I do optimal structure (and bit) packing without much thought because it is an old habit. As the article states, I have noticed that the only other people that do habitual careful structure optimization these days have been doing low-level and high-performance code as long as I have. Most programmers are oblivious to it.

The reasons you would do it today are different than a decade ago and the rules have changed because the processors have changed. To add two clarifying points to the original article:

- The main reason to do optimal structure packing today is to reduce cache line misses. Because cache line misses are so expensive it is a big net performance gain in many cases to have the code do a little more work if it reduces cache line fills; optimal structure packing is basically a "free" way of minimizing cache misses.

- On modern Intel microarchitectures, alignment matters much less for performance than it used to. One of the big changes starting with the i7 is that unaligned memory accesses have approximately the same cost as aligned memory accesses. This is a pretty radical change to the optimization assumptions for structure layout. Consequently, it is possible to do very tight memory packing without the severe performance penalty traditionally implied.

What constitutes "optimal" structure packing is architecture dependent. The original C structure rules were designed in part to allow the structures to be portable above all else. If you design highly optimized structures for a Haswell processor, code may run much more slowly or create a CPU exception and crash on other architectures, so keep these tradeoffs in mind. The article is discussing basic structure packing which typically has easily predictable behavior almost anywhere C compiles.