speed of low-level C funcs: example of memmove

[spir]

On 04/09/2011 07:08 PM, spir wrote:
> Hello,
>
> To insert of delete an array slice, I tried to use C's memmove, thinking it
> would be far faster than "manually" copying bit per bit (by any kind of magic).
> But I still wrote a D versions just to check what the actual speed gain is. To
> my great surprise, the C-memmove and D-manual versions perform *exactly* at the
> same speed (considering measure imprecision).
> Note: this remains true when elements are bigger; speed slows down slowly (eg
> dchar's take only 1/3 more time).

Correction: memmove can be from 5 to 10 times faster on big arrays. For 
instance, with 1_000_000 char array:

void chrono () {
     char[] s;
     d_time t;
     enum N = 100;

     // C memmove
     s = ("0".mult(1_000_000)).dup;
     t = getUTCtime();
     foreach (_ ; 0..N) {
         s.shiftEndPartC(3,5);
         s[3..5] = "--";
         s.shiftEndPartC(5,3);
     }
     t = getUTCtime() - t;
     writefln("C time: %s", t);

     // D manual
     s = ("0".mult(1_000_000)).dup;
     t = getUTCtime();
     foreach (_ ; 0..N) {
     s.shiftEndPartD(3,5);
     s[3..5] = "--";
     s.shiftEndPartD(5,3);
     }
     t = getUTCtime() - t;
     writefln("D time: %s", t);
}

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