Re: [eigen] Best way of initializing a matrix of fixed size

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2017-03-08 18:02, Matthieu Brucher wrote:
> Well, the elements are not simple numbers, they are results of different
> expressions like these:
> https://github.com/mbrucher/AudioTK/blob/master/ATK/Preamplifier/Triode2Filter.cpp#L85

Some suggestions (no guarantee that any of them will work in your case):
* For a Vector4d (or Vector4f or anything smaller) you can also use the 
corresponding constructor:
    Vector F(f1,f2, g1,g2);
* If you use the comma-initializer don't initialize your matrix with 
Zero() before (your compiler may be smart enough to optimize it away, 
but that operation is not necessary).
* Sometimes it may be easier for the compiler if you initialize your 
matrix coefficient-wise (reduces readability though ...), i.e.,
    Matrix M; // don't zero-initialize unless most entries are 0
    M(0,0)= -(Ib_Vbe /* + ... */ );
    M(1,0) = M(0,1) = M(1,3) = M(3,1) = 0.0;
    M(2,1) = -( M(1,2) = (Ib_Vce + Ic_Vce));
    // ...

But as Gael said, make sure you compile with full optimization and check 
your assembly ...


Christoph



> 2017-03-08 13:56 GMT+00:00 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>
> > Strange, it usually compiles to trivial code, e.g.:
> >
> > void foo(Matrix4f &m)
> > {
> >   m <<  1,  2,  3,  4,
> >         5,  6,  7,  8,
> >         9, 10, 11, 12,
> >        13, 14, 15, 16;
> > }
> >
> > compiles to:
> >
> > movaps LCPI0_0(%rip), %xmm0    ## xmm0 = [1065353216,1084227584,
> > 1091567616,1095761920]
> > movups %xmm0, (%rdi)
> > movaps LCPI0_1(%rip), %xmm0    ## xmm0 = [1073741824,1086324736,
> > 1092616192,1096810496]
> > movups %xmm0, 16(%rdi)
> > movaps LCPI0_2(%rip), %xmm0    ## xmm0 = [1077936128,1088421888,
> > 1093664768,1097859072]
> > movups %xmm0, 32(%rdi)
> > movaps LCPI0_3(%rip), %xmm0    ## xmm0 = [1082130432,1090519040,
> > 1094713344,1098907648]
> > movups %xmm0, 48(%rdi)
> > popq %rbp
> > retq
> >
> > make sure you compiled with full optimization. If so, check the respective
> > assembly.
> >
> > gael
> >
> > On Tue, Mar 7, 2017 at 1:57 PM, Matthieu Brucher <
> > matthieu.brucher@xxxxxxxxx> wrote:
> >
> > > Hi,
> > >
> > > I'm wondering what is the best way of initializing a 4x4 matrix. I'm
> > > currently using the comma initializer, but it shows up quite high on a
> > > valgrind profile compared to taking the inverse of the same matrix or
> > > preprocessing the coeffs (pow, ln...).
> > > I'm using Eigen 3.2.4, but I can upgrade.
> > >
> > > Cheers,
> > >
> > > Matthieu
> > > --
> > > Information System Engineer, Ph.D.
> > > Blog: http://blog.audio-tk.com/
> > > LinkedIn: http://www.linkedin.com/in/matthieubrucher

--
 Dipl. Inf., Dipl. Math. Christoph Hertzberg

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