Let Go, Standard Library From Community

[Don Clugston]

Dan wrote:
> Frits van Bommel Wrote:
> 
>> Dan wrote:
>>> Oh my...
>>>
>>> I went through tango.core.Array and tango.math.Math and personally found several rather junior mistakes like:
>>>
>>> int abs(int x){
>>>   return x > 0? x : -x;
>>>  // should be: return x &= 0x7FFF_FFFF;
>>> }
>> ...
>>
>> That's *so* wrong...
>> You may want to save yourself some embarrassment by checking code before 
>> you post it here:
> 
> Bah... you know what I meant.  Unpredicted branches on an x86 cost roughly 6 cycles, not including the neg; we assume for both cases that we're inlining the function (no call overhead)
> 
> What I intended to show was that you could do it better with a touch of bit math.
> 
> I think the right way is actually to use something like asm's:
> 
> rol EAX, 1; // 1000_0001 becomes 0000_0011
> shr EAX, 1; // 0000_0011 becomes 0000_0001
> 
> Of course, I can't test this.  I'm at work, not at home with my development box. I may have also used the wrong right shift instruction, or used the rotate that goes through the sign bit in EFLAGS.
> 
> Surely one of the asm gurus has presented this already?
> 
> So I looked around a bit.
> 
> Some further reading:
> http://www.azillionmonkeys.com/qed/2scomp.html
> http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs

BTW I'm mentioned on that second page <g>.

Seriously, the emphasis to this point in Tango has been getting the 
interface and the implementation correct, rather than low-level 
optimisation (since it can be done later). There's a lot still to be 
done. Any contributions are very welcome.