How do I choose the correct primative?

[Marco Leise]

C compilers like D compilers will pack a struct of two 16-bit
words into a 32-bit type if you don't force an alignment:
http://dlang.org/attribute.html#align
What you should avoid is having a data type start at an
address that is not a multiple of its size, especially when it
comes to SIMD.
Working with 16-bit values is not really supported in todays
x86 CPUs though, and integer math in D typically yields ints
even when you use smaller data types, reflecting what happens
on the hardware. Usually I use uint, size_t, real for things
that will go to CPU registers and the smallest data type that
will work for storage in memory.
Keep in mind that RAM access is slow compared to how fast CPUs
run. It can be beneficial to have "slower" data types if they
allow more data to fit into the CPU cache.

Typically you sort the fields of a struct by size with the
larger ones (e.g. pointers) at the top followed by ints,
shorts and finally bytes if you want to conserve memory. There
is even a template to do that for you, but I think it is more
of a toy, when you can easily do that manually without the
clutter:
http://dlang.org/phobos/std_typecons.html#.alignForSize