Counterexample: the struct in the article. It's "large enough that you care about shaving padding" in aggregate. The 2 optimal arrangements eliminate padding so

Counterexample: the struct in the article. It's "large enough that you care about shaving padding" in aggregate. The 2 optimal arrangements eliminate padding so it uses 1/16th of an x86-64 cache line. This guarantees the whole struct is on one cache line and uses the least total cache, so the 2 arrangements with padding are strictly worse, regardless of which fields are hot or cold. A struct of arrays form may be better still, but that's more than rearranging fields and beyond the compiler to produce. More generally, there are surely cases where it's better to arrange all the hot fields together at the expense of increasing the struct size, but I'm unsure it's the common case, and my bullshit alarm fires when people say so without evidence. (Even more so if they're also saying programmers are commonly arranging the fields optimally by hand.)