Re: Help with Hand-Optimized Assembly

[Terje Mathisen]

Bill Woessner wrote:
> On Jan 13, 4:59 am, Terje Mathisen<"terje.mathisen at
> tmsw.no"@giganews.com>  wrote:
> > I'll second James' suggestion about SSE2!
>
> I'm open to using SSE2.  The only reason I used x87 is that I started
> with the assembly code that g++ generated.  By default, it generates
> x87 instructions.  But I'm certainly willing to try with SSE2
> instructions.
>
> > Anyway, it seem that what you are trying to do is to take the
> > difference between two angles and then make sure that said
> > difference will be in the [-pi .. pi>  range, right?
>
> That's it exactly.  It's such a simple thing, but I can't come up with
> a really elegant way to do it.  The code generated by g++ involves a
> jump.  But I think this should be possible without a jump by using a
> conditional move.
>
> > Anyway, trying your original algorithm:
>
> I'll give your implementation a try.  A big part of the challenge (for
> me, at least) is figuring out how to get this in to a form that g++
> will understand.  I don't really care where or how this is
> implemented, but I need to be able to call it from C++.  And it really
> should be inline, as well.  Otherwise, all the efficiency gained by
> tweaking the assembly will be lost.  :-p

Inline C isn't too hard to write:

inline double delta(double th1, th2)
{
    static double pi = 3.14159265357989;
    static double zero_or_twopi[2] = {0, 3.14159265357989*2};
    static double absmask = (double) 0x7fffffffffffffff;
    static double signmask = -0.0;

    double diff = th1-th2;
    double adiff = (double) ((int64_t) diff & (int64_t) absmask);
    int toolarge = adiff > pi;

    adiff -= zero_or_twopi[toolarge];

    adiff = (double) ((int64_t) adiff ^
       ((int64_t) signmask & (int64_t) diff));

    return adiff;
}


--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"