No-one cares anymore about ancient computers or weird specialist systems of course, but don't neglect my important requirements by breaking compatibility with a

No-one cares anymore about ancient computers or weird specialist systems of course, but don't neglect my important requirements by breaking compatibility with any the platforms I am relying on at any point in time. We also need all of the most aggressive optimisations that compiler writers can come up with—this is high-performance code, after all!—but we certainly don't have time to deal with any breaking changes that would force revisions to our big important codebases. Make sure we can realise significant performance gains with just a simple recompilation. But remember to keep everything straightforward and close to the machine: we really hate all that weird UB which it's so easy to trigger by making an obvious, reasonable assumption which turns out to be wrong for some inexplicable reason.