[gsg]

Constness of pointer or reference parameters has no optimisation effect because it doesn't convey any reliable information. It doesn't indicate whether the thing is written to in the body of the function (the compiler already has that information). It doesn't indicate whether the thing can be written to through another pointer (const parameters may be aliased). And it doesn't indicate whether the thing is written to through the same pointer by a callee (because the language contains const_cast).

Since the very thing that const is supposed to indicate is "no writes", and it doesn't do that, const annotations provide zero information. Thus they add no scope for optimisation.

constexpr isn't relevant to this issue. Since sane programs don't spend any appreciable time calculating constants, it's also rather uninteresting for the purpose of making programs run fast. As far as I can see its only practical purpose is to expand the set of fixed terms allowed as template arguments.

[dchichkov]

Would you guarantee that optimizers wouldn't use heuristics at IPO stage?

Either way, you are right, and my comment should have been: 1) constraint 2) providing clues to developers 3) providing clues to optimizer.

[gsg]

Without yielding useful information, const annotations can't hope to have any positive effect on compiler optimisations, fancy or not.

I agree that const is useful for the other reasons you mention.