Re: [ggt-devel] GMTL: Vector expression template code checked in

[Allen B. <al...@vr...>]

Patrick Hartling wrote:
> What is the fix for this:
> 
> gmtl/TriOps.h:60: error: no match for 'operator*' in 'gmtl::operator
> +(const
>    gmtl::VecBase<T, SIZE, R1>&, const gmtl::VecBase<T, SIZE, R2>&) [with
> T =
>    double, unsigned int SIZE = 3, R1 =
>    gmtl::meta::VecBinaryExpr<gmtl::VecBase<double, 3,
>    gmtl::meta::DefaultVecTag>, gmtl::VecBase<double, 3,
>    gmtl::meta::DefaultVecTag>, gmtl::meta::VecPlusBinary>, R2 =
>    gmtl::meta::DefaultVecTag]((+(+tri)->gmtl::Tri<DATA_TYPE>::operator[]
> [with
>    DATA_TYPE = double](2))) * one_third'
> 
> The code that is not compiling is this (from gmtl/TriOps.h modified to
> include gmtl/VecOps.h):
> 
> template< class DATA_TYPE >
> Point<DATA_TYPE, 3> center( const Tri<DATA_TYPE>& tri )
> {
>    const float one_third = (1.0f/3.0f);
>    return (tri[0] + tri[1] + tri[2]) * one_third;
> }
> 
> It seems as though the operator* overload in gmtl/VecOps.h isn't
> matching the above for some reason, but I can't figure out why.  I've
> tried calling the gmtl::Point<DATA_TYPE,3> constructor explicitly inside
> the parentheses performing the point addition, but that doesn't help.
> Based on what you say below, the above shouldn't be affected at all by
> the changes you just checked in, but I don't have a very good
> understanding of this stuff in general.

Strange, try using:

DATA_TYPE(one_third) and see what happens.

Teh strange part of this is the operator* above seems to have two 
VecBase params.  This should not be allowed.

-Allen

> 
>  -Patrick
> 
> 
> On Thu, 2004-09-16 at 14:51 -0500, Allen Bierbaum wrote:
> 
>>I just merged my work on expression templates from the work branch into 
>>the CVS head of GMTL.
>>
>>This is a *major* feature addition and re-working of the VecBase (Vec & 
>>Point) classes and all operators that work on them.
>>
>>This change uses expression templates to remove temporary objects from 
>>expression evaluation and potentially allow compilers to better optimize 
>>the code.
>>
>>For example, with the previous code:
>>
>>vec1 = vec2 + vec3 + (vec4 * 6)
>>
>>that would have created 3 temporary vectors.  In the new code the 
>>expression templates output code that directly computes the final value 
>>from the given operands.  This has shown at least a 2x performance 
>>increase in my tests.
>>
>>The code is far from perfect though and could definitely be optimized 
>>much further.  (especially by someone that knows intel assembly and has 
>>some patience)
>>
>>There is one common compiler error to be aware of.  It is not possible 
>>to pass expression templates where VecBase/Vec/Point objects are expected.
>>
>>Details:
>>
>>GMTL makes extensive use of expression templates.  These templates come 
>>into play when evaluating statement such as:
>>
>>gmtl::Vec3f vec1, vec2, vec3;
>>vec1 = vec2 + (vec3 * 6.0);
>>
>>When evaluating the expression on the second line, GMTL creates an 
>>expression template tree to fully evaluate this expression when = is 
>>called.  The details of this process are normally hidden from the user, 
>>but can come to the forefront when a method expects a Vec or Point 
>>argument and is passed an expression.  For example:
>>
>>void myMethod(gmtl::Vec3f vec)
>>{ ... }
>>
>>gmtl::Vec3f vec1, vec2;
>>
>>myMethod(vec1);           // Works correctly
>>myMethod(vec1 + vec2);    // Fails to compile
>>
>>The reason the second line fails to compile is that the type for "vec1 + 
>>vec2" is not a Vec3f.  It is an expression template that describes how 
>>to sum two Vec3f's. (You may expect an autoconversion to happen, but it 
>>does not because of template argument deduction rules).
>>
>>There are two solutions to this, either explicitly construct a temporary
>>object to pass to the method (which is what an auto conversion would do 
>>anyway) or write the method to handle expression templates.  This second 
>>option is not difficult, but does require a further understanding of 
>>GMTL then is documented here.
>>
>>Here is how the first option could be used:
>>
>>myMethod(gmtl::Vec3f(vec1 + vec2));     // Creates a temporary to pass in
>>
>>
>>
>>In closing: Give the code and try and let me know what you think. :)
>>
>>
>>-Allen


-- 
-- Allen Bierbaum                         al...@vr...
-- Research Assistant and PhD Candidate
-- VR Juggler Team                              www.vrjuggler.org
-- Virtual Reality Applications Center       www.vrac.iastate.edu