My coding spring project here at Sage Days 24 was/is to write a new library for dense linear algebra over small extensions of GF(2).

The main idea is to represent matrices over GF(2^e) internally as M4RI matrices over GF(2) in a row major flat representation. This allows to  re-use many of the M4RI functions such as addition of rows and matrices, stacking, augmenting etc.

For the elimination and multiplication base  cases we use a simple trick inspired by M4RI: Assume we found a pivot, now we need to rescale it and then add the right multiple of it to every row with a nonzero entry in the same column. If we have an n x n matrix we expect to perform n^2 multiplications and n^2 additions per pivot. However, if the field is small then we can precompute all possible multiplies of our pivot row and store them in a table. Then, we use this table as a lookup to perform the elimination of other rows. This costs 2^k * n multiplications and  n^2 additions per pivot. Note that M4RI has efficient functions to perform the look-up + elimination part.

We haven’t implemented any asymptotically fast methods yet but I hope to finish with Strassen multiplication this week. Asymptotically fast elimination requires slightly more infrastructure (TRSM, taking care of  offsets which are not zero % 64 etc.) If anybody is interested in helping out that would be  greatly appreciated. Also, I guess one should also try M4RI (the algorithm) for GF(2^2).

Some first results

In these plots red means that Magma is faster by a factor of e^abs(red) and blue means that our code is faster by a factor of e^abs(blue). More details can be found on the SD24 wiki page.

To get a sense for absolute numbers,  here are some preliminary benchmarks for row echelon forms of dense  random n x n matrices over GF(2^4)

|| n    || Sage 4.5 || NTL/2 || Magma || M4RIE (old)|| M4RIE (new) ||
|| 1000 ||    49.49 || 18.84 || 0.090 || 0.097      || 0.046       ||
|| 2000 ||   429.05 || 149.11|| 0.510 || 0.529      || 0.28        ||
|| 3000 ||  1494.33 ||526.57 || 1.640 || 2.315      || 1.00        ||

Bottom line it is much much faster (> 1,000x) than what we have in Sage right now (which is the generic implementation). Also note that switching to NTL would not improve things much.

I should point out that the strategy for multiplication in Tom and  Robert’s paper Bitslicing and the Method of Four Russians Over Larger Finite Fields is likely to be better.  Judging from the timings in that paper we are about a factor of two  behind them. I plan to implement/port their very cool trick for finite  extension fields at some point in the future. The trick is limited to  multiplication as far as I understand it thus it might make still sense to consider my representation for the elimination base case etc.

One thought on “M4RIE

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