NMatrix getter and setter way slower than Matrix

[lokeshh]

I'm using NMatrix for competitive programming for a while now and I have found NMatrix to be slower than Matrix for most of my programs. For example methods such as [], []= which are frequently used in my programs are slower in NMatrix.

Here's a benchmark I ran and following are the results:

       user     system      total        real
Matrix getter  3.700000   0.020000   3.720000 (  3.744560)
NMatrix getter 12.930000   0.060000  12.990000 ( 13.065677)
Matrix setter  2.920000   0.020000   2.940000 (  2.965361)
NMatrix setter 16.600000   0.060000  16.660000 ( 16.889194)

[v0dro]

Damn that sucks. Need some urgent performance fixes for nmatrix.

[wlevine]

@lokeshh Good catch! I think this is due to a lot of heap allocation of SLICE objects, which can be avoided. I will try to have a patch soon.

[wlevine]

For #[]=, I notice that we're spending a lot of time in nm::dense_storage::set(), allocating and freeing stuff. I'm not going to look into this any more at the moment, just noting this for future reference.

[translunar]

Thanks for looking into this, @wlevine. I concur with your explanation.