On average there would be less than 4 bytes remaining to compare. So a simple straightforward byte comparison should do the job efficiently.

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 didn't check assembly for '=='. What I have seen is that struct comparison in dmd is implemented as byte per byte compare even if the struct is 64bit long (e.g. Rebindable).  I suppose dmd uses this strategy because struct/array may not be 64bit aligned, or they could have different alignment.

In order to use the suggested optimization we need a good planet alignment.  The effort to check that, or enforce it, is not worth the benefit on average.

There could be a benefit in specific use cases where the alignment is ensured.

I would be interested in such optimization with Rebindable. Can the 'is' operator be overloaded ?


Le 30/05/2016 11:28, ag0aep6g via Digitalmars-d-learn a écrit :
> 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 `==`?

-- 
Bien cordialement,

Ch.Meessen

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?