struct-layout-analysis

# Struct layout: measure with pahole, pad with a multiplier ## Measuring — pahole, never throwaway sizeof programs The developer build (`-Og -ggdb`) already carries DWARF; read it instead of compiling sizeof/offsetof snippets: ```sh pahole -C <struct_name> build/lib/dns/libdns*.so.p/<file>.c.o pahole --holes -C <struct_name> <object-or-binary> ``` pahole prints member offsets, padding holes, total size, and cacheline boundaries — complete layout information the compiler already computed, with no boilerplate to write. Any object file under `build/` or the final binary works as input. ## Padding to cachelines — union arm with a plain multiplier Pad a struct to a whole number of cache lines with a union arm sized by a plain multiplier, always paired with a static assert: ```c struct foo { union { struct { /* members */ }; uint8_t __padding[ISC_OS_CACHELINE_SIZE * 6]; }; }; STATIC_ASSERT(sizeof(struct foo) % ISC_OS_CACHELINE_SIZE == 0, "struct foo size must be a multiple of the cacheline size"); ``` Pick the smallest `n` that fits; when members outgrow the arm to a non-multiple size, the STATIC_ASSERT fires and prompts a bump of `n`. Do NOT use the enumerated-sizeof trailing formula `uint8_t __padding[CACHELINE - (sizeof(a) + sizeof(b) + ...) % CACHELINE]` (the historical qpzone style): it must enumerate every member's sizeof, so adding a member silently miscounts unless the formula is updated in lockstep. The multiplier form has no such coupling and is easier to read.