Bit Aligning for Space and Performance Boosts

Question

In the book Game Coding Complete, 3rd Edition, the author mentions a technique to both reduce data structure size and increase access performance. In essence it relies

Answer 1

Such hand-optimizations are generally long dead. Alignment is only a serious consideration if you're packing for space, or if you have an enforced-alignment type like SSE types. The compiler's default alignment and packing rules are intentionally designed to maximize performance, obviously, and whilst hand-tuning them can be beneficial, it's not generally worth it.

Probably, in your test program, the compiler never stored any structure on the stack and just kept the members in registers, which do not have alignment, which means that it's fairly irrelevant what the structure size or alignment is.

Here's the thing: There can be aliasing and other nasties with sub-word accessing, and it's no slower to access a whole word than to access a sub-word. So in general, it's no more efficient, in time, to pack more tightly than word size if you're only accessing, say, one member.