2019-08-27-ModernCpp

Morton order matrices in C++

Rupert Nash

r.nash@epcc.ed.ac.uk

Source for this can be obtained from Github. Get a new copy with:

git clone https://github.com/EPCCed/2019-08-27-ModernCpp/

or update your existing one with

git pull

then you can

cd 2019-08-27-ModernCpp/exercises/morton-order

The Morton ordering (or z-ordering) of a matrix lays out the elements along a recursive z-shaped curve, as shown in the figure of four iterations of the Z-order curve (from Wikipedia).

You can compute the Morton index z from the x- and y-indices (i and j respectively) by interleaving their bits. An example is shown in the table.

	0	1	2	3
0	0	1	4	6
1	2	3	5	7
2	8	9	12	13
3	10	11	14	15

Mapping between x-y indexes and Morton index for a 4 by 4 matrix. Decimal on the left and binary on the right.

	00	01	10	11
00	0000	0001	0100	0101
01	0010	0011	0110	0111
10	1000	1001	1100	1101
11	1010	1011	1110	1111

Mapping between x-y indexes and Morton index for a matrix of size 4-by-4. Decimal on the left and binary on the right.

The advantage of laying out data in this way is that it improves data locality (and hence cache use) without having to tune a block size or similar parameter. On a modern multilevel cache machine[^1], this means it can take good advantage of all the levels without tuning multiple parameters.

(E.g. an ARCHER node has L1, L2, and L3 caches, and the RAM is divided into two NUMA regions. If using a PGAS approach one can view local RAM as a cache for the distributed memory - i.e. 6 levels!)

This exercise will walk you through a simple implementation.

I have included implementations of the functions that do the “bit-twiddling” for translating between a two-dimensional x-y index and the Morton index, in the file bits.hpp. These are reasonably fast, but can be beaten if you are interested to try!

In what follows each section corresponds to a subdirectory with the same number.

Implement the underlying data storage and element access

Go to the step 1 directory:

cd  2019-08-27-ModernCpp/exercises/morton-order/step1

Using the partial implemenation in matrix.hpp, your task is to implement the allocation (and release!) of memory to store the data and to use the helper functions from bits.hpp to allow element access. You will need to implement a number of member functions (marked in the source with \\ TODO) and add whatever data members are needed (marked in the same way).

There is a test program test_matrix_basic.cpp which runs a few sanity checks on your implementation (and similarly with test_bits.cpp). The supplied Makefile should work.

Implement a basic iterator to traverse the matrix in order

Go to the step 2 directory:

cd  2019-08-27-ModernCpp/exercises/morton-order/step2

I have a potential solution to part 1 here, but feel free to copy your implementation into this.

The exercise here is to complete the matrix_iterator class template that I have started. I’ve provided most of the boilerplate to have this work as a “bidirectional iterator”. See http://en.cppreference.com/w/cpp/concept/BidirectionalIterator for full details of what this means, but basically it’s one that can move forward and backward through the data.

Again, the things that need added are marked with \\TODO. The most important thing to think about is how you will refer to the current position and be able to traverse through it efficiently in Morton order - the performance should be identical to looping over a raw pointer!