X[]==Y[] is 7X slower than it should be -- why?

[janderson]

Don wrote:
> I've been doing some timing tests, and found that the built-in array 
> comparison is amazingly slow.
> 
> Given:
> uint [2000] X, Y;
> 
> bool b = (X[] == Y[]);
> 
> on my Pentium M, this takes ~30000 cycles in the case where both arrays 
> are equal.
> It's not hard to write an optimal asm routine; this takes 4000 cycles 
> (7.5 times faster).
> But, it's also possible to perform the comparison in simple D code, 
> something like:
> 
> int compareArrays(uint [] x, uint y[])
> {
>     for (int i=0; i<x.length; ++i) {
>         if (x[i] != y[i]) return x[i]-y[i];
>     }
>     return 0;
> }
> 
> and this takes 6000 cycles when compiled with -release -O, 5X better 
> than the built-in version.
> 
> So, why is the built-in compare so slow? Using the CPU performance 
> counters:
> X[]==Y[] --> 10000 branches, 35000 memory loads, 30000 cycles.
> for loop --> 4000 branches, 6000 memory loads, 6000 cycles.
> asm      --> 4000 branches, 4000 memory loads, 4000 cycles.
> 
> What's it doing? Is it doing a byte-by-byte comparison? (it shouldn't do 
> that even for strings). Is it doing bounds checking for every array 
> element?? (even with -release -O)
> 
> There's significant optimisation potential here.

I haven't looked but this is probably somewhere in the front end code. 
You could always summit a patch.

-Joel