BZDEV: Temp variables in template metaprograms

[Walter L. <la...@na...>]

Hi,

I was looking at the example in the documentation of Blitz of ray
reflection.  It claims that if you start with

     typedef TinyVector<double,3> ray;
     ray x,y,n;

     y = x - 2 * dot(x,n) * n;

it becomes

    double _t1 = x[0]*n[0]+x[1]*n[1]+x[2]*n[2];
    double _t2 = _t1 + _t1;
    y[0] = x[0] - _t2 * n[0];
    y[1] = x[1] - _t2 * n[1];
    y[2] = x[2] - _t2 * n[2];

I've looked through the blitz library and it seems like what Blitz
really generates is

    y[0] = x[0] - 2*(x[0]*n[0]+x[1]*n[1]+x[2]*n[2]) * n[0];
    y[1] = x[1] - 2*(x[0]*n[0]+x[1]*n[1]+x[2]*n[2]) * n[1];
    y[2] = x[2] - 2*(x[0]*n[0]+x[1]*n[1]+x[2]*n[2]) * n[2];

That is, it does not seem to actually create a temporary variable.  Do
I just not understand Blitz correctly, or is it that the temporary is
an optimization performed by the compiler?  If so, is there any hope
that the compiler will be able to do such an optimization if the
objects in the array are more complicated (specifically, if I repeated
the above code with a TinyVector<Vector<double>,3>)?

Thanks,
Walter Landry
la...@sp...

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