Compiler Flags, Layout, and the Machine

What the optimizer may and may not do

At -O2 the compiler assumes no undefined behavior: signed overflow won't happen, pointers won't alias char buffers illegally, in-bounds access is guaranteed. UB lets it delete your "safety" checks — which is why UB hunting (module 12) is a performance topic too. It may reorder, vectorize, and inline — but it cannot fix O(n²).

Flags that matter, briefly

-O2 default shipping; -O3 marginal, sometimes worse; -march=native for your machine only; -fsanitize=… never in production; LTO links the optimizer across translation units (module 15 territory).

Layout micro-wins that are actually macro

• SoA over AoS when loops touch field subsets (module 10).
• std::vector over std::list almost always — contiguous beats node-based even with extra copying.
• Smaller structs → more records per cache line → fewer misses.
• Sorting before scanning improves branch prediction and lets early-exit algorithms fire.

None of these require cleverness — only measuring and choosing.