[adzm]

> Which worked well unless compiled with `strict-aliasing` gcc optimizations enabled

I can't imagine enabling this by default instead of opting in with __restrict or equivalent. Just so many things that could go wrong if every little piece of code was not written with aliasing in mind.

[deathanatos]

The GCC flag is `-fno-strict-aliasing`, unless there is one I'm unaware of, which tells the compiler to essentially assume code might make aliasing mistakes.

> Just so many things that could go wrong if every little piece of code was not written with aliasing in mind.

You should always be writing "with aliasing in mind". It is a rule of the language, which specifies the "strict aliasing" that flag refers to, and it's UB to alias in ways which are not allowed. (Some aliasing is permitted by C. It's mostly type-punning that isn't.)

For computing the packet checksum, I'm not sure how you'd manage to run afoul of the strict-aliasing rule (you're just iterating over an array of octets … right?), but C is one of those "assume nothing" languages…